The Facts On File

DICTIONARYof

ECOLOGYand

THE ENVIRONMENT

The Facts On File

DICTIONARYof

ECOLOGYand

THE ENVIRONMENT

Edited byJill Bailey

®

The Facts On File Dictionary of Ecology and The Environment

Copyright © 2004 by Market House Books Ltd

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CONTENTS

Preface vii

Entries A to Z 1

Appendixes

I. SI Units 246

II. Webpages 247

Bibliography 248

vii

PREFACE

This dictionary is one of a series covering the terminology and concepts usedin important branches of science. The Facts on File Dictionary of Ecologyand The Environment is planned as an additional source of information forstudents taking Advanced Placement (AP) Science courses in high schools. Itwill also be helpful to older students taking introductory college courses.

This volume covers the topics important for an understanding of the basicprinciples of ecology - the scientific study of the relationships between or-ganisms and their natural environment. It also deals with the wider subjectof how human populations interact with and affect the environment as awhole. Environmental science is an area of study involving a number of dif-ferent disciplines besides biology – chemistry, geology, meteorology, humangeography, etc. There is particular interest in the effects of pollution and onconservation and the management of habitats. The definitions are intendedto be clear and informative and, where possible, we have provided helpfuldiagrams and examples. The book also has a selection of short biographicalentries for people who have made important contributions to ecology andenvironmental science. The appendixes contain a short list of useful web-pages and an informative bibliography.

The book will be a helpful additional source of information for anyonestudying the AP Environmental Science course, and also to students of APBiology. However, we have not restricted the content to this syllabus. Ecol-ogy and the environment are important to everyone, and we have tried tocover these subjects in an interesting and informative way.

ACKNOWLEDGMENTS

Consultant editor

Stewart Thompson B.Sc., Ph.D.

Contributors

John Clark B.Sc.Robert Hine B.Sc.Eve Daintith B.Sc.

abaptation The process by which evolu-tionary forces acting on ancestral formshave helped determine present-day adapta-tions of organisms to their environment.

abiotic Nonliving. Abiotic factors arethe physical and chemical aspects of an or-ganism’s environment, such as light, tem-perature, water, oxygen, and carbondioxide. They include climatic, EDAPHIC

and PHYSIOGRAPHIC factors. Compare bi-otic.

abscission The controlled separation ofa part of an organism from the rest. For ex-ample, the shedding of leaves in the fall andthe dropping of fruits is triggered bychanges in the balance of plant growth sub-stances (hormones).

absolute humidity See humidity.

absorbed dose The amount of energyabsorbed by a tissue or other substancefrom incident radiation. It is measured ingrays (Gy): one gray = the transfer of onejoule of energy to one kilogram of material.

absorption 1. The retention of radiantenergy by an object, e.g. by the pigments ofa photosynthetic organism.2. The uptake of small nutrient moleculesinto the body of an organism. In animalsthis takes place after digestion of largerfood molecules. In plants it includes the up-take of water and solutes.3. See adsorption.

abundance A measure of the number ofindividual organisms in an area. An assess-ment of FREQUENCY (the relative abundanceof each species present in an area), gives a

quick subjective estimate. COVER and abun-dance may be combined for a subjectivecommunity description (see Braun-Blan-quet scale). Quantitative measures ofabundance include DENSITY (the number ofindividuals in a given area), cover, or thefrequency of occurrence of a species in ran-domly placed QUADRATS.

abyssal zone The ocean-floor environ-ment between 4000 and 6000 meters indepth. It is characterized by extremely highwater pressure, low temperatures and nu-trient levels, and an almost total absence oflight. See also benthic zone.

acclimation See acclimatization.

acclimatization A reversible change inthe physiology or morphology of an organ-ism in response to changes in its environ-ment. The term is generally used in thecontext of the natural environment. Theterm acclimation tends to be used in a lab-oratory context.

acid A compound that acts as a protondonor in aqueous solution, i.e. it releaseshydrogen ions or protons, giving a pH lessthan 7. Acids turn litmus paper from blueto red, and react with alkalis (bases) toyield neutral salts. See pH. Compare alkali.

acid rain Rain with a very low pH(below pH 5.6 and often below pH 4), dueto pollution from oxides of nitrogen andsulfur released by the burning of fossil fuelsor other industrial emissions. These pollu-tants combine with water in the atmos-phere, forming nitric and sulfuric acids, aprocess that may be catalyzed by other pol-lutants such as ammonia, hydrogen perox-

1

A

ide, or ozone. Acid rain lowers the pH ofsoil, lakes, and rivers. The sulfuric acid re-acts with minerals in the soil to produceammonium sulfate, which liberates toxicions of aluminum and certain heavy met-als, which in turn inhibit metabolic reac-tions, cause the leaching out of certainplant nutrients from the soil, and inhibitthe uptake of others. Acid rain also has adirect effect on plant life, breaking downlipids and damaging membranes in leaves.When these ions leach into waterways,they damage the gills of fish and contami-nate drinking-water supplies.

acid soil A soil with a pH less than 7.0.Soils with pH below 5 are said to be ‘veryacid’, e.g. the surface horizons of someBROWN EARTH soils. High soil acidity maybe due to LEACHING of soluble salts, such ascalcium carbonates, in areas of high rain-fall. Also, in such climates very little clay(derived from the weathering of thebedrock) moves to the upper horizons ofthe soil because rain is continually perco-lating downward, so there are few cationsto exchange with the hydrogen ions. Alter-natively, acidity may be caused by the ac-cumulation of HUMIC ACIDS in waterloggedsoils. ACID RAIN has led to the acidificationof many soils. In cold regions acid soils re-sult from the accumulation of organicacids due to the slow decomposition of leaflitter in coniferous forests.

acre A unit of area. 1 British statute acre= 0.4047 hectare, or 4840 square yards. Informer times, different parts of the BritishIsles, and even different forests, had differ-ent standards for the acre, all larger thanthe current statute acre.

acrotelm The upper layer of blanketPEAT that forms in areas where the rainfallis so high that decomposition of plant re-mains is seriously retarded. The acrotelm isabove the average level of the WATER TABLE,so is saturated only from time to time. As aresult, air can enter it, triggering active de-composition of the peat, and the break-down of the plant remains into humicacids. When not saturated, water can flowreadily through the actrotelm, but heavy

surface RUNOFF may cause peat erosion ifthe peat gets very dry and loose. The layerbetween the acrotelm and the impermeablemineral subsoil below is called thecatotelm. It is permanently saturated, withno water flow and very little oxygen. Thepeat here is very dense and remains unde-composed. The boundary between acro-telm and catotelm varies with the level ofthe water table, which determines whetherthe peat is breaking down or building up.See blanket bog.

Actinobacteria (actinomycetes; ray fungi)A phylum of EUBACTERIA that includesrodlike coryneform bacteria and branchingfilamentous true actinobacteria. They pro-duce thick-walled resistant resting sporescalled actinospores. Most actinobacteriaare aerobic SAPROBES, but some species arePATHOGENS of humans, other animals, andplants. Some are involved in lichenlikesymbioses. See lichens.

actinomycetes See Actinobacteria.

activated carbon Charcoal that hasbeen heated in a vacuum to drive off ab-sorbed gas. Activated carbon can absorblarge quantities of gases, and is used to re-move solvent vapors and clarify liquids; itis also used in gas masks.

activated sludge process A SEWAGE

treatment process during which sludgemixed with sewage is aerated by means ofmechanical agitation or compressed air(see aerator). The aeration encourages thegrowth of microorganisms that breakdown the sludge into a suspension of or-ganic solids called activated sludge, andabsorb organic compounds from thesewage (primary effluent). After severalhours, the mixture is allowed to flow intoa further settling tank, the clarifier, wherethe activated sludge settles out and theclear water above is disinfected and dis-charged as secondary effluent. Some of thesludge is recirculated into the aerator, andmixed with more raw sewage. The remain-der of the sludge is treated and disposed of.

active transport The transport of mol-

acid soil

2

ecules or ions across a cell membraneagainst a concentration gradient, with theexpenditure of energy. The mechanism in-volves a carrier protein that spans themembrane and transfers substances in orout of the cell by changing shape.

acylglycerols See lipid.

adaptation 1. A genetically determinedcharacteristic, such as a structure, physio-logical process or behavior, that makes anorganism better suited to its environment,thereby increasing its survival chances.NATURAL SELECTION will act on the underly-ing GENOTYPE: individuals possessing fa-vorable adaptations will leave moresurviving offspring in successive genera-tions, so the adaptation will graduallyspread through the population.2. Changes in form and/or behavior of anorganism during its lifetime in response toenvironmental stimuli. For example, thedevelopment of sun and shade leaves onthe same tree. See also acclimatization.3. Changes in the excitability of a senseorgan in response to continuous stimula-tion. For example, continued repetition ofa stimulus may lead to a decrease in re-sponsiveness.

adaptive function A mathematicalfunction that combines phenotypic FITNESS

in different environments to give a measureof the overall fitness of a PHENOTYPE in aheterogeneous environment. Where organ-isms encounter patches of habitat of type iin direct proportion to their frequency (pi),the overall fitness of a phenotype (W) is theaverage of its fitnesses in each habitat typeweighted by their frequencies:

W = p1W1 + p2W2If a graph of fitness in each of two envi-

ronments is plotted, the adaptive functionforms a line of equal fitness whose positionon the graph is determined by the relativefrequencies of the two environments. Theslope of the adaptive function will varyfrom place to place as the relative frequen-cies of the different types of HABITAT

PATCHES vary.The above equation is appropriate in

spatially different environments. When ap-

plied to temporally varying environments,where habitat changes with time ratherthan space, the adaptive function is mostaccurately represented by a geometricmean, since fitness in this case is expressedas changes in numbers of individuals witheach phenotype from one generation to thenext. The average fitness of a phenotype isthen:

W = W1p1W2p2See patch dynamics; patchiness.

adaptive radiation The process bywhich new species and varieties adapted tospecialized modes of life arise from a com-mon ancestor. This may happen when aspecies is introduced into a new environ-ment, when competitors are eliminated bya catastrophe, as happens after mass ex-tinctions, or populations become isolatedfrom competition by forces such as moun-tain-building, inundation, or continentaldrift. Examples include the evolution oflemurs from primitive primates on the is-land of Madagascar, and the diversifica-tion of primitive mammals into terrestrial,arboreal, and aquatic forms during theTertiary period.

adenosine triphosphate See mitochon-drion; respiration.

adiabatic Without gain or loss of heat,i.e. without exchange of heat with the sur-roundings.

adiabatic cooling and heating Thecooling of rising air associated with its ex-pansion as atmospheric pressure declineswith increasing altitude. For example, inrising columns of air, the volume of air in-creases as the pressure of the atmosphereabove it decreases with increasing altitude.This cooling effect is usually about 1°C per100 m for dry air, 0.5°C per 100 m for sat-urated air (because moisture condenses outof the air, releasing latent heat). This tem-perature change affects the rate at whichthe air rises relative to adjacent bodies ofair, the stability of the air column, andcloud formation. The law of adiabatic ex-pansion or compression states that a gaswill cool if allowed to expand freely from a

3

adiabatic cooling and heating

higher pressure to a lower pressure withoutthe transfer of external energy to the gas.Similarly, it will heat if compressed from alower to a higher pressure in the absence ofenergy transfer from the gas.

ADP See respiration.

adsorption The adhesion of moleculesto the surface of solids or liquids. Inadsorption, the molecules remain on thesurface. In ABSORPTION, the molecules per-meate into the bulk of the material.

advection The horizontal movement ofa mass of air or liquid (e.g. air or water cur-rents), or the transport of substances (e.g.pollutants) by such movement. Transportof energy (such as heat) in this way is calledadvective energy.

adventitious roots Roots that do notarise in the normal position on the plant.For example, they may arise from the stem,as in the rooting of runners and bulbs, orfrom the leaf, as in certain begonias, whichcan propagate from leaf cuttings.

aeolian See eolian.

aerator A tank, usually of concrete,through which air is passed during thetreatment of sewage by the ACTIVATED

SLUDGE PROCESS. Compressed air may bepassed through porous diffusers at the baseof the aerator, in which case the bubblingof the air mixes and oxygenates the mix-ture, or from the atmosphere, mixed intothe sewage using mechanical propellers atthe surface.

aerenchyma A kind of tissue found inmany aquatic plants, such as water lilies(Nymphaea spp), which contains manylarge intercellular air spaces that aid thediffusion of oxygen to the roots and mayalso increase buoyancy.

aerial root A root that arises above thelevel of the soil. Most aerial roots are ad-ventitious roots, arising from stem tissue,rather than true lateral roots. Aerial rootsare typical of EPIPHYTES, and may hang

down into the air to absorb moisture fromit, as in some orchids; the epidermal layersof these roots develops a sheath of emptycells, the velamen, that acts rather like asponge. The prop roots or STILT ROOTS ofplants such as maize and some palms,which arise above the soil and grow downinto it, provide extra support, as do BUT-TRESS ROOTS. Breathing roots, or pneu-matophores, found in some MANGROVES

and other plants of waterlogged areas, arerich in lenticels and serve to get oxygeninto the root. Some of these are true aerialroots, arising above the ground, whereasothers are lateral roots that grow up intothe air.

aerobe An organism that uses aerobicRESPIRATION and can live and grow only inthe presence of free oxygen. Compareanaerobe.

aerobic 1. Describing an organism thatrequires oxygen for respiration.2. Describing an environment that containsfree oxygen.

aerobic respiration See respiration.

aerodynamic method A techniqueused to estimate potential EVAPOTRANSPIRA-TION from a surface (for example, a lake oran area of vegetation) that takes into ac-count both the supply of heat energy andthe transportation of water vapor awayfrom the evaporating surface by air move-ments.

aerosol 1. A dispersion of liquid or solidparticles in a gas, e.g. mist, smoke.2. A pressurized container with a spraynozzle use to dispense aerosols such as de-odorants and insecticides. The propellentgases used in many such aerosols are po-tent POLLUTANTS.

aesthetic injury level The level of vis-ible injury to plants or other commoditiesat which it is deemed desirable to applypesticides.

afforestation The establishment of aforest on land not previously forested. This

ADP

4

may be by natural colonization by trees orby deliberate planting.

age class The individuals in a popula-tion of a particular age.

age distribution The number of indi-viduals of each age in a population. Whenthe birth rate and survival rates for eachage remain unchanged for a long period oftime, the population acquires a stable agedistribution and the population will growor decline at a constant rate per head.When the population is not changing insize, the stable age distribution is called thestationary age distribution. Such a popula-tion will usually have more older and feweryounger individuals than a growing popu-lation.

Agent Orange A defoliant used in theVietnam War. Composed of esters of tri-chlorophenoxyacetic acids, it was heavilycontaminated with PCDDs (polychlori-nated dibenzodioxins) and PCDFs (poly-chlorinated dibenzofurans). These have aharmful effect on human reproduction, de-velopment, and immunity to disease. Theyare also suspected to be carcinogenic.Agent Orange is a persistent POLLUTANT,especially in the marine environment. Seealso dioxin.

age structure The relative proportionsof a population in different age classes. Agestructure affects the rate at which a popu-lation grows; a population with only asmall proportion of individuals of repro-ductive age will grow relatively slowly. Thegrowth rate of the population is the sum ofthe growth rates of the individuals in eachage class weighted by the proportion ofeach age class in the population. Thesmaller the range of age in each age class,the more accurate the growth rate calcu-lated in this way.

aggregate 1. A rock consisting of min-eral or rock fragments.2. A clump of soil particles, ranging frommicroscopic granules to small crumbs,which forms the basic structural unit ofSOIL.

aggregated dispersion See dispersion.

aggregate size distribution The pro-portions of surface soil aggregates of dif-ferent sizes in a given soil. The aggregatesize distribution is affected by the applica-tion of fertilizers, the soil tillage system,and by wind erosion. It affects soil proper-ties such as aeration, drainage, and waterretention. See soil erosion; soil structure;soil texture.

aggregate species A group of species soclosely related that it is very difficult to dis-tinguish between them in the field. Suchspecies usually arise through HYBRIDIZA-TION between closely related species. Anexample is the blackberry (Rubus frutico-sus agg.).

aggregation of risk A distribution ofrisk of attack from predators in which insome habitat patches the prey are at greateror lesser risk of predation than would beexpected by chance alone. The term is usu-ally applied to the risk of a host speciesbeing attacked by a PARASITOID. See alsoaggregative response.

aggregative response The response ofa predator to prey density in which itspends more time in habitat patches withhigher densities of prey, leading to higherpredator densities in these patches, or inpatches with lower densities of prey, lead-ing to lower densities of predators in thesepatches. See also aggregation of risk.

aggression A behavior pattern that in-timidates or injures another organism, ex-cept for the purpose of predation.Aggression ranges from fighting over dom-inance in social animals to the singing of abird in defense of its territory.

aggressive mimicry See mimicry.

agonistic Describing behavior that aidssurvival, including AGGRESSION and defen-sive and avoidance behaviors. Such behav-ior as defense of territory, for example,may help an animal obtain sufficient foodor access to mates, both vital to survival. In

5

agonistic

its purest sense, agonistic behavior im-proves the chances of survival of the indi-vidual’s genes, so it can also include certainkinds of cooperative behavior between re-lated individuals. See territoriality.

agricultural potential See land-use ca-pability.

agricultural potential maps See map.

Agricultural Revolution The changesome 10–12 000 years ago from a nomadicpastoral way of life to a more settledlifestyle based on the cultivation of crops,especially cereals. This began in the MiddleEast and western Asia, and in the Nile val-ley. It led to the growth of towns and cities,and to the development of more complex,less portable tools and technologies, morecomplex societies, and the more rapid evo-lution of human culture.

agroforestry An integrated system offarming in which herbaceous crops andtree crops are cultivated simultaneously onthe same patch of land.

agronomy A branch of agriculture thatdeals with the theory and practice of cropproduction and the management of soils.

air The mixture of gases that forms theATMOSPHERE.

air pollution See pollution.

air temperature The temperature of theatmosphere at a specific altitude in a spe-cific location, or the temperature of air in aconfined space such as a room or a reactionvessel. On a large scale, the NASA GISSsurface temperature is a measure of thechanging global surface temperature de-rived from data from many meteorologicalstations around the world as well as (morerecently) satellite data for sea-surface tem-perature. See also global warming; green-house effect.

alarm responses Animal responses todanger that serve to warn other animals.They may be visual, e.g. the flash of the

white underside of the tails of rabbits orwhite-tailed deer as they run for cover. Au-ditory signals such as the alarm calls ofbirds are often recognized by many differ-ent species and function even in dense for-est. Injured animals may release alarmchemicals; for example, minnows andaquatic snails release chemicals into thewater that alert other members of the samespecies.

alary polymorphism See polymor-phism.

albedo The fraction of incident lightthat is reflected by a surface or body, suchas a cloud or planet. Albedo is an impor-tant parameter in calculations of energybalance.

aldrin A chlorinated hydrocarbon usedas an insecticide. Its use is being discontin-ued because it is toxic to warm-bloodedanimals through its effects on the centralnervous system.

algae A diverse group of photosyntheticeukaryotes, most of which are not highlydifferentiated into tissues and organs whencompared with lower plants. They lack dis-tinguishable roots, stems, or leaves andthere is no true vascular system. The keyfeatures distinguishing them from plants(kingdom PLANTAE) is the lack of a layer ofsterile cells surrounding the reproductiveorgans, and lack of an embryo stage duringdevelopment. The algae range from single-celled organisms to large seaweeds over 50m long showing distinct tissue differentia-tion. They are placed in the kingdom PRO-TOCTISTA.

algal bloom See bloom.

alkali A substance that dissolves inwater to give a solution of pH greater than7. Alkalis are usually soluble hydroxides orcompounds that release hydroxyl (OH–)ions in solution. They turn litmus paperfrom red to blue and react with ACIDS toyield neutral salts. See pH.

alkaline soil A soil with a pH greater

agricultural potential

6

than 7.0. Soils with pH above 9 are said tobe ‘strongly alkaline’. Such soils are usuallyrich in calcium ions, and are often derivedfrom rocks such as limestone or chalk,which are mostly composed of calcium car-bonate.

alkylmercury fungicides Mercury-basedorganic compounds once widely used asFUNGICIDES, especially on seeds, paint, andpaper. They are skin irritants, and can alsoaffect the central nervous system, some-times with fatal results. Mercury from thedegradation of such fungicides can accu-mulate in fish including edible species suchas salmon, swordfish, and tuna. The use ofalkylmercury fungicides has been prohib-ited in the United States since the early1970s. See also bioaccumulation.

allele (allelomorph) One of several alter-native forms of a particular gene. Differentalleles often have different effects on thephenotype. Where more than one allele ispresent in an individual, the phenotypicoutcome depends on which alleles are re-cessive or dominant: a dominant allele isexpressed regardless of which other allelesare present, whereas a recessive allele is ex-pressed only in the absence of dominant al-leles. In a few cases, more than one allelemay be dominant, and the phenotypic out-come is a blend of the expression of thetwo alleles. For example, codominant alle-les for white and red flower color respec-tively may, when present together, give riseto pink flowers. See gene; phenotype.

allelochemical See allelopathy.

allelopathy The release by a plant of achemical (allelochemical) that poisons orinhibits the growth of nearby plants, so re-ducing COMPETITION.

Allen’s rule The generalization thatmammals from cold climates have shorterextremities (ears and limbs) that otherwisesimilar mammals from warmer climates.See also Bergmann’s rule.

alliance See association.

allochthonous Describing materialstransported into a community or ECOSYS-TEM from outside, for example mineralsand organic matter carried by streams,lakes, and oceans but derived from adja-cent terrestrial ecosystems.

allogenic succession See succession.

allopatric speciation See speciation.

allopolyploid See polyploidy.

alluvial Found in or derived from riveror stream deposits. For example, an allu-vial fan is a fan of debris dropped by astream or river where it experiences achange of gradient and flow rate slows, sothe water can no long carry so large a load.Alluvial soil is formed on river floodplainsand deltas where new sediment is de-posited on the surface at each successiveflooding. The repeated addition of nutri-ents in this way can lead to very fertilesoils.

alpha decay The spontaneous emissionfrom a radioactive isotope of an alpha par-ticle – a positively charged particle consist-ing of the nucleus of a helium atom,comprising 2 neutrons and 2 protons.

alpine tundra See tundra.

alternation of generations The occur-rence of alternating haploid and diploidindividuals in the life cycle of an organ-ism. In bryophytes, vascular plants, many algae, and some fungi a haploid ga-mete-producing sexual phase, the gameto-phyte, alternates with a diploid spore-producing asexual phase, the sporophyte.In bryophytes the dominant generation isthe gametophyte, which forms the mainplant body. In vascular plants the sporo-phyte is dominant. Sporophyte andgametophyte may or may not be morpho-logically similar, depending on the species.Among animals, many invertebrates, suchas jellyfish and flatworms, show alterna-tion of sexual and asexual generations, butthere are no distinct haploid and diploidgenerations.

7

alternation of generations

alternative energy Energy derived fromrenewable sources, such as wind and wavepower, solar energy (see solar power), hy-droelectric power, geothermal energy, hy-drogen-based FUEL CELLS, biomass energy,and wood energy. These technologies donot draw on finite resources and are gener-ally perceived to be nonpolluting.

ambient Present on all sides; e.g. theambient temperature is the temperature ofthe surrounding air.

amensalism An interaction betweentwo individuals or species in which one isadversely affected by the other. An exam-ple is the production of antibiotics by cer-tain molds to kill or inhibit the growth ofnearby bacteria. Such associations betweenmicroorganisms are termed antibiosis.Compare commensalism; mutualism.

amino acids Organic acids that containan acidic carboxylate group (–COOH) anda basic amino group (–NH2) in their mol-ecules. Proteins are formed from chains ofamino acids and there are twenty of thesethat occur naturally. In adult humans tenof the twenty amino acids can be synthe-sized by the body itself. Since these are notrequired in the diet they are known asnonessential amino acids. The remainingten cannot be synthesized by the body andhave to be supplied in the diet. They areknown as essential amino acids. Variousother amino acids fulfill important roles inmetabolic processes other than as con-stituents of proteins. For example, or-nithine and citrulline are intermediates inthe production of urea.

ammonia NH3. A colorless pungentgas, highly soluble in water and alcohol. Itis highly reactive in solution, reacting withmost acids to form salts, and with metals toform nitrides. Ammonia is a by-product ofthe breakdown of proteins and aminoacids in the body. Highly toxic to cells, it isexcreted directly by fish and many am-phibians, dissolving in the surroundingwater. Terrestrial animals convert it to the less toxic urea (CO(NH2)2) during de-amination in the liver. Ammonia is formed

naturally during the decomposition of pro-teins, purines, and urea by bacteria. In thesoil, nitrifying bacteria (see nitrification)such a Nitrosomonas convert ammonia tonitrites, an essential stage in the NITROGEN

CYCLE. See also Haber process.

Amphibia The class of vertebrates thatcontains the most primitive terrestrialtetrapods – the frogs, toads, newts, andsalamanders. Amphibians have four pen-tadactyl limbs, a moist skin without scales,and a middle-ear apparatus for detectingairborne sounds. They are cold-blooded(exothermic) and the adults have lungs andlive on land but their skin, also used in res-piration, is thin and moist and body fluidsare easily lost, therefore they are confinedto damp places. In reproduction, fertiliza-tion is external and so they must return towater to breed. The eggs are covered withjelly and the aquatic larvae have gills forrespiration and undergo metamorphosis tothe adult.

amphidromous See diadromous.

Amphipoda (amphipods) An order ofsmall crustaceans (see Crustacea), manywith flattened shrimplike bodies and longhairy antennae. They breathe by means ofgills, and are found in lakes, rivers, oceans,muddy parts of the ocean floor, sandybeaches, caves, and moist tropical islandhabitats. Amphipods include sand fleas(sand hoppers) and scuds. Most species areherbivores or scavengers, but some havepiercing and sucking mouthparts and areexternal parasites of larger invertebratessuch as cnidarians and sponges. They areimportant sources of food for larger inver-tebrates, fish, seabirds, shore birds, smallwhales, seals, and sea lions.

anabolism The metabolic synthesis ofcomplex molecules from simpler ones.Compare catabolism.

anadromous See diadromous.

anaerobe An organism that is able tolive in the absence of oxygen. Obligateanaerobes are unable to live if free oxygen

alternative energy

8

is present. Facultative anaerobes are ableto live in the presence of free oxygen butcan survive in anaerobic conditions byusing anaerobic RESPIRATION. Of these,some (e.g. denitrifying bacteria) never useoxygen for respiration, whereas others useit if it is available (e.g. yeasts). Compareaerobe.

anaerobic respiration See respiration.

analysis of variance (ANOVA) A sta-tistical analysis that tests the significance ofdifferences between experimental samples.In its strict sense, the term is applied onlyto tests for differences between more thantwo variables. Observed differences be-tween the samples are compared with thevalues that would obtain if the differencesbetween the samples had arisen purely bychance (the NULL HYPOTHESIS). While T-TESTS, correlation analysis, and regressionanalysis can be applied to two samples,other tests are needed where more thantwo variables are involved. Frequency dis-tributions – plots of the number of sampleswith each given value – can be tested by theCHI-SQUARED TEST or by multiple correla-tion or multiple regression analysis.

anatomy See morphology.

anchorage A means of securing an or-ganism to a substrate, e.g. the holdfasts ofseaweeds such as kelps or the adhesive diskof sea anemones.

anemometer See wind.

Angiospermophyta (angiosperms; flow-ering plants) A phylum of vascular seedplants that bear flowers. Most also pro-duce fruits (seeds enclosed in carpels). Thedominant generation is the sporophyte,which may be herbaceous or woody. Inmost angiosperm species the XYLEM con-tains vessels and the PHLOEM has compan-ion cells associated with the sieve tubeelements. The flower bears the reproduc-tive organs, and often has associatedbrightly colored sepals and/or petals. Thegametophyte generation is highly reduced,consisting of the female embryo sac inside

the ovule and the male cells of the pollengrain, contained in the anthers of the malestamens. The pollen may be transferredfrom one flower to another by wind, water,insects, or other animals (see pollination).The ovary, which contains the ovules, con-sists of one or more carpels, thought tohave evolved from folded leaflike struc-tures. A specialized extension of eachcarpel, the stigma, bears a receptive surfaceon which the pollen germinates, growing apollen tube down to the ovule. The non-motile male gametes are released into theovule, and double fertilization takes place:one male gamete fertilizes the ovum, whilethe other fuses with two of the haploid fe-male nuclei, the polar bodies, giving rise toa triploid tissue (having three sets of chro-mosomes per nucleus), the endosperm. Theovules develop into seeds, which containstored food in the form of endosperm orswollen first leaves (cotyledons). The ovarywall and/or other flower parts may bemodified to form specialized fruits adaptedfor animal, wind, water, or explosive dis-persal. The production of the embryo in-side a tough resistant seed coat and theelimination of the need for water to enablethe male sex cells to reach the females en-abled the angiosperms to become highlysuccessful terrestrial plants. Angiospermsare divided into two major classes: di-cotyledons (characterized by having twocotyledons in the seed and net-veinedleaves) and monocotyledons (characterizedby having one cotyledon in the seed andleaves with parallel veins).

angiosperms See Angiospermophyta.

anhydrobiosis (cryptobiosis) The abil-ity to survive the drying up of the habitatby means of dormancy or estivation.

Animalia The kingdom that containsanimals. Animals are heterotrophic, multi-cellular EUKARYOTES. They are often motileand respond to stimuli rapidly. They lackcellulose cell walls and chlorophyll andgrowth is usually limited. See heterotroph.

anion A negatively charged ion, e.g. thehydroxyl ion (OH–). Compare cation.

9

anion

Annelida A phylum of segmentedworms that includes the earthworms, bris-tle worms, lugworms, and leeches. Theyare found in aquatic, marine, and terres-trial environments. Annelids are distin-guished by series of external ringlikesegments running the length of the longsoft cylindrical body. The body is coveredin a cuticle of chitin, and most annelidshave segmentally arranged bristles calledchaetae, which are used in locomotion.The body wall contains layers of longitudi-nal and circular muscle and the body cav-ity (coelom) isolates the gut from the bodywall. The gut runs from the mouth to anus,there are well-developed blood and ner-vous systems, and nephridia for excretion.Many annelids are hermaphrodite.

The class Oligochaeta comprise the ter-restrial earthworms, and many freshwaterspecies. Ranging in length from a few mil-limeters to over 3 m they have relativelyfew chaetae. All oligochaetes are hermaph-rodite. Most are scavengers, feeding on de-caying organic matter. Earthworms (e.g.Lumbricus) live in burrows and eat soil,extracting the digestible matter and ex-pelling mineral particles. Their burrowsand feeding habits are extremely importantin maintaining aeration and drainage insoils, turning over soil layers and mixingnutrient-rich and oxygenated layers withthe rest of the soil. Some aquatic speciesalso feed on living microscopic organisms.

annual 1. Occurring every year or oncea year.2. Covering a 12-month period, as in rain-fall statistics, for example.3. A plant species that completes its lifecycle within 12 months, especially a plantthat germinates, flowers, sets seed, and dieswithin a single year or growth season.Compare biennial; ephemeral; perennial.

annual rings Rings of wood and barkproduced by a tree during the growing sea-son each year. In temperate climates, theproduction of new wood begins in springand ends in the fall. Wood is made up ofXYLEM, a water-conducting tissue whosecells lose their protoplasm at maturity, be-coming hollow. In many species of north

temperate trees the xylem vessels producedat the start of the growing season are muchlarger than those developed toward the endof the season, their larger lumens allowingmore rapid conduction of water. This dif-ference is partly the result of better wateravailability and lower transpiration rates,and partly because of hormone changes.Thus, early wood has large diameter cellswith thin cell walls and later wood the con-verse. This gives rise to alternating lightand dark rings of wood respectively, onelight and one dark band usually represent-ing one year’s growth. Annual rings givevaluable information about past climatesand may also be used to date wood. Seedendrochronology; dendroclimatology.

ANOVA See analysis of variance.

antagonistic pleiotropy See senes-cence.

antagonistic resource See resource.

Antarctic The entire area of the Earth’ssurface south of latitude 66°30′ S. Com-pare Arctic.

Antarctic Circle The line of latitudearound the Earth at 66°30′ S. It marks thenorthern limit of the area within which forone day or more each year the sun does notset (winter solstice, December 21 or 22) orrise (summer solstice, June 21 or 22). Theperiod of continuous day or night increasessouthward from the Antarctic Circle,reaching six months at the South Pole.Compare Arctic Circle.

Anthozoa A class of the phylumCnidaria containing the corals and seaanemones, in which the polyp is the onlybody form. Sea anemones are solitary ani-mals that trap food using a ring of featherytentacles that eject stinging cells to para-lyze their prey. Corals are colonial, polypsbeing embedded in a gelatinous, horny, orcalcareous matrix. Accumulations of colo-nial corals with calcareous skeletons formreefs in warm shallow waters.

anthropogenic Resulting from the ac-

Annelida

10

tions of humans. The term is usually ap-plied to environmental changes such ashabitat change or pollution.

anthropogenic extinction See extinc-tion.

anthropology The study of humans, in-cluding their diversity of culture and phys-ical characters, evolutionary history andphylogeny, and geographical distribution.

antibiosis See amensalism.

antibiotic A specific chemical producedby a living organism, usually a microor-ganism, that inhibits the growth of or killsanother microorganism.

anticyclone A circulating air mass inwhich atmospheric pressure increases to-ward the center, around which winds cir-culate in a clockwise direction in anorthern hemisphere and a counterclock-wise direction in the southern hemisphere.Anticyclones give rise to calm fine weather,perhaps with fog in winter. Compare de-pression.

antifouling paints Paints applied to thehulls of boats to prevent build up of algaeand marine invertebrates. Some of these,such as TRIBTUYL TIN OXIDE (TBTO), arecauses of reproductive disruption in ma-rine invertebrates.

antioxidant A compound that preventsor delays the oxidation of other com-pounds. Industrial antioxidants includevegetable oils and phenol derivatives usedto reduce the speed of drying of paints ordelay the oxidation of plastics, rubbers,drugs, and other synthetic substances aswell as foodstuffs.

ants Small insects of the order Hy-menoptera, family Formicidae, distributedworldwide, but especially in warm cli-mates. Most ants are social insects living incolonies of nonbreeding workers domi-nated by a reproductive queen ant. Malereproductives are produced usually once ayear, and die after the mating flight. The

fertilized female removes her wings andfounds a new nest. Larger soldier antsguard the colony and the foraging workers.Some ants build moundlike nests of soil,sand, plant material, and even dung. Oth-ers live in complex underground nests.

Ants form more than half of the totalinsect biomass in the world, and almost athird of the total animal biomass in tropi-cal rainforest. In certain habitats, they arethe main predators, scavengers, and turn-ers of soil, playing an important role in de-composition and in maintenance of soilaeration, drainage, and fertility.

aphids Small sap-sucking, soft-bodiedinsects of the order Hemiptera, familyAphididae, of average length about 2.54mm. They include greenfly and blackfly.Aphids can reproduce rapidly by PAR-THENOGENESIS. In most species wingless fe-males produce live female young byparthenogenesis throughout the summer.When overcrowding occurs, winged formsarise and colonize new plants. Aphids areserious economic pests of crops and green-house plants, stunting plant growth or in-hibiting flower and fruit production, andtransmitting pathogenic bacteria and fungior viruses. They can be controlled by insec-ticides, or by biological control, usually in-volving ladybirds, the larvae and adults ofwhich prey on aphids.

aphotic zone The part of a body ofwater in which light intensity is too low forphotosynthesis to take place. Comparephotic zone.

apical meristem See meristem.

apomixis 1. Any kind of ASEXUAL RE-PRODUCTION, including VEGETATIVE PROPA-GATION and PARTHENOGENESIS.2. The production of seeds without priorfertilization. This may occur by means ofthe formation of a diploid embryo or em-bryo sac by a somatic (non-reproductive)cell, or from an ovum that has not under-gone meiosis. A plant that undergoesapomixis is termed an apomict. Apomixisis common among polyploid plants (seepolyploidy) (e.g. triploids in which ho-

11

apomixis

mologous chromosomes cannot pair prop-erly at meiosis) that are unable to repro-duce sexually; these are obligate apomicts,e.g. some cultivated varieties of banana(Musa) and garlic (Allium). It also occursin species near the edge of their geographi-cal range, where environmental conditionssuch as daylength are not favorable for sex-ual reproduction or where insect pollina-tors are absent; these are facultativeapomicts, e.g. some Potentilla species.Apomixis also permits the formation ofHYBRID SWARMS. See asexual reproduction;hybrid.

aposematism The means by which apoisonous or otherwise dangerous organ-ism advertises its dangerous nature to po-tential predators. The commonest form iswarning coloration: bright contrasting col-ors such as black and yellow, orange, orred makes it easy for a potential predatorto recognize the organism as unfavorableprey, deterring it from attacking. Somespecies instinctively avoid animals withsuch coloring.

apostatic prey selection See preda-tor–prey relationships.

apparent competition See competi-tion.

aquaculture The culture of fish, shell-fish, or underwater plants in natural orcontrolled freshwater or marine environ-ments. Aquaculture may take place in en-closed shallow coastal areas, in cages at orbelow the surface of the open sea, in fresh-water ponds, or in large tanks. It is a com-mercially important method of producingseafood such as mussels, oysters, clams,lobsters, crawfish, and shrimps, and fishsuch as salmon, tilapia, and catfish. Sea-weeds may be grown in similar situations.The method of growing plants in nutrientsolution instead of soil is called HYDRO-PONICS.

aquatic 1. Living or taking place inwater.2. A plant or animal that lives in water.

aquatic biome see biome.

aquiclude A mass of impermeable rockthat forms a barrier to the movement ofgroundwater. Compare aquifer.

aquifer A body of permeable rock ordrift saturated with GROUNDWATER andthrough which groundwater may move.An aquifer that is overlain by an imperme-able rock layer is said to be confined.Where the path from the aquifer to the at-mosphere is through permeable material,the aquifer is aids to be unconfined.Aquifers are important sources of waterfor drinking, irrigation, and industry, butunless the rate of extraction is equal to orless than the rate of replenishment fromprecipitation the supply will eventually be-come exhausted.

Archaea (archaebacteria) A kingdom orDOMAIN of PROKARYOTES that includes thehalophilic and methanogenic bacteria andthe thermoacidophilic bacteria – the EX-TREMOPHILES. They are found in extremeenvironments, such as hot springs, boilingmuds, volcanic craters, saline sediments,and deep-sea (hydrothermal) vents. TheArchaea are distinguished from the BAC-TERIA by the nucleotide sequences of theirribosomal RNA; their ribosomes, whichhave a distinctive shape and resemble thoseof EUKARYOTES rather than prokaryotes;cell walls lacking a peptidoglycan layer;and the composition of their lipids andRNA polymerase enzyme. See also hy-drothermal vent.

archaebacteria See Archaea.

Archean (Archeozoic) See Precambrian.

archipelago A group of islands or anexpanse of water with many islands scat-tered across it.

Arctic The region north of the northern-most limit of tree growth (the treeline) inthe northern hemisphere, which includesGreenland (Kalaallit Nunaat), Svalbardand other polar islands, the northernmostparts of Canada, Alaska and Siberia, Scan-

aposematism

12

dinavia, and Iceland, and the Arctic Ocean.Most of the region consists of TUNDRA veg-etation, permanent ice or snow cover, orocean, most of which freezes in winter. Onland, the ground below a certain depth re-mains permanently frozen. It experiencesextremely long days in summer (24 hoursat the pole), and long nights in winter (24hours at the pole). The term is sometimesrestricted to the area north of the ARCTIC

CIRCLE (66°30′N), in which there is at leastone 24-hour period each year when the sundoes not set and one when it does not rise.See also Arctic-alpine species. CompareAntarctic.

Arctic-alpine species A species foundboth in the Arctic and on high mountainsin temperate latitudes such as the Rockies,Alps, and Himalayas, e.g. least willow(Salix herbacea). This relict distribution in-dicates that these species were once part ofthe TUNDRA vegetation that was wide-spread during the PLEISTOCENE: as the cli-mate warmed they survived only in coolerlatitudes and altitudes.

Arctic Circle The line of latitudearound the Earth at 66°30′ N. It marks thesouthern limit of the area within which forone day or more each year the sun does notset (winter solstice, December 21 or 22) orrise (summer solstice, June 21 or 22). Theperiod of continuous day or night increasesnorthward from the Arctic Circle, reachingsix months at the North Pole. CompareAntarctic Circle.

arctic vegetation See tundra.

area effect The effect of HABITAT area onspecies diversity and on the number of in-dividuals of each species present. In gen-eral, the number of species decreases witha decrease in the area of the habitat patch.See island biogeography theory.

arid Describing an area where rainfall isinsufficient to support agriculture. In mostarid areas, annual PRECIPITATION is lessthan 25 centimeters. Areas with 25–51 cmof annual precipitation are termed semi-arid.

arithmetic mean See mean.

arrested succession See succession.

Arrhenius, Svante August (1859–1927) Swedish physical chemist. Arrheniusis noted for his work in chemistry on thetheory of electrolytes and the rates ofchemical reactions. He was also the first tospeculate on the effect of carbon dioxideon atmospheric temperature (what is nowknown as the GREENHOUSE EFFECT). In 1895he calculated that a doubling in the level ofcarbon dioxide in the atmosphere wouldlead to a 10°C rise in average temperature.

arsenic (As) A chemical element foundboth free and combined in many minerals,usually associated with silver and anti-mony. Arsenious oxide, A2O3, formed byroasting arsenic-containing metal ores, ishighly poisonous and is used in pesticides.Arsenic pentoxide, A2O5, is used in herbi-cides and insecticides. In parts of India andBangladesh, arsenic compounds in wellwater cause major health problems, espe-cially cancers. These chemicals derive fromarsenic-containing rocks through whichthe deep well water passes. Similar arseniccontamination of groundwater also occursin other parts of the Far East and theUnited States.

artesian well A spring into which waterflows without the need for pumping. Arte-sian wells obtain their water from AQUIFERS

in which the GROUNDWATER is trapped buyimpermeable rock layers above and belowthe water-holding strata. If the aquifer isbeing topped up by water entering theaquifer at a level higher than the artesianwell, the hydrostatic pressure of the waterin the aquifer forces water to the surface.

Arthropoda The largest phylum of theAnimal kingdom, with segmented bodies,jointed appendages, and a hard externalskeleton (exoskeleton) made of chitin. Itincludes the crustaceans, insects, spiders,scorpions, mites, ticks, horseshoe crabs,centipedes, and millipedes. In the FiveKingdoms system of classification, theArthropods are subdivided into three dis-

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Arthropoda

tinct phyla – the Chelicerata (spiders, scor-pions, mites, ticks, harvestmen or daddy-longlegs, and horseshoe crabs), in whichthe anterior (front) pair of appendages(chelicerae) are clawed, and which have nomandibles or antennae; the Mandibulata,(insects, millipedes, and centipedes), whosebodies are divided into distinct head,thorax, and abdomen, and which havemandibles; and the Crustacea, which havetwo pairs of antennae on the head.

Arthropods are found in almost everyhabitat on the planet, and are of great eco-nomic importance. Shrimps, lobsters, andcrabs are important sources of food world-wide. Planktonic crustaceans and insectlarvae are major links in the food chains ofboth freshwater and marine habitats, in-sects are vital pollinators of crops, and de-composers aid the formation of humus,enhancing the fertility of the soil. How-ever, many arthropods are serious pests oflivestock, crops, and stored food: insectsand mites attack crops and timbers, marinearthropods parasitize farmed fish, sap-sucking insects drain the host plants of re-sources and may transmit pathogenicbacteria, fungi, or viruses, and ticks and in-sects such as mosquitoes and flies spreaddiseases through the human populationand its livestock.

artifact An object created by humans oran attribute characteristic of or resultingfrom a human activity or institution, e.g. apiece of prehistoric pottery.

artifical rain Rain produced by nonnat-ural means. A number of experiments havebeen tried using chemicals distributed fromaircraft or rockets, either to encouragecloud formation or to induce rain forma-tion from existing clouds.

artificial selection Manipulation byhumans of the breeding between differentindividuals of a population in order to pro-duce a desired evolutionary outcome.

asbestos A fibrous mineral used to makeheat-resistant products such as insulationmaterials, brake linings, electrical insula-tion, and fireproof fabrics. Asbestos dust is

highly toxic when inhaled, giving rise tomesothelioma, lung cancer, and asbestosis.Blue asbestos is the most toxic form, andits use has been banned in many countries.

asexual reproduction The formationof new individuals from a single parentwithout the production of gametes or spe-cial reproductive structures. It occurs inmany plants, usually by vegetative propa-gation or spore formation; in unicellularorganisms usually by fission; and in multi-cellular invertebrates by fission, budding,fragmentation, etc.

aspect Morphological appearance, suchas shape, size, color, and pattern. Aspect isparticularly important in cryptic speciesseeking to avoid detection by PREDATORS. Ifthe HABITAT contains a variety of back-grounds against which an animal might beviewed, evolution may lead to an increasein aspect diversity in the species livingthere, each being camouflaged against aparticular feature of the background. Forexample, certain moths have evolved tomatch the pattern of bark of the treespecies they frequent, the shape of brokentwigs, dead or living leaves complete withpatches of decay, and so on. Thus an in-crease in aspect diversity may reflect the di-versity of the habitat, and is related to thecharacteristics of the available restingplaces and the searching techniques of thepredator(s).

assemblage A collection of plant andanimal species typical of a particular envi-ronment. Species assemblages can serve asindicators of environmental conditions.

assimilation The process of incorpora-tion of simple molecules of food that hasbeen digested and absorbed into living cellsof an animal and conversion into the com-plex molecules making up the organism.

association A group of species living inthe same place. More specifically, a largeCLIMAX COMMUNITY of plants usuallynamed after the dominant plant species(usually of the uppermost layer of vegeta-tion), e.g. heath association. Associations

artifact

14

usually have more than one dominantspecies. The term alliance is used to de-scribe a community with only one domi-nant species.

assortative mating Reproduction ofanimals in which the males and females ap-pear not to pair at random but tend to se-lect partners of a similar phenotype.

atavism The reappearance of a charac-teristic, known as a throwback, in an indi-vidual after several generations of absence,usually caused by a chance combination ofparticular genes.

Atlantic period The period from about6000 to 3000 BC that was characterized bya warming of the climate worldwide andan increase in rainfall in north temperateregions. During this period deciduousbroad-leaved forests spread northward.The early Atlantic period was the warmestphase so far of the present interglacial pe-riod.

atmometer An instrument that meas-ures the evaporating capacity of the air bymeasuring water loss from a porous non-living wet surface.

atmosphere 1. The gaseous envelopesurrounding the Earth, a mixture of invisi-ble, odorless and tasteless gases containingalmost constant amounts of nitrogen (N2)and oxygen (O2) with lesser amounts of he-lium (He), methane (CH4) (derived mainlyfrom ruminant animals), hydrogen (H2),nitrous oxide (N2O), and the inert gasesargon (Ar), neon (Ne), krypton (Kr), andxenon (Xe). Other components present invariable concentrations include watervapor (H2O), carbon dioxide (CO2), ozone(O3), sulfur dioxide (SO2), nitrogen diox-ide (NO2), various anthropogenic pollu-tants such as chlorofluorocarbons (CFCs)and chlorine (Cl2), and microscopic solidand liquid particles in suspension. The at-mosphere extends thousands of kilometersabove the Earth, and is retained by theEarth’s gravity. Its composition is more orless constant up to about 100 km above theground, although its density decreases with

distance from the ground – hence the short-age of oxygen for mountaineers at high al-titudes. At 100 km there is a high level ofincident solar radiation, and an increasingproportion of charged particles (ions), giv-ing rise to the auroras.

The atmosphere has several distinct lay-ers. The lowermost, extending up to15 km, is the troposphere. In this layerconvection currents mix air warmed at theEarth’s surface with high air, maintaining avertical temperature gradient of about 6°C(10.8°F) per kilometer. The troposphere isthe zone in which water vapor is found andweather occurs. At the top of the tropos-phere is the tropopause, where the temper-ature falls to about –60°C (–76°F). Abovethis the stratosphere extends for some50 km, a dry layer lacking strong convec-tion currents, with a marked increase intemperature with increasing altitude. In theupper part of this zone is the OZONE LAYER,formed by action of ultraviolet light fromthe Sun on oxygen molecules (O2), whichbreak down, allowing oxygen atoms to re-combine as ozone (O3). Ozone forms a bar-rier restricting the passage of harmfulultraviolet radiation to the Earth’s surface.Above the stratopause (mesopeak) at 50km is the mesosphere, where temperaturesonce again decline with altitude. Its outerlimit is the mesopause, at a temperature of

15

atmosphere

COMPOSITION OF AIR(% by volume)

nitrogen (N2) 78.08oxygen (O2) 20.95carbon dioxide (CO2) 0.03argon (Ar) 0.93neon (Ne) 1.82 × 10–3

helium (He) 5.24 × 10–4

methane (CH4) 1.5 × 10–4

krypton (Kr) 1.14 × 10–4

xenon (Xe) 8.7 × 10–5

ozone (O3) 1 × 10–5

nitrous oxide (N2O) 3 × 10–5

water (H2O) variable, up to 1.00

hydrogen (H2) 5 × 10–5

about –85°C. Beyond this, in the thermos-phere, temperature rises with altitude.Higher still the density of the gases de-creases still further as they merge with thesolar wind, a stream of charged particlesemanating from the Sun.2. (standard atmosphere) A unit of pres-sure approximating to the mean atmos-pheric pressure at sea level or to thepressure exerted by a vertical column of

mercury 760 mm high. It is defined as101 325 pascals (about 14.7 lbs per squareinch).

atoll A coral reef surrounding a lagoon.Many atolls are derived from the craterrims of sunken volcanoes, coral growth onthe rim keeping pace as a volcano sinksbelow the surface of the ocean to enclosethe lagoon.

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mesopause

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temperature °Fpressure(hPa)

stratosphere

troposphere

mesosphere

thermosphere

ozonelayer

Atmosphere: atmospheric layers

atomic energy See nuclear energy.

ATP See respiration.

auger A device used to sample soils. Thecommonest type is the corkscrew auger,which resembles a large corkscrew that istwisted down into the soil, then with-drawn. Ideally, an auger is marked at in-tervals, so the depth of the source of eachpart of the sample is known. A corkscrewauger can also be used horizontally to ex-tract a sample from a tree trunk for datingpurposes (see dendrochronology).

augmentation A form of PEST CONTROL

in which extra individuals of an indigenousnatural predator are released to help sup-press a rapidly-growing pest population.

Australasia One of the six main zoo-geographical areas, composed of Australiaand the islands of its continental shelf, Tas-mania, New Guinea, and New Zealand.Marsupial (pouched) and monotreme (egg-laying) mammals are particularly charac-teristic, but many other unique vertebratesand invertebrates are also found.

autecology (species ecology) The studyof the interaction of a single species or in-dividual organism with its environment. Itinvolves a detailed study of the life cycle,coupled with measurements of the variousliving and nonliving environmental factorsthat affect the species. Compare synecol-ogy.

autochthonous Describing material pro-duced within a community or ecosystem,such as organic matter production or min-eral cycling.

autocidal control A form of PEST CON-TROL in which the pest contributes to itsown demise. An example is the eradicationof the screw-worm fly (Cochliomyia ho-minivorax) from the southern USA by re-leasing sterile (irradiated) males intopopulations to compete with fertile malesand decrease the birth rate.

autogamy See sexual reproduction.

autogenic succession See succession.

autopolyploid See polyploidy.

autotroph An organism that requiresonly simple inorganic compounds forgrowth. For example, most plants andalgae are autotrophs, using carbon dioxideand water to synthesize organic com-pounds with the aid of light energy (photo-autotrophs). Some photosynthetic bacteriause hydrogen sulfide or other inorganiccompounds instead of water as the hydro-gen source/electron donor. Other bacteria(chemoautotrophs) derive the energy forsynthesis from inorganic compounds. Au-totrophs are primary producers, formingthe base of FOOD CHAINS. Compare het-erotroph. See producer.

autotrophic succession See succession.

average reproductive rate See repro-ductive rate.

Aves The class of vertebrates (phylumChordata) that contains the birds, most ofwhose characteristics are adaptations forflight. Birds are endothermic (warm-blooded) vertebrates with a body coveredin feathers, a four-chambered heart, fore-limbs modified to form wings, the sternummodified to form a keel for the attachmentof the wing muscles, the pelvic girdle andhind limbs modified so as to support thebird when standing on two legs, a hornybill with no teeth, and a yolked egg with acalcareous shell.

Birds have a well-developed social life,including territorial and courtship dis-plays, nesting, parental care, and song.Many undertake long migrations. Thereare 28 orders. The perching birds (passer-ines) account for 60% of all birds and theremaining flightless birds (ratites) tend tobe swift runners.

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Aves

Bacillariophyta See diatoms.

backcross A cross between an individ-ual and one of its parents or an individualof identical genotype to that of one of itsparents. See heterozygous; homozygous;plant breeding.

background extinction See extinction.

background radiation RADIATION com-ing from sources other than that under in-vestigation. The background radiationmay increase at altitude, owing to thegreater intensity of cosmic rays, and inareas where the rock or soil contains ra-dium, whose decay causes release of theradioactive gas radon. See radioactivedecay.

Bacteria In the Five Kingdoms system ofclassification, a KINGDOM containing all thePROKARYOTES, formerly called the Prokary-otae or Monera. Most taxonomists nowdivide the prokaryotes into two distinctkingdoms or domains (a taxonomic levelabove kingdom), the Bacteria and the AR-CHAEA.

Bacteria are found over almost all theplanet, from the interstices of rocks deepunderground to icebergs, mountains,deserts, deep oceans, and the insides ofother organisms. Most are single cells,ranging from rounded cocci to rods andspirochetes. A few, such as the cyanobac-terium Anabaena, form filaments of cells,clusters, or dense colonies (e.g. thecyanobacteria that form stromatolites).Bacteria play an important role in the recy-cling of nutrients in soil and water, in DE-COMPOSITION, and in NITROGEN FIXATION

(e.g. the symbiotic bacterium Rhizobium inthe root nodules of legumes).

Bacterial Code of Nomenclature SeeInternational Codes of Nomenclature.

bacteriophage (phage) Any VIRUS thatinfects bacteria. Phages have protein coats(capsids) consisting of a head containingthe nucleic acid (double- or single-strandedDNA or RNA), and a helical tail throughwhich the virus injects nucleic acid into itshost. RNA-containing phages are alwayslethal to the host cell, but some DNAphages may become integrated into thehost DNA, being replicated when it repli-cates. At a later time the host may be trig-gered to reproduce new viruses, whichescape by causing the breakdown of thehost cell wall (lysis), killing the host.Phages can sometimes transfer bacterialgenes between bacteria, when segments ofnucleic acid become incorporated into thephage genome, a process called transduc-tion. Phages are used in GENETIC ENGINEER-ING to transfer nonviral DNA (deliberatelyinserted into the phage nucleic acid) to bac-teria.

bacteroid A Rhizobium bacterium inthe root nodule of a legume after it has in-creased in size about 40-fold and changedshape to a Y- or X-shaped cell, becomingcapable of NITROGEN FIXATION. In thisform, the bacterium is incapable of inde-pendent growth, and is dependent on itshost for part of its nutrition. See symbiosis.

balanced polymorphism See polymor-phism.

bar chart See graph.

barren A fairly level area of land withlittle vegetation and few trees, usually onsandy or serpentine soils. Plants are often

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B

smaller and stunted when compared to in-dividuals of the same species from morefertile habitats, and there are often groupsof specialized endemic species. The term isused especially in the eastern United States,for example, the pine barrens of New Jer-sey.

basal metabolic rate (BMR) The mini-mum amount of energy an animal needs tosustain itself for a given time. It is some-times expressed as the minimal quantity ofheat produced (by metabolism) by an indi-vidual at complete physical and mental rest(but not sleeping) 12–18 hours after eating,expressed in milliwatts per square meter ofbody surface.

base 1. A substance that reacts with anACID to form a salt and water. In solution,it releases ions that can combine with hy-drogen ions.2. A nitrogenous (nitrogen-containing)molecule that forms part of a nucleic acid,and is the basic unit of the genetic code.Such bases are either PURINES or PYRIM-IDINES.

baseflow See dry weather flow.

Batesian mimicry See mimicry.

bathymetry The measurement of waterdepth, especially in the oceans. This is usu-ally done by echo sounding, sometimessupplemented by the use of underwatercameras. See echo sounder.

baumgrenze See tree line.

bearing The horizontal angle between asurvey line and a specific reference direc-tion.

Beaufort scale A numerical scale rang-ing from 0 (calm) to 12 (hurricane) thatwas introduced by Admiral Sir FrancisBeaufort (1774–1857) in 1805 to describevariations in wind force at 10 m (30 ft)above the ground and originally based onthe effect of wind on a fully rigged man-of-war. See table overleaf.

becquerel (Bq) A unit of radioactivity:one becquerel is the activity of a quantityof a radioactive substance in which there isone spontaneous atomic disintegration persecond. The becquerel is an SI unit, and su-persedes the curie (Ci): 1 becquerel = 2.7 x10–11 Ci. See radioactive decay.

bedding plane A surface that indicateswhere successive phases of deposition of arock occurred. Many SEDIMENTARY ROCKS

show successive parallel bedding planesthat represent times when depositionstopped then started again, or when thesize of particles deposited changed signifi-cantly. Visually bedding planes may be de-tected as cracks at the surface of a rockexposure, or changes in color or rock type.The angle of the bedding planes may indi-cate whether the rocks have been tilted orfolded since their formation. Rocks cleavemost readily along their bedding planes.

bees Insects of the order Hymenoptera.Unlike the related wasps, they feed theirlarvae on a mixture of pollen and honey,and some of their hairs are branched orfeathered to trap pollen. Bees feed onpollen and nectar, and store some of thenectar as honey. Some species, such as hon-eybees (Apis mellifera), have basketlikestructures on their rear legs in which tocarry pollen from the flower to the nest.The main economic importance of bees isas pollinators: without them many majorcrops would fail.

beetles Insects of the order Coleoptera.Their forewings are modified to form hardleathery wing-cases (elytra) that protect theabdomen and membranous hind wingswhile resting. The head has biting mouth-parts. Beetles undergo complete metamor-phosis, the eggs hatching into legless grubsor caterpillar-like larvae, which pupate be-fore emerging as adult insects. Beetles arefound in a wide range of terrestrial andfreshwater habitats. Many are predators,but some feed on plant material (includingwood) and others on detritus or carrion(e.g. carrion beetles). Some are seriouspests of crops (e.g. Colorado potato bee-tle), trees (e.g. elm bark beetle), stored

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beetles

goods (e.g. grain weevils), and householdgoods (e.g. woodworm). Others are benefi-cial – ladybirds (Coccinella) feed on aphidsand are used in the BIOLOGICAL CONTROL ofpests in greenhouses.

behavioral ecology See ecology.

bell curve See normal distribution.

benthic zone The sediments at the bot-tom of streams, ponds, lakes, and oceans.The benthic zone is an extremely variableenvironment, depending on the latitude,

the depth of the water (and hence temper-ature, light intensity, and pressure), itssalinity, and the nature of the sediments(whether they are well aerated or anoxic).In shallow waters light supports photosyn-thesizing organisms, including diatoms andphotosynthetic bacteria, but in deeperwater the main source of nutrients is therain of detritus from above – the remainsand excreta of plankton and larger animalsand plants, as well as the products of thedecomposition of larger carcasses that sinkdown to the sediments. This fallout varies

behavioral ecology

20

THE BEAUFORT SCALE ON LAND

Beaufort Descriptive Wind speed Visualnumber term knots mph effect

0 calm < 1 < 1 smoke from factories risesvertically

1 light air 1 – 4 1 – 4 wind direction shown bysmoke, but not by aweather vane

2 light breeze 4 – 6 4 – 7 wind felt on face3 gentle breeze 7 – 10 8 – 12 broken twigs from trees are

in constant motion4 moderate breeze 11 – 16 13 – 18 dust and loose paper raised

from sidewalk5 fresh breeze 17 – 21 19 – 24 swaying of small trees6 strong breeze 22 – 27 25 – 31 large branches of trees in

motion7 near gale 27 – 33 32 – 38 whole trees vibrate8 gale 34 – 40 39 – 46 twigs detached from trees9 strong gale 41 – 47 47 –54 chimney pots and roof tiles

fall from houses10 storm 48 – 55 55 – 63 trees uprooted; much struc-

tural damage11 violent storm 56 – 63 64 – 72 considerable damage. Usu-

ally experienced along thecoast; very infrequent in-land

12 hurricane > 64 > 73 mostly confined to the trop-ics. Disastrous; loss oflife, towns flattened

(As the Beaufort scale is subjective, wind speeds assigned to each Beaufort numbervary; as a result some values appear in more than one wind force class.)

with the seasons and currents, and withcertain weather patterns.

benthos The organisms that live on or inthe floor of a stream, pond, lake, or ocean.The benthos is made up of the many mi-croorganisms and invertebrates living inand on the surface layers of the sediments,and includes: bacteria, diatoms, fora-miniferans, ciliates, amebas, flagellates,copepods, and other small crustaceans; at-tached forms such as crinoids, hydroids,sea anemones, sponges, sea squirts (ascidi-ans), and bivalves; burrowing animals suchas polychaete worms and nematodes;creeping animals such as starfish, brit-tlestars, sea urchins, snails, and flatworms(platyhelminths); bottom-dwelling fishsuch as flounders; and plants and algaewith roots or holdfasts fixed in the sedi-ment, such as pondweeds, water lilies, andmany seaweeds (e.g. kelps). Organismslarger than 1 mm are called macrobenthos,those between 0.1 and 1 mm meiobenthos,and those smaller than 0.1 mm microben-thos. Some organisms filter detritus fromthe water, or from water drawn throughburrows in the sediments; others scavenge,prey on the detritus feeders, or graze onbottom-rooted plants or algae. See detri-tus. See also benthic zone; hydrothermalvent.

Bergmann’s rule The generalization thatwarm-blooded animals of the same speciesliving in cold regions tend to be larger thanthose living in warmer climates. It is sug-gested that the smaller ratio of surface areato volume in the larger animals reduces therate of heat loss, thus saving energy. Seealso Allen’s rule.

beta decay (β-decay) The disintegrationof a radioactive element (with the emissionof an electron or positron) that results inthe mass number of the nucleus remainingunchanged, but the atomic number (protonnumber) increasing or decreasing by one.Beta radiation is more penetrating thanalpha radiation. See radioactive decay.

beta diversity See species diversity.

Bhopal A town in central India, wherein 1984 a catastrophic release of some 44tonnes of methyl isocyanate (MIC) oc-curred at the Union Carbide factory. MICwas an ingredient of the insecticide car-baryl, which was produced at the factory.Water seeping into a storage tank of MICtriggered a runaway chemical reaction thatblew open the safety valve on the tank, re-leasing MIC. The wind carried a cloud oftoxic vapor over an area housing some100 000 people. At least 2500 people died,and 50 000 others were disabled for a timeby respiratory and eye problems.

bias A nonrandom distortion in meas-urements due to such factors as poorly cal-ibrated instruments, changes in a specimenduring an experiment, or the subjectivejudgment of the experimenter. Personalbias can be eliminated by ‘blind trials’, inwhich the identities of individual speci-mens are not known to the person makingthe measurements or observations. Othertypes of bias can be detected by means oftrial runs using different methods to meas-ure single variables and comparing the re-sults.

bicentric distribution A geographicalDISTRIBUTION of a species in which there aretwo separate centers of distribution. It is anexample of a disjunct distribution. It maybe explained by the contraction of therange of a formerly more widespreadspecies because of changing environmentalconditions, often because of climatechange. In cases in which the centers arenot too far apart it may be caused by theability of the species to disperse its off-spring across unfavorable areas to reachareas where conditions are favorable forsurvival.

biennial A plant that takes two years tocomplete its life cycle. In the first year itgrows vegetatively, storing food in peren-nating organs (storage organs that enablethe plant to survive an unfavorable seasonsuch as winter or a dry season). In thesecond year it uses this stored food to pro-duce more leaves, and also flowers and

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biennial

seeds. Compare annual; ephemeral; peren-nial.

bimodal distribution A frequency dis-tribution that has two major peaks. Thesedo not have to be of equal height.

binary fission See asexual reproduc-tion.

binomial distribution See distribution.

binomial system of nomenclature Asystem of classification of living organismsdevised by the Swedish botanist Carl Lin-naeus (1707–78), in which each species isdefined by two names. The first, writtenwith a capital letter, is the genus name, andthe second is the species name. Both namesare in Latin or are Latinized, and are nor-mally printed in italic type. For example,humans belong to the genus Homo andspecies sapiens, so the species is Homosapiens. Such a two-part name is called abinomial. See International Codes ofNomenclature.

binomics (bioecology; environmental bi-ology) The study of the relationships be-tween organisms and their environment.

bioaccumulation The accumulation ofPOLLUTANTS and other toxins in the tissuesof living organisms. For example, the in-secticide DDT and its derivatives accumu-late in the fatty tissues of mammals andbirds. These toxins are passed up the foodchain, accumulating in greatest concentra-tions in top predators such as mammalsand birds of prey. Certain species or vari-eties of plants accumulate toxic ions suchas cadmium, lead, nickel, and zinc and areable to tolerate them, and a few species,called hyperaccumulators, can accumulatemore than 100 times more toxins thanmost plants (hyperaccumulators are de-fined as organisms that can accumulateover 10 000 mg of the toxic element perkilogram). See also bioremediation; bio-concentration; biomining.

bioassay A experimental technique fordetermining the strength of a biologically

active chemical by its effect on a living or-ganism or group of organisms. Examplesof this include the use of caged fish to de-termine the rate at which they accumulatepoisons or even die in polluted waters – anindication of the level of pollution (thisprocess is called biomonitoring); the use ofsimple animals as indicators of the hor-mone activity in extracts from mammalianendocrine glands; and the use of the rate ofincrease in numbers of cultured bacteria tomeasure the activity of vitamins.

bioaugmentation The reinforcing ofnatural biological processes using additionorganisms. For example, decomposition oforganic material such as sewage is aug-mented by the addition of various speciesof bacteria. It is essentially the concentra-tion of organisms that grow naturally inthe environment, not the addition of genet-ically modified organisms. Bioaugmenta-tion is important in converting human orindustrial waste to nonpolluting products.

biochemical oxygen demand See bio-logical oxygen demand.

biocide A substance that kills living or-ganisms. See fungicide; herbicide; insecti-cide; pesticide.

bioclimatology The study of climate inrelation to the environments of living or-ganisms.

bioconcentration The increase in bio-accumulated chemicals in the tissues of or-ganisms as they pass up the FOOD CHAIN. Itis sometimes used specifically to identifythe net accumulation of a chemical directlyfrom the aquatic environment by gills orother epithelial tissue and its subsequentretention in the tissues, as opposed to up-take through the ingestion of a chemical infood or water. See also bioaccumulation.

biodegradable Describing a substancethat can be broken down by microorgan-isms.

biodiesel A vegetable oil, such as rape-seed oil, that can be used as a fuel in mod-

bimodal distribution

22

ified diesel engines. The methyl ester ofrapeseed oil (rapeseed methyl ester, orRME) can be used in unmodified engines.

biodiversity The variety of organismspresent in the living world. More specifi-cally, species biodiversity is the number ofspecies present in a particular area orecosystem. In general, biodiversity tends tobe highest in complex and highly produc-tive ecosystems, such as topical rainforests.Biodiversity is often used as an indicator ofthe health of such ecosystems.

bioecology See binomics.

bioenergetics The study of energy trans-formations in living organisms. Energytransfer from the environment to living or-ganisms and from organism to organismvia the FOOD CHAIN is inefficient: about 2%of the energy in incident radiation is con-verted to chemical energy in photosynthe-sis, and when heterotrophic organismsconsume other organisms, only about10–20% of the energy is passed on fromlevel to level in the food chain. See energytransformation; trophic level; respiration.See also bioenergy.

bioenergetic web See food web.

bioenergy The energy contained in liv-ing organisms. The term is used to meanenergy, fuels, or energy-related productsderived from renewable biological re-sources such as plants (but not FOSSIL

FUELS), microorganisms, and biologicalwastes. Typical sources are fast-growingcrops and trees grown as a fuel sources,aquatic plants, agricultural feed crops,waste and residues from timber mills andrelated industries, animal wastes, and mu-nicipal and industrial wastes. These may beburned in coal- or wood-fired boilers togenerate electricity, or in high-efficiencygasification plants that drive turbines, andfuel cells.

biofilms Thin layers of bacteria andother microorganisms, including pro-toozoans, algae, and fungi, that adhere tosurfaces in aqueous environments by

means of gluelike mucilages. They arefound naturally coating rocks, stones, andother objects in streams and rivers andclogging drains. They are also the maincomponent of plaque on teeth. Biofilmscost industry large sums of money, becausethey not only clog up working parts of ma-chinery, but also produce corrosive chemi-cals that damage metal, plastic, and othersurfaces.

biofuel A fuel derived from renewableorganic resources such as fast-growingcrops and trees; animal, industrial, andmunicipal wastes; and the reactions of mi-croorganisms (see bioenergy). They includeBIODIESEL, BIOGAS, and GASOHOL.

biogas Gas produced by the degradationof organic material by microorganisms inthe absence of oxygen. It is a mixture ofmethane and carbon dioxide. Biogas re-sults from the treatment of organic waste,and can be a problem in garbage dumpsand septic tanks if not piped away. Biogasis also produced in the digestive systems ofanimals – the biogas emitted by livestockworldwide has a significant effect onGLOBAL WARMING, since both carbon diox-ide and methane are greenhouse gases. Bio-gas from garbage dumps can be piped topower electricity generation for localhomes and businesses, and small-scale di-gesters are available for home use.

biogeochemical cycle The cycling ofchemical elements between organisms andtheir physical environment, for examplethe cycling of nitrogen in the NITROGEN

CYCLE. Various feedback controls operateto control biogeochemical cycles, and thesemay be disrupted by human activity, lead-ing to pollution. For example, the presentrapid accumulation of carbon dioxide inthe atmosphere is in part due to the burn-ing of fossil fuels, which unbalances theCARBON CYCLE.

biogeographical region Any geographi-cal region that has a distinctive flora andfauna.

biogeography The study of the distrib-

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biogeography

ution of plants (including world vegetationtypes) and animals past and present, and ofthe changing relationship between humansand their natural environment.

biohazard A biological agent (e.g. apathogenic microorganism) or condition(e.g. inadequate laboratory procedures)that poses a risk to life or health. There isan internationally recognized biohazardwarning symbol.

biological clock The internal mecha-nism of an organism that regulates CIRCA-DIAN RHYTHMS and various other periodiccycles.

biological conservation Active man-agement aimed at preserving habitat andspecies diversity and the genetic diversitywithin species. It also involves manage-ment of the processes upon which speciesdiversity depends, such as biogeochemicalcycling, and the control of biotic and abi-otic factors that affect diversity and thesurvival of species and varieties. Biologicalconservation may include allowing the sus-tainable use of resources. See also conser-vation; sustainable development.

biological control The control of pestsand diseases by introducing or substan-tially increasing the numbers of theirnatural predators or pathogens, or by in-terfering with their natural processes. Forexample, ladybirds (Coccinella) or para-sitic wasps may be introduced to green-houses to control aphid populations, andspores of fungal diseases are sprayed on in-vasive waterweeds such as the water hy-cainth (Eichhornia). Introduced predatorsor disease organisms may be geneticallyengineered to make them more potentpredators/pathogens. Sterile males may be released in large numbers to reduce thereproductive success of some wild insectpests, or sex-attractant chemicals (phero-mones) may be used to trap males or fe-males. Biological control may also beindirect, as in the addition of organic nu-trient supplements to soils to increase thenumbers of saprotrophic microorganisms

to combat damping-off fungi (Pythium,Rhizoctonia). See also autocidal control.

biological efficiency See ecological ef-ficiency.

biological oxygen demand (biochemicaloxygen demand; BOD) The standardmeasurement for determining the level oforganic pollution in a sample of water. It isthe amount of oxygen used by microorgan-isms feeding on the organic material over agiven period of time, usually 5 days, typi-cally expressed as milligrams of oxygen perliter of water. Sewage effluent must be di-luted to comply with the statutory BODbefore it can be disposed of into cleanrivers.

biological species concept The defini-tion of a SPECIES as a population or groupof populations whose members are capableof interbreeding to produce fertile off-spring. In practice, some so-called speciesthat are normally separated by geographi-cal barriers may interbreed with each otherif brought together. The definition alsobreaks down in areas where HYBRID SWARMS

occur, leading to AGGREGATE SPECIES, espe-cially where subsequent POLYPLOIDY ren-ders the offspring fertile. See geographicalisolation; reproductive isolation; specia-tion.

bioluminescence The emission of lightby living organisms. It may be produced byinternal chemical reactions or by the re-emission of absorbed energy as light radia-tion. In certain deep-sea fish light isproduced by symbiotic bacteria living inspecial tissues. A wide range of organismsexhibit bioluminscence, from bacteria,fungi, algae, and plants to insects, squid,and fish. In deep-sea animals it can serve toilluminate prey or to conceal an animal’ssilhouette from predators below. Theflashing luminescence of fireflies is used inspecies recognition when courting.

biomagnification The increase in con-centration of accumulated chemicals in thetissues of organisms at higher levels of theFOOD CHAIN due to the processes of BIOAC-

biohazard

24

CUMULATION and BIOCONCENTRATION. Forexample, the concentration of organochlo-rines in sea water is in picograms per literto a few nanograms per liter, in marine in-vertebrates this rises to tens of nanogramsper liter, in mussels several milligrams perliter, and in the fatty tissues of marinemammals and predatory birds, hundredsof milligrams per liter. See trophic level.

biomanipulation The management ofan ECOSYSTEM by manipulating the top endof the FOOD CHAIN (top-down control)rather than the nutrient input. For exam-ple, EUTROPHICATION can be ameliorated byremoving plankton-eating fish, thus allow-ing herbivorous zooplankton that graze onalgae to increase in numbers. The decreasein numbers of algae in turn allows greaterlight penetration, promoting the growth ofaquatic plants and the reoxygenation ofthe water. Compare bottom-up control.

biomarkers Physiological, histological,or biochemical changes in organisms thatcan be used as indicators of exposure totoxic chemicals. For example, the concen-tration of vitamin A in the livers of otters(Lutra) shows strong negative correlationwith the concentration of PCBs (polychlo-rinated biphenyls) in the waters they livein. See hazard indicator; indicator species.

biomass The total mass of all the livingorganisms in a particular ECOSYSTEM orarea or at a particular TROPHIC LEVEL in aFOOD CHAIN. It is usually expressed as dryweight per unit area. The ratio of biomassto annual production is called the biomassaccumulation ratio:

biomass (g m–2)/net productivity (g m–2 yr–1)

See net primary production.

biome A major regional communitycharacterized by distinctive vegetation andanimal forms. Biomes are related to cli-mate, latitude, topography, and soils, andmerge into one another at their bound-aries. They are the largest geographicalunits, and are often named after the domi-nant type of vegetation in the case of ter-restrial biomes, e.g. grassland. Aquatic

biomes include coral reefs, lakes, and themarine rocky shore.

biometry The statistical analysis of bio-logical phenomena.

biomining The use of genetically engi-neered microorganisms to recover toxic oreconomically important metals. See alsobioremediation.

biomonitoring See bioassay.

bioremediation The use of living or-ganisms to break down pollutants orwastes, such as industrial effluents, miningspoil, or oil spills, and to restore contami-nated ecosystems. Plants may be used (phy-toremediation) to extract heavy metalsfrom contaminated soils and water. Somecrop species can be genetically modified toaccumulate toxic ions, e.g. Arabidopsis hasbeen altered to express the enzyme mer-curic ion reductase, which converts Hg2+ toHg, which is volatilized and released intothe atmosphere. Uptake may also be as-sisted by the use of chemical chelatingagents to immobilize the toxins. Contami-nated water is treated by RHIZOFILTRATION

using plants with high transpiration ratesand extensive root systems, such as wil-lows (Salix) or reeds (Phragmites), or bythe use of aquatic plants that are removedand destroyed once they have extracted thetoxins. Organic wastes are usually tackledby bacteria and protozoans, and occa-sionally fungi (certain fungi are capable of breaking down POLYCHLORINATED BI-PHENYLS).

biosensor An analytical device that usesbiological interactions to provide qualita-tive or quantitative data about chemical orbiological molecules or processes. Biosen-sors are composed of biological materialsuch as enzymes, antibodies, nucleic acids,cells, or microorganisms, usually inti-mately associated with a transducer thatoutputs electrochemical, optical, or othersignals proportional to the concentrationof the substance being analyzed or tochanges in it. Such sensors have wide ap-plications in clinical diagnosis and medical

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biosensor

and veterinary research and in drug analy-sis and detection. They are also used in thecontrol of industrial effluents and in themonitoring and control of pollution.

biosphere See ecosphere.

biosphere reserve A type of nature re-serve of global importance, designated byUNESCO’s Man and the Biosphere Pro-gramme. Typically, a biosphere reserve hasa zoned region of management consistingof a highly protected core area. This is sur-rounded by buffer zones of varying degreesof restriction on human activity in whichland use such as forestry, agriculture, orgrazing may be permitted, provided theyhave no significant effects on the core area.Around the periphery are zones of cooper-ation, zones of influence, and transitionareas where human activity may affect thereserve. By preserving a relatively undis-turbed core area, the effects of land use andhuman activity on the remainder of theecosystem can be assessed.

biosynthesis The chemical reactions bywhich a living cell builds up its necessarymolecules from other molecules present.See anabolism.

biota The flora and fauna of a particularregion or geological period.

biotechnology The use of biologicalprocesses for industrial, agricultural, andmedical purposes. Examples are: the pro-duction of antibiotics by bacteria such asPenicillium; the use of FERMENTATION byyeasts and bacteria in the production ofbeer, wine, and yogurt; and the geneticmodification of farm crops and livestock,including the production of human hor-mones and blood proteins by geneticallyengineered animals, and the production ofmonoclonal antibodies by microorgan-isms. See genetic engineering.

biotelemetry The remote electricalmeasurement and recording of physiologi-cal variables and actions of organisms andsubstances, allowing continuous monitor-ing of animal movements or physiological

variables such as body temperature, bloodpressure, heart rate, food and water intake,and nerve activity. The data are usuallytransmitted by radio, infrared, or ultra-sonic signals from a remote site, which mayrange from a few centimeters to thousandsof kilometers away. It includes the trackingof wild mammals, birds, fish, and other an-imals. Distant signals, for example frommigrating birds, may be transmitted viasatellite.

biotic Relating to life, especially describ-ing living components of the environmentthat through their presence or activities af-fect the life of organisms in that environ-ment or alter other aspects of theenvironment. For example, the presence oftall trees in a forest affects abiotic (nonbi-otic) factors such as temperature, light in-tensity, humidity, and wind speed. Thepresence of several similar species leads tocompetition between them for resourcessuch as nutrients and water.

biotic climax See climax community.

biotic potential The maximum capac-ity of an organism or population to in-crease under optimum, nonlimitingenvironmental conditions. It is determinedby the age of sexual maturity, the fre-quency of reproduction, and the number ofoffspring born at a time. Optimum envi-ronmental conditions are those in whichfood, space, water, and other abiotic fac-tors are not limiting and there are nopredators, parasites, or diseases. Devia-tions from these conditions constitute envi-ronmental resistance.

biotope A geographic region character-ized by certain environmental conditionsand populated by a characteristic flora andfauna. The term is applied particularly tomicrohabitats, such as cow droppings in apasture.

biotype 1. A naturally occurring groupof individuals all with the same geneticcomposition, i.e. a clone of a pure line.Compare ecotype.

biosphere

26

2. A physiological race or form within aspecies that is morphologically identicalwith it, but differs in genetic, physiological,biochemical, or pathogenic characteristics.

birds See Aves.

birth rate The frequency of live births ina population, usually expressed as thenumber of live births per 1000 individualsper year, or as the average number of off-spring produced per individual per unit oftime. This may be expressed as a functionof age (bx). Compare natality rate.

bite density The number of bites a her-bivore takes while foraging. This is af-fected by the density of the plants in thatarea, and is directly related to the rate offood intake and the method of foraging.Where plant density is high, bite size be-comes more important than bite density indetermining food intake.

black earth See chernozem.

blanket bog A peat-forming plant com-munity that occurs, even on slopes, in areasof high rainfall, which causes a constantdownward movement of water, so thatminerals from the soil or rock below areprevented from reaching the vegetation atthe surface. This appears to prevent the es-tablishment of trees and shrubs. It is thus aclimatic climax vegetation type (see climaxcommunity). Blanket bogs are dominatedby bog mosses (Sphagnum spp.), often as-sociated with cotton grass (Eriophorum)or species of Scirpus. Low-growing erica-ceous plants (Ericaceae), rushes (Jun-caceae), and sedges (Cyperaceae) may alsobe common. In parts of north temperatureregions blanket bogs have been growingsince the start of the ATLANTIC PERIOD, andsome have accumulated great thicknessesof highly acid PEAT, often with a pH ap-proaching 4, even over limestone. See bog.

blastula See Animalia.

blight One of a range of plant diseasesthat causes sudden, serious leaf damage.The cause may be a pathogenic fungus,

oomycete, or bacterium. For example earlyblight of potatoes is due to the oomycetePhytophthora infestans, late blight to thefungus Alternaria solani, and fire blight ofpears to the bacterium Erwinia amylovora.

bloom (water bloom; algal bloom) Avisible sharp increase in the numbers of aspecies of plankton, usually an alga (e.g. di-atoms) or dinoflagellate (Dinomastigota).Blooms of diatoms often occur in spring,when nutrients are plentiful. As the growthof increasing numbers of algae in the wateruses up the nutrients (especially the silicaneeded for diatom tests), reproductiverates slow and numbers decline. Otherblooms may be due to EUTROPHICATION.

blow-out An area of DUNES where thevegetation has been eroded away, usuallyby human feet or vehicles, leading to fur-ther erosion by the wind and sometimes thedestruction of the dune or dune system. Incoastal areas this may be exacerbated byhigh tides or storm tides, which allow thesea to rush the blow-out and flood the hin-terland, further eroding the dunes as it doesso. Such destabilized dunes may also beginto migrate, burying the vegetation (andsometimes also human settlements) in theirpath.

blue-green algae See Cyanobacteria.

blue-green bacteria See Cyanobacte-ria.

BMR See basal metabolic rate.

BOD See biological oxygen demand.

body residue The total amount of aparticular chemical in an individual organ-ism.

bog A climax plant community with notrees, in which organic matter accumulatesas PEAT. Bogs are found in regions ofpoorly drained and permanently wet landin areas of high year-round rainfall and hu-midity. Decomposition rates are slow, sopeat builds up. The vegetation is oftendominated by bog mosses (Sphagnum spp.)

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bog

and sedges; other plants may include suchspecies as bog myrtle (Myrica gale), bogrosemary (Andromeda polifolia), Labradortea (Rhododendron tomentosum), and var-ious insectivorous plants. Mats of bog veg-etation may develop around the shores oflakes and large pools, gradually extendingover the water surface, often to a depth ofseveral feet. These can be dangerous, be-cause what appears to be solid vegetationin fact covers deep water. There are severaltypes of bogs: BLANKET BOG develops inareas of constant high rainfall. Raised bogsform from fens, where rain leaches nutri-ents from the top of the fen peat, making itacidic, especially in the center, so peat ac-cumulates faster than at the edges. Valleybogs occur in mountain valleys where run-off and snow meltwater accumulate. Kettlebogs are found where bog mats creep overthe surface of lakes formed by the meltingof buried blocks of ice stranded at the endof the last glacial period.

boom and bust cycle The cycle ofadaptation that drives the evolution of par-asites when attempts are made to cultivateresistant varieties of plants. Initially, a dis-ease-resistant crop variety is widelyadopted, but then a new variant of thepathogen arises and the resistant varietysuffers a crash in numbers. A new resistantstrain of crop is then developed, and even-tually the pathogen evolves a new resistantrace, and so the cycle continues.

boreal forest (cold forest; taiga; northernconiferous forest) A major BIOME southof the TUNDRA dominated by tall evergreenconifers, mainly spruce, fir, and pine. Bo-real forest is found in Canada and in north-ern Eurasia, stretching from Scandinaviaacross to Siberia. The summers are cooland the winters are very severe with tem-peratures as cold as –60°C. The conifers re-tain their leaves throughout the year andare adapted to cold conditions but in thenorthernmost areas bordering the tundradeciduous larches and birches replacethem. In these areas, the temperatures aretoo low and the landscape too exposed forconifers to survive. The needlelike leaves ofthe boreal forest create a nutrient-poor

PODZOL. The low levels of light result infew plant species on the forest floor, butthe deep leaf litter supports a more diversecommunity of invertebrates and microor-ganisms. Reindeer and wolves migratesouthward from the tundra into the borealforest in winter. Mountain coniferous for-est and pine forest for the south-eastUnited States, the Andes, the Alps, and theHimalayas are also boreal forests.

boron (B) An element found in low con-centration as boric acid in the soil solutionand in plant tissues. It is an essential el-ement for plant growth; alfalfa yellows andheart-rot of beets have been attributed toboron deficiency. Its role in plants is poorlyunderstood, but it is thought to be involvedin carbohydrate transport.

Botanical Code of Nomenclature SeeInternational Codes of Nomenclature.

botanical insecticide (botanical) AnINSECTICIDE derived from plant material.Botanicals are accepted for use in organiccrop production, but they can leaveresidues and may be disruptive to naturalpredators of pests and also be toxic to hu-mans. Pyrethrum, a broad-spectrum insec-ticide derived from the dried flower headsof Chrysanthemum cinerarifolium, dis-rupts an insect’s nervous system on con-tact. Many botanicals degrade rapidly insunlight and may cause allergic reactionsand dermatitis in humans. Some are moretoxic, e.g. rotenone, derived from the rootsof various tropical legumes, including Der-ris, a broad-spectrum insecticide. This issafe for honeybees, but does kill someother beneficial insects, and can be fatal ifinhaled by mammals.

bottleneck (population) A temporarydramatic reduction in population size, re-sulting in a decrease in genetic diversityand gene frequency in succeeding genera-tions, due partly to loss of individuals fromthe population and partly to the increase innumber of deleterious homozygous reces-sives that are produced by subsequent in-breeding. An example is the cheetah, whichtoday has very little genetic variation. A

boom and bust cycle

28

bottleneck causes the population to divergefrom other populations of the samespecies, and may be the first step towardspeciation or extinction. The decrease ingenetic diversity may restrict the popula-tion’s ability to adapt to future environ-mental change. See inbreeding; inbreedingdepression.

bottom-up control The regulation of aFOOD WEB or ECOSYSTEM by the availabilityof resources, whereby organisms on thesame TROPHIC LEVEL experience limitedavailability of food, so are controlledmainly by competition for resources withother organisms in the same trophic level,rather than by predation. Thus the avail-ability of organisms in the lower trophiclevels regulates the numbers of organismsin the levels above. This is a useful ap-proach when considering the productivityof each trophic level. See also biomanipu-lation.

brackish Describing water that is moresaline than fresh water, but less saline thansea water.

Braun-Blanquet scale A system for de-scribing an area of vegetation in terms ofABUNDANCE, COVER, and association de-vised by J. Braun-Blanquet in 1927. Scalesof 1–5 are assigned to numbers of individ-uals and the proportion of the area coveredby them, the degree of clumping of individ-uals, and the degree of presence of an indi-vidual plant (what proportion of the areassampled it occurs in). It is now used mainlyin a modified form as the Domin scale,which contains more divisions and is moreaccurate.

breeding system (mating system) Thepattern of matings between individuals in apopulation, including the method of mateselection or pollination, number of simul-taneous mates and permanence of pairbond, degree of inbreeding, genetic deter-mination of sex and incompatibility, andany other factors that place limits on thegenetic diversity of the offspring.

broad-leaved evergreen forest Seetemperate deciduous forest.

broken stick model See random nichemodel.

bromomethane See methyl bromide.

brood parasitism The act of leavingeggs or young to be reared by an individualthat is not the parent – usually an individ-ual of another species. For example, manycuckoos lay their eggs in the nests of otherbirds.

brown earth A type of SOIL found underdeciduous forests in temperate regions. It istypically rich in humus and slightly acid,and is a fertile soil for agriculture.

brown tree snake A species of snake(Boiga irregularis), native to Asia, whichwas accidentally introduced to the islandof Guam in a military cargo and caused theEXTINCTION there of 12 species of birds and6 species of lizards. See exotic.

browser A vertebrate herbivore thatfeeds on trees and shrubs, e.g. giraffe (Gi-raffa camelopardalis) and tapir (Tapirus).

Bryophyta (mosses) A phylum of sim-ple plants, the mosses, mostly found inmoist habitats. They have distinct gameto-phyte and sporophyte generations, the ga-metophyte being the dominant stage.

bryophytes (Bryata) A general term formosses (Bryophyta), liverworts (Hepato-phyta), and hornworts (Anthocerophyta),plant phyla that were formerly grouped to-gether in the phylum or division Bryo-phyta. In the Five Kingdoms classification,the term Bryophyta is reserved for themosses, the other groups being placed inseparate phyla.

bryozoans (moss animals; ectoprocts)Members of the phylum Bryozoa – a groupof aquatic invertebrates that form coloniescomposed of units called zooids. They arewidely distributed in both fresh and saltwater, mainly in shallow water, anchored

29

bryozoans

to the seabed or forming crusts on rocks,shells, ships, seaweeds, and the shaded pil-ings of breakwaters. The zooids are littlehydralike animals with a cilia-covered ten-tacle-bearing organ, the lophophore, filledwith fluid, which can be extended to pro-pel tiny food particles into the mouth.

budding 1. The production of buds onplants.2. See asexual reproduction.

buffer A solution that resists any changein acidity or alkalinity (i.e. a change in H+

concentration). Buffers are important inliving organisms because they guardagainst sudden changes in pH. They in-volve a chemical equilibrium between aweak acid and its salt or a weak base andits salt. In biochemistry, the main buffersystems are the phosphate (H2PO4

– /HPO4

2-) and the carbonate H2CO3/HCO3–) sys-

tems. They are also useful for controllingpH in in vitro experiments and cultures.See pH.

bulb See perennating organ.

bulk density The ratio of the mass of asample to its external volume (which in-cludes the volume of air and water thatmay be contained within it).

buttress root A kind of PROP ROOT (seeaerial root; stilt root) that is asymmetri-cally thickened, forming a planklike above-ground outgrowth at the base of the tree,providing extra support. Buttress roots arecommon in many tropical trees, includingmany species of fig (Ficus). See also man-grove.

budding

30

cactus Any member of the large an-giosperm family Cactaceae, which aremainly succulent xerophytic dicotyledonsfound in warm dry parts of the Americas.In many species the leaves are reduced tospines, which reduces transpiration, detersherbivores, and helps reflect the heat of thesun. Most species have succulent photo-synthetic stems that store water. The rootsystems are shallow and spreading, exploit-ing occasional showers. See xerophyte.

cadmium (Cd) A bluish-white relativelysoft metallic element found in traceamounts in the Earth’s crust, and also inzinc ores and certain other ores. It is highlytoxic if inhaled or ingested. Discharges ofcadmium into air or water are strictly lim-ited by law in most countries.

Caenozoic See Cenozoic.

Cainozoic See Cenozoic.

calcareous 1. Consisting of or contain-ing calcium carbonate.2. Growing on limestone or chalk or in soilrich in calcium carbonate.

calcareous soil (alkaline soil; basicsoil) A SOIL with a pH greater than 6.0.Such soils are found where chalk (calciumcarbonate) accumulates in the surface lay-ers, usually derived from calcareousbedrock such as limestone, or from a cal-careous sand deposit near coasts. Calcare-ous soils are commonest in regions of lightrainfall, and are rich in calcium ions andoften in other elements.

calcicole Describing a plant that thriveson neutral to alkaline soils high in calciumcarbonate. Compare calcifuge.

calcifuge Describing a plant that growson sandy or peaty soils, and cannot growon basic substrates, such as most chalk andlimestone soils. Compare calcicole.

calcium (Ca) A silver-white metallic el-ement that does not occur free in nature,usually forming compounds such as car-bonates (limestone and chalk) and sulfates(gypsum). The human body is 2% calcium,because calcium is a major constituent(70% by weight) of bones and teeth. Theelement is also involved in many metabolicand physiological processes. It also occursin the shells of many animals, such ascorals, mollusks, echinoderms, foramini-ferans, and some planktonic algae. Themain source of dietary calcium is milk andother dairy products, leafy green vegeta-bles, and oily fish.

calcium carbonate (CaCO3) A whiteor colorless compound commonly found innature as chalk, limestone, and calcite. Itmakes up about 4% of the Earth’s crust.The incorporation of calcium carbonate inthe shells of marine plankton and corals,mainly as the mineral aragonite, removescarbon dioxide from the atmosphere.

calorie A unit of energy equal to theamount of heat required to raise the tem-perature of one gram of water by 1°C atone atmosphere pressure. It has now beenreplaced by the SI unit the joule (1 calorie= 4.1855 J), but is still often used to desig-nate the energy-producing potential offoods, where the usual unit is the kilocalo-rie or Calorie (with capital letter ‘C’).

calorimetry The measurement of theamount of heat absorbed or evolved duringa chemical or physical reaction, change of

31

C

state, or dissolving of a solid or gas. For ex-ample, the calorific value of foods (theamount of heat released if the food is com-pletely oxidized) is measured using a BOMB

CALORIMETER.

Calvin–Benson cycle See photosynthe-sis.

Cambrian The earliest period of the Pa-leozoic era, about 590–510 million yearsago. It is characterized by the appearanceof algae and a proliferation of marine in-vertebrate animal forms, including ances-tors of most modern animals – theso-called Cambrian explosion. See also ge-ological time scale.

camouflage The color, pattern, orshape of an animal that has evolved so thatthe animal can blend with its background.For example, many moths take on thecolor and pattern of the bark on whichthey rest (see industrial melanism). Otherforms of camouflage include breaking upthe recognizable body outline by large,bold blocks of color or stripes, concealingshadows by having darker undersides thanback (countershading), or adorning thebody with pieces of debris from the sur-rounding environment (e.g. decoratorcrabs). Such concealment is also calledcrypsis.

CAM plant See Crassulacean acid me-tabolism.

campos See savanna.

cannibalism The eating of an animal byanother member of the same species. Thismay serve various purposes. For example,if their temporary pool starts to dry up andfood runs short, the tadpoles of desertspadefoot toads will start to eat each other;this ensures that at least some survive tobreed. Male lions taking over a pride of fe-males often kill and eat the existing young;this rapidly brings the mothers into repro-ductive condition, and the new pride malescan sire their own young. In crowded con-ditions, animals such as aquarium guppies

will eat their young, a means of controllingpopulation size.

canopy In a woodland, forest, or shrubcommunity, the uppermost layer of vegeta-tion, formed from the branches of thetallest plants. In complex forests, severalzones of the canopy are recognized – thelower zone, the upper zone consisting ofthe intermeshed crowns of trees, and theemergent zone, where the crowns of a fewtaller species emerge into the light abovethe general canopy. Compare groundlayer.

Cape Province An area of southernAfrica with one of the richest floras in theworld, containing some 8550 species offlowering plants in more than 1500 genera,with a high proportion (30%) of endemicgenera.Many ornamental plants and green-house plants originated in this region.

capillarity See surface tension.

captive breeding The breeding of ani-mals in captivity, usually in order to pre-serve an endangered species or to improvethe breeding stock by mating animals ofdifferent genetic backgrounds. If sufficientnumbers can be bred in captivity, animalsmay be released into suitable habitats inthe wild, helping re-establish populationsin areas from which the species has dis-appeared, e.g. the black-footed ferret(Mustela nigripes) of the United States, andthe Hawaiian goose (Branta sandvicensis)in Hawaii. In other cases, species may beprotected by captive breeding while threatssuch as introduced predators are removedfrom their native habitats. Certain endan-gered species of partulid tree snails, ren-dered extinct in their native habitat by therosy glandina snail, which was introducedto control yet another introduced predator,the giant African snail, survive only incaptivity. Where there are insufficientnumbers to establish a viable breedingpopulation, eggs or young may be fostered– placed with parents of a closely relatedspecies.

capture–recapture A method of esti-

Calvin–Benson cycle

32

mating population size or density of mo-bile animals. This involves catching a ran-dom sample of a population, marking theindividuals in some distinctive way, thenreleasing them back to mix with the rest ofthe population. At a later date another ran-dom sample is captured. Population size isestimated from the proportion of this sec-ond sample that bear the mark. Roughlyspeaking, the proportion of marked ani-mals in the second sample is inversely pro-portional to the size of the population.

carbamates Esters or salts of carbamicacid (NH2COOH), certain of which areused as insecticides or parasticides. Theseact by inhibiting cholinesterase, whichbreaks down the neurotransmitter acetyl-choline, vital to the transmission of manynerve impulses. Carbamates are contact in-secticides that kill larvae, nymphs, andadults. Although high levels of exposure tothese pesticides may cause carbamate poi-soning in humans and livestock, they breakdown more quickly than organophosphatepesticides.

carbohydrates A class of organic com-pounds that occur widely in nature andhave the general formula (Cx(H2O)y). Car-bohydrates are generally divided into twomain classes: sugars (saccharides) and poly-saccharides. They are both stores of energy(e.g. starch and glycogen) and structural el-ements in living systems (e.g. the celluloseof the plant cell wall).

carbon (C) A nonmetallic elementfound in all organic compounds. It is thebasis of all living matter and an essential el-ement in plant and animal nutrition. Car-bon enters plants as carbon dioxide and isassimilated during photosynthesis into car-bohydrates, proteins, and fats, forming thebackbones of such molecules. The elementcarbon is particularly suited to such a rolebecause it can form stable covalent bondswith other carbon atoms, and with hydro-gen, oxygen, nitrogen, and sulfur atoms. Itis also capable of forming double and triplebonds as well as single bonds and is thus aparticularly versatile building block. Car-bon, like hydrogen and nitrogen, is far

more abundant in living materials than inthe Earth’s crust. See also carbon cycle.

carbonates Compounds derived fromcarbonic acid or carbon dioxide, whichcontain the carbonate ion (CO2

3–). Theyare weak bases: in solution, the carbonateion can accept a hydrogen ion from water,forming a bicarbonate ion (HCO3

–) and ahydroxyl ion. They react with acids, form-ing a salt and releasing gaseous carbondioxide.

carbon cycle The circulation of carbonfrom the atmosphere into living organismsand, after death, back into the atmosphereagain. Plants, algae, and photosyntheticbacteria take in carbon dioxide from theatmosphere and during the process of PHO-TOSYNTHESIS fix the carbon into living tis-sues. Carbon is returned to the atmospherein carbon dioxide emitted by respiring or-ganisms or resulting from decay. The at-mospheric carbon dioxide concentration isalso affected by the burning of FOSSIL FUELS,which release carbon fixed millions ofyears ago, so adding to current levels. Car-bon dioxide is also given off during vol-canic eruptions. The anaerobic breakdownof peat, especially the enhanced rates ofbreakdown in the tundra as a result ofglobal warming, releases METHANE, an-other source of atmospheric carbon; evenmore significant is the release of methaneby livestock. Smaller amounts are releasedby decomposition in landfill sites. The in-corporation of carbon dioxide into calciumcarbonate in the shells of marine planktonand invertebrates traps carbon that is notrespired back into the atmosphere. See il-lustration overleaf.

carbon dating (radiocarbon dating) Amethod of estimating the age of organicmaterial; it is based on the ratio of radio-active carbon atoms to stable carbonatoms in the material. The radioactive car-bon-14 (14C) is formed by the interactionof cosmic rays with atmospheric nitrogen.Together with the other two carbon iso-topes, it is present in living organisms; oncean organism dies it ceases to absorb 14C,which steadily decreases, decaying at the

33

carbon dating

known rate of the half-life of 5730 ± 30years. An estimate of the age of a materialcan be obtained from the residual 14C pre-sent. It gives reasonably accurate resultsfor about 40,000–50,000 years. However,since the method was originally developedin 1946–47 by Willard F. Libby (1908–80), uncertainties have arisen as to the ac-tual 14C content of the atmosphere duringpast ages. It is now accepted that the con-tent varies in line with fluctuations in thecosmic ray activity that produces the 14C.Corrections to dates within the recent pastare now made through calibration withdates obtained through dendrochronology(tree-ring dating).

carbon dioxide (CO2) A colorlessodorless gas, heavier than air, that makesup at least 0.03% by volume in the atmos-phere. It is taken in by plants during PHO-TOSYNTHESIS and released during theRESPIRATION of living organisms and by theDECOMPOSITION of organic matter. It is alsoreleased during combustion and by vol-canic activity.

Carbon dioxide is soluble in water,forming a weak solution of carbonic acid(H2CO3). The oceans contain largeamounts of dissolved carbon dioxide, andact as buffers to counteract changes in at-mospheric carbon dioxide, such as in-creases due to the burning of FOSSIL FUELS

and forest trees. Carbon dioxide is aGREENHOUSE GAS, so contributes to GLOBAL

WARMING. The contribution of differenttypes of vegetation to atmospheric carbondioxide levels can be demonstrated bymeasuring the carbon dioxide flux – the netmovement of carbon dioxide between theland/soil surface and the atmosphere. Thiscan vary with the season as well as with thevegetation type.

carbon dioxide equivalent (CDE) Ameasure used to compare the emissions ofdifferent greenhouse gases: the amount ofcarbon dioxide by weight that would causethe same amount of radiative forcing as agiven weight of the other greenhouse gasbeing measured. Carbon dioxide equiva-lents are generally calculated by multiply-ing the mass of the gas of interest (inkilograms) by its estimated GLOBAL WARM-ING POTENTIAL.

carbon dioxide flux See carbon diox-ide.

carbon equivalent (CE) A metric meas-ure used to compare the emissions ofdifferent greenhouse gases. In the US,greenhouse gas emissions are usually ex-pressed as million metric tons of carbonequivalents (MMTCE). Carbon equivalent(CE) units can be converted from CARBON

DIOXIDE EQUIVALENT (CDE) units by multi-plying the CDE by the ratio of the molecu-lar weight of carbon to carbon dioxide (i.e.12/44).

carbon dioxide

34

organiccompoundsin animals

feedingfeedingfeedingorganiccompoundsin green plants

fossil fuels(coal and peat)

combustioncombustioncombustionrespirationrespirationrespirationand decayand decayand decayphotosynthesisphotosynthesisphotosynthesis

CO2free carbondioxide in theatmosphere

respirationrespirationrespirationand decayand decayand decay

Carbon cycle

carbonic acid (H2CO3) A weak acidformed in small amounts when CARBON

DIOXIDE dissolves in water:CO2 + H2O → H2CO3 → H+ + HCO3

–

Carbonic acid occurs in rainwater. Al-though it is a relatively weak acid, in runoffwater it is capable of dissolving CALCIUM

CARBONATE in rocks such as limestone toform caves.

Carboniferous The second most recentperiod of the Paleozoic era, some 355–280million years ago. It is named for the ex-tensive coal deposits that formed from theremains of vast swamp forests whichthrived in the warm, humid climate. In theUnited States the Carboniferous is often di-vided into the Mississippian or Lower Car-boniferous and the Pennsylvanian orUpper Carboniferous. In the United Statescoal formation was restricted to the Penn-sylvanian period. On land, amphibians anda few primitive early reptiles evolved, andin addition to treelike forms of clubmossesand giant horsetails, the first true trees ap-peared – primitive gymnosperms. Aquaticlife included sharks and coelacanths. Seegeological time scale.

carbon monoxide (CO) A colorlessflammable highly toxic gas produced bythe incomplete combustion of carbon, es-pecially during the burning of FOSSIL FUELS

in vehicle engines. Motor vehicles may ac-count for up to 98% of CO emissions inurban areas. Emissions can be reduced byfitting car exhausts with suitable CAT-ALYTIC CONVERTERS. Vehicles running onliquefied petroleum gas also have lowerCO emissions than gasoline-fueled ones.Carbon monoxide forms a complex withthe red blood pigment hemoglobin, pre-venting the red blood cells from carryingoxygen, hence its toxicity.

carbon sequestration The net removalof carbon dioxide from the atmosphereinto long-lived pools of carbon in terres-trial, marine, or freshwater ecosystems.Such pools (called carbon sinks) may beliving biomass in soils and vegetation or in-organic forms of carbon in soils and rocks

and in the calcareous shells of marineplankton and invertebrates.

carbon sink See carbon sequestration.

carbon tetrachloride (CCl4) (tetra-chloromethane) A colorless toxic liquidused in some dry-cleaning processes and incertain fire extinguishers. Under the MON-TREAL PROTOCOL, its use is being phasedout, because its vapor is a GREENHOUSE GAS.See global warming.

carbon to nitrogen ratio (C:N ratio)The ratio of carbon to nitrogen in organicmatter. When organic material is decom-posed in the soil, the C:N ratio has an ef-fect on the rate of DECOMPOSITION andhence on the amount of HUMUS formed andthe rate at which soil nitrogen is released orimmobilized. The rate of decompositionincreases as the C:N ratio becomes smaller,as microorganisms in general require aratio of about 30–35:1 for efficient diges-tion of compost or other organic material,materials with higher ratios having insuffi-cient nitrogen for rapid decomposition.Decomposition of material with high C:Nratios results in release of nitrogen from thedecomposing organic material into the soil.Most fresh plant material contains about40% carbon, but species differ in their ni-trogen content. During decomposition, upto 35% of the carbon present will be con-verted into humus if there is sufficient ni-trogen present, the remainder beingrespired as carbon dioxide. The C:N ratioof humus is about 10:1. See also carboncycle; nitrogen cycle.

carcinogen Any substance or agent thatcauses living tissues to become cancerous.Chemical carcinogens include many or-ganic compounds and certain inorganiccompounds such as asbestos. Physicalagents that can cause cancer include radio-active materials and x-rays. Many carcino-gens achieve their effect by causingmutations: they are mutagens.

Carnivora An order of mammals (seeMammalia) that contains flesh-eatingmammals such as dogs (Canis), foxes and

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Carnivora

wolves (Vulpes), cats (Felis), otters (Lutra),badgers (Meles), and bears (Ursus). Theirteeth are specialized for biting and tearingflesh and they have well-developed claws.A few members of the order are omnivo-rous, e.g. bears. The giant panda (Ail-uropoda melanoleuca) is a herbivore.

carnivore Any animal that eats mostlyflesh, especially a mammal of the orderCARNIVORA. Insect-eating plants are alsodescribed as carnivorous. Compare herbi-vore; omnivore.

carnivorous plant See insectivorousplant.

carr A subclimax stage in the SUCCESSION

of a HYDROSERE or HALOSERE, in which peathas built up but the soil is too wet for largetrees to grow. Carr is dominated by plantstolerant of waterlogging, such as alders(Alnus) and willows (Salix). See fen; sere.

carrying capacity (K) The maximumpopulation size that can be supported in-definitely by the available resources of agiven environment.

Carson, Rachel Louise (1907–64) USscience writer, who worked as a genetic bi-ologist (1936–52) and later as editor forthe US Fish and Wildlife Service. Herbooks, notably The Sea around Us (1951)and Silent Spring (1962), greatly increasedpublic awareness of the natural environ-ment and warned of the dangers of pollu-tion.

caste A specialized form of a social in-sect that performs a particular function inthe colony. Castes are distinguished bystructural and functional differences. Forexample, honeybees have three castes: thequeen (a fertile female) reproduces; work-ers (sterile females) gather food; drones(males) mate with the queen. Ants and ter-mites often have in addition special soldiercastes for defense. The caste system is anexample of POLYMORPHISM in animals.

CAT See control action threshold.

catabolism The breakdown by livingorganisms of complex molecules into sim-pler compounds, as happens in respiration.The main function of catabolic reactions isto provide energy, which is used in the syn-thesis of new substances, for work (e.g.muscle contraction), for transmission ofnerve impulses, and for maintenance offunctional efficiency. Compare anabolism.

catadromous See diadromous.

catalyst A substance that changes therate of a chemical reaction without beingaltered or used up in the reaction. Enzymesare highly specific biochemical catalysts.

catalytic converter A device fitted tovehicle exhaust systems that contains oneor more catalysts that reduce the emissionof pollutant gases such as carbon monox-ide, nitrogen oxides, and unreacted hydro-carbons. Basic converters first reduce theexhaust gases to eliminate oxides of nitro-gen, then oxidize them with additional airto eliminate carbon monoxide and un-burned hydrocarbons. Modern ‘three-wayconverters’ use different catalysts to speedup the reactions with different types ofgases, making it possible to deal simultane-ously with different emissions. In manycountries it is compulsory to fit catalyticconverters to all new cars. See pollution.

catastrophe A major change in the envi-ronment that causes widespread damageand the death of many organisms. Somecatastrophes are one-off events, such as theimpact of a large asteroid on global cli-mate, which may have contributed to themass EXTINCTION at the end of the CRETA-CEOUS period. Others recur, but at intervalssufficiently far apart in time that speciesare unable to adapt to them by natural se-lection, for example large volcanic erup-tions such as those of Mount St Helens inthe United States.

catchment area (drainage basin) Thearea of land from which water runs off ordrains into any given river valley or reser-voir.

carnivore

36

catch per unit effort (CPUE) In theharvesting of a natural resource such asfish, the total catch (biomass or numbers)divided by the total effort required (e.g. thenumber and size of ships and the numberof days spent fishing).

cation A positively charged ION, formedby removal of electrons from atoms ormolecules, e.g. H+, Ca2+. In electrolysis,cations are attracted to the negativelycharged electrode (the cathode). Compareanion.

cation exchange capacity (CEC) Thecapacity of a soil to hold CATIONS (posi-tively charged ions), usually expressed inmilliequivalents per 100 grams (meq/100g) of soil. It is an indication of the soil’sability to hold plant nutrients. A plant ob-tains the nutrients by exchanging onecation for another at the point soil colloidsmeet root colloids in the soil solution. CECchanges with pH, increasing with risingpH. The efficiency of CEC depends on therelative concentration of the cations, andtheir valences and diffusion rates. Highcation exchange capacities are typical ofclay minerals, in which spaces in the crys-tal lattices allow cations of similar size tobe incorporated into the lattice. Clay soilshave greater CECs than sandy soils. Or-ganic matter also has a high CEC, so soilswith plenty of HUMUS have high CECs.

catotelm See acrotelm.

CBD See Convention on Biological Di-versity.

CDE See carbon dioxide equivalent.

CE See carbon equivalent.

CEC See cation exchange capacity.

cell The basic unit of structure of all liv-ing organisms, excluding viruses. Prokary-otic cells (typical diameter 1 µm) aresignificantly smaller than eukaryotic cells(typical diameter 20 µm). Some organismsconsist only of single cells, e.g. amebae anddiatoms. The largest cells are egg cells (e.g.

ostrich, 13 cm in diameter), and the small-est are mycoplasmas (about 0.1 µm indiameter). All cells contain, or once con-tained, genetic material in the form of DNA,which controls the cell’s growth and someof its activities; in eukaryotes this is en-closed in the membrane-bounded nucleus.All are filled with cytoplasm, containingvarious organelles (membrane-bound struc-tures enclosing a specific environment inwhich certain cell processes are carried out,e.g. mitochondria are the sites of respira-tion, and chloroplasts are sites of photo-synthesis), and are surrounded by a plasmamembrane (cell membrane), which con-trols entry and exit of substances. Plantcells and most prokaryotic cells are sur-rounded by rigid cell walls, structures de-posited outside the plasma membrane. Inmulticellular organisms cells become spe-cialized for different functions; this iscalled differentiation. Within the cell, fur-ther division of labor occurs between theorganelles.

Celsius scale See temperature.

Cenozoic (Caenozoic, Cainozoic) Thepresent geological era, beginning some 65million years ago, and divided into two pe-riods, the Tertiary and the Quaternary. It ischaracterized by the rise of modern organ-isms, especially mammals and floweringplants. See also geological time scale.

center of diversity (gene center) A re-gion where a certain taxon shows greatergenetic diversity than it does anywhereelse. The term is often used in relation tocrop plants, especially to the region inwhich the particular crop is thought tohave originated. For example, numerouswild relatives of wheat and other relatedgrain crops occur in the Middle East, and itis believed that wheat was first domesti-cated there. Secondary centers of diversityrepresent areas where the crop has beencultivated for a long time, but where thereare no wild relatives. Centers of diversityare important sources of genes for the de-velopment of new cultivars. See genetic re-sources.

37

center of diversity

centigrade scale See temperature.

central-place foraging Foraging inwhich the foragers radiate out from a cen-tral base. For example, the sorties of leaf-cutter ants (Atta spp.), which have acentral underground nest. See foragingstrategy.

Cephalochordata See Chordata.

cereal A plant that yields a starchy grainused as food for animals and humans. Ce-reals (e.g. barley) are also used in the brew-ing industry. Most cereals are grasses, e.g.barley, corn (maize), sorghum, oats, rice,rye, and wheat. Wheat is the world’s mostwidely grown cereal, a staple food in tem-perate regions and one of the oldest cerealsknown. Milling of wheat dates back atleast 75 000 years. Rice is the secondlargest cereal crop, grown mainly in tropi-cal and subtropical regions, especially inAsia. Cereals grain are rich in carbohy-drates but relatively low in protein.

cesium (Cs) A rare silvery-white softmetallic element, with a very low meltingpoint (28°C). The weakly radioactive iso-tope cesium-137 has a half-life of 33 years,and is a waste product of nuclear powerstations processing uranium and pluto-nium, and is also used as a coolant in nu-clear reactors. If released into theatmosphere, it is carried into the soil byrainfall, and taken up by plants and graz-ing livestock, being found in their milk.

CFCs (chlorofluorocarbons) A group ofhydrocarbon-based compounds in whichsome of the hydrogen atoms are replacedby chlorine or fluorine atoms. They arechemically inert and were formerly widelyused as refrigerant liquids (freons) in re-frigerators (especially CFCl3 and CF2Cl2),aerosol propellants, and blowing agents inthe manufacture of plastics. However,there is strong evidence that when they es-cape to the upper ATMOSPHERE they causedepletion of the OZONE LAYER, hence theiruse had been phased out since the late1980s. Unlike most gases, CFCs are notbroken down as they rise through the tro-

posophere, but once they reach the stratos-phere the stronger light intensities therecause them to dissociate, releasing chlorineatoms that take part in the catalytic de-struction of ozone. CFCs are also GREEN-HOUSE GASES, with global warmingpotentials 3000–13000 times greater thanthat of carbon dioxide.

chalk A white soft fine-grained lime-stone consisting of calcium carbonate andthe remains of microscopic fossil planktonsuch as coccoliths and foraminifera. Somechalk deposits contain up to 99% pure cal-cite. Sponge spicules, diatoms, and radio-larian tests and grains of quartz contributesmall amounts of silica. Extensive chalkoutcrops of the upper Cretaceous periodoccur in western Europe, England, and theUnited States from South Dakota to Al-abama and south to Texas. Soils over chalkare alkaline, and support a characteristicCHALK GRASSLAND in areas of grazing bysheep or rabbits. See also calcareous soil.

chalk grassland A species-rich grass-land formed on thin CALCAREOUS SOILS

overlying CHALK, where it forms a subcli-max maintained by grazing, usually bysheep or rabbits. The flora is rich in herbs,especially rosette plants and other plantsthat grow close to the ground. It is foundmainly in southern and eastern England,northern France, and a few other parts ofwestern Europe.

chamaephyte See Raunkiaer’s life-formclassification.

chaparral See maquis.

Chapman cycle A cycle of reactionsproducing OZONE in the stratosphere byhigh-energy ultraviolet radiation from theSun. When ultraviolet rays strike moleculesof oxygen (O2), they split the moleculesinto separate oxygen atoms. The free Oatoms can then combine with oxygen mol-ecules to form ozone (O3) molecules.Ozone is a relatively unstable molecule,and if it absorbs certain wavelengths of ul-traviolet or visible light it regenerates oxy-gen molecule and oxygen atoms, releasing

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heat and raising the temperature of the at-mosphere locally. Ozone production ispowered by ultraviolet radiation:

O2 + UV → 2OO + O2 → O3

O3 + UV → O + O2or

O3 + O → O2 + O2See ozone layer.

character (trait) 1. In genetics, any rec-ognizable attribute of the PHENOTYPE of anorganism. Characters may be the result ofthe action of one or many genes. Heritabledifferences in a character shown by differ-ent individuals in a population are due todifferent forms (ALLELES) of the gene(s).2. In taxonomy, attributes such as form,anatomy, and physiology that are used bytaxonomist to compare taxa and constructphylogenies. Examples include leaf shapeand the arrangement of petals in a flower.

character displacement The diver-gence of the characteristics of two similarspecies in areas where their ranges overlap.This reduces COMPETITION as each speciesevolves to exploit a different ecologicalniche. For example, the nuthatches Sittaneumayer and S. tephronata show differ-ences in beak size in areas where theirranges overlap, allowing them to avoidcompetition by feeding on seeds of differ-ent sizes. This also occurs within a com-munity, when similar species in the samecommunity are compared. This is termedcommunity character displacement. Seealso adaptive radiation.

chart datum The lowest mean waterlevel measured over a period of time for agiven area. Chart datum is used as a refer-ence point against which to measure theheight of tides, the chart datum point being0.

checkerboard distribution In ISLAND

BIOGEOGRAPHY THEORY a species distribu-tion in which two or more ecologically sim-ilar species have mutually exclusive butinterdigitating distributions such that anyone island supports only one of the species,e.g. the cuckoo-doves Macropygia mackin-

layi and M. nigrirostris in the Bismarckarchipelago off the coast of New Guinea,and the distribution of nocturnal species oflorises (Nycticebus) and tarsiers (Tarsius)on small islands west of the WALLACE’S LINE

in Asia. If they occurred together, suchspecies would probably compete for thesame resources. These distributions are rel-atively rare.

chelating agent A chemical that formsring complexes with metal ions, especiallyheavy metal ions, for example, EDTA,which is use to treat heavy metal poison-ing. Chelating agents may be used in BIORE-MEDIATION to bind toxic heavy metals tosoil particles, a process known as seques-tration.

Chelicerata See Arthropoda.

chemical competition See competi-tion.

chemical oxygen demand (COD) Theamount of oxygen consumed in the oxida-tion of organic and oxidizable inorganicmatter in a sample of water, typically ex-pressed as milligrams of oxygen per liter ofwater. COD is usually determined by incu-bating known volumes of water withknown quantities of chemical reagents atabout 150°C until oxidation is complete,then determining the amount of thereagent changed by means of colorimetryor spectrophotometry. COD is used in in-dustrial and municipal laboratories dealingwith industrial waste and chemically pol-luted water. Compare biological oxygendemand.

chemiluminescence See phosphores-cence.

chemoautotroph An organism that de-rives energy for the synthesis of organicmolecules from the oxidation of inorganiccompounds. Only certain specialized bac-teria are chemoautotrophs. For example,Nitrobacter is a chemoautotroph that oxi-dizes nitrite to nitrate.

chemoheterotroph An organism that

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chemoheterotroph

derives energy for the synthesis of organicmolecules from the oxidation of organiccompounds. All animals are chemo-heterotrophs.

chemosterilant 1. A chemical used tosterilize an object or surface, such as surgi-cal instruments, laboratory benches, ani-mal housing, and air ducts. Seeds and soilsmay also be chemically sterilized. Chemo-sterilants are substances that kill bacteriaand other pathogenic organisms. High-level sterilants are capable of killing bac-teria, fungi, viruses, and spores. Somechemosterilants are available for house-hold use, e.g. sodium hypochlorite (house-hold chlorine bleach).2. A chemical used to sterilize (in the re-productive sense) pests and thus reduce thenumber of offspring in the subsequent gen-eration. For example, chemically sterilizedlaboratory-reared male fruit flies may bereleased into the wild population to com-pete for mates with normal males.

chemotaxonomy The classification oforganisms according to their chemicalmakeup. The compounds involved are usu-ally primary and secondary products ofmetabolism.

chemotroph An organism that derivesenergy for the synthesis of organic require-ments from chemical sources. Mostchemotroph organisms are HETEROTROPHS

(i.e. CHEMOHETEROTROPHS) and their en-ergy source is always an organic com-pound; animals, fungi, and most bacteriaare chemoheterotrophs. In autotrophs (i.e.CHEMOAUTOTROPHS) the energy is obtainedby oxidation of an inorganic compound;for example, by oxidation of ammonia tonitrite or a nitrite to nitrate (by nitrifyingbacteria), or oxidation of hydrogen sulfideto sulfur (by colorless sulfur bacteria). See autotroph; chemoautotroph; chemo-heterotroph; heterotroph. Compare photo-troph.

Chernobyl nuclear accident A seriousaccident that occurred during a badlyplanned experiment at a nuclear powerplant at Chernobyl, near Kiev, in the

Ukraine, on April 25, 1986. Following anexplosion, fire, and partial meltdown,radioactive substances, including radio-active isotopes of cesium, cobalt, iodine,krypton, and xenon, were released into theatmosphere and carried over a wide areaon the wind. Some 30 workers and fire-fighters at the Chernobyl plant died withina few hours, 200 became acutely sick, andothers exposed to the radioactive cloudhave died since (some estimates put thedeaths so far at 120 000). Farther from thecenter of the pollution, in the UK, pro-jected excess deaths are only 50 over the 40years following the accident. The area formiles around Chernobyl was heavily cont-aminated and crops can no longer begrown there. 135 000 local residents in a30-mile radius of the plant have been per-manently relocated. The toxic cloudreached Scandinavia, the UK, and conti-nental Europe as far south as Italy, and thechemicals were brought down into the soilby rain, especially in upland areas, render-ing livestock (including reindeer) and theirmilk unsafe for human consumption. Pas-ture remains contaminated in places today.Humans were exposed to gamma radiationfrom the pollutant cloud, gases, and conta-minated particulates from the air, andother contaminants in food. The reactorhas been buried in a concrete sarcophagusto contain the radioactivity. See also nu-clear energy.

chernozem (black earth) A freely drain-ing calcareous soil found in regions withhigh summer rainfall, characterized byhumus distributed fairly evenly throughoutthe profile. The upper horizon of the soil isvery thick and black with humus; it mergesinto a brown lime-rich horizon below.Chernozems are rich in plant nutrients andhave a good crumb structure, allowingdrainage and aeration. They are associatedwith grasslands in temperate climates suchas the prairies of North America, the pam-pas of South America, and the Russiansteppes, where they support highly produc-tive agricultural systems.

chill factor See wind chill.

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chimera An individual or part of an in-dividual in which the tissues are a mixtureof two genetically different tissues. It mayarise naturally due to mutation in a cell ofa developing embryo, producing a line ofcells with the mutant gene and hence dif-ferent characteristics compared to sur-rounding cells. It may also be inducedexperimentally. For example, two mouseembryos at the eight-cell stage from differ-ent parents can be fused and develop into amouse of normal size. Analysis of the geno-types of the tissues and organs of such amouse reveals that there is a random mix-ture of the two original genotypes. Inplants, chimeras between different speciesmay arise as a result of grafting: a bud maydevelop at the junction between the scionand stock with a mixture of tissues fromboth. Many variegated plants are alsochimeras: a mutation has occurred in a sec-tor of tissue derived from a particular layerof the shoot apex, resulting in subsequentchlorophyll deficiency. For example, in awhite-edged form of Pelargonium, the out-ermost layer is colorless, indicating a lackof chlorophyll due to a mutation, but incontrast to animal chimeras the geneticmixing is confined to this region, and doesnot extend throughout the plant.

chi-squared test (χ2 test) A statisticalmethod for testing how well a set of exper-imental values fits the set of values thatwould be expected from some particularhypothesis. If a particular observed value iso and the corresponding expected (hypoth-esis) value is e, then for each observedvalue o one calculates (o – e)2/e. These re-sults are added for all values in the set ofobservations to give the parameter χ2. Thegoodness of fit can be obtained from avail-able tables of χ2. Chi is the Greek letter χ.

chlorinated hydrocarbons Hydrocar-bons in which one or more hydrogen atomsare replaced by chlorine atoms. Some chlo-rinated hydrocarbons are contact poisons,and are used as INSECTICIDES, e.g. aldrin,dieldrin, chlordane, DDT, and methoxy-chlor. Use of these insecticides was bannedin most Western countries after the discov-ery that they persisted through the food

chain in the fatty tissues of animals andwere toxic, particularly to birds of prey,which declined sharply in numbers. Theyare still used in some tropical countries. Seealso CFCs.

chlorination The treatment of drinkingwater and swimming pools with chlorineor chlorine compounds such as sodiumhypochlorite or calcium hypochlorite todisinfect the water, promote coagulation,or control tastes and odors. For small vol-umes of water, these compounds may beadded in tablet form; for larger volumesgaseous chlorine is bubbled through thewater from pressurized steel containers.

chlorine (Cl) An element found in traceamounts in plants and one of the essentialnutrients in animal diets. Common tablesalt, an important item of the diet, is madeup of crystals of sodium chloride. The chlo-ride ion (Cl–) is important in bufferingbody fluids and, because it can pass easilythrough cell membranes, it is also impor-tant in the absorption and excretion of var-ious cations.

chlorofluorocarbons See CFCs.

chlorophyll One of a group of photo-synthetic pigments that absorb blue-violetand red light and reflect green light, im-parting the green color to green plants.Chlorophylls are involved in the light reac-tions of PHOTOSYNTHESIS, absorbing lightenergy and initiating electron transport.Photosynthetic bacteria have a slightly dif-ferent group of chlorophyll pigments, bac-teriochlorophylls, similar to chlorophyll a,but absorbing at slightly longer wave-lengths, including far-red and infraredlight.

chloroplast A photosynthetic organellecontaining CHLOROPHYLL and other photo-synthetic pigments. Chloroplasts are foundin all photosynthetic cells of plants andprotoctists, but not in photosyntheticPROKARYOTES (i.e. bacteria). A chloroplasthas a membrane system containing the pig-ments, on which the light reactions ofPHOTOSYNTHESIS occur. The surrounding

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chloroplast

gel-like ground substance, or stroma, iswhere the dark reactions occur.

chlorosis The loss of chlorophyll fromplants, resulting in yellow (chlorotic)leaves. It may be the result of the normalprocess of senescence, lack of light, lack ofkey minerals for chlorophyll synthesis(particularly iron and magnesium), or dis-ease. See deficiency disease.

cholera An acute infection of the smallintestine caused by the bacterium Vibriocholerae. The bacterium is taken into thebody with contaminated food or drink,and is especially common in areas wherethere is poor sanitation and no clean waterfor drinking and cooking. Bacterial toxinsreleased in the intestine trigger severe wa-tery diarrhea, vomiting, and rapid dehy-dration. The skin pales, blood pressurefalls, and severe muscular cramps set in.This is followed by coma and death. Thedisease can be rapidly reversed by adminis-tering salt solutions, further aided byantibiotics. Vaccination provides only par-tial, short-lived protection.

Chordata A major phylum of bilaterallysymmetrical metamerically segmentedcoelomate animals, characterized by thepossession at some or all stages in the lifehistory of a dorsal supporting rod, the no-tochord. The dorsal tubular nerve cord liesimmediately above the notochord and anumber of visceral clefts (gill slits) are pre-sent in the pharynx at some stage of the lifehistory. The post-anal flexible tail is themain propulsive organ in aquatic chor-dates. The phylum includes the subphylumCraniata (Verebrata), in which the noto-chord is replaced by a vertebral column(the backbone). There are two other sub-phyla, the Urochordata and the Cephalo-chordata (sometimes known collectively asthe Acrania or Protochordata).

CHP plant See combined heat andpower plant.

chromatid One of a pair of replicatedchromosomes found during the prophase

and metaphase stages of MITOSIS and MEIO-SIS.

chromium (Cr) A hard silvery metalthat occurs naturally in the mineralchromite (FeO.Cr2O4). Chromium is a mi-cronutrient for animals, needed in traceamounts; it is involved in glucose metabo-lism. Dietary sources of chromium includemeat, dairy products, pulses, and grains.Chromium can reach waterways andsewage sludge via effluents from metal-processing industries and tanneries, and istoxic above levels of about 2 mgl–1, espe-cially when also in the presence of othermetals such as nickel.

chromosome One of a group of thread-like structures of different lengths andshapes in nuclei of eukaryotic cells. Theyconsist of DNA with RNA and protein(mostly histones) and carry the GENES.Each chromosome contains one DNA mol-ecule, which is folded and coiled. The namechromosome is also sometimes given to thegenetic material of bacteria, though thesehave a rather different structure and, withthe exception of a few Archaea, lack his-tone-like proteins. During nuclear divisionthe chromosomes are tightly coiled and areeasily visible through the light microscope.After division, they uncoil and may be dif-ficult to see. Along the length of the un-coiled chromosomes are bead-likestructures called nucleosomes, highly orga-nized aggregations of DNA and histones.The number of chromosomes per nucleusis characteristic of the species (e.g. humanshave 46).

chromosome mutation See mutation.

ciliates See Ciliophora.

Ciliophora A phylum of protoctistscontaining some of the best known proto-zoans. All have cilia for locomotion, a con-tractile vacuole, and a mouth. Most havetwo types of nuclei, the meganucleus con-trolling normal cell metabolism, and thesmaller micronucleus controlling sexual re-production (conjugation). Binary fissionalso takes place. Some (e.g. Paramecium)

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are covered with cilia. Others (e.g. Vorti-cella) have cilia only round the mouth, andin some (e.g. Stentor) these cilia are spe-cialized for feeding.

circadian rhythm A daily rhythm ofvarious metabolic activities in animals andplants. Such rhythms persist even when theorganism is not exposed to 24-hour cyclesof light and dark, and are thought to becontrolled by an endogenous biologicalclock. Circadian rhythms are found in themost primitive and the most advanced oforganisms. Thus Euglena (a single-celledprotoctist) shows a diurnal rhythm in thespeed at which it moves to a light source,while humans are believed to have at least40 daily rhythms.

CITES (Convention on InternationalTrade in Endangered Species) The Con-vention on International Trade in Endan-gered Species of Wild Fauna and Flora. It isan agreement signed between some 160countries, prohibiting commercial trade inendangered species and products derivedfrom them. As more species have becomeendangered, the list of protected specieshas lengthened. CITES sets out degrees ofprotection for different species, allowingthreatened but not ENDANGERED SPECIES tobe exported and imported under license.

clade In CLADISTICS, a branch of a lineagethat results from the splitting of an earlierlineage. The split results into two newTAXA, each of which is represented as aclade (branch) in a phylogenetic diagram.See phylogeny.

cladistics A method of CLASSIFICATION inwhich the relationships between organismsare based on selected shared characteris-tics. These are generally assumed to havebeen derived from a common ancestor, inthe evolutionary process of cladogenesis,although the ‘transformed cladists’ believethat shared characteristics alone provide alogical basis for classification without pos-tulating evolutionary relationships. Thepatterns of these shared characteristics aredemonstrated in a branching diagramcalled a cladogram. The branching points

of the cladogram may be regarded either asan ancestral species (as in an evolutionarytree) or solely as representing shared char-acteristics.

Cladistics assumes that the closeness ofa relationship depends on the recentness ofcommon ancestry, indicated by the numberand distribution of shared characteristicsthat can be traced back to a recent com-mon ancestor. Cladistics also regards theonly true natural groups as those contain-ing all the descendants of a common ances-tor.

cladogenesis See cladistics.

cladogram See cladistics; dendrogram.

clarification The removal of suspendedor colloidal material from a liquid. For ex-ample, sewage sludge is clarified by settlingin tanks, followed by the FLOCCULATION ofsuspended and colloidal matter. An alter-native method, used in the treatment of in-dustrial wastes, is FLOTATION, in whichsuspended particles, such as metal hydrox-ides, are carried to the surface of the liquidby gas bubbles that adhere to the particles.The scum is then skimmed off. FILTRATION

is seldom a suitable option, because the fil-ters tend to clog up quickly, but it may beused to further clarify a liquid that has al-ready been processed by the above proce-dures.

class A taxonomic rank that is subordi-nate to a phylum (or sometimes a divisionin plant taxonomy) and superior to anorder.

classification The grouping and arrange-ment of organisms into a hierarchical order(hierarchical classification). Each level oforganization is called a TAXON. In the mostwidely used classification systems for livingand fossil organisms, the major taxa, in de-scending rank, are DOMAIN, KINGDOM, PHY-LUM, CLASS, ORDER, FAMILY, GENUS, SPECIES,and RACE or VARIETY. The aim of classifica-tion is usually to aid in identification of or-ganisms or to represent their phylogeneticrelationships or, ideally, both. An impor-tant aspect of classifications is their predic-

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classification

tive value. For example, if a characteristicis found in one member of a group ofplants, then it is also likely to be found inthe other members of that group eventhough the characteristic in question wasnot used in the initial construction of theclassification. The validity of existing clas-sification systems is now being tested bymRNA studies. See Five Kingdoms classifi-cation; phylogeny.

clay An extremely fine-textured soilmade up of small mineral particles lessthan 0.002 mm in diameter, formed mainlyfrom hydrated aluminum and magnesiumsilicates. Clay soils are derived fromgranitic rocks. Clay minerals have highcation exchange capacities: spaces in thecrystal lattices allow cations (positive ions)of similar size to be incorporated into thelattice. However, there is no such capacityto hold or exchange anions (negative ions)such as nitrate (NO3

–) and sulfate (SO42-),

and these tend to leach from clay soils. Themain source of anions in clay soils ishumus. Clay soils are poorly aerated, sincethe air spaces between the particles are verysmall. They may also become very stickyand difficult to work when wet and caneasily become waterlogged. Nutrient avail-ability to plants can be a problem becausethe nutrients may become chemicallybound to the surfaces of the particles. Seealso soil structure; soil texture.

Clean Air Act Legislation initially in-troduced in the USA in 1970 that aimed toreduce air pollution, especially sulfur diox-ide emissions, from industries and motorvehicles. It was initially administered bythe then newly formed ENVIRONMENTAL

PROTECTION AGENCY (EPA). The act identi-fied air pollutants and set out primary andsecondary standards for each. For eachpollutant, there are specific limits, proce-dures for reducing levels, and timescalesfor compliance. The primary standard pro-tects human health, and the secondarystandard encompasses potential damage toproperty and the environment. SimilarClean Air Acts were introduced in the UKin 1956 and 1958, outlawing the burningof coal in defined city areas; these made a

significant impact on reducing London’sinfamous smogs. 1990 amendments to theUSA’s Clean Air Act permitted transfer-able-allowance schemes that allow compa-nies to buy and sell pollution credits (thelegal rights to produce specific amounts ofpollution, in particular sulfur dioxide, ni-trogen oxides, carbon monoxide, lead,ozone, and particulate matter, as well ashazardous air pollutants (HAPs) such asthe vapors released by dry cleaning plants,chemical plants, and printing plants). Bypurchasing these credits from cleaner com-panies, companies with high emissions canremain in business while they convert tocleaner technologies. The Clean Air Actsthat apply today control emissions by fac-tories, motor vehicles, and aircraft, andalso the production and use of chemicalsthat impact on air quality, the OZONE

LAYER (e.g. CFCS), and ACID RAIN deposi-tion.

clean fuels Fuels that cause relativelylow levels of polluting emissions. They in-clude such power sources as FUEL CELLS,natural gas, alcohol, and BIOFUELS.

cleaning station A location to whichfish come so that one or more individualsof another species (usually a fish or shrimp)may seek out and remove dead surface ma-terial and external parasites, includingthose from inside their mouths. This is asymbiotic relationship, and is often accom-panied by specific displays by which thecleaner indicates its willingness to clean ordisplays coloration that the ‘customer’ canrecognize, and the ‘customer’ signals its de-sire to be cleaned. This enables even preda-tory fish to be cleaned by potential preyspecies. See symbiosis.

clean technology A means of providinga human benefit that, overall, uses fewerresources and causes less environmentaldamage than alternative means with whichit is economically competitive. For exam-ple, the use of CLEAN FUELS to generate elec-tricity or automotive power.

clean-up technology (end-of-pipe tech-nology) Technology used to clean up

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pollution, in contrast to clean technology,which is technology designed to generateless pollution in the first place. Flue gasdesulfurization is an example; it involvesgreater costs than using fuels that releaseless sulfur dioxide, and also reduces the ef-ficiency of the process. Compare cleantechnology.

Clean Water Act A legislative measureintroduced in the United States in 1972 ‘torestore and maintain the chemical, physi-cal, and biological integrity of the nation’swaters’ so that they can support ‘the pro-tection and propagation of fish, shellfish,and wildlife and recreation in and on thewater.’ The Act aims to regulate dischargesof POLLUTANTS into waterways, financetreatment plants for municipal wastewater,and manage polluted runoff. It does notdeal directly with groundwater issues. Arecent development has been trials of emis-sions trading for nutrients such as phos-phates, nitrates, and ammonia dischargesfrom farms and industrial premises (seeClean Air Act).

clear-cutting The felling of timber inwhich all the trees are removed from a site.This greatly increases runoff and theLEACHING of nutrients, especially nitrates.

clear-felling The cutting down of all thetrees in the forest.

Clements, Frederic Edward (1874–1945) American ecologist, noted for hisoriginal work Research Methods in Ecol-ogy (1905), in which he described the maintechniques of field work.

climate The average pattern of WEATHER

at a place, including solar radiation, tem-perature, humidity, precipitation, wind ve-locity, and atmospheric pressure. Climatedepends on the variation in atmosphericconditions at a location over a period ofyears. As well as the conditions that mightbe expected at different times of year, italso encompasses the extremes reachedand the frequency of less common weatherevents. Thus the precise climate of a place

depends on the timescale over which it isbeing considered.

Solar radiation drives the atmosphericcirculation, which is modified by the CORI-OLIS FORCE (the effect of the Earth’s rota-tion) and topography. Solar radiationvaries with latitude: at the equator the Sunis overhead, and day length varies little allyear round. At the poles the Sun’s raysstrike the Earth at a considerable angle, soincoming solar radiation is less, anddaylength varies from 24-hour daylight inmidsummer to 24-hour night in midwinter.Because of the tilt of the Earth’s axis, solarradiation at a given site varies through theyear, and the farther from the equator, thegreater the seasonal variation in radiation.Altitude also affects climate, since atmos-pheric pressure decreases, solar radiationincreases, and wind speed is usually greaterat higher altitudes. Warm air holds moremoisture than cold air, and this affects hu-midity. Humidity is also affected by dis-tance from the sea.

Land and water have different specificheats: they heat up and cool down at dif-ferent rates, land warming and coolingbeing much faster than that of water. Thusthe continental interiors have more ex-treme climates than coastal areas, withhotter summers and colder winters. Dis-tance from the sea also affects precipita-tion, as does the distribution andorientation of mountain ranges. Majorocean currents also affect climate. For ex-ample, the presence of the GULF STREAM, awarm current that flows across the northAtlantic and warms the coast of westernEurope, especially the British Isles, meansthat temperatures in London, for instance,are warmer than those in Montreal in win-ter, although London is farther north thanMontreal.

The Köppen climate classification sys-tem is widely used and is based on themajor vegetation zones and their climaticrequirements. It recognizes five major cli-mate types, which are further subdividedon the basis of precipitation quantity anddistribution through the year. These are:A Tropical moist climates: the averagemonthly temperature remains above 18°C(65°F) all year round, and there are no sea-

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sons. Annual rainfall is greater than 1500mm, and is greater than total evaporation,so there is high humidity. These climatesare between the equator and 15-25° lati-tude. They include tropical wet climates,where rainfall occurs all year; tropicalmonsoon climates, when most of the rain-fall is during the hottest months and thereis a dry season of 3-5 months, and tropical

wet and dry (savanna) climates, with along, dry winter.B Dry climates: on average, evaporationexceeds precipitation throughout the year,so there are no permanent streams. Theseclimates are found between 20° and 35° oflatitude.C Mild subtropical mid-latitude (warmtemperate) climates: The coldest month

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has an average temperature below 18°C(65°F) but above –3°C (27°F), and at leastone month has an average above 10°C(50°C). There are pronounced summer andwinter seasons. These climates are foundbetween 30° and 50° of latitude. They in-clude the humid subtropical climate withhot warm summers with precipitationfrom frequent thunderstorms and mildwinters with precipitation from cyclones;Mediterranean climates with rainfallmainly in winter from cyclones and verydry summers due to the sinking air of sub-tropical highs; and marine (oceanic) cli-mates that are humid, with mild winters,during which heavy precipitation fallsfrom depressions, and short dry summers,as along the western coasts of continents inthe mid-latitudes.D Moist continental mid-latitude (cooltemperate) climates: the coldest month hasan average temperature below –3°C (27°F)but more than –30°C, and the warmestmonth has an average above 10°C (50°C).These climates are found in the mid-lati-tudes north or south of 50°. The wintersare severe, with heavy snow and strongwinds blowing from continental or polarregions.E Polar climates: the average temperatureof the warmest month is below 10°C(50°C). There are no marked seasons. Inthe polar tundra there is permafrost – soilfrozen permanently to depths of hundredsof meters; the polar ice caps are perma-nently covered in ice and snow.

climate change In general, the long-term fluctuations in the Earth’s climate.More particularly the term is used to de-scribe a significant change from one cli-matic condition to another on a globalscale; for example the shifts from ice agesto warmer periods that have taken placeduring the Earth’s geologic history. Suchchanges occur naturally, sometimesabruptly but at other times over consider-able timescales, from changes, for exam-ple, in the Earth’s orbit, the energy outputfrom the Sun, volcanic eruptions, orogeny(mountain building), meteor strikes, orlithospheric motions. Over considerablyshorter timescales more recent climate

changes include the periods of warmer-than-average temperatures and cooler thanaverage periods. More recently, the studyof climate change has sometimes beendominated by the concept of GLOBAL

WARMING and other climate changescaused by human activities.

climate zone A division of the globeinto a sequence of geographical regions de-fined by their climates, in particular thetemperature and amount of precipitationand their distribution through the year.These zones are typically associated withparticular types of natural vegetation.

climatic climax A climax communitythat is directly due to the influence of cli-mate. See climax community.

climatic factor An influence that is dueto climate, for example, temperature, hu-midity, wind speed, precipitation, atmos-pheric pressure, solar radiation. Aspects ofclimate affect living organisms. See alsoedaphic factor.

climatic optimum The period of high-est prevailing temperature since the last ICE

AGE. In most parts of the world this oc-curred about 4000–8000 years ago.

climax adaptation number A phy-tosociological ordering system in whichspecies are assigned a value in the range 1to 10 according to their importance values(the sum of each species’ relative density,relative frequency, and relative abundanceexpressed as percentages). The importancevalue may range from 0 to 300 and enablesspecies to be ranked according to their phy-tosocoiological importance.

climax community The final COMMU-NITY in a SUCCESSION of natural plant com-munities in one area under a particular setof conditions. For example, the temperaterainforests of the Pacific northwest of theUnited States are probably climax commu-nities. A climax is in a self-perpetuatingsteady state, at least for a time, althoughthere are constant small changes, such aswhen a tree dies and pioneering plants in-

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climax community

vade or a different species replaces it. A cli-max may be temporarily disrupted by nat-ural disasters such as hurricanes orvolcanic activity. If there are changes in theenvironment (for example, changes in tem-perature, weather patterns, or drainage) orin the local species pool (e.g. if exoticspecies invade), a different climax commu-nity will evolve. Succession to a climaxmay also be arrested and held at any stageby human intervention; for example,heathlands and many grasslands resultfrom and are maintained by grazing, whichprevents the growth of trees and shrubs.Such an equilibrium is called a subclimax,plagioclimax, or biotic climax.

Early proponents of the climax conceptthought that a single climax community –a monoclimax – would dominate in a givenclimatic region. However, the nature of theclimax community is also affected by localsoil types, topography, and aspect, and thefrequency of natural phenomena such asfire, and the composition of the localspecies pool. So a single climatic regionmay contain a mosaic of several differentclimax communities, often grading intoone another – a polyclimax. See clisere. Seealso climax theory; climate zone.

climax theory The theory that vegeta-tion SUCCESSION eventually reaches a stateof equilibrium with its environment. Thereare two basic versions of the theory: mon-oclimax, proposed by F. E. Clements in1904 and 1916, which predicts a specificend-point determined by climate, and poly-climax, proposed by A. G. Tansley in 1916and 1920, which predicts a CLIMAX COM-MUNITY determined by edaphic conditionsor other conditions such as fire. The poly-climax version is most widely acceptedtoday, since the climate rarely remains suf-ficiently stable for long enough to enable asingle vegetational climax to be reached.

cline A gradual change in phenotype, es-pecially morphology, shown by a species ofother related groups of organisms acrossits geographical range, usually along a lineof environmental or geographical transi-tion. It reflects underlying changes in geno-type.

clint See limestone pavement.

clisere A succession of CLIMAX COMMU-NITIES in a given area over time, each givingway to the next as a result of climaticchanges. See sere.

clonal dispersal In a MODULAR ORGAN-ISM, the growth or movement away fromeach other of the component modules. Thismay or may not involve separation fromthe parent. For example, the polyps of acoral; the buds of a yeast colony; or theseparation of successive whorls of leaveson a stem as the internodes elongate; or thegrowth of runners (e.g. strawberry) thatproduce shoots at intervals, which becomeindependent plants. Such plants may coverlarge areas – a single clone of aspen has beknown to cover 14 hectares.

clone 1. A group of organisms or cellsthat are genetically identical. In nature,clones are derived from a common ances-tor by mitotic cell division, asexual repro-duction, or parthenogenesis.2. Identical copies of a segment of DNAthat has been cloned in a cloning vectorsuch as a plasmid, bacteriophage, or bac-terium. Such DNA clones form the basis ofDNA libraries (GENE BANKS).

cloning The production of geneticallyidentical organisms (CLONES) or cells froma parent organism or cell. This may happennaturally by asexual reproduction, or itmay be done by tissue culture, embryo cul-ture, and other techniques. Cloning is animportant part of GENETIC ENGINEERING.

cloning vector See vector.

closed canopy A CANOPY in which thecrowns of individual trees overlap to forma continuous layer.

closed community See communitystructure.

closed population A population inwhich OUTBREEDING with individuals fromother populations is prevented by a geneticbarrier. This is an early step in SPECIATION.

climax theory

48

cloud A suspension of tiny waterdroplets or ice particles in air, produced bythe condensation of water vapor. Conden-sation happens around condensation nu-clei – solid particles (e.g. dust) or ions.These include dust, smoke, ash, ammo-nium salts from vehicle exhausts, oceansalt, sulfate particles produced by phyto-plankton, and organic vapors from forests.Most are less than 0.2 µ in diameter, butsome are greater than 1 µ. Condensationnuclei affect the cloud droplet size, whichaffects both the amount of precipitationand the radiation transfer properties of thecloud.

Clouds form when moist air is cooledbelow its dew point (see dew). This com-monly happens as warm moisture-laden airrises and is cooled by expansion as, for in-stance, when air warmed by the hot groundrises by convection (e.g. in the tropics,where clouds form by midday and rain fallsin the afternoon); where a cold front nearthe ground pushes up a mass of warmer airas it moves forward; or where warm andcold air currents mix.

cloud forest Tropical forest, usuallyfound above 1000 meters elevation, whererainfall is heavy and moisture-laden air isdeflected upward by the mountain, form-ing clouds for most of the day. The highhumidity and abundant moisture leads tolow tree growth, often 5–8 meters, but sup-ports dense growth of EPIPHYTES, especiallymosses, ferns, filmy ferns, clubmosses, andlichens. In the forest clearings, ferns andherbs such as begonias often reach largesizes.

clumped dispersion See dispersion.

cluster sampling See sampling.

Cnidaria A large phylum of aquatic,mostly marine invertebrates – the mostprimitive of the multicellular animals.Cnidarians are radially symmetrical anddiploblastic, the body wall having two lay-ers separated by a layer of jelly (mesoglea)and enclosing the body cavity (coelen-teron). The single opening (mouth) is sur-rounded by a circle of tentacles, which are

used for food capture and defense and maybear stinging cells (cnidoblasts). Twostructural forms include the sedentarypolyp (e.g. Hydra, sea anemones), and thecolonial CORALS and the mobile medusa(jellyfish); either or both forms occur in thelife cycle. See also Anthozoa.

coagulation The clumping of particlesinto larger particles. It is an importantstage in the processing of SEWAGE, in whichsmall light suspended particles that will nototherwise settle out are first clumped to-gether using chemical coagulants such asaluminum sulfate to form lumps calledflocs. This process is called flocculation.After coagulation, the suspension is trans-ferred to a sedimentation tank, and the(now larger) particles will settle out.

coal A carbonaceous deposit formedfrom the remains of fossil plants. Most ofthe world’s coal deposits formed about300 million years ago during the Carbonif-erous (Pennsylvanian) period, under warmtropical or subtropical climates when vastswamp forests flourished. The stages in theformation of so-called humic or woodycoals pass through partially decomposedvegetable matter such as PEAT, through lig-nite, subbituminous coal, semianthracite,to anthracite. During this process, the per-centage of carbon increases and volatilecomponents and moisture are graduallyeliminated. Sapropelic coals are derivedfrom algae, spores, and finely divided plantmaterial. See Carboniferous; fossil fuel.

COD See chemical oxygen demand.

codon A group of three nucleotide basesin a messenger RNA (mRNA) moleculethat codes for a specific amino acid or sig-nals the beginning or end of the message(start and stop codons). Since four differ-ent bases are found in nucleic acids thereare 64 (4 × 4 × 4) possible triplet combina-tions. The arrangement of codons alongthe mRNA molecule constitutes the geneticcode. When synthesis of a given protein isnecessary the segment of DNA with the ap-propriate base sequences is transcribedinto messenger RNA (see transcription).

49

codon

When the mRNA migrates to the ribo-somes, its string of codons is paired withthe anticodons of transfer RNA molecules,each of which is carrying one of the aminoacids necessary to make up the protein.The term codon is sometimes also used forthe triplets on DNA, which are comple-mentary to those on messenger RNA butcontain the base uracil instead of hymine.

coefficient of haze (COH) A measureof the amount of dust and smoke in the at-mosphere. COH units are defined as thequantity of particulate matter that pro-duces an optical density of 0.01 on a paperfilter. A COH value of less than 1.0 is con-sidered to represent relatively clean air.Values greater than 2.0 occur where thereis a high proportion of combustion-gener-ated particles, such as those from vehicleexhausts, forest fires, or the burning of fos-sil fuels. The COH is a measure of thelight-absorbing capacity of the particles,which is due mainly to carbon produced bycombustion. To determine the COH, am-bient air is drawn through a paper filter,and a photometer is used to detect changesin the amount of light transmitted throughthe paper, producing an electrical signalproportional to the optical density.

coevolution The complementary evolu-tion of two or more ecologically interde-pendent species, such as a flowering plantand its pollinators. An adaptation in onespecies triggers the evolution of a comple-mentary adaptation (coadaptation) in theother(s). For example, the evolution oflong tubelike flowers favors the evolutionof birds with long beaks and tongues toreach the nectar deep inside. These birdsare more efficient pollinators, since theyare more likely to visit similar flowers,while they benefit from reduced competi-tion with birds with shorter beaks. See alsomutualism; pollination; predator–prey re-lationships.

coexistence The living together of twoor more species or organisms in the samehabitat, such that none is eliminated by anyof the others, i.e. there is no competitionbetween them. For example, this may

occur when predators or parasites limit thecompetitive power of certain species thatmight otherwise be more effective competi-tors. See competitive exclusion principle;niche; predator–prey relationships.

COH See coefficient of haze.

cohort A group of individuals thatbegan life at the same time in the same pop-ulation.

cohort generation time See genera-tion.

cohort life table See life table.

cold acclimation See cold tolerance.

cold desert See boreal forest; desert.

cold forest See boreal forest.

cold hardening See cold tolerance.

cold tolerance (cold acclimation; coldhardening; hardening; frost tolerance) Thedevelopment of an ability to tolerate verylow temperatures, especially temperaturesbelow freezing, in advance of those tem-peratures actually occurring. Most plantsand animals accumulate free amino acids(mainly plants), polyhydroxyalcohols orpolyols (mainly animals), or other organiccompounds (e.g. oligosaccharides) in theircells when exposed to a period of low (butnot damaging) temperatures, helping pre-vent damage to cell membranes when tem-peratures fall still further. In some insects,most of the animal’s glycogen stores areconverted to polyols as temperatures fall,and these allow the formation of ice be-tween cells, but protect the membranesfrom damage due to dehydration as wateris withdrawn from the cells to form the ice.The degree of cold tolerance shown by aspecies depends on the normal range oftemperatures of the habitat in which it hasevolved. The most cold-tolerant species arecertain mosses and lichens of polar regions,which can tolerate temperatures of –70°C.See vernalization.

coefficient of haze

50

coliform bacteria Gram-negative rod-shaped bacteria that can obtain energy aer-obically or anaerobically by fermentingsugars to produce acid or acid and gas.They live in soil, water, or as pathogens ofplants, but most are found in the intestinesof warm-blooded animals (fecal col-iforms), e.g. Escherichia coli. Many, suchas Salmonella, are pathogenic to humans.The presence of fecal coliforms in water in-dicates that the water is contaminated withfeces, so may also contain other pathogenicorganisms. The Coliform Index gives a rat-ing of water purity based on the fecal bac-teria count.

Coliform Index See coliform bacteria.

colloid A dispersion of microscopicfinely divided insoluble solid particles in aliquid. They remain suspended as a resultof their small size and electrical charge: theparticles have identical electrical charge, sothey repel each other and cannot formclumps that would settle out. Colloid par-ticles range from 10-7 to 10-3 cm in diame-ter.

colonization The movement and subse-quent spread of a species into a habitat, lo-cation, or population from which it wasabsent.

combined heat and power plant (CHP)The generation of heat and power (electric-ity) from waste. This may involve inciner-ating the waste, or using the methane thatforms in landfills. The heat from the com-bustion process can be used to provide hotwater for houses in the vicinity, and/orelectricity for local use or for the nationalgrid. While not a highly efficient energyproduction process, CHP can substitute forenergy derived from FOSSIL FUELS, so reduc-ing undesirable emissions.

combined sewer A sewer system thatcarries both SEWAGE and storm-waterrunoff. Under normal conditions, its entireflow passes through a waste treatmentplant, but during heavy rain storms theextra volume of water may cause some of

this sewage and storm water mixture tooverflow, bypassing the treatment system.

combustion chamber The compart-ment in which waste is burned in an incin-erator.

commensalism An association betweentwo organisms in which one, the commen-sal, benefits while the other (the host) re-mains unaffected either way, e.g. thesaprophytic bacteria in animal intestinesand EPIPHYTES on the branch of a tree.Compare amensalism; mutualism; para-sitism; symbiosis.

commercial waste Solid waste pro-duced by businesses such as shops, offices,restaurants, and places of entertainment.

community A group of POPULATIONS ofdifferent species living together in a certainenvironment and interacting with eachother. See association; consociation.

community character displacementSee character displacement.

community structure The pattern ofspecies ABUNDANCE and population interac-tions within the community. This is deter-mined by the interactions between thedifferent species and between the speciesand the ABIOTIC environment. Communitystructure encompasses the number ofspecies present (see species richness), theirrelative abundance, feeding relationships,and the way the resources of the local envi-ronment are partitioned between thespecies. When considering a community ona larger regional scale encompassing di-verse habitats, interactions between popu-lations and patterns of habitat selection arealso important. Longer-term communitystructure will also be affected by SPECIA-TION, which will affect species richness.Thus community structure depends onboth gradients of environmental condi-tions and phylogenetic history, as well ason the local abundance of particular preda-tors, herbivores, and parasites. Where dataon community structure is plotted againstgradients of environmental conditions (e.g.

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community structure

moisture, temperature, light intensity,salinity) or topography, the boundaries be-tween different communities may be sharp(closed communities) or blurred (opencommunities). In open communities thereis a gradient of species abundance andsome population interaction across com-munity boundaries.

comparative risk assessment (CRA)The assessment or ranking of proposedprojects, especially environmental manage-ment strategies, according to predictions oftheir potential environmental effects.

compensation depth See photic zone.

compensation level See sublittoral.

compensation point (light compensationpoint) The light intensity at which therate of PHOTOSYNTHESIS is exactly balancedby the combined rates of RESPIRATION andPHOTORESPIRATION, so that there is no netexchange of oxygen and carbon dioxideand the rate of synthesis of organic ma-terial equals the rate of breakdown byrespiration. At normal daylight light inten-sities the rate of photosynthesis exceeds therate of respiration. Shade plants tend toreach their compensation points fasterthan sun plants, as they are better able tomake use of dim light. The point at whichphotosynthesis ceases to increase with in-crease in light intensity is termed the lightsaturation point. This point occurs atmuch higher light intensities in C3 PLANTS

than in C4 PLANTS. Low compensationpoints indicate high photosynthetic effi-ciency.

competition The utilization of the sameresources by one or more organisms of thesame species (intraspecific competition) orbetween different species living together ina community (interspecific competition),when the resources are not sufficient to fillthe needs of all the organisms. The closerthe requirements of two species, the lesslikely it is that they can live in the samecommunity, unless they differ in behav-ioral ways, such as periods of activity orfeeding patterns. The presence of one or

more competing species will depress aspecies’ birth and/or growth rate and/or in-crease its death rate.

Competition may be aggressive, e.g. byexcluding others from resources or by theuse of toxins (e.g. some plants and bac-teria), which is termed direct competition,or it may simply exclude others by con-suming so much of the resource that it re-duces their availability to others – indirectcompetition.

Competition may be simply competi-tion for a renewable resource (consumptivecompetition), for space in which to estab-lish (preemptive competition), or for terri-tory in which to feed or to compete forfemales (territorial competition). Oneplant species may outcompete another bygrowing on or over it, so depriving it oflight or of water containing nutrients(overgrowth competition), or it may pro-duce a toxic chemical that preventsseedling establishment or larval settlement(chemical competition), as happens withsome plants, which produce toxic root ex-udates.

Where competition involves access to aresource, such as food, rather than the levelof the resource itself, it is called contestcompetition: animals compete with eachother for access to the resource, limitingthe number that can utilize it at any onetime and using up energy. The converse isscramble competition or exploitation, inwhich competition is directly for the re-source itself. See also niche.

competitive exclusion principle Theprinciple that when members of differentspecies compete for the same resources (i.e.occupy the same ecological NICHE) in a sta-ble environment, it is likely that one specieswill be better adapted and outcompete theothers, which will consequently decline innumbers and eventually die out. See com-petition.

competitive release The expansion ofthe NICHE of a species due to the absence ofcompetition with other species. For ex-ample, it may occur on an island wherecompetitors are absent, or a recent envi-

comparative risk assessment

52

ronmental event such as a hurricane mayhave removed other species (e.g. trees).

competitive strategy A life strategyadopted by organisms that live in habitatswhere resources are abundant and distur-bance rare. The main issue for survival iscompetition for resources as populationnumbers increase and the habitat becomescrowded. See Grime’s habitat classifica-tion.

complementary resources See re-source.

compliance monitoring The collectionand evaluation of data on POLLUTANT con-centrations and loads in permitted dis-charges in order to ascertain whether ornot they fall with within the limits allowedby the discharge permit.

compost An organic fertilizer madefrom the breakdown by bacteria and othermicroorganisms of garden rubbish, kitchenvegetable waste, and other biodegradablematerial. Bacteria in soils are mixed withthe waste and the mixture turned regularlyto admit air. The organic material breaksdown to a relatively stable humus-like ma-terial.

concentrate and contain (containment)A method of waste management that in-volves concentrating potential pollutantsand enclosing them in escape-proof con-tainers. For example, the sealing of radio-active substances and their burialunderground, or the depositing of com-pressed waste in sealed and covered landfillsites.

concentration The relative amount of asubstance in a given area or volume. Forexample, the mass of a solute per unit vol-ume of solution, or the concentration of agas in parts per million by volume.

condensation The conversion of a gasor vapor into a liquid or solid by cooling.

condensation nuclei See cloud; fog.

cone of depression A depression in theWATER TABLE that forms around a well aswater is pumped out of it. It is usuallycone-shaped, and its cross-section indicatesthe scope of the influence of the well on thewater table.

confidence interval In statistical data,a range of values for a given variable, suchthat the range has a specific probability ofincluding the true value of the variable.The upper and lower values of this rangeare called the confidence limits. See proba-bility.

confidence limits See confidence inter-val.

confined aquifer See aquifer.

confounding factor In epidemiologicalstudies, the level of exposure to a pathogenor disease may appear to be correlated withthe incidence of the disease, when in factboth are related to a third factor – a con-founding factor. For example, a popula-tion living in an industrial area may have ahigh level of respiratory disease that ap-pears to be related to levels of pollution,when it is due to the poor living conditions,which may also contribute to the incidenceor severity of the disease.

conglomerate A coarse-grained sedi-mentary rock made up of more or lessrounded fragments larger than 2 mm in di-ameter, e.g. pebbles or boulders.

Coniferophyta The largest phylum ofGYMNOSPERMS, comprising about 560species of evergreen trees and shrubs, withmany important species, e.g. Pinus (pine),Picea (spruce), Taxus (yew), and Abies(fir). They dominate the vast boreal forestsof the northern hemisphere. Most are ever-green, but Larix (larches) and Taxodium(swamp cypresses) are deciduous. Thewood lacks xylem vessels. Conifers havesimple linear or scale-like leaves, and themale and female reproductive structuresare borne on the undersides of scalesarranged in cones. Conifers produce pollengrains and after fertilization seeds develop.

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Coniferophyta

coniferous forest See boreal forest.

conifers See Coniferophyta.

connectance The total number of linksin a food web divided by the number ofpossible links.

connectedness See food web.

conservation The management of wildplants and animals or other natural re-sources to ensure their survival for use byfuture generations. This may involve themaintenance of particular natural habitatsand the control of environmental quality.Modern conservation has to consider peo-ple as part of natural ecosystems, and tobalance the needs of the wild fauna andflora against the social and economic needsof local people to promote the sustainableuse of resources. Conservation usuallyaims to preserve BIODIVERSITY, but may onoccasion focus on the survival of a particu-lar ENDANGERED SPECIES, including suchmeasures as captive breeding or seed andgene banks. Where the habitat being con-served is a subclimax (see climax commu-nity), it may be necessary to intervene toprevent the natural succession running itscourse. For example, reedbeds may be cutback to maintain areas of open water. Seebiosphere reserve; CITES.

conservation biology The applicationof population ecology and genetics toproblems of decreasing biodiversity.

consistency The absence of BIAS in ex-perimental procedures or data analysis.Consistency can be improved by the use ofproperly calibrated instruments and by theuse of blind trials, where the identity of in-dividual samples is not known to the ex-perimenter.

consociation A CLIMAX COMMUNITY ofnatural vegetation dominated by one par-ticular species, such as oakwood, domi-nated by the oak tree, or Calluna heathlanddominated by the heather, Calluna vul-garis. Many consociations together mayform an ASSOCIATION, for example oak-

wood, beechwood, and ashwood consocia-tions together make up a deciduous forestassociation. Consociations are usually rel-atively small climax communities.

conspecific Describing individuals thatbelong to the same SPECIES.

constant (constant species) A speciesfound in a particular ASSOCIATION or COM-MUNITY but not confined to it, as comparedto a faithful species, which is seldom foundoutside a particular association or commu-nity. See phytosociology.

consumer An organism that feeds uponanother organism, e.g. animals, parasiticand insectivorous plants, and many het-erotrophic protoctists. A primary con-sumer feeds on plants or other primaryproducers. A secondary consumer feeds onprimary consumers, and a tertiary con-sumer feeds on secondary consumers, andso on. See food chain; trophic level. Com-pare decomposer; producer.

consumption efficiency The percent-age of the available energy that is actuallyconsumed at a given TROPHIC LEVEL. For aprimary consumer (herbivore) it is the per-centage of the net PRIMARY PRODUCTIVITY

that is ingested, the remainder being left todecompose. For higher level CONSUMERS, itis the percentage of the productivity of theprey that is eaten by a carnivore. See pro-ductivity.

consumptive competition See compe-tition.

contact pesticide A herbicide or insecti-cide that kills a plant or insect on contactrather than needing to be absorbed or in-gested, e.g. paraquat (herbicide), pyrethrin(insecticide). Compare systemic.

contagious dispersion See dispersion.

containment See concentrate and con-tain.

contest competition See competition.

coniferous forest

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continental drift The theory that pre-sent-day continents have arisen by thebreaking up and drifting apart of a previ-ously existing ancient land mass (Pangaea).There is much evidence to support thetheory, and it serves to explain the distrib-ution of contemporary and fossil plantsand animals. Continental drift is now be-lieved to reflect the movement over geolog-ical time of underlying plates in the Earth’scrust – the theory of PLATE TECTONICS.

continental crust See crust.

continentality A measure of the effectof isolation from marine influences uponclimate. Different temperatures prevailingin January and July are generally used asthe indicator. With increasing distancefrom the oceans, seasonal temperature dif-ferences tend to become more extreme.

continental shelf The shallow, gentlysloping submarine plain around a conti-nental margin, formed by the submergenceof the edge of the continent. The shelf isnormally covered in mud, silt, or sand. It isseldom deeper than 100–200 m, and usu-ally ends abruptly in a steep slope to theoceanic abyss, the continental slope. Con-tinental shelves vary in width, but are usu-ally about 65 km wide. For example, onthe western side of the USA the shelf is onlyabout 32 km wide, but along the easterncoast it is in places more than 120 kmwide.

continuous sampling The continuousmonitoring of the composition of a sub-stance, such as an effluent, by removingsamples for analysis. In an industrial ortreatment plan, for example, part of the ef-fluent stream may be diverted to a pointwhere samples are taken. This may happenat different points in the processing, pro-viding information about the effluent at allstages of processing.

continuous variation See quantitativevariation.

continuum The gradual change inspecies composition of overlapping POPU-

LATIONS in a large COMMUNITY along an en-vironmental gradient, such as rainfall. Thiscan be quantified as a continuum index:the populations are assigned arbitrary val-ues according to how closely their speciescomposition approaches either end of thecontinuum. Increasing values of the contin-uum index correspond to seral stages lead-ing to the mature CLIMAX COMMUNITY. Thelower values may also be regarded as rep-resenting local climax communities forparticular topographic or soil conditions,rather than as populations in a larger com-munity. The concept was originally basedon work in southern Wisconsin, where thecontinuum was of woodland along a mois-ture gradient from dry sites dominated byoak and aspen to moist sites dominated bysugar maple, basswood, and ironwood. Seesere.

contour line A line drawn on a maplinking points of equal elevation/altitude.

contour plowing A method of farmingin which plowing and planting are carriedout along the contours, i.e. at right anglesto the slope rather than up and down it.This reduces soil erosion.

contour strip farming A method offarming in which strips of crops are alter-nated with strips of close-growing foragecrops such as grass, hay, or forage grains,to further reduce soil erosion. See contourplowing.

control In experimental technique, theuse of an untreated sample – the control –as a standard against which to comparetreated samples. For example, the controlmay remain free of chemical treatment, ad-dition of nutrients, or physical changes inits environment.

control action threshold (CAT) Thelevel of pest density and the density of thepest’s natural enemies above which it isnecessary to intervene to prevent the pestpopulation rising above acceptable levels.

convenience sampling See sampling.

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convenience sampling

Convention on Biological Diversity(CBD) An international convention,signed at the United Nations Rio Summitin 1992, that aims to conserve BIODIVER-SITY, the sustainable use of natural re-sources, and the equitable sharing of thebenefits arising from exploitation of ge-netic resources. It covers all aspects of bio-diversity from genetic resources toecosystems, and aims to conserve resourceswhile promoting sustainable development,and to encourage the sharing of costs andbenefits between developing and developedcountries. The United Nations Environ-ment Program (UNEP) responded by com-missioning the Global BiodiversityAssessment, to collect and review data,theories, and opinion relating to current is-sues worldwide, which provided valuableinformation on the rapid rate at which en-vironmental change is taking place andnatural ecosystems are being modified.

Convention on International Trade inEndangered Species See CITES.

Convention on Wetlands of Interna-tional Importance (Ramsar Convention)An international convention betweenmember states of the United Nations thataims to coordinate international action toconserve wetlands of international impor-tance. It does so by restraining the removalof wetlands by drainage, cultivation, andhuman settlement, and recognizing the roleof wetlands as regulators of the hydrologi-cal regime and as habitat for plants and an-imals, as well as their scientific, economic,cultural, and recreational value. A list ofWetlands of International Importance(Ramsar sites) is maintained, and theWorld Conservation Union holds a registerof the wildlife and management informa-tion relating to these sites. The conventionwas signed at a meeting in Ramsar, Iran, in1971, and amended in 1982 and 1987. Sig-natures may or may not be followed by rat-ification with UNESCO (United NationsEducational, Scientific and Cultural Orga-nization). See World Conservation Union.

convergence The occurrence of SPECIES

with similar morphology and adaptations

in similar habitats in different parts of theworld. For example, the similarity of cactiand euphorbias in the deserts of NorthAmerica and Africa.

convergent evolution The independentdevelopment of similar (analogous) struc-tures in unrelated organisms as a result ofliving in similar ecological conditions andselection pressures. An example is the evo-lution of similar succulent forms in theCactaceae and the Euphorbiaceae, inwhich the leaves are reduced to pale spines.The wings of vertebrates and insects are anexample in which quite distinct groups ofanimals have independently adapted in asimilar way to life in the air.

cooling tower In electricity-generatingplants, including coal-, oil-, and nuclear-powered plants, a large structure that pro-vides a large internal surface area acrosswhich the heat from the exhaust steam(from turbines) is used to preheat water forthe boilers.

cool temperate rainforest See temper-ate deciduous forest.

cooperative breeding The involvementof individuals other than the mating pair inthe breeding of the species. For example, insocial insects, the workers feed, clean, andcare for the young, and in some species ofbirds and mammals, nonbreeding relativeshelp rear the young. In social insects theworkers share 75% of their parents’ genes,whereas their own offspring would haveonly 50% of their genes, so cooperationhelps them perpetuate their genes.

copper (Cu) A metal trace element (seemicronutrient), essential for plant growth.Copper deficiency in plants results inchlorosis, suggesting that copper may beinvolved in the synthesis of chlorophyll. Ithas a role in certain metabolic activities inanimals. Dietary sources of copper includeliver, seafood, fish, nuts, whole grains, andlegumes. Copper sulfate is used as a fungi-cide in Bordeaux mixture. Copper com-pounds are often toxic to living organisms.

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coppice (copse) A woodland managedfor wood production by cutting trees backto ground level at regular intervals (usually10–15 years) and allowing adventitiousshoots to grow up from the base. Theyoung shoots, which are often verystraight, are used for fencing, charcoalburning, and firewood. This labor-inten-sive practice has largely died out, andtoday coppicing is done mainly for conser-vation purposes.

coprophagy The habit of feeding ondung. Examples of coprophagous organ-isms include dung beetles and fungi such asPilobolus. Compare ruminants.

copse See coppice.

coral bleaching See corals.

coral reef See corals.

corals Colonial animals (members of thephylum CNIDARIA), class ANTHOZOA thatconsist of polyps embedded in a gelatinous,horny, or calcareous matrix. They are sus-pension feeders, trapping tiny organismsand organic detritus with their tentacles.Many corals contain symbiotic zooxan-thellae (see dinoflagellates), whose pig-ments give corals their brilliant colors.These symbionts are photosynthetic, andcontribute to the corals’ nutrition. In theabsence of zooxanthellae, the rate of cal-cium carbonate deposition is much slower,with serious implications of the mainte-nance of coral reefs. Temperatures greaterthan 30°C cause corals to expel their zo-oxanthellae and die, causing coral bleach-ing. Pollution by sediment, such as thatcarried into coastal areas by untreatedsewage or by rivers passing through areasof deforestation and soil erosion, reduceslight penetration and prevents the zooxan-thellae from photosynthesizing.

Many coral species show asexual repro-duction by budding. Buds remain attachedto the parent polyp, so building up acolony. In time, the older polyps die, butthe matrix surrounding them remains,building up a coral reef. Accumulations ofcolonial corals with calcareous skeletons

form substantial reefs in warm shallow wa-ters. Solitary corals may also contribute tothe bulk of the reef, some reaching diame-ters of 25 cm, as do red algae that are en-crusted with calcium carbonate. Coralreefs have a complex topography, and pro-vide many ecological niches for other ani-mals and algae; their biodiversity has beenlikened to that of rainforests.

core The central part of the Earth, lyingbelow the crust and mantle at a depthbelow 2900 km. It is thought to be com-posed mainly of molten nickel and iron.

corer A device use to extract a relativelyundisturbed sample of a vertical section ofa sediment or of ice. It usually comprises arotating cutter that isolates a column ofsediment.

Coriolis force In a rotating object, aforce of inertia that acts to the left of the di-rection of movement in clockwise rotation,and to the left in a counterclockwise rota-tion. It causes an apparent deflection of anobject from its path when moving in a ro-tating system, especially when moving in alongitudinal direction, an effect called theCoriolis effect. Thus an object moving in alongitudinal direction will appear to be de-flected to the left in the southern hemi-sphere and to the right in the northernhemisphere because the Earth rotates east-ward, and because the velocity of theEarth’s rotation is greater at the equatorthan at the poles.

The Coriolis force affects the directionof prevailing winds, such as the Westerliesin the northern hemisphere and the easterlytrade winds in the southern hemisphere. Italso affects the rotation of ocean currents,which circulate clockwise in the northernhemisphere and counterclockwise in thesouthern hemisphere.

corm See perennating organ.

correlated extinction See extinction.

correlated response A response of thePHENOTYPE of one character to selectionacting upon another character. For exam-

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ple, selection for increased body weight inchickens results in increased egg size butdecreased rates of laying.

correlation The relationship betweentwo or more qualitative or quantitativevariables. On a graph, the drawing of astraight line through a scattering of pointsso as to minimize the STANDARD DEVIATION

of all the points from the line (a line of bestfit), thus allowing quantification of theway in which the two variables are relatedto each other. The closer the points fit theline, the stronger the correlation betweenthe two variables. This may be done sub-jectively, or it may be done statistically,giving confidence limits for the correlation.See correlation coefficient.

correlation coefficient The strength ofthe linear relationship between two vari-ables can be expressed as a correlation co-efficient, which equals the covariancedivided by the square root of the productof the variances of the two variables. Thevalues of the correlation coefficient varyfrom –1 to +1, a value of +1 indicating apositive relationship, a value of –1 a nega-tive relationship, and a coefficient of 0 in-dicating that there is no linear relationshipbetween them. The closer the correlationcoefficient is to +1 or –1, the less the scat-ter of points about a line of best fit (seecorrelation). However, the correlation co-efficient does not imply any dependency ofone variable upon the other, or give any in-formation about the nature of the relation-ship between the two.

corridors Connections between frag-ments of similar HABITAT in a patchy land-scape. For example, a thin strip ofwoodland may link two much greaterareas of woodland, allowing mixing ofspecies and individuals between the twocommunities. In the conservation of BIODI-VERSITY, such corridors are vital to preservea good level of OUTBREEDING and geneticdiversity (and hence breeding success andthe ability to adapt to environmentalchange) in populations or communitiesthat are becoming fragmented (and there-fore smaller) because of habitat loss due to

human activity or climate change. Inecosystems where certain species need tomigrate seasonally, as for example in theAfrican savannas, where grazing herbi-vores need to migrate to areas where recentrainfall has led to new grass growth, orwhere a species has distinct breeding areasaway from its normal range, corridorsallow this to happen.

cost-benefit analysis A quantitativeevaluation of the economic, social, and en-vironmental costs that would be incurredin implementing a project or regulationversus the social, economic and, some-times, environmental benefits.

courtship The specialized patterns ofbehavior that are preliminary to matingand reproduction. Its function is to syn-chronize precisely the activities of male andfemale so that copulation can occur. It isalso important in enabling the partners toidentify each other as potential mates – i.e.as of the same species and opposite sex. Inthis, courtship also serves as an isolatingmechanism, preventing different butclosely related species from interbreeding.See reproductive isolation.

covariance A statistical measure of cor-relation of the fluctuation in two variables(i.e. the deviation from their means), basedon the original units of measurement of thevariables. Covariance differs from correla-tion in that it is based on the actual units ofthe original measurements of the variables,not on standardized units. Compare corre-lation.

cover The proportion of the ground oc-cupied by plants of a given species, as de-termined by a perpendicular projection onto the ground of their aerial parts. It is usu-ally expressed as a percentage of the totalarea, and gives an estimate of plant abun-dance. It may be estimated visually usingscales such as the Domin scale (see Braun-Blanquet scale). Visual assessment may beaided by the use of a wire grid, or samplingmay be made more objective by taking anumber of sample points using a pointquadrat (a quadrat with pins hanging

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down at intervals; in which each hit by apin is recorded). Where leaves overlap, thiscan give cover values greater than 100%.For this reason, visual estimates are oftenmore meaningful in dense vegetation. Seeabundance; frequency.

cover crop A crop grown to protect del-icate seedlings of a later crop, or to provideprotection against soil erosion betweencrop plantings or in certain seasons.

C3 plant A plant in which the first prod-uct of PHOTOSYNTHESIS is a 3-carbon acid,glycerate 3-phosphate (phosphoglycericacid, PGA). Most plants are C3 plants.They are characterized by high carbondioxide COMPENSATION POINTS owing toPHOTORESPIRATION. Compare C4 plant.

C4 plant A plant in which the first prod-uct of PHOTOSYNTHESIS is a 4-carbon dicar-boxylic acid, oxaloacetic acid. C4 plantshave evolved from C3 plants by a modifi-cation in carbon dioxide fixation, leadingto more efficient carbon dioxide uptake.The modified pathway is called the Hatch-Slack pathway or C4 pathway. In theleaves, the mesophyll cells surrounding thevascular bundles (bundle sheath cells) con-tain the carbon dioxide-fixing enzymephosphoenolpyruvate carboxylase (PEPcarboxylase) in their cytoplasm. This has ahigher affinity for carbon dioxide thandoes ribulose bisphosphate (RuBP) car-boxylase. The product of carbon dioxidefixation is oxaloacetate, which is rapidlyreduced to the C4 acid malate or transami-nated to the C4 acid aspartate. These C4acids are transported to the bundle sheathcells encircling the leaf veins, where theyare decarboxylated, releasing CO2, whichis then refixed as in C3 plants.

C4 plants are mainly tropical or sub-tropical, including many tropical grasses(e.g. maize, sugar cane, and sorghum).They are more efficient than C3 plants,producing more glucose per unit leaf area.They have up to double the maximum rateof photosynthesis of C3 plants and lose lesswater by transpiration and have lowercompensation points. See photorespira-tion. Compare C3 plant.

CPUE See catch per unit effort.

CRA See comparative risk assessment.

crab See Crustacea.

Craniata See Chordata.

Crassulacean acid metabolism (CAM)A form of photosynthesis in which the lightand dark reactions are separated in time:carbon dioxide taken in through the stom-ata at night is converted into an organicacid for storage, then converted back tocarbon dioxide for use in photosynthesisduring the day. Instead of initially combin-ing with ribulose bisphosphate, the carbondioxide combines with the 3-carbon phos-phoenolpyruvate (PEP) to form ox-aloacetic acid, which is then converted tomalic acid for storage in the vacuole. Thefollowing day, when the stomata areclosed, the malic acid is broken down andthe carbon dioxide is released to take partin the light reactions of photosynthesis.CAM allows plants of arid environmentsto keep their stomata closed to conservewater during the day, opening them atnight to exchange gases. When stressed bydrought, CAM plants can last for days oreven weeks without opening their stomata,conserving water and living on stored re-sources. CAM was first discovered in thefamily Crassulacaeae and has since beenfound in many other succulent plants, suchas cacti. Its disadvantage is that it is a slowform of photosynthesis, resulting in ratherslow growth. See photosynthesis. CompareC4 plant; C3 plant.

creep The slow movement of unconsoli-dated weathered material, including rockfragments, mineral grains, and soil, downslopes as a result of gravity. It may be aidedby alternate freezing and thawing, freezingcausing a raising of the material followedby a down-slope movement on thawing;expansion and contraction caused bychanges in temperature; the burrowing ofanimals; and the lubrication by water (seesolifluction). Compare rain-wash.

Cretaceous The most recent period of

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Cretaceous

the Mesozoic era, 145–66 million yearsago. It was marked by continued domina-tion of land and sea by dinosaurs until arapid extinction toward the end of the pe-riod. The marine ammonites and aquaticreptiles also became extinct in this period.Primitive mammals were present, but wererelatively insignificant in number, size, and variety until the Cenozoic era, whichfollowed. Birds and fishes evolved intostructurally modern forms during the Cre-taceous. The dominant vegetation on landin the early Cretaceous consisted of forestsof cycads, conifers, ginkgoes, and ferns.

Most modern-type ferns, gymno-sperms, and angiosperms arose during theCretaceous, and by the end of the periodthe flowering plants had replaced the gym-nosperms as the dominant terrestrial vege-tation, forming vast broad-leaved forestswith magnolias, figs, poplars, sycamores,willows, and herbaceous plants and a greatvariety of insect pollinators. The Creta-ceous is named for the large amounts ofchalk (fossilized plankton) found in rocksof the period. See also geological timescale.

crista See mitochondrion; respiration.

criteria pollutants In the United States,certain pollutants known to be hazardousto human health for which the Environ-mental Protection Agency is obliged underthe 1970 Clean Air Act to set NationalAmbient Air Quality Standards.

critical day length See photoperiodism.

critical load A quantitative estimate ofthe level of exposure to pollutants belowwhich significant harmful effects on speci-fied elements of the natural environmentdo not occur.

cropping-off The harvesting (by hu-mans or by natural predator or grazing) ofa particular species, trophic level in a FOOD

CHAIN, or link in a FOOD WEB. For example,in fish farming, nitrogenous waste may beremoved by allowing algae to grow on it,then adding fish that will remove the algaeby eating them (i.e. cropping them off).

crop rotation The practice of plantingdifferent crops on the same land in succes-sive years. Many plant diseases and pestsare species-specific, and this avoids the per-petuation of sources of infection from yearto year, reducing the build-up of pest pop-ulations. It also avoids the removal of ex-actly the same balance of minerals from thesoil year after year, as different crops havedifferent requirements. It is common to in-clude a leguminous crop in the rotation toreplenish the nitrates in the soil, sincelegumes contain nitrogen-fixing bacteria innodules on their roots. The crop may bemay plowed into the soil to fertilize it.

cross 1. The act of cross-fertilization.See backcross; pollination; plant breeding.2. An organism resulting from cross-fertil-ization.

cross-contamination 1. The movementof underground contaminants such as tox-ins, pollutants, or pathogen-infested soilfrom one level or area to another as a resultof burrowing or other activity that mixesthe soil or invades or permeates the rocks.2. The transfer of pathogens or toxins fromone substance to another as a result of badstorage procedures. For example, the infec-tion of an animal by picking up bacteria orother pathogens from ground, straw, orother bedding soiled by an infected animal.

crossing over The exchange of geneticmaterial between homologous chromatidsby the formation of chiasmata duringMEIOSIS, which results in the new combina-tions of alleles on the daughter chromo-somes. This is the main source of geneticvariation during sexual reproduction.

cross-pollination See pollination.

crown-of-thorns starfish A large starfish (Acanthaster) that preys on certainspecies of CORAL. In the 1960s and 1980spopulation explosions of this starfish in thePacific Ocean, and especially on Australi-a’s Great Barrier Reef, killed up to 90% ofthe corals in some areas. The coral skele-tons became overgrown with algae, andreef fish and other residents disappeared.

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Once the starfish numbers declined, thecorals recovered. Opinion differs as towhether this is a natural phenomenon orone triggered by human disturbance.Crown-of-thorns starfish on reefs off thePacific coast of Central America particu-larly consume a species of coral, Pocillo-pora, that tends to crowd out other species,so helping maintain species diversity on thereef.

crumb structure The texture of a SOIL

in terms of the size of the soil particles inthe surface horizons and how they interact,often determined simply by rubbing thesoil with fingers. Some soils, such as sandysoils, have a loose crumb structure withgood drainage whereas in clay soils theparticles are small and stick together togive a dense crumb structure and poordrainage.

crust The outermost layer of the LITHO-SPHERE, which consists of relatively lightrock and comprises less than 1% of thetotal mass of the Earth. On average, itextends 35 km below the surface. There are two types: continental crust consistsmainly of granitic rock, with a veneer ofsedimentary rocks near the surface, thick-est in high mountain belts; oceanic crust ismainly formed of basalt, which forms fromlava that wells up along mid-oceanicridges. In places, a thick layer of sedimen-tary rock is building up on the deep oceanfloor. The crust comprises a series of dis-tinct plates (see plate tectonics) that moverelative to each other as a result of convec-tion forces in the semiliquid mantle below.

Crustacea A large group of arthropodscontaining the mostly aquatic gill-breath-ing crawfish, crabs, lobsters, barnacles,water fleas, etc., and the terrestrialwoodlice and sowbugs. The body is di-vided into a head, thorax, and abdomen.The head bears compound eyes, two pairsof antennae, and mouthparts composed ofa pair of mandibles and two pairs of max-illae. The thorax is often covered with adorsal carapace (crust-like shell). The ap-pendages are typically forked and special-ized for different functions. The sexes are

usually separate and development is indi-rect, via one or more larval stages. In theFIVE KINGDOMS CLASSIFICATION the Crus-tacea are given the rank of phylum, but insome other classification systems they con-stitute a class of the phylum ARTHROPODA.

Crutzen, Paul (1933– ) Dutch meteo-rologist. In 1970 Crutzen argued that ni-trous oxide, arising from the use ofnitrogen-rich fertilizers and the combus-tion of fossil fuels, could cause depletion ofthe OZONE LAYER. Soon after, Crutzen’swarnings were overshadowed by thegreater threat from the chlorofluorocar-bons, which was first identified in 1974 by ROWLAND and MOLINA, with whomCrutzen shared the 1995 Nobel Prize forchemistry. Crutzen was also one of the firstscientists to warn (1982) of the dangers ofa NUCLEAR WINTER.

crypsis See camouflage.

cryptophyte See Raunkiaer’s life-formclassification.

cultivar Any agricultural or horticul-tural variety of a species. The term is de-rived from the words cultivated variety.See variety.

cumulative exposure The total expo-sure of an organism to a chemical or formof radiation over a given period of time: thesum of repeated exposure events, ex-pressed in quantitative terms; for example,the number of millisieverts of radiation.

cumulative frequency The frequencywith which an observed variable takes on avalue equal to or less than a specified value.Such frequencies are often plotted as his-tograms to show cumulative frequency dis-tributions.

current A directional movement ofwater whose speed is measured in volumepassing a particular point in a particulartime, for example, liters or gallons per sec-ond. Oceanic curents are large, relativelypermanent movements of water due to dif-ferences in temperature and/or salinity,

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both of which affect the density of thewater. The direction of these currents is de-termined by the ocean bed topography, thepositions of the continents and continentalshelves, and the CORIOLIS FORCE. Changesin the strength and position of these cur-rents can have dramatic effects on globalclimate (see El Niño; La Niña).

Cyanobacteria A phylum of EUBACTE-RIA containing the blue-green bacteria (for-merly called blue-green algae) and thegreen bacteria (chloroxybacteria). Bothgroups convert carbon dioxide into or-ganic compounds using photosynthesis,generally using water as a hydrogen donorto yield oxygen, like green plants. Undercertain circumstances some use hydrogensulfide instead of water, yielding sulfur.The main storage product is starch orstarch-like compounds. These bacteria arespherical or form long microscopic fila-ments of individual cells. The cyanobacte-ria are an ancient group, and their fossils(stromatolites) have been dated at up to3500 million years old. They reproduceasexually by binary fission, or by releasingsporelike propagules or filament frag-ments. Today, most species are found insoil and fresh water. Many species, e.g.Nostoc and Oscillatoria, have certain cellsthat are non-photosynthetic and fix nitro-gen. They are important sources of nitratesin rice paddies. Some nitrogen-fixingspecies form symbioses with plants andlichens. It is thought that certain ancientcyanobacteria became permanent sym-bionts of ancestral algae and green plants,taking up residence in their cells as photo-synthetic organelles (plastids), which even-tually evolved into chloroplasts.

Cycadophyta (cycads) A phylum oftropical and subtropical cone-bearing

GYMNOSPERMS with palmlike compoundleaves and special coralloid roots at or nearthe ground surface, which contain symbi-otic nitrogen-fixing CYANOBACTERIA. Thestems are thick and woody and the leavesare arranged in a dense rosette at the base.The bases of dead leaves provide importantmechanical support. Cycads are slow-growing and are pollinated by wind andbeetles. The female cones and ovules areunusually large, except in the genus Cycas.

cyclic climax A community in whichthere is a cyclical pattern of dominantspecies and associated biota. For example,where heaths growing in exposed locationsreach a certain size, they tend to sufferwind damage and split open, allowing thewind access to other parts of the vegeta-tion. The damage spreads through thearea, as regeneration or recolonization be-gins on the parts that were damaged first.

cyclic succession See succession.

cyclone See depression.

cytoplasm The living contents of a CELL,excluding the nucleus and large vacuoles,in which many metabolic activities occur.It is a colorless substance enclosed withinthe plasma membrane and contains or-ganelles and various inclusions (e.g. crys-tals and insoluble food reserves). Thecytoplasm is about 90% water. It is a truesolution of ions (e.g. potassium, sodium,and chloride) and small molecules (e.g.sugars, amino acids, and ATP); and a col-loidal solution of larger molecules (e.g.proteins, lipids, and nucleic acids). It canbe gel-like, usually in its outer regions (ex-oplasm), or sol-like (endoplasm).

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damped oscillation A fluctuation inthe number of individuals of a populationthat decreases in size as the population ap-proaches its equilibrium.

dark bottle A bottle covered to excludelight, used to estimate the respiration ratesof aquatic phytoplankton in PRODUCTIVITY

experiments. A sample of water is placed inthe bottle and suspended at the depth fromwhich the sample was obtained. Changesin oxygen concentration inside the darkbottle are due to the respiration of mi-croorganisms and phytoplankton. Whenpaired with a light bottle, in which photo-synthesis can proceed, gross PRIMARY PRO-DUCTIVITY can be estimated (assuming thatthe respiration rates are the same in bothlight and dark bottles) by adding the oxy-gen consumed by respiration in the darkbottle to the change in oxygen concentra-tion in the light bottle. The change in oxy-gen in the light bottle alone represents thenet productivity of the aquatic organisms –the balance between oxygen uptake due tophotosynthesis and oxygen depletion dueto respiration. This is called the oxygenmethod.

dark reaction See photosynthesis.

darwin A measure of the rate of evolu-tion, expressed as units of change per unittime.

Darwinism Darwin’s explanation ofthe mechanism of evolutionary change,which basically states that in any variedPOPULATION of organisms only the individ-uals best adapted to that environment willtend to survive and reproduce. Less well-adapted individuals will tend to perish

without reproducing. Hence the unfavor-able characteristics, possessed by the lesswell-adapted individuals, will tend to dis-appear from a species, and the favorablecharacteristics will become more common.Over time the characteristics of a specieswill therefore change, eventually resultingin the formation of new species. Darwincalled this process of selective birth anddeath natural selection. It derived from hisobservations that the individuals of aspecies show variation, and that althoughmore offspring are produced that are re-quired to replace their parents, not all sur-vive and population numbers tend toremain fairly stable, which suggests com-petition for survival. The main weakness ofDarwin’s theory was that he could not ex-plain how the variation, upon which nat-ural selection acts, is generated, because atthe time it was believed that the character-istics of the parents become blended in theoffspring. This weakness was overcomewith the discovery of Mendel’s work andits description of particulate inheritance,the later discovery of genes and chromo-somes, and the modern science of genetics,which led to a modification of Darwin’stheory known as NEO-DARWINISM.

day degrees The number of degrees bywhich the average daily temperature of agiven location differs from a standard,such as the minimum temperature neededfor growth or flowering. It can be used tocompare growing seasons in different yearsor in different places, for example.

day-neutral plant See photoperiodism.

DDT (dichlorodiphenyltrichloroethane)A synthetic ORGANOCHLORINE insecticide

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D

introduced in the late 1930s and subse-quently widely used to control insect carri-ers of diseases, such as malaria, typhus,and yellow fever. It is highly toxic to in-sects at low concentrations, both on con-tact and when ingested, affecting thenervous system. However, many species ofinsects evolve resistance to DDT. Its persis-tence in the environment (it has a half-lifeof 15 years) led to widespread poisoning ofcertain animals, especially fish, birds, andmammals, which accumulated high con-centrations of DDT in their fatty tissues.DDT thus accumulates in the food chain,affecting especially top predators. Thebreakdown product DDE causes thinningof eggshells in birds. DDT is widely presentin human milk, and can cause prematureshortening of the lactation period in nurs-ing mothers. It is also suspected of being acarcinogen and possibly harming the im-mune system. Its sale and use is nowbanned in many countries.

death rate The number of deaths in apopulation per unit time.

decay constant (λ) The probability thatan atom of a RADIOACTIVE ISOTOPE willdecay within a particular period of time (t).The rate of disintegration is given by:

(dN/dt) = λNwhere N is the number of radioactiveatoms present. The total lifetime of a radio-active parent atom is theoretically infinite,but its persistence in the environment isusually expressed as its half-life (T): thetime it will take for half the quantity of theparent material to decay. T is related to thedecay constant by T = 0.6993/λ.

decay products The chemical elementsor compounds that result from the radio-active disintegration of a substance. Forexample, the isotopes argon-40 and cal-cium-40 are produced by decay of theradioactive isotope potassium-40.

deciduous Describing plants that sea-sonally shed all their leaves, for examplebefore the winter or a dry season. It is anadaptation to prevent excessive water loss

by transpiration when water is scarce.Compare evergreen.

decomposer An organism that feedsupon dead organisms, breaking themdown into simpler substances. Decom-posers recycle nutrients, making themavailable to producer organisms. Bacteriaand fungi are important decomposers inmost ecosystems. DETRITIVORES, by break-ing down partially decomposed materialstill further, may also be considered to bedecomposers. See also food chain.

decomposition The gradual break-down of dead organic material to simpleinorganic substances of lower energy con-tent. This may be entire carcasses, shedparts of bodies (e.g. leaves, root caps, oldcoral polyps), or feces. Decompositiontakes place by both chemical and physicalprocesses; eventually organisms such asbacteria, fungi, and DETRITIVORES attackand respire the last products of disintegra-tion, releasing inorganic nutrients, water,and carbon dioxide. Decomposition in-volves both the release of energy and thedemineralization of the nutrients – theirconversion from an organic form to an in-organic form. This makes possible the re-cycling of nutrients in the ecosystem, asnutrients are released back into soil orwater for uptake by other organisms.Under aerobic conditions, early stage de-composers respire aerobically, releasingcarbon dioxide. However, if oxygen is inshort supply, fermentation (anaerobic res-piration) takes place, producing by-prod-ucts such as alcohol and organic acids,increasing the acidity of the local environ-ment and affecting the succession of de-composers. In terrestrial habitats, fungidominate in acidic conditions, but in theocean sediments specialist bacteria such asdenitrifying bacteria, sulfate-reducing bac-teria, and METHANOGENS are important.See also decomposer.

decontamination The removal of un-wanted bacterial, chemical, radioactive, orotherwise harmful materials or pests froman area.

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deep scattering layer A layer of waterin the oceans that reflects/scatters soundwaves, e.g. from an echo sounder. It is astratified layer, up to 200 m thick, contain-ing dense concentrations of fish and zoo-plankton.

deep-well injection The disposal ofhazardous waste by pumping into deepwells, where it remains trapped in the poresof permeable rocks.

deficiency disease A disease caused bydeficiency of a particular essential nutrient,such as a VITAMIN or MICRONUTRIENT, usu-ally with a characteristic set of symptoms.For example, lack of vitamin C in mam-mals causes scurvy, with a number ofsymptoms, including bleeding gums, looseteeth, painful joints, internal bleeding,anemia, and poor wound healing.

deflected climax A CLIMAX COMMUNITY

that is maintained by the activity of variousorganisms, e.g. by grazing, browsing, orhuman interference.

deflocculating agent A substanceadded to a suspension to prevent settling.See clarification; coagulation.

defluoridation The removal of excessfluoride from water to prevent the discol-oration of teeth.

defoliation The removal of leaves fromplants. Certain kinds of HERBICIDES, calleddefoliants, act by causing leaf ABSCISSION.

deforestation The clearing of forests.The consequences of the removal of largetracts of forest leads to an increase in soilerosion by winds and/or heavy floods andresults in a decrease of BIODIVERSITY. Forexample, the tropical RAINFORESTS, whichare the most diverse BIOMES in the world,are in great danger as a result of deforesta-tion. See also Dust Bowl.

degenerate phase In the cyclical patternof succession in grassland and heathlandcommunities, the stage in which lichensmove into the mature hummocks and ero-

sion of the hummocks begins. In grasslandsthis phase starts with the colonization ofgrass hummocks by lichens; in heathlandsit begins when 20–30-year-old bushes ofling (Calluna vulgaris) become full of gapsand older central stems are colonized bylichens and mosses. The phase ends whennew grass or heath plants invade the gap.See hummock and hollow cycle.

degradative succession (heterotrophicsuccession) A short-lived SUCCESSION ofDETRITIVORE communities on dead organicremains, each releasing further nutrients tothe environment for the next stage to uti-lize.

deimatic behavior Animal behaviorthat warns off potential predators. For ex-ample, the hissing of a snake before itstrikes, or the puffing up of the body intoads. See also aposematism.

deionized water See water.

deme A subpopulation of a SPECIES: adiscrete interbreeding group of organismswith recognizable cytological or geneticcharacters that is spatially distinct fromother such groups, although it may be ad-jacent to them. Within the deme there is anequal chance of all possible male and fe-male pairings, but cross-breeding withmembers of other demes is rare.

demersal Living on or near the seabed,but being able to swim actively.

demography The statistical study ofhuman populations. It encompasses popu-lation size and density, distribution, birthand death rates, immigration and emigra-tion, marriage and divorce rates, and theinteraction of populations with externalfactors such as the urban environment andthe economy.

dendrochronology A method of ar-chaeological dating by the ANNUAL RINGS oftrees, used when the lifespans of living andfossil trees in an area overlap. Exact datesfor sites can be calculated and the methodis more accurate than radioactive dating

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techniques (see isotopic dating). Some ex-tremely long-lived trees, such as bristleconepines, which can live for up to 5000 years,have been used in this work. For trees thatare not so old, ring patterns can bematched between samples from treeswhose ages overlap.

In addition to dating, the study of an-nual rings gives a record of past climate,since ring width correlates with availabilityof water and with the current weather.Traces of pollutants such as lead may alsobe detected in the rings.

dendroclimatology The study of therecord of past climates found in the AN-NUAL RINGS of trees. The radial growth ofthe wood is affected by the weather duringa particular year, and is especially sensitiveto seasonal changes in water availability,which affect the date at which growth com-mences in that year, the rate of growth, andthe size of the cells. Climatic stress maygive rise to incomplete or missing rings, orto distortions at certain points on the cir-cumference or at different heights up thetree. Drought may give rise to an extra ring(‘false ring’) that does not show the usualgradation of increasing cell wall thicknessand decreasing cell diameter. The numberof cells in each ring, the diameter of theirlumens, and differences in the thicknessand composition of the cell walls all giveinformation about previous climatic condi-tions.

dendrogram A branched diagram usedin taxonomy to demonstrate relationshipsbetween species, families, or other taxa, orto show the relationships between individ-uals, as in family trees. See also cladistics.

dendrograph An instrument that con-tinuously records the circumference of atree. It is used, for example, to record dailyfluctuations in girth caused by net changesin water content.

denitrification The loss of nitrogen ornitrogen compounds, especially the chemi-cal reduction of nitrates or nitrites by bac-teria, usually with the release of gaseousnitrogen. Denitrifying bacteria such as

Pseudomonas, Micrococcus, and Clostrid-ium use nitrate as the terminal electronacceptor in anaerobic respiration, so deni-trification occurs mainly in environmentsthat have low levels of oxygen, such as wa-terlogged soils and marine and freshwatersediments. The process is important interms of soil fertility because some of theproducts of denitrification (e.g. nitrites andgaseous nitrogen) cannot be used by plantsas a nitrogen source. See nitrogen cycle.Compare nitrification.

denitrifying bacteria See denitrifica-tion.

density In ecology, the number of a spe-cific type of organism per unit area. Den-sity is usually measured by countingindividuals in a series of randomly placedQUADRATS. It gives a measure of ABUN-DANCE, and allows comparison of differentspecies or different areas. Relative densityis sometimes used to give a measure of thenumerical importance of one species rela-tive to others of similar habit in a commu-nity, and is often expressed as a percentage.

density dependence The regulation ofpopulation size by mechanisms that arethemselves controlled by the density of thepopulation: the rate of growth in numbersof the population depends on the numberalready present in the population. It can beseen clearly in a population in a new en-vironment, which may show a typical S-shaped growth curve, with growth pro-ceeding slowly at first, then increasingrapidly, before declining to a more or lessstable peak, which represents the carryingcapacity of the environment for that or-ganism. The birth or growth rate in thepopulation decreases or the death rate in-creases as the population density increases.It is thought that most populations experi-ence density-dependent regulation at somestage.

density independence The tendencyfor the birth, growth, or death rates in apopulation to remain unaffected as thepopulation density increases. The popula-tion size is being regulated by mechanisms

dendroclimatology

66

that are independent of the population sizeor density until the final population crash.In a new environment, such populationstypically increase rapidly then stopabruptly as factors such as the end of thebreeding seasons or changing seasons limitgrowth, producing a J-shaped growthcurve. Algal blooms are an example.

deoxyribonucleic acid See DNA.

dependent variable See variable.

depensation The increase in parasitismand predation that occurs when the num-bers in a population fall below a criticalthreshold. Depensation further reducesnumbers, making it unlikely that the popu-lation will recover, e.g. for some species ofcommercially overfished stock such as theAtlantic cod.

deposit feeder An animal that feeds onthe sediment of the seabed, ingesting mudor sand and extracting organic materialfrom it.

deposit gauge A device for collectingand measuring solid and liquid atmos-pheric POLLUTANTS, especially dust.

depression (cyclone) A circulating bodyof air in which the atmospheric pressuredecreases to a minimum at the center. Inthe northern hemisphere, the winds in a de-pression circulate in a counter-clockwisedirection; in the southern hemisphere theycirculate in a clockwise direction. Depres-sions are usually about 1000 to 4000 km indiameter. The winds are generally strongerthan those associated with anticyclones.Depressions are usually associated withstormy, unsettled weather, and occurmainly in the mid-latitudes. They tend totravel mainly across the oceans, becausethe water surface offers less resistance totheir strong winds than does land. Theytherefore affect particularly the maritimeparts of continents. Their occurrence is af-fected by large land masses and especiallyby mountain ranges. Cyclones that form inlatitudes 10° to 15°, called tropical cy-clones, are of much smaller diameter, only

100–500 km across, but are much moreviolent, with wind speeds up to 60 metersper second or 216 kph (almost 134 mph) – twice the speed of mid-latitude cyclones.In the Atlantic Ocean they are called hurri-canes, and in the Pacific Ocean typhoons.Compare anticyclone.

dermal toxicity See contact pesticide.

desalination The removal of salt frombrine to produce fresh water. Desalinationis used to irrigate arid regions in which seawater is available, especially if solar powercan be used as an energy source. Severalmethods are employed, the most commonbeing evaporation of the sea water by heator by reducing the pressure on it (flashevaporation). The vapor is condensed toform relatively pure water. Freezing is an-other technique; pure ice forms from brineas it freezes. The method theoretically re-quires less energy than evaporation but theprocess is slower and technically more dif-ficult. Reverse OSMOSIS is another methodused. Pure water and salt water are con-tained on either side of a permeable mem-brane. The pressure of the salt water israised above the osmotic pressure, causingwater to pass from the brine to the pureside. Because the osmotic pressure requiredis about 25 atmospheres there are difficul-ties with large-scale application. In electro-dialysis, the ions are subjected to an electricfield instead of increased pressure, the pos-itive and negative ions being filtered offthrough separate membranes. Anothermethod used for low salinities is ion ex-change; in this method the salt ions arechemically removed from the solution. Seeillustration overleaf.

desert A major BIOME characterized bylow rainfall and thus supporting little or novegetation. A hot desert may form in anyclimate where average annual precipitationis less than 250 mm and intermittent. Atrue desert has no higher plant life. A hotsemidesert, in which plant and animal lifeis sparsely distributed and adapted to longperiods of drought, has an annual rainfallof less than 400 mm. The rain falls in briefheavy showers and varies from year to

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year. There are few sparsely distributedshrubs. The perennial plants consist of suc-culent xerophytic trees, shrubs, and herbssuch as euphorbias and cacti. There arealso EPHEMERAL plants with seeds that liedormant until a brief shower of rain thatprompts them to germinate, flower, and setseed in a short period of time, as little astwo weeks in some cases. Snakes andlizards are common, as are bees, butter-flies, and ants. Larger animals face particu-lar problems with scarcity of food as wellas water, very cold nights, and difficultiesmaking permanent burrows and shelters inthe shifting sand. Camels are well adaptedto this harsh environment.

Cold deserts include the TUNDRA andother regions permanently covered with iceand snow.

desertification The process of desertformation, which may be due to climatechange or to overgrazing or other humanactivities such as the removal of vegetationand concomitant soil erosion, overcultiva-tion, or the overextraction of water for ir-rigation, or for domestic or industrial use.During desertification, the water tablefalls, and the topsoil and groundwater be-come increasingly saline as water evapo-

rates from the bare soil, leaving salt be-hind. The reduction of surface watercauses increased soil erosion, and the lossof soil and salinization cause a loss of veg-etation. This alters the ALBEDO of the landsurface, encouraging the climate to changetoward increasing dryness. An example isthe southward spread of the Sahara Desertinto former areas of steppe grassland andscrub woodland in the Sahel region ofAfrica as a result of severe drought coupledwith overgrazing, overcultivation, and theremoval of valuable plant cover for fire-wood.

desert pavement A thin layer of graveland stones left on the ground after windand water have removed the smaller min-eral particles from the surface of a desert.

desulfurization The removal of sulfurfrom FOSSIL FUELS to reduce pollution.

determinate growth See growth.

detritivore An organism that feeds onDETRITUS. Examples are various kinds ofworms, snails, slugs, insects, mites, bac-teria, and fungi. Detritivores are DECOM-POSERS: the larger species break down the

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35000

10000

5000

2000

1000

750

300

200

feed-watersalinityin partsper million

sea

Baltic/estuarial

brackishborehole

polluted river

river

pretreatment +

evaporation

freeze (temperate zones)

freeze

reverse osmosisreverse osmosis

electrodialysis

reverse osmosis

pretreatment + ion exchange

product-watersalinity inparts per

million

500

50

2

1

dri

nka

ble

feed

-wat

erfo

rb

oile

rs

+io

nex

chan

ge

Desalination

detritus by maceration or by grinding it intheir intestines into smaller particles withrelatively large surface areas that pass outin the feces and are then consumed by thesmaller detritivores in the litter and soil. Inaquatic ecosystems many detritus feedersfilter particles of detritus from the water,or sieve the sediments using tentacles orfine mucus nets.

detritus Finely divided fragments ofdead material, such as dead or partially de-cayed plants and animals, leaf litter, feces,and products of the breakdown of organicmaterial by DECOMPOSERS. In forest ecosys-tems most of the plant production ends upas detritus, because wood is difficult forherbivores to digest.

developmental response A response toenvironmental factors that results in thedevelopment of alternative forms of an or-ganism according to the particular condi-tions under which it grows and develops.For example, arrowhead (Sagittaria sagitti-folia) has linear straplike leaves below thewater, which offer little resistance to flow-ing water, but leaves that develop abovethe water are arrow-shaped; the buck’shorn plantain (Plantago coronopus) hashairier leaves when growing in windy situ-ations. European water striders (Gerris),small freshwater insects that hunt on thesurface film of ponds and lakes, show four different wing lengths according towhether they inhabit large, permanentlakes (short wings or none, because they donot need to disperse and growing wingscosts energy), or temporary ponds (longwings so they can disperse to other bodiesof water to breed). In some Gerris speciesthe late summer generation develop longwings, so they can move to the shelter ofwoodlands to overwinter.

developmental threshold The mini-mum body temperature of an organismthat is necessary for development to occur.

Devonian The geological period some405–355 million years ago, between theSilurian and the Carboniferous periods ofthe Paleozoic era. It was characterized by

an enormous number and variety of fish,most of which have become extinct with-out leaving any modern relatives. Duringthe late Devonian, primitive amphibianswere evolving from crossopterygians (lobe-finned fish). Terrestrial fauna included in-sects and spiders. During this period thefirst major invasion of the land by plantsoccurred. The very first land plants datefrom late Silurian but during the Devonianmany vascular land plants appeared, suchas the rhyniophytes (all now extinct), club-mosses (Lycophyta), horsetails (Spheno-phyta) and early ferns (Filicinophyta). Theperiod is named after rocks found inDevon, England. See geological time scale.

dew Moisture that condenses from theair onto the surface of cool objects, espe-cially at night. It forms on clear calm nightswhen exposed surfaces lose heat by radia-tion faster than the air does. The cooledsurface cools the adjacent air, and if itcools below its dew point (the temperatureat which water vapor begins to condense,which depends on the degree of saturationof the air with water vapor), water vaporcondenses out onto the surface. If the ex-posed surface is colder than the freezingpoint of water, the vapor condenses out ashoar frost. See also fog; frost.

dew point See dew.

diadromous (amphidromous) Describ-ing fish that migrate regularly betweenfresh water and salt water. Fish that spendmost of their lives in the sea, but migrate tofresh water to spawn (e.g. the Atlanticsalmon, Salmo salar) are termed anadro-mous, whereas those such as the Europeaneel (Anguilla anguilla), which migrate fromfresh water to the sea to spawn, are termedcatadromous.

diapause In some insects and mites, aperiod of DORMANCY in the life cycle duringwhich growth and development cease andmetabolism is greatly decreased. It is oftenseasonal, occurring, for example, in winteror during a dry season; but in some cases itmay last for several years. It enables the in-

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sect to survive unfavorable environmentalconditions.

diatomaceous earth See diatoms.

diatoms A phylum of the Protoctista(formerly called the Bacillariophyta)whose members are unicellular algae foundin freshwater, the sea, and soil. Much ofPLANKTON is composed of diatoms andthey are thus important in aquatic FOOD

CHAINS. Diatoms have silica cell walls (frus-tules) composed of two overlapping valvesornamented with perforations, which arearranged differently in each species. Di-atoms are typically pillbox-shaped (cen-tric) or coffin-shaped (pennate). The frus-tules of dead diatoms are highly resistant todecay, and form deep sediments on thefloors of oceans and lakes called diatoma-ceous earth or kieselguhr.

dicotyledons See Angiospermophyta.

dieldrin An organochlorine contact PES-TICIDE used to control insects in the soil andto mothproof carpets and other furnish-ings. It is also formed in the body by thebreakdown of the pesticide ALDRIN. It is apowerful carcinogen, and persists in theenvironment. Dieldrin has been banned bymost countries for some time, but there arestill remnants in trade and in waste tips.

diesel A fuel derived from PETROLEUM

consisting of various alkanes used as a fuelfor diesel (compression-ignition) engines.Diesel-powered vehicles are up to 40%more fuel efficient and have lower emis-sions of carbon dioxide, carbon monoxide,and hydrocarbons but higher emissions ofnitrogen oxides and small particulates(PM10) than gasoline-fueled vehicles.Diesel-powered vehicles can be fitted withan oxidation catalyst to reduce emissionsof carbon monoxide, hydrocarbons, andparticulates (see catalytic converter). Ultra-low sulfur diesel (USLD) is highly refinedand has a sulfur content of less than 15parts per million.

differential resource utilization Theuse of different resources by different

species in the same habitat or the use of thesame resource at a different time or place,or in a different manner. The term is gen-erally used in the context of interspecificCOMPETITION. For example, in a rainforest,different species feed at different levels ofvegetation, and some are nocturnal andsome diurnal. Specialized diets may meanthat while some insect species feed on thenectar or pollen of flowers, others may feedon the foliage of the same tree.

differentiation A process of changeduring which cells with generalized formbecome morphologically and functionallyspecialized to produce the different celltypes that make up the various tissues andorgans of an organism.

diffuse competition See competition.

diffusion The movement of moleculesalong a concentration gradient from areasof high concentration to areas of lowerconcentration as a result of random move-ment. Gaseous exchange in plants andanimals takes place by diffusion. See tran-spiration. Compare active transport; os-mosis.

diffusion tube A tube, usually about 7 cm× 1 cm in size, sealed at one end, whichhouses a chemical absorbent, used to mon-itor gaseous POLLUTANTS, especially nitro-gen oxides.

digester A strong closed vessel or tankin which complex organic substances arebiologically degraded (‘digested’) by an-aerobic microorganisms. For example,SEWAGE SLUDGE may be broken down tomethane, carbon dioxide, and water, withthe release of energy. The energy releasedmay be trapped and used to generate elec-tricity, and the methane (BIOGAS) may alsobe piped off for use in power generation.

The reaction vessel in which woodchips are heated with chemicals to separatethe fibers by dissolving LIGNIN during theprocess of pulp and paper manufacture isalso called a digester.

digestion 1. The process by which en-

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zyme-mediated chemical reactions breakdown ingested food into simpler, solublesubstances that can be assimilated by thetissues.2. The decomposition of organic matter bymicroorganisms, usually carried out indus-trially in large chambers or vats called DI-GESTERS.

dilute and disperse A method of deal-ing with POLLUTANTS in which the pollutantis discharged into a large body of water orair that will reduce its concentration andcarry it away from its source, further re-ducing its concentration on the way. Ex-amples are the emission of gases frompower stations burning FOSSIL FUELS andthe discharge of small quantities of un-treated sewage into the sea. Compare con-centrate and contain.

dilution ratio The ratio of the volumeof water entering a stream to the total vol-ume of water in the stream. It gives an in-dication of the capacity of the stream todilute and assimilate waste.

dilution series A series of progressivelymore dilute solutions of known concentra-tion. Dilution series are used to preparestandard solutions and can also be used todetermine the efficiency of removal of asubstance such as a dissolved POLLUTANT,by comparing the solution after extractionwith a dilution series of known concentra-tions.

dimictic Describing a lake in whichthere are two seasonal periods (spring andfall) of vertical mixing when water circu-lates throughout the volume of the lake. Insummer, there is thermal STRATIFICATION asthe surface waters warm and become lessdense, floating on top of and not mixingwith the colder layers below. In the fall, thesurface waters cool to the temperature ofthe waters below, and mixing may also beaided by the surface friction generated byfall storms. In winter, when the surface wa-ters cool below 4°C, they expand, againbecoming less dense than the water below,giving a reverse stratification. In spring, thesurface waters warm, becoming denser

than the water below, so they sink andmixing occurs. Compare monomictic.

dimorphism The existence of two dif-ferent forms of an organism. An example issexual dimorphism in animals. See POLY-MORPHISM.

dinoflagellate A marine or freshwaterprotoctist of the phylum Dinoflagellata(Dinomastigota) that swims in a twirlingmanner by means of two undulipodia (fla-gella). These lie at right angles to eachother in two grooves within the organism’srigid body wall (test). Many possess sting-ing organelles that they discharge to catchprey; and some produce potent toxins thatare capable of killing fish. Roughly half ofall known dinoflagellates are capable ofphotosynthesis.

dioxin Polychlorinated dibenzo-p-diox-ins, found as contaminants in commercialproducts and generated by certain indus-trial processes, such as the manufacture ofherbicides and disinfectants. They areknown to be highly toxic and persistentPOLLUTANTS, with effects on reproduction,development, and the immune system; theymay also be carcinogenic. Dioxins were ac-cidentally released by an explosion at achemical plant in Seveso, Italy in 1976,causing widespread mortality in animals,and among humans the disfiguring skindisease chloracne. See also Agent Orange.

diploid A cell or organism containingtwice the haploid number of chromosomes(i.e. 2n). In animals the diploid condition isgenerally found in all but the reproductivecells and the chromosomes exist as ho-mologous pairs, which separate at meiosis,one of each pair going into each gamete. Inplants exhibiting an alternation of genera-tions the sporophyte is diploid, whilehigher plants are normally always diploid.Exceptions are those species in which poly-ploidy occurs.

direct competition See competition.

directional selection A type of selec-tion that acts on the PHENOTYPES for a par-

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directional selection

ticular characteristic in a population (andhence on the ALLELES that give rise to them)by moving the mean phenotype towardone extreme of its expression. Directionalselection is often a response to a pro-gressive change in the environment (e.g.climate change). Compare disruptive selec-tion; stabilizing selection.

dirty dozen A popular name for thetwelve toxic substances the United NationsEnvironment Program (UNEP) has tar-geted in a legally binding internationalconvention: ALDRIN, chlordane, DIELDRIN,endrin, DDT, dioxins, furans, HEPTACHLOR,hexachlorobenzene, mirex, POLYCHLORI-NATED BIPHENYLS (PCBs), and toxaphene.

disaggregation The process of breakingup material into smaller pieces or units.

disassortative mating Negative assor-tative mating. See assortative mating.

discontinuous variation See variation.

discrete generations A series of genera-tions of an organism in which one genera-tion dies before the next begins. Forexample, annual plants in a desert flowerand set seed after rain, then die as anotherdrought begins. Their seeds do not germi-nate until the next time rain falls, whichmay be long after the parent plant has died.

dispersal The tendency of an organismto reproduce in areas some distance awayfrom its breeding site. In plants, fungi, andmicroorganisms dispersal is passive, andthe degree of dispersal depends on the na-ture of the seeds, fruits, and spores, and theagents that disperse them, which may beanimals, wind, or water. Dispersal is im-portant in the COLONIZATION of new areas,and in the recolonization of areas wherevegetation has been lost. Many animalspecies have special behaviors to promotedispersal. For example, at a certain ageyoung male lions are driven away fromtheir pride and wander a wide area untileventually competing for or establishingprides of their own.

dispersal barrier (ecological barrier)An area of unsuitable habitat separatingtwo areas of suitable habitat, of a sufficientsize that a particular species is unable tocross it to colonize suitable areas on theother side. See also corridors.

dispersal polymorphism The existenceof more than one type of dispersal struc-ture in a species or among the progeny ofan individual. Dispersal polymorphism isfound in both animals and plants. Certainplants produce two kinds of seeds: one thatremains near the parent plant, exploitingthe conditions that favored the growth ofits parent, and the other that disperses far-ther away, to colonize new habitats or es-cape any local disaster that may befall thehabitat of its parent. Plant hoppers, whichare usually wingless, develop winged prog-eny in crowded conditions when resourcesare under strain.

dispersant A chemical, typically a deter-gent, used to break up oil into smallerdroplets in clearing oil spills.

dispersion The pattern of distributionof organisms in an environment. There arethree main dispersion patterns. In randomdispersion there is an equal chance of find-ing an individual at any place at a giventime. This happens when individuals areunaffected by the presence of other indi-viduals or by patchiness in the environ-ment. In regular dispersion individuals aremore localized than would be expected onpurely statistical grounds. This may be dueto a tendency to avoid other individuals, orto a failure to survive if too close to others.Regular dispersion is also referred to aseven, uniform, or spaced dispersion or asoverdispersion. In aggregated dispersion(clumped or contagious dispersion or un-derdispersion) individuals are closer to-gether than expected. This may be due toan attraction to other individuals or to cer-tain parts of a patchy environment, or to apattern of reproduction in which individu-als give rise to offspring close to them (as,for example, in many forms of asexual re-production in plants). See also distribution.Compare dispersal.

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dispersive mutualism See mutualism.

disposables Objects that are intended tobe thrown away after a single use or a fewuses.

disruptive coloration Coloration thatdisrupts an animal’s outline, making itmore difficult for a predator to see it. Ex-amples include the stripes of a zebra, whichmake it hard to distinguish individualswhen seen in a large group, as for example,when fleeing from a predator. Many an-gelfish of coral reefs have striking blocks ofcolor, so do not present the image of a fishshape.

disruptive selection A type of selectionthat acts on the phenotypes for a particularcharacteristic in a population (and henceon the alleles that give rise to them) to pro-gressively move them toward two oppositeextremes. In other words, there is selectionfor two different alleles of the gene. Thismay happen, for example, where a popula-tion is located at the boundary betweentwo distinct habitats, which exert differentselection pressures; or where a populationadapts to two different pollinators. Disrup-tive selection can lead to SPECIATION if di-vergence becomes so pronounced thatindividuals exhibiting opposite extremes ofphenotype become unable to interbreed.Compare directional selection; stabilizingselection.

dissimilation A biochemical transfor-mation that results in the oxidation of or-ganic compounds and their conversion toinorganic compounds or elements, so thatthey no longer form part of the biotic envi-ronment. See denitrification.

dissolved load The part of a river’s loadthat is carried in solution. It usually con-sists mainly of calcium, sodium, bicarbon-ate, sulfate, and chloride ions.

dissolved oxygen (DO) The quantity ofoxygen that is dissolved in an aqueous so-lution. It is usually expressed as mgL–1 oras a percentage saturation. It represents theoxygen available to fish and other aerobic

organisms, and indicates the water’s abilityto support aquatic life. Dissolved oxygencan be measured by means of a dissolvedoxygen electrode or a fiber optic oxygensensor.

distilled water See water.

distribution The way in which aspecies, population, or other ecologicalunit. This is distributed over a particulararea.

disturbance An event or change in theenvironment that changes the compositionof a community and may also affect thesuccession of communities in the area byremoving individuals and opening up newspaces for colonization. Examples includehurricanes, fires, or changes in land usecaused by agricultural development orurban settlement.

diurnal rhythm See circadian rhythm.

divergence 1. A horizontal flow ofwater in a different direction away from aparticular area, e.g. the spreading of oceansurface water in areas of UPWELLING.2. The differential diversification and seg-regation of certain parts of a TAXON to pro-duce new species, genera, families, orders,or classes. For example, the diversificationof the ancestors of horses into zebras,horses, and asses. See divergent evolution.

divergent evolution The evolution ofdifferent forms from a single basic struc-ture in response to different selection pres-sures.

diversity See species diversity.

diversity index A mathematical expres-sion of SPECIES DIVERSITY in a community,taking into account the relative ABUNDANCE

of different species. It is used to comparethe structure of different communities andto assess the effects of environmental fac-tors.

divide (drainage divide; watershed) Adividing ridge between adjacent CATCH-

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divide

MENT AREAS. Water on one side of the ridgedrains into one catchment area (drainagebasin), and water on the other side of theridge drains into the adjacent catchmentarea.

DNA (deoxyribonucleic acid) A sub-stance mainly found in the chromosomes,that contains the hereditary information oforganisms. The molecule is made up of twohelical chains coiled around each other togive a double helix. Phosphate moleculesalternate with deoxyribose sugar moleculesalong both chains and each sugar moleculeis also joined to one of four nitrogenousbases – adenine, guanine, cytosine, orthymine. The two chains are joined to eachother by bonding between these bases. Thesequence of bases along the chain makes upa code – the genetic code – that determinesthe precise sequence of amino acids in pro-teins. The two purine bases (adenine andguanine) always bond with the pyrimidinebases (thymine and cytosine), and the pair-ing is quite specific: adenine with thymineand guanine with cytosine. DNA is thehereditary material of all organisms withthe exception of RNA viruses. Togetherwith RNA and histones it makes up thechromosomes of eukaryotic cells.

DO See dissolved oxygen.

doldrums The region on either side ofthe equator from 30°S to 30°N, betweenthe two belts of trade winds, in which in-tense heating of the land and sea causeswarm, most air to rise, creating low pres-sure, high humidity, and cloudy weatherwith light, variable winds.

domain A taxonomic grouping of or-ganisms that ranks higher than kingdom.There are generally considered to be threedomains: the ARCHAEA, BACTERIA, and Eu-karya (see eukaryotes). The Five Kingdomssystem of classification recognizes onlytwo ‘superkingdoms’: the Prokarya (whichincludes the single kingdom Bacteria com-prising both archaea and the bacteria) andthe Eukarya.

domestication The selective breeding of

plant and animal species by humans so thatthey are exploited in some way. Domesti-cation is a form of imposed DIRECTIONAL

SELECTION. See hybridization; vegetativepropagation. Compare natural selection.

dominance In vertebrate societies, astrict hierarchy (dominance hierarchy) inwhich each individual occupies a particularposition that is recognized by others in thegroup. High-ranking individuals are usu-ally aggressive toward subordinates. Estab-lishment of the hierarchy often involvesfighting or ritualized contests, or displays.Once the hierarchy is established, subordi-nate animals usually avoid threateningdominant animals and perform submissivedisplays to avoid being threatened bythem. Disruption of the hierarchy usuallyoccurs only if new animals move into thepopulation or if an animal dies. In some ex-treme hierarchies, such as wolves (Lupus),only one pair of animals in a pack, thedominant pair, breeds. Among domesticpoultry, where dominant individuals tendto peck at subordinates, the hierarchy issometimes termed the peck order.

dominant 1. Describing a species that isthe most important member of a commu-nity. See dominance.2. An allele that, in a heterozygote, pre-vents the expression of another (recessive)allele at the same locus. Organisms withone dominant and one recessive allele thus appear identical to those with twodominant alleles, the difference in theirgenotypes only becoming apparent on ex-amination of their progenies. The domi-nant allele usually controls the normalform of the gene, while mutations are gen-erally recessive.

Domin scale See Braun-Blanquet scale.

dormancy A period of minimal meta-bolic activity of an organism or reproduc-tive body. It is a means of surviving aperiod of adverse environmental condi-tions, e.g. cold or drought, and occurs inboth plants and animals.

dosage 1. The measured amount of a

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chemical or form of radiation to which anorganism is exposed or which it ingests (itsexposure dose).2. The amount of an absorbed substancethat actually reaches specific tissues and isavailable to interact with metabolicprocesses, for example, to elicit a toxic re-sponse (the target dose).

dose equivalent (HT) The dose of ioniz-ing radiation absorbed by an organismmultiplied by a factor that takes into ac-count the difference in the biologicaleffects of different kinds of radioactive par-ticle having the same energy. It is expressedin sieverts (Sv). 1 sievert is the radiationdose delivered in one hour at a distance ofone centimeter from a point source of 1 mgof radium enclosed in platinum 0.5 mmthick. For beta-particles, gamma-rays, andx-rays this factor is 1, but for alpha-parti-cles it is 20. See also dosage.

downstream drift The drifting down-stream of inorganic and organic materialand organisms as a result of the flow ofwater. It affects especially bottom-dwellinginvertebrates such as the larvae of mayflies,caddisflies, and midges in streams andrivers. For some of these species down-stream drift is part of a regular cycle of mi-gration, the larvae drifting downstreamand the adults flying back upstream to laytheir eggs. It allows for the rapid repopula-tion of parts of the stream after floods orpollution have removed the original inhab-itants, and also gives immature insects thechance to avoid overcrowding. Down-stream drift is most marked at night, andduring spring and summer.

drainage The movement of water fromland, either naturally or artificially, as itflows over the surface and through rocksand soil under the influence of gravity,eventually reaching the sea, an inland lake,or underground reservoir.

drainage basin See catchment area.

dredging The removal of mud or othersediments to deepen channels or keep wa-terways and harbors clear. The sediments

removed by dredging may be used for landreclamation. Dredging can have deleteri-ous effects on the environment, releasingtoxic chemicals and heavy metals frombottom muds, and stirring up silt that killsaquatic life and benthos.

drift edition See map.

drought A prolonged period whenwater loss by evaporation and transpira-tion exceeds precipitation, resulting in de-pletion of groundwater and soil moistureand reduction of stream flow, and a short-age of water. A drought cycle is a tempo-rary but repeated period of drierconditions in a particular environment. Forexample, in the North American grass-lands droughts occur at approximately 22-year intervals.

drought resistance Physiological mech-anisms that enable living organisms tocope with drought conditions. These mayrange from adaptations to conserve water(e.g. water storage in the tissues of succu-lents or in the bladders of desert tortoises,extra-thick cuticles on plant surfaces), toadaptations to minimize heat stress in situ-ations where evaporative cooling is nolonger possible or much reduced (e.g. theproduction of heat-stress proteins). In ex-treme cases the organism may be able totolerate drying out, as in the resurrectionplants of the American deserts, which dryout and blow around the deserts, then takeroot and turn green again after rain.

dryfall The settling out of suspendedparticles from the atmosphere during peri-ods without rain. Dryfall is a source of nu-trients for living organisms. Comparewetfall.

dry-matter production PRODUCTIVITY

expressed in terms of the dry weight of or-ganic matter produced per unit area perunit time.

dry season A regularly occurring periodwithout precipitation or with much re-duced precipitation that occurs at a par-ticular time of year every year. In

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dry season

Mediterranean, west-coast, and subtropi-cal climates the dry season occurs in sum-mer, and is exacerbated by high rates orevaporation and transpiration. In the trop-ics close to the Equator, there are often twodry seasons a year, corresponding to themigration north and south of the equator-ial rain belts. In other parts of the tropics,such as many parts of India, the dry seasonoccurs in winter.

dry weather flow (DWF; baseflow) Ina stream or river, the flow of water derivedfrom the seepage of groundwater and fromthe flow of water through the upper soilhorizons. During periods of drought, mostof the river flow may be due to dry weatherflow, but during peak flow periods it isonly a very small proportion of the totalflow.

dune A ridge or hill of sand typically be-tween 1 and 50 m high deposited there bythe wind. Sand dunes usually migratedownwind with time, as sand is picked upby the wind on the windward slope, thendropped as the wind speed decreases whenit reaches the downwind side of the dune.Dunes are formed in sandy deserts andalong coastlines. Coastal dune systemsform important barriers against the sea,protecting land and settlements in the hin-terland.

Along sandy coasts the main band ofbare sand dunes facing the shore are calledforedunes. In time grasses such as marramor beachgrass colonize the foredunes, theirstems and roots trapping more sand. Asvegetation dies and decays, soil starts tobuild up, and more species of plants movein. The hollows between dunes may beeroded by the wind down to the watertable, so they contain pools or lakes, andoften support diverse flora. Such wet hol-lows are called dune slacks. Once the vege-tation cover is more or less complete, thedunes are called fixed dunes, and will notmigrate unless the vegetation cover is seri-ously broken, by off-road vehicles or stormseas, for example. At the shoreward edgeof a dune system – and in deserts – smallembryo dunes form as sand piles up againstobjects washed up by the tide or small tufts

of vegetation. In some parts of the world,large mature dune systems often have adune heath vegetation dominated by low-growing ericaceous shrubs, relativelydrought-tolerant plants.

Dust Bowl An area in the USA, extend-ing across western Kansas, Oklahoma, andTexas, and into Colorado and New Mex-ico. During the 1930s droughts and over-farming caused topsoil erosion.

Dutch elm disease A serious disease ofall species of elm (Ulmus) caused by thefungus Ceratocystis ulmi. The fungusspreads through the XYLEM vessels, and theplant responds by producing tyloses (blad-derlike ingrowths of the adjacent paren-chyma cells that penetrate the xylemvessels through the pits) that seal off the af-fected vessels. Symptoms include yellowingand curling of the leaves, wilting and rapiddeath of branches and eventually of thewhole tree. The fungus is spread by elm-bark beetles (usually Scolytus scolytus). Inthe 1970s an especially virulent strain ofthe disease was introduced to Britain fromCanada on imported timber, and virtuallyall the hedgerow elms of England (manymillions) were killed, dramatically chang-ing the landscape.

dynamic-composite life table See lifetable.

dynamic equilibrium A state of anECOSYSTEM whereby there is a relativelystable balance between populations of or-ganisms and the environment despite con-stant small-scale change and disturbance.Such an equilibrium is due to the action ofopposing forces (e.g. numbers of births anddeaths) that proceed at approximatelyequal rates.

dynamic life table See life table.

dysphotic zone See photic zone.

dystrophic Describing lakes whosewater contains a high content of organicmatter. Compare eutrophic; oligotrophic.

dry weather flow

76

earthquake A sudden movement of theEarth (seismic activity) due to release ofstress in the CRUST, e.g. by movement alonga fault line. The movement is propagatedthrough the rocks and soil as a series ofshock waves (seismic waves), causing theground to shake. Earthquakes range indepth from near-surface events to (rarer)movements as deep as 720 km. The stressescausing earthquakes may be associatedwith volcanism (volcanic eruptions and in-trusions of magma into the crust) and/orwith movements of crustal plates (see platetectonics). Movements of the ground at thesurface and below ground are measured bya seismograph. A seismometer is an elec-tronic device that detects, amplifies, filters,and records the motions of the earth in aparticular direction. The magnitude of anearthquake is usually measured on theRichter scale.

Earth Summit A United Nations Con-ference on Environment and Developmentin Rio de Janeiro, Brazil, in 1992 to discussways of achieving SUSTAINABLE DEVELOP-MENT on a global scale. It produced the RioDeclaration on Environment and Develop-ment, which defined sustainable develop-ment as development that ‘meets the needsof present generations without compromis-ing the ability of future generations to meettheir own needs,’ and outlined key policiesto help individual countries devise nationalstrategies for sustainable development. Italso gave rise to two legally binding con-ventions: the CONVENTION ON BIOLOGICAL

DIVERSITY and the FRAMEWORK CONVEN-TION ON CLIMATE CHANGE, as well as aStatement of Forest Principles. A secondEarth Summit took place in Johannesburg,South Africa, in 2002.

earthworms See Annelida.

ebb tide The TIDE when it is going out.

eccritic temperature The preferredbody temperature of ECTOTHERMS, usuallymaintained by ‘shuffling’ between exposedsunny areas and those providing shade.Compare thigmotherm.

ecesis The GERMINATION and establish-ment of colonizing plants in an area, thisbeing the first stage in a SUCCESSION. Theassociated verb is ecize. See colonization;pioneer species.

Echinodermata A phylum of marineand intertidal invertebrates containing thestarfish, sea urchins, feather stars, sea cu-cumbers, and brittlestars. Some are preda-tors, while others feed on detritus. Thephylum includes both commercially impor-tant edible species and important predatorsof commercially harvested shellfish. Mostechinoderms have a radial symmetry, typi-cally with five rays radiating from a centraldisk. All have calcareous skeletal platesand most have spines. Part of the coelom(central body cavity) is modified as thewater vascular system, a kind of hydro-static skeleton that extends into hydraulictube feet, used typically in locomotion. Thewater vascular system has an externalopening, the madreporite. The nervous sys-tem is simple and there are no excretory or-gans. Echinoderms produce one or morestages of planktonic larvae. See alsocrown-of-thorns starfish.

echolocation A method of navigationused by some animals, such as bats anddolphins. They emit high-pitched sounds,often inaudible to humans, which are re-

77

E

flected back off nearby objects and de-tected by the ears or other sensory recep-tors.

echo sounder (sonar device) A devicethat sends pressure waves from pulses ofsound to the sea bed, then interprets theechoes to give a depth profile, the delay be-tween sending and receiving the wavesbeing a measure of depth. Echo sounding isalso used to detect shoals of fish, the posi-tion of underwater objects such as wrecks,and the depth of polar ice. The procedureis sometimes called sonar, by analogy toradar, which works in air.

ecize See ecesis.

ecocline A CLINE that is due to a specificenvironmental factor, such as a gradient ofheavy metal concentration in the soil.

ecoengineering The incorporation of anatural process in a controlled environ-ment to provide a useful ‘end-product’, forexample, the use of reed beds to removeheavy metals from sewage farms.

ecological amplitude The range of tol-erance by a species to a particular environ-mental condition(s). When mapped, thisprovides a bell-shaped curve, the actualshape of which can be used to identify eco-logical indicator species.

ecological diversity See biodiversity;species diversity.

ecological efficiency (biological effi-ciency) The efficiency of energy transferbetween one TROPHIC LEVEL and the next,usually expressed as the percentage of theenergy of the BIOMASS produced by thedonor trophic level.

ecological energetics The study of theenergy transformations in ecosystems.

ecological indicator See indicator.

ecological isolation The separation ofPOPULATIONS or subpopulations as a resultof changes in the environment or in their

ecology. Isolation leads to restricted GENE

FLOW between the populations, and to fur-ther divergence due to NATURAL SELECTION

pressures in the local environment. The re-sulting differences in gene frequencies mayeventually lead to SPECIATION. See geneticdrift. Compare geographical isolation; re-productive isolation.

ecological niche See niche.

ecological range The range of environ-mental conditions under which individualsof a species can survive.

ecological sustainability See sustain-ability.

ecology The study of the relationshipsof organisms to one another and to theirliving (BIOTIC) and nonliving (ABIOTIC) envi-ronment.

economically optimum yield (EOY)The best YIELD that can be obtained in agiven situation in terms of economic valuerather than BIOMASS. It is defined as the dif-ference between the gross value of the har-vested crop and the sum of the fixed costs(such as finance, insurance, and adminis-trative overheads), and the variable costssuch as fuel and labor, which increase withthe effort put into harvesting the crop.Compare maximum sustainable yield.

economic injury level (EIL) The levelof abundance of a pest below whichputting more effort into controlling it willcost more than is saved by pest control.Above this level, it is cost effective to con-trol the pest.

ECOPATH An ecosystem MODEL thatcalculates energy flow and biomass pro-duction in various conditions for a steady-state ecosystem containing many species. Itis used particularly for aquatic ecosystems,especially for estimating sustainable levelsof cropping.

ecophysiology See physiological ecol-ogy.

echo sounder

78

ecoregion A geographic area defined byits natural vegetation, fauna, soil type,physical features, and climate. Comparebiogeographical region; biome.

ecosphere (biosphere) The global eco-system; the part of the Earth and its atmos-phere that is inhabited by living organisms.

ecosystem A unit made up of all the liv-ing and nonliving components of a partic-ular area that interact and exchangematerials with each other. The concept ofthe ecosystem differs from that of the COM-MUNITY in that more emphasis is placed onabiotic factors. The term can be applied onvarious scales, from small ponds to thewhole planet.

ecotone A transition zone between twoor more different communities, e.g. at theboundary between land and water, orwhere rocks and soils change. It is some-times known as an edge habitat. Such habi-tats are usually species-rich. See edgeeffect.

ecotourism Tourism that promotestravel to natural, ‘unspoiled’ habitats toobserve wildlife or indigenous peoples.Ecotourism can be an important source ofincome to local people as part of a SUS-TAINABLE DEVELOPMENT program, but alltoo often it generates disturbance and pol-lution, destroying the very qualities it setout to promote.

ecotoxicology The study of the adverseeffects of manmade substances on popula-tions and communities, including humans.

ecotype A subpopulation of individualsgenetically adapted to the combination ofenvironmental factors in their local habi-tat, but still able to mate with other eco-types belonging to the same species andproduce fertile offspring. Differences be-tween ecotypes may be physiological ormorphological. Compare biotype. See alsoadaptive radiation; speciation.

ectomycorrhiza See mycorrhiza.

ectoparasite See parasitism.

ectotherm An animal that keeps itsbody temperature within fairly narrowlimits by behavioral means, such as bask-ing in the sun or moving into shade, e.g.snakes and lizards. See poikilothermy.Compare endotherm.

ectotrophic mycorrhiza See mycor-rhiza.

edaphic factors The physical, chemical,and biological characteristics of the SOIL

that together form an important compo-nent of the habitat because of their influ-ence on plant distribution. The mainedaphic factors are water content, PH, or-ganic matter, and SOIL TEXTURE.

edge effect 1. The sampling errors thatoccur at the edges of sampling plots/areas.This may be due to items at the edge of theplots experiencing significantly differentconditions from those in the center, or be-cause items overlap the periphery. This ef-fect is especially pronounced in smallsampling areas.2. The tendency for a transitional zone be-tween communities (an ecotone) to containa greater variety of species and more densepopulations of species than the communi-ties surrounding it.

edge habitat See ecotone.

effective dose equivalent See doseequivalent.

effective population size (Ne) The av-erage number of breeding individuals in apopulation that contribute genes to thenext generation. This is often less than theobserved number of breeding individuals.There may be a number of causes includingimbalances in the numbers of individualsof each sex, a BREEDING SYSTEM in which ahigher proportion of one sex may mate,differences in fertility between individualsand, especially in small populations, loss ofheterozygosity (see heterozygous) and IN-BREEDING DEPRESSION. The effective popula-tion size determines the rate at which

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effective population size

genetic VARIATION is lost as a result of GE-NETIC DRIFT.

effluent Waste material that is dis-charged into the environment, for exam-ple, from sewage outfalls and factorychimneys.

EIA See Environmental Impact Assess-ment.

EIL See economic injury level.

EIS See Environmental Impact State-ment.

electrochemical sensor A sensor thatuses electrodes specific to particular ions todetermine the quantity of substances in so-lution.

electromagnetic energy (electromagneticradiation) Energy propagated by vibrat-ing electric and magnetic fields. It can bethought of as being in the form of waves oras streams of photons (units of light). Theenergy carried depends on the frequency.The frequency and wavelength are relatedby the equation:

λv = cwhere c is the speed of light, λ the wave-length, and v the frequency. The electro-magnetic spectrum ranges from low-frequency radio waves to high-frequencygamma rays.

electromagnetic radiation See electro-magnetic energy.

electromagnetic sense A sense used bysome fish (particularly cartilaginous spe-cies) to detect subtle changes in magneticimpulse, which they use to locate prey.Some insects, e.g. fire ants, are also at-tracted to the electromagnetic fields pro-duced by underground cables, which theysubsequently attack.

electromagnetic spectrum See electro-magnetic energy.

electron transport chain See photo-synthesis.

electrophoresis The migration of elec-trically charged particles toward oppo-sitely charged electrodes in solution underan electric field – the positive particles tothe cathode and negative particles to the

effluent

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10–15 –

10–14 –

10–13 –

10–12 –

10–11 –

10–10 –

10–9 –

10–8 –

10–7 –

10–6 –

10–5 –

10–4 –

10–3 –

10–2 –

10–1 –

1 –

10 –

102 –

103 –

104 –

105 –

– 1020

– 1019

– 1018

– 1017

– 1016

– 1015

– 1014

– 1013

– 1012

– 1011

– 1010

– 109

– 108

– 107

– 106

– 105

– 104

– 103

– 102

– 10

– 1

gamma rays

X-rays

ultravioletradiation

visible light

infrared(heat)

radiation

EHF

SHF

UHF

VHF

HF

MF

LF

VLF

radiofrequencies

freq

uen

cy/k

Hz

wav

elen

gth

/m

Electromagnetic radiation: the electromagneticspectrum

anode. The rate of migration varies withmolecular size and shape. The techniquecan be used to separate or analyze mixturesof organic molecules, such as proteins ornucleic acids. The medium used may bewetted filter paper, a gel of starch or a sim-ilar polymer (gel electrophoresis), or a sim-ilar inert porous medium.

element A substance that cannot bechemically decomposed into simpler sub-stances. The atoms of an element all havethe same proton number, and thus thesame number of electrons, which deter-mines its chemical activity. Compare com-pound; ion.

elfin forest (Krummholz) Forest domi-nated by dwarfed and deformed trees andshrubs found between the timberline andthe tree line on tropical mountains. It isnormally between 1 and 3 m tall. The zoneof elfin forest is termed the Kampfzone.

El Niño (El Niño-Southern OscillationEvent; ENSO) A warm ocean currentthat flows southward along the Pacificcoast of tropical South America, approxi-mately every seven years. It is associatedwith a change in the atmospheric circula-tion known as the SOUTHERN OSCILLATION.El Niño flows during the southern mid-summer when the prevailing trade winds

81

El Niño

increasedsurfacetemperatures

AUSTRALIA

I N D O N E S I AI N D O N E S I AI N D O N E S I A

warmwarmwarm

waterswaterswaters

normal trade winds

normal ocean currents

AUSTRALIA

I N D O N E S I AI N D O N E S I AI N D O N E S I A

weakenedtrade winds

El Niño currents

NORTH AMERICANORTH AMERICANORTH AMERICA

SOUTHAMERICA

NORTH AMERICANORTH AMERICANORTH AMERICA

SOUTHAMERICA

El Niño

weaken, allowing the Equatorial counter-current to strengthen; warm surface watersare no longer driven westward, but floweastward instead, to overlie the cold waterof the northward-flowing PERU CURRENT.Severe El Niño events prevent the up-welling of nutrient-rich cold water alongthe Pacific coast and cause widespreaddeath of plankton and declines in fish. Theclimatic effects of El Niño are felt through-out the Pacific and beyond, causing in-creased rainfall in desert areas fringing thePacific coast of South America, butdroughts in parts of Africa. See also LaNiña.

El Niño Southern Oscillation EventSee El Niño.

Elton, Charles Sutherland (1900–91)British ecologist. Elton joined an OxfordUniversity expedition to Spitzbergen(1921), where he carried out ecologicalstudies of the region’s animal life. FurtherArctic expeditions were made in 1923,1924, and 1930, leading to his appoint-ment as biological consultant to the Hud-son’s Bay Company, for which he carriedout investigations into variations in thenumbers of fur-bearing animals using trap-per’s records dating back to 1736. In 1932Elton helped establish the Bureau of Ani-mal Population at Oxford – an institutionthat subsequently became an internationalcenter for information on and research intoanimal numbers and their ecology. In thesame year he became editor of the newJournal of Animal Ecology, launched bythe British Ecological Society. Elton wasone of the first biologists to study animalsin relation to their environment and otheranimals and plants. His demonstration ofthe nature of food chains and cycles, aswell as such topics as the reasons for dif-ferences in animal numbers, were discussedin Animal Ecology (1927). In 1930 he pub-lished Animal Ecology and Evolution, inwhich he advanced the idea that animalswere not invariably at the mercy of theirenvironment but commonly, perhapsthrough migration, practiced environmen-tal selection by changing their habitats.

EMAP (Environmental Monitoring andAssessment Program) A research pro-gram developed by the United States ENVIRONMENTAL PROTECTION AGENCY toprovide tools for monitoring and assessingthe status and trends of national ecologicalresources. It aims to further scientific un-derstanding to assist the use of monitoringdata in assessing current ecological statusand forecasting future risks to natural re-sources.

embryo dune See dune.

emergent 1. An individual tree or groupof trees that rises significantly higher thanthe continuous CANOPY of tropical RAIN-FORESTS.2. An aquatic plant that has most of its veg-etative parts above water.

emergent poverty Poverty that hasonly recently arisen in a particular area.For example, the migration from the coun-tryside to the cities in many parts of theworld has created new urban poor; redis-tribution of land or the introduction ofmechanization may lead to poverty in ruralareas as people lose their jobs or theirsource of free food.

emigration The movement of individu-als out of a POPULATION or from one area toanother. Compare immigration.

emission A discharge of gaseous, partic-ulate, or soluble waste material or otherPOLLUTANTS into air or water. Vehicle ex-haust gases are an example. Emissions thatare trapped by emission control systemsare called primary emissions. Secondaryemissions are those not catered for by suchcontrols, for example emissions throughleaks in pipes.

emission credit See emissions trading.

emissions trading The control of totalemissions by allowing overproducers ofemissions to purchase emission creditsfrom companies that produce less thantheir quota of emissions, or by allowingcompanies that reduce their emissions sub-

El Niño Southern Oscillation Event

82

stantially to save credits against futureemissions.

emulsifier A device or chemical thataids the suspension of one liquid in an-other, for example an appropriate organicchemical in an aqueous solution.

emulsion The dispersion of one liquid ina second immiscible liquid.

encounter competition See competi-tion.

endangered species A species that is indanger of EXTINCTION. See CITES; Endan-gered Species Act; Red Data Book.

Endangered Species Act A federal actpassed in 1973 to protect the United States’endangered and threatened species, endan-gered species being those in imminent dan-ger of extinction throughout their range,e.g. the black-footed ferret, the blue whale,and the California condor, and threatenedspecies being likely to become endangeredin the foreseeable future, e.g. the northernspotted owl. Many states also recognizerare species, with low or declining num-bers, shrinking habitats, and so on. Endan-gered species and their habitats areprotected by law, and states are required toset up recovery plans.

endemic Describing a population orspecies that is restricted geographically.Some endemics have evolved in geographi-cal isolation on islands or mountaintops;others may be relics of once widespreadspecies that have become restricted owingto climate change, geological change, orhuman activity.

endobiont (endosymbiont) A symbioticorganism that lives within the body of itshost. See also endosymbiont theory.

endocrine disrupter See endocrine tox-icity.

endocrine toxicity Toxicity that affectsthe endocrine systems of animals – theirhormones – often by blocking receptors for

certain hormones. Such endocrine-disrupt-ing chemicals are thought to be involved infertility problems in humans as well asother animals, and especially in fish andother aquatic organisms. They are also im-plicated in some cancers, such as breastand testicular cancer. Natural estrogensand synthetic estrogens used in hormonetreatments and contraceptives are found inSEWAGE effluent. Other endocrine dis-rupters include tributyl tin oxide, whichcauses female dog whelks to become mas-culinized, and certain industrial detergents,which cause male trout to produce vitel-logenin, an egg yolk protein normally pro-duced only by females.

end-of-the-pipe Pollution or wastetreatment processes or devices located im-mediately at the outlet of a collecting sys-tem, e.g. at a sewage outfall. The pollutantsare treated before they are discharged tothe environment.

endogenous rhythm Cyclical, physical,and biochemical processes that occur in anorganism in response to internal stimuli,e.g. the opening and closing of stomata,which usually coincides with dawn anddusk respectively, but which continueseven if kept in continuous darkness. Seecircadian rhythm.

endomycorrhiza See mycorrhiza.

endoparasite See parasitism.

endoparasitoid See parasitoid.

endoplasmic reticulum See nucleus.

endosperm The nutritive tissue that sur-rounds the embryo in angiosperms. Manyendospermic seeds (e.g. cereals and oilseeds) are cultivated for their food reserves.

endosymbiont See endobiont.

endosymbiont theory The theory thateukaryotic organisms evolved from symbi-otic associations between bacteria. It pro-poses that integration of photosyntheticbacteria, for example purple bacteria and

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endosymbiont theory

Cyanobacteria, into larger bacterial cellsled to their permanent incorporation asforerunners of the plastids (e.g. CHLORO-PLASTS) seen in modern eukaryotes. Simi-larly, other symbiotic aerobic bacteria gaverise to the MITOCHONDRIA. Chloroplastsand mitochondria contain some circularDNA and small ribosomes, both more sim-ilar to those of bacteria than those ofeukaryotes, and a double membrane enve-lope. The chloroplasts of some groups ofalgae have a triple envelope, suggestingmore than one symbiotic event. See Bac-teria; eukaryote; symbiosis.

endotherm An animal that can generateheat internally to raise its body tempera-ture, e.g. birds and mammals. See homoio-thermy. Compare ectotherm.

energetic efficiency The ratio of energystored or useful work done to energy in-take.

energy The capacity for doing work. En-ergy has several forms, some of which areconvertible; they include chemical energy(energy stored in chemical compounds),ELECTROMAGNETIC ENERGY, potential en-ergy (the capability of doing work), kineticenergy (movement), mechanical energy, ra-diant energy (light), and thermal energy(heat). Energy is measured in JOULES.

energy budget The comparison be-tween the energy entering and the energyleaving the body of an animal, a particularTROPHIC LEVEL, or an industrial process. It

is an indication of the efficiency of energyuse. See also production efficiency.

energy conservation The strategy forreducing the energy usage by individuals,households, industrial units, or modes oftransport. For example, the reduction inenergy use for heating by installing insula-tion, double-glazing, and energy-efficientboilers and the use of low-energy lighting.The strategy can also be applied to the un-necessary use and discarding of objectsthat use energy in their manufacture, suchas plastic bags.

energy efficiency The use of processesand fuel-consuming devices designed to en-sure that they make efficient use of energy:that the energy input is low in relation toenergy output. For example, the introduc-tion of condensing boilers for heating,COMBINED HEAT AND POWER PLANTS thatburn waste, low-energy light bulbs, and thedevelopment of vehicles that make efficientuse of fuel. See also energy budget.

energy flow web See food web.

energy recovery The conversion of or-ganic wastes to usable energy. For exam-ple, microbes may be used to digest organicwastes and generate methane gas, refusemay be burned to produce steam power, orheated to produce oil. See combined heatand power plant.

energy subsidy The use of energy that isnot derived from an organism’s own me-tabolism.

energy transformation The transfor-mation of one kind of energy into another.For example, during photosynthesis, lightenergy is transformed into chemical en-ergy, in respiration chemical energy istransformed into heat energy, and in loco-motion chemical energy is converted to me-chanical energy.

enhanced treatment process See di-gestion (def. 2).

endotherm

84

hydroelectric and otherrenewable sources

7.0%

oil37.5%

coal25.1%

naturalgas

24.0%

nuclear6.4%

Energy: primary sources (World 1993)

enriched uranium See enrichment (def.3).

enrichment 1. The planting of youngtrees in a forest, e.g. after timber extrac-tion.2. The addition of nutrients, for example,from agricultural runoff or sewage efflu-ent, to surface water. It greatly increasesthe growth of algae and aquatic plants. Seeeutrophication.3. The processing of uranium to increasethe proportion of the fissionable form U-235 in relation to U-238, or to add pluto-nium P-239 to natural uranium, for use innuclear reactors or nuclear weapons. En-riched uranium contains up to 3% U-235,as compared with 0.7% in natural ura-nium. See radioactive isotope.

ENSO See El Niño.

entropy Symbol: S. In a given system,the availability of energy to do work. Inany system that undergoes a reversiblechange, the change of entropy is defined asthe heat absorbed divided by the thermo-dynamic temperature:

dS = dQ/TThe concept of entropy has been widenedto take in the general idea of disorder – thehigher the entropy, the more disorderedthe system. For example, a chemical reac-tion involving polymerization may exhibita decrease in entropy because there is achange to a more ordered system.

environment The complete range of ex-ternal conditions under which an organismlives, including physical, chemical, andbiological factors, such as temperature,light, the availability of food and water,and the effects of other organisms.

environmental biology See binomics.

environmental hazard One or morefactors in the environment that pose athreat to air, water or soil quality, floraand fauna, or human health and safety, forexample spills of oil or toxic chemicals.

Environmental Impact Assessment

(EIA) An attempt to predict the effects ofproposed industrial and other building de-velopments, land-use changes or legislationon the natural and human environments,and on human health and wellbeing. TheUnited States Environmental ProtectionAgency requires a written EIA prior to fed-eral activities such as the building of high-ways or airports and, if it is still in doubtabout the impact of the proposed activity,it will request an ENVIRONMENTAL IMPACT

STATEMENT.

Environmental Impact Statement (EISor ES) A written analysis prepared for orby the United States Environmental Protec-tion Agency that identifies and analyzes indetail the environmental impacts, bothpositive and negative, of a proposed ac-tion. See also Environmental Impact As-sessment.

environmentalism A social movementthat attempts to understand the impact ofhuman activity on the environment.

environmental load The concentration(or other measure of quantity) of one ormore chemicals (e.g. waste gases from afactory chimney) or physical fluxes (e.g.heat or sound) emitted by a source into theenvironment.

environmental management Proce-dures and controls aimed at conserving thestatus of an environment. It may involveactive intervention to maintain a particularhabitat, for example, a particular stage inplant succession. Alternatively it may aimto balance the conservation of a particularnatural resource with the needs of localhuman communities to promote sustain-able development.

environmental monitoring Continu-ous or periodic measuring of environmen-tal parameters and/or the amounts ofparticular chemical or physical pollutantsto prevent deterioration of the environ-ment. Monitoring may also include at-tempts to predict changes to the ecosystemand take preventive action. See EMAP.

85

environmental monitoring

Environmental Monitoring and As-sessment Program See EMAP.

environmental noise Background sig-nals that derive from outside the environ-ment, but which interfere with themeasurement or detection of environmen-tal processes or biotic factors.

environmental polymorphism See poly-morphism.

environmental potential See land-usecapability.

environmental potential maps Seemap.

Environmental Protection Agency(EPA) The United States Governmentbody responsible for the monitoring andcontrol of pollution, such as hazardouswastes, pesticides and other toxic sub-stances, radioactive materials, and thequality of air, water, and drinking water. Ithas the power to enforce its regulations.The EPA also carries out research in theeffects of pollution and human activities onthe environment.

environmental resistance See bioticpotential.

environmental stochasticity Randomvariation in the ABIOTIC environment.

enzootic Describing a disease con-strained by geographic boundaries.

Eocene The second oldest epoch of theTERTIARY PERIOD, 55–38 mya. It was char-acterized by predominance of early hoofedmammals. Many other mammals (e.g. car-nivores, bats, and whales) and birds werealso present. It was a period of widespreadtemperate and subtropical forests. The ex-tensive grasslands of the late Tertiary hadnot yet developed, but large hoofed grazingmammals, such as horses and elephants,were evolving rapidly. See also geologicaltime scale.

eolian Caused or deposited by wind.

EOY See economically optimum yield.

EPA See Environmental ProtectionAgency.

ephemeral A plant that has a very shortlife cycle, and may complete more than onelife cycle within a year. Examples are shep-herd’s purse (Capsella bursa-pastoris) andcertain desert plants that grow, flower, andset seed in brief periods of rain. Compareannual; biennial; perennial.

epidemic A relatively sudden and wide-spread outbreak of a disease, affecting ei-ther a wide area or a large number ofpeople or animals in a small area.

epidemiology The study of the inci-dence, distribution, and control of diseasesaffecting large numbers within a popula-tion. These include both epidemics of in-fectious diseases and diseases associatedwith environmental factors and dietaryhabits (e.g. lung cancer, some forms ofheart disease, etc.).

epifauna Organisms living on the sur-face of a substrate.

epilimnion In a stratified lake (a lake inwhich there are distinct layers of differenttemperature and/or density), the upperlayer of warm water. It usually has a fairlyuniform temperature, because it undergoesmixing by wind action. See stratification.

epipelagic The oceanic zone extendingfrom the surface to about 200 m, whereenough light penetrates to allow photosyn-thesis.

epiphyte (air plant) Any plant growingupon or attached to another plant or objectmerely for physical support.

epoch See geological time scale.

Equator An imaginary line at latitude 0°that girdles the Earth and divides it into thenorthern hemisphere and southern hemi-sphere.

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equatorial forest See rainforest.

equilibrium In population ecology, thestate in which the numbers of births anddeaths are equal, so there is no fluctuationin population numbers. See also dynamicequilibrium; equilibrium theory.

equilibrium hypothesis See equilib-rium theory.

equilibrium isocline (zero net growthisocline) On a graph that shows the ef-fect of the availability of two potentiallylimiting resources on population numbers,a line that indicates zero net populationgrowth rate. It represents the balance be-tween the consumption and renewal of theresources. In a situation where two speciescompete for the same resources, by plot-ting the equilibrium isocline for both pop-ulations, the species whose isocline liesclosest to the resource axis will eventuallycompetitively exclude the other speciesfrom occupying the same NICHE. See com-petitive exclusion principle.

equilibrium species A species whosemain survival strategy is competitive abil-ity rather than reproductive rate or disper-sal success. Such species are typical ofstable environments. In unstable environ-ments, such species rely on surviving unfa-vorable conditions by saving energy,entering a sate of DORMANCY or ESTIVATION

with minimal metabolic demands, ratherthan relying on, in plants, for example,producing seeds that will germinate whenconditions improve. Compare fugitive spe-cies.

equilibrium theory (equilibrium hy-pothesis) The theory that a COMMUNITY ina defined area is in a state of DYNAMIC EQUI-LIBRIUM, the number of species at a giventime being determined by the balance be-tween IMMIGRATION and EXTINCTION. Fol-lowing a disturbance such as a naturalCATASTROPHE, the community will tend toreturn to equilibrium. This theory is basedon the assumption the organisms occupy-ing the same NICHE compete for resources,and that when resources are limiting, one

species will inevitably exclude another (seecompetitive exclusion principle). Nonequi-librium theory, by contrast, is concernedwith the temporary behavior of a systemwhen it moves away from the equilibriumpoint, and in particular its variation overtime.

equinox The period of the year whenthe Sun is overhead at local solar time atthe Equator. It happens twice a year andday and night are equal time periods oftwelve hours. In the northern hemispherethe vernal or spring equinox occurs on 20or 21 March and the fall or autumnalequinox on 22 or 23 September. This is re-versed in the southern hemisphere.

equitability A measure of the evennesswith which species are distributed in acommunity. For example, if out of a com-munity of 100 organisms, 95% were of onespecies, and the other 5% of three otherspecies, this would be an uneven distribu-tion.

equivalent dose See dose equivalent.

era See geological time scale.

erg A large expanse of sand (sand sea) ina hot desert. The sand usually accumulatesin an enclosed basin, where sediments de-rived from the surrounding rocky desertwere deposited by rivers or in a lake.

ericaceous Of, relating to, or being aheath or of the heath family of plants,which are mostly shrubby, dicotyledonous,and often evergreen plants that thrive onopen, barren soil that is usually acidic andpoorly drained.

erosion 1. The wearing away of rocks,soil, or buildings by physical or chemicalprocesses. Physical processes include theeffects of PRECIPITATION, rivers, RUNOFF,frost, changes in temperature, and wind.Chemical processes include corrosion bywater containing dissolved substances, including ACID RAIN, and by gases such asthose containing pollutants. These pro-cesses are often exacerbated by human ac-

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tivities such as land-clearance, logging, in-dustrial pollution, or diversion of water-ways. The products of erosion are oftentransported by wind or water and de-posited elsewhere, e.g. in the sea. See soilerosion; weathering. See also solifluction;runoff.2. (genetic erosion) The depletion of ge-netic diversity in domestic crops and live-stock.

essential amino acid See amino acid.

essential resource A RESOURCE that lim-its the numbers of a consumer populationindependently of other resources.

estivation A period of inactivity seen insome animals during the summer or dryhot season. For example, lungfish respondto the drying up of water by burying them-selves in the mud bottom. They re-emergeat the start of the rainy season. See also HI-BERNATION.

estuary A complex ECOSYSTEM that oc-curs where fresh water draining off theland meets salt water, for example, at themouths of rivers, SALT MARSHES, bays, andLAGOONS. The boundary of tidal estuaryoccurs where the fresh water starts, or atthe point in a river to which the highestspring tides penetrate. Estuarine habitatsare often subject to cyclical fluctuations insalinity.

ethanol A CLEAN FUEL (used as an alter-native fuel for motor vehicles) that can beproduced from grain crops, and frompaper and wood wastes. Because it is de-rived from recently grown crops, it doesnot contribute to carbon dioxide accumu-lation as do FOSSIL FUELS. Ethanol andethanol/gasoline blends (GASOHOL) havebeen used for many years. Methanol, an-other alcohol produced from wood andcoal, has similar benefits, and provides amuch better performance for motor vehi-cles than ethanol.

ethene See ethylene.

Ethiopian One of the six zoogeographi-

cal regions of the earth. It encompassesAfrica south of the Sahara Desert, thesouthern half of Arabia, and (according tocertain authorities) Madagascar. The ani-mals characteristic of this region are thegorilla, chimpanzee, African elephant, rhi-noceros, lion, hippopotamus, giraffe, cer-tain antelopes, ostrich, guinea fowl,secretary bird, and, in Madagascar, thelemur.

ethnobotany The study of the use ofplants by humans.

ethology The study of the behavior ofanimals in their natural environment.

ethylene (ethene) A gaseous hydrocar-bon produced in varying amounts by manyplants, which functions as a plant hor-mone. Ethylene is involved in the control ofGERMINATION, cell growth, fruit ripening,ABSCISSION, and SENESCENCE; it inhibits lon-gitudinal growth and promotes radial ex-pansion. In ripening fruit a rapid rise inethylene production precedes respirationto reach the climacteric ripeness; produc-tion of ethylene stimulates further produc-tion of ethylene. Thus ripe fruits stimulateother fruits to ripen quickly; this processcan be controlled to some extent for fruitstorage and transport.

ethylene dibromide A colorless volatileorganic liquid used as a fuel additive to re-move LEAD.

Eubacteria A DOMAIN or a major sub-kingdom of the BACTERIA, containing alarge, diverse, and widespread group ofbacteria, principally distinguishable fromthe other major subkingdom, the ARCHAEA,by differences in the RNA sequences oftheir RIBOSOMES, which are thought to re-flect the very early evolutionary divergencebetween the two groups. Most are single-celled organisms that divide by binary fis-sion. The group includes photosyntheticforms, some utilizing water and carbondioxide, as do plants, but others derivingenergy from a range of inorganic sub-strates. Heterotrophic types include bothaerobic and anaerobic forms, and many

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important DECOMPOSERS and nitrogen fix-ers. Some phyla, such as the ACTINOBACTE-RIA, are important sources of antibiotics.See nitrogen fixation. Compare Archaea.

eucalypts Australian hardwood trees ofthe genus Eucalyptus (family Myrtaceae).Widely cultivated as fast-growing timbertrees, they produce a strong-smelling resinthat is highly flammable, so are susceptibleto fire. Many eucalypts have characteristicpeeling bark. They include jarrah (E. mar-ginata) and karri (E. diversicolor), and themountain ash (E. regnatus), probably thetallest tree in the world.

eucaryote See eukaryote.

Eukarya See eukaryote.

Eukaryota See superkingdom.

eukaryote Any member of all the livingkingdoms except the Bacteria (Archaeaand Eubacteria), which are the prokary-otes. Eukaryotes are defined by the pres-ence of a much more elaborate cell than the prokaryotes. Features not found inPROKARYOTES include the genetic materialpackaged in chromosomes within a mem-brane-bound nucleus; possession of mito-chondria (see mitochondrion) and, inphotosynthetic eukaryotes, chloroplasts; aquite different and much more elaboratedmembrane structure including internalmembranes such as the endoplasmic retic-ulum and Golgi body; different-sized ribo-somes and complex undulipodia composedof arrays of microtubules. Some of thesefeatures probably arose through endosym-biosis of prokaryotes. In the FIVE KINGDOMS

CLASSIFICATION, the eukaryotes constitute asuperkingdom or DOMAIN, the Eukarya,containing the kingdoms Animalia, Fungi,Plantae, and Protoctista. See endosymbionttheory.

Eulerian measurements See water-flow measurements.

Eulerian models See Lagrangian mod-els.

euphotic zone See photic zone.

euryecious Describing an organism thatis able to live in a wide range of habitats.

euryhaline Describing organisms thatare able to tolerate wide variations of saltconcentrations (and hence osmotic pres-sure) in the environment. For example, theeel (Anguilla) can live in both fresh waterand salt water. Compare stenohaline.

eusociality The complex social organi-zation of certain groups of insects, espe-cially termites, ants, bees, and wasps, inwhich only a few members of the commu-nity breed, while the other CASTES are ster-ile or non-breeding workers, and wherethere is an overlap of generations, so thatthe offspring can help rear their youngersiblings.

eutrophic 1. Describing lakes or pondsthat are rich in nutrients and consequentlyare able to support a dense population ofplankton and littoral vegetation. See eu-trophication. Compare oligotrophic.2. Describing fen peats that contain highconcentrations of minerals and bases.

eutrophication The process that resultswhen an excess of nutrients enters a lake,for example, as sewage or from waterdraining off land treated with fertilizers.The nutrients stimulate the growth of thealgal population, giving a great concentra-tion or BLOOM of such plants. When thesedie they are decomposed by bacteria,which use up the oxygen dissolved in thewater, so that aquatic animals such as fishare deprived of oxygen and die from suffo-cation.

evaporation The changing of a liquid toa vapor by the expenditure of heat energy,which is drawn from the surroundings,thus cooling them. Compare transpiration.See also evapotranspiration.

evaporimeter An instrument for meas-uring the rate of evaporation.

evaporites See salt pan.

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evapotranspiration The loss of watervapor by EVAPORATION and TRANSPIRATION.

evergreen Describing plants that retaintheir leaves throughout the year or throughseveral years. Compare deciduous.

evolution The process of genetic changethat occurs in populations of organismsover a period of time. It manifests itself asnew characteristics in a species, and mayeventually lead to the formation of newspecies. See Darwinism; Lamarckism; nat-ural selection.

evolutionary tree A branching diagramshowing the phylogenetic relationships be-tween different TAXA.

excreted energy The part of the energyassimilated by an animal that is eliminatedin the form of nitrogenous waste.

excretion The process by which excess,waste, or harmful materials, resulting fromthe chemical reactions that occur withinthe cells of living organisms, are eliminatedfrom the body. The main excretory prod-ucts in animals are water, carbon dioxide,salts, and nitrogenous compounds.

exon A segment of a GENE that is bothtranscribed and translated and hence car-ries part of the code for the gene product.Most eukaryotic genes consist of exonsinterrupted by noncoding sequences (in-trons). Both exons and introns are tran-scribed initially, but the introns are thenremoved, leaving mRNA, which has onlythe essential sequences and is translatedinto the protein. Bacteria do not have in-trons.

exotic A non-native species, often intro-duced into surroundings that it finds hos-pitable and lacking in population-controlmechanisms such as disease or predators.Compare indigenous.

expectation of further life (ex) The av-erage remaining lifetime of an individual ofage x.

experiment A procedure carried outunder controlled conditions in order to es-tablish or verify a hypothesis or law. Ex-periments involve exploring or testingcausal relationships between variables, andusually involve a control procedure againstwhich the others are compared, random-ization of design, and repeats to ensure va-lidity or assign significance. See control;null hypothesis; sampling.

exploitation 1. Direct competition for aresource.2. The removal of BIOMASS, including indi-viduals, from a population by consumers.

exponential growth A type of GROWTH

in which the rate of increase in numbers ata given time is proportional to the numberof individuals present. Thus, when thepopulation is small multiplication is slow,but as the population gets larger, the rateof multiplication also increases. An expo-nential growth curve starts off slowly andincreases faster and faster as time goes by,so becoming J-shaped (when plotted on alogarithmic scale, it is linear). However, atsome point factors such as lack of nutri-ents, accumulated wastes, etc. limit furtherincrease, when the curve of number againsttime begins to level off and the curve be-comes sigmoid (S-shaped).

exposure indicator An environmentalcharacteristic that can be measured to pro-vide evidence of exposure to, and some-times also the magnitude of, a chemical orbiological stress.

exposure limit The threshold concen-tration in workplace air of a chemicalabove which it is deemed to be harmful toworkers.

exposure meter See light meter.

expression The occurrence in the PHE-NOTYPE of characteristics related to a par-ticular ALLELE or alleles. See genotype.

extinction The complete disappearanceof a species from the planet; or from the

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whole or a particular part of its naturalrange (compare extirpation).

The natural, slow replacement ofspecies that occurs as ecosystems change istermed backround extinction. Anthro-pogenic extinctions are due to human ac-tivity, e.g. the dodo (a flightless bird thatlived on the island of Mauritius in the In-dian Ocean) was rendered extinct byhuman hunters and predation by and com-petition with introduced animals. Sec-ondary extinction occurs where one speciesbecomes extinct as a consequence of an-other becoming extinct. For example, thedodo was the sole distributor of the seedsof a species of the tree Sideroxylon (for-merly Calvaria major), consuming thefruits and passing out the seeds in its drop-pings. Following the demise of the dodo,the tree also declined to extinction. Some-times extinction is due to a situation wherepopulation density is correlated with envi-ronmental factors (correlated extinction).For example, if the numbers of individualsin adjacent populations decline as a resultof environmental factors, such as drought,and if emigration rates are related to popu-lation density, the density may become toolow to trigger emigration, so both popula-tions may become extinct.

At intervals in the past there have beenextinctions of large numbers of species as aresult of natural catastrophes – mass ex-tinctions. Some authorities consider that asimilar mass extinction is happening at thepresent time as a result of human activityand unsustainable consumption of naturalresources. The current rate of extinctions isestimated to be several thousand speciesevery year. Such mass extinctions have oc-curred at least three times in the past 750

million years, one at the end of the Permianperiod, associated with large movements ofthe tectonic plates bearing the continentalland masses, resulted in the loss of some90% of all the species on Earth, while over50% were lost in the Cretaceous mass ex-tinction some 65 million years ago, whenthe dinosaurs were among those life formsthat vanished. The effect on global climateof a large asteroid impacting the Earth isthought to have been the cause.

extinction vortex A cascade of extinc-tions that spreads from an initial speciesthrough the community of which it was apart. This is especially likely to happen if aKEYSTONE SPECIES becomes extinct. See ex-tinction.

extirpation The loss of a species fromcertain areas but not from the whole of itsrange. Compare extinction.

extrapolation The use of data derivedby observation to deduce estimated valuesfor variables or conditions that have notbeen observed. For example, the continua-tion of a line of best fit to the origin of aGRAPH, even though there are no valuesplotted in that part of the graph.

extremophile A microorganism thatlives in extreme environments, such as hot springs or highly saline water. Seehalophile; methanogen; thermophile.

extrinsic factors Factors acting fromoutside the organism – the physical andchemical impacts of the environment andthe effects of other organisms.

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facilitation 1. In a community SUCCES-SION, the facilitating of the establishment oflater species in a succession by the action ofearlier species, which change the local en-vironment, making it more favorable forlater species to colonize. For example, inthe colonization of newly exposed ground,early pioneers often include species such asalder (Alnus), which possess root nodulescontaining symbiotic bacteria that fix ni-trogen. Decomposition of these species re-leases soluble nitrogen compounds into thesoil, increasing its fertility.2. The reinforcement of a particular behav-ior due to the presence of other individualsof the same species. For example, increasednumbers of foraging birds in a flock maylead to increased rates of discovery of newfood sources or, by improving the chancesof detecting predators, may increase thetime available to each individual for feed-ing.

factorial experiment An experiment inwhich the material is divided into a numberof groups, such that every possible combi-nation of treatments can be tested sepa-rately on at least one group.

facultative Describing an organism thatcan utilize certain conditions but is not de-pendent on them; i.e. an organism that canadopt an alternative mode of living. For ex-ample, a facultative anaerobe is an organ-ism that can grow under anaerobicconditions but is also able to survive in aer-obic conditions. A facultative annual maycomplete its life cycle in a single year butmay contain a small proportion of individ-uals that do not breed until their secondsummer, while the rest of the populationbreeds in the first summer. Compare oblig-ate.

facultative mutualism See mutualism.

fairy ring A ring of dark green grasscaused by the presence of the mycelia andfruiting bodies of certain FUNGI. Themycelium grows outward from the center,depleting the soil as it grows. When themycelium dies, it rots and releases nutri-ents into the soil, so the vegetation behindit may show increased growth and a darkergreen color. Rings may continue growingfor hundreds of years, and can reach a di-ameter of over 200 m.

faithful See constant.

fall bloom A BLOOM of algae that occursin the fall (autumn) when a decrease of airtemperature and wind activity stirs up thewater of a lake, bringing up nutrients fromdeeper waters where they have beentrapped all summer by STRATIFICATION.This vertical mixing of layers of water iscalled fall overturn.

fallout Particulate matter that is trans-ported in the atmosphere by turbulence,but eventually reaches the land surface inrain or settles out as dust. The term is usedespecially for radioactive material releasedby nuclear explosions.

family In classification, a TAXON thatcomprises a collection of similar genera.Families may be subdivided into subfami-lies, tribes, and subtribes. Plant familynames generally end in aceae whereas ani-mal family names usually end in idae. Sim-ilar families are grouped into orders. Seetaxonomy.

Farman, Joe (1930– ) British atmos-pheric chemist. Farman had been engaged

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in the study of the Antarctic atmospheresince 1957. In 1982, investigating atmos-pheric ozone, he first noted a very lowreading in the ozone level. Initially he sus-pected that his readings were caused by in-strumental or other errors, but, whensimilar low levels were recorded in the fol-lowing season, Farman investigated fur-ther. An examination of past recordsrevealed that there had been a decline inozone levels since 1977. By the end of the1984 season Farman was convinced thathe had detected experimental evidence fora persistent seasonal fall in Antarctic ozonelevels of about 40%. Farman was aware ofthe work of ROWLAND and began to suspectthat ozone depletion was connected withatmospheric CFCs. He published his re-sults in May 1985.

fast breeder reactor A nuclear reactorthat uses fast neutrons to convert uraniumto plutonium. The process is up to 120times more efficient than energy generationby other types of nuclear reactor, but itproduces excess plutonium, which can berecycled, and RADIOACTIVE WASTE. See nu-clear energy.

fats Triglycerides of long-chain car-boxylic acids (fatty acids) that are solidbelow 20°C. They commonly serve as en-ergy storage material in higher animals andsome plants. See also lipid.

fatty acid See lipid.

fault A fracture in a body of rock alongwhich there has been some displacement ofone part relative to the other. Faults maybring one rock type adjacent to another,causing changes in soil type, fauna, andflora. Where a permeable rock is broughtup against an impermeable one, a springline may result. Movements along faultlines are common causes of EARTHQUAKES.The stresses that cause such movements areoften related to tectonic movements. Seeplate tectonics.

fauna The animal life of a region or ageological period. See biogeographicalrealms.

fecundity The capacity of an organismto produce offspring. Many organismsshow such enormous fecundity that thesize of their population would rapidly in-crease if all of them survived. In practicemost offspring do not survive. See also nat-ural selection; fertility.

fecundity table A table showing thenumber of offspring (for example, thenumber of bird eggs) produced by individ-uals of different ages in a population. Suchtables are used to compare fecundity fromyear to year, or in different seasons, as wellas showing the effects of maturity or agingon fecundity.

feedback mechanism A mechanism inwhich the product of a process affects therate of the process. For example, an in-crease in the number of herbivores in anECOSYSTEM leads to overgrazing and reduc-tion of the herbivore population by starva-tion or increased susceptibility to disease,which allows the plants to grow back, andso on – the population of both species fluc-tuates about a mean. This is an example ofnegative feedback, opposing the change inthe system. Positive feedback has the op-posite effect, reinforcing a change in thesystem.

fell An open mountainside with shortvegetation.

female choice A form of SEXUAL SELEC-TION in which the female chooses her mate.It may lead to exaggerated male character-istics, such as the peacock’s tail, with fe-males choosing the male with the mostimpressive display. It is thought that sincethe growth of such features uses a lot of en-ergy, this is a way of selection for FITNESS.In such species, the males are usually moreshowy than the females.

fen An area of wet PEAT that has resultedfrom the silting up of an open stretch ofwater. Fen is a subclimax community in aHYDROSERE, prevented from developing tothe CLIMAX COMMUNITY by the wet condi-tions, which prevent the growth of trees.See sere; succession.

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feral Describing formerly domesticatedplants or animals that have escaped intothe wild, or their descendants.

fermentation The breakdown of or-ganic substances, particularly carbohy-drates, under anaerobic conditions. It is aform of anaerobic respiration and is seen incertain bacteria and in yeasts. The incom-pletely oxidized products of alcoholic fer-mentation – ethanol and carbon dioxide –are important in the brewing and bakingindustries.

ferns See Filicinophyta.

fertility 1. The degree to which a SOIL

possesses sufficient quantities of the el-ements necessary for vigorous plantgrowth. See macronutrient; micronutrient.2. The number of fertilized eggs (ova) pro-duced by an organism. In plants fertility isalso measured by the number of seeds pro-duced.

fertilization The fusion of a male GA-METE with a female gamete to form a ZY-GOTE, the essential process of SEXUAL

REPRODUCTION. External fertilization oc-curs when gametes are expelled into a wa-tery substrate from the parental bodiesbefore fusion; it is typical of aquatic ani-mals and spore-bearing plants. Internalfertilization takes place within the body ofthe female and complex mechanisms existto place the male gametes into position. In-ternal fertilization is an adaptation to lifein a terrestrial environment, although it isretained in secondarily aquatic organisms,such as pondweeds or sea turtles.

Internal fertilization allows terrestrialorganisms to become independent of exter-nal water for reproduction. It also allows aconsiderable degree of nutrition and pro-tection of the early embryo, which is seenin both mammals and seed plants.

fertilizer A substance applied to land toincrease soil fertility and hence plantgrowth. Fertilizers include ‘organic’ prod-ucts such as MANURE, COMPOST, bone meal,fishmeal, and GUANO, and inorganic chem-

icals such as nitrates, phosphates, andpotash.

fetch The length of water surface overwhich wind blows, causing waves. Thelength of fetch dictates the degree of waveactivity.

field capacity The point at which thesoil contains all the water it can hold bycapillary and chemical attraction. Anymore water added to soil at field capacitywould drain away by gravity. Soil underthese conditions has a high water potential(see osmosis). Water content at field capac-ity is usually expressed as a percentage ofthe weight or volume of oven-dry soil. Thisis affected by the SOIL TEXTURE, CLAY SOIL

having a high field capacity and SAND a lowfield capacity. See capillarity.

field layer The herb and shrub layer of a plant community. Compare canopy;ground layer.

field work (field studies) Work carriedout in a particular habitat, i.e. ‘IN THE

FIELD.’

FIFRA The Federal Insecticide, Fungi-cide and Rodenticide Act, which requirescertain pesticide ingredients to be regis-tered, and regulates the manufacture,usage, and labeling of pesticides in theUnited States.

Filicinophyta (ferns) The largest phy-lum of spore-bearing VASCULAR PLANTS,comprising some 12 000 species of ferns.There are many fossil genera, some datingback to the DEVONIAN period. Ferns havelarge spirally arranged leaflike fronds bear-ing spores on their margins or undersur-faces. The spores give rise to heart-shapedhaploid prothalli, delicate plates of cellsthat bear archegonia and antheridia.Motile male gametes from the antheridiafertilize the ova in the archegonia. The fer-tilized ovum develops into the diploid fernplant while still embedded in the prothal-lus, deriving nutriment from it for a while.Ferns do not show secondary thickening,but in large ferns such as tree ferns bands

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of sclerenchyma (tissue containing fibersreinforced with lignin) and the overlappingbases of the fronds help support the plant.Many ferns spread by means of rhizomes.Most grow on the ground, but manyspecies are epiphytes, especially in the trop-ics. See also alternation of generations.

filter feeding The method of feeding ofsome aquatic animals, especially inverte-brates, in which small suspended food par-ticles are strained from the surroundingwater. Many animals simply allow waterto flow over or through them, but othersactively produce a current, often by meansof cilia. The type of filter used varies fromminute hairlike cilia to the fringes of bris-tles on the legs of shrimps, the sticky netsof radiolarians, the serrated bills of flamin-goes, and the large horny baleen plates ofcertain whales. Small filter feeders are DE-TRITIVORES, whereas the larger ones areCARNIVORES. See ciliates.

filter strip An area of vegetation used toremove organic matter, pollutants, or sedi-ments from runoff and waste water. Seebioremediation.

filtration The removal of suspendedparticles from a fluid by passing or forcingit through a porous material (the filter).The fluid that passes through the filter isthe filtrate. In the treatment of water, sandor filters are used to remove solid particlesthat may contain pathogens.

fire ant Solenopsis invicta, a pest intro-duced to the southern United States. Theyare small red or yellowish ants that have anextremely painful sting. They cause dam-age to grain crops and attack poultry.

fish See Pisces.

fission 1. See asexual reproduction.2. See nuclear energy.

fitness In an evolutionary context, theability of an organism to produce a largenumber of offspring that survive to a re-productive age. This is Darwinian fitness, ameasure of evolutionary success – the abil-

ity to pass on genes to the next generation.POPULATION GENETICS is concerned with rel-ative fitness – the relative contribution ofthe individual organism to the GENE POOL

of the next generation. This may be ex-pressed as a fitness set – a graph plottingthe different phenotypes in a populationand their values for various components offitness, such as reproductive rate or sur-vivorship. The sum of an organism’s Dar-winian fitness and that of its relatives iscalled inclusive fitness. Fitness is thus ameasure of the degree to which an organ-ism has successfully adapted to its environ-ment. The phrase ‘survival of the fittest’summarizes the principles of the theory ofnatural selection. See Darwinism; naturalselection; population genetics.

Five Kingdoms classification A classi-fication system that recognizes five king-doms containing at least 96 phyla. Itcomprises two superkingdoms, the Pro-karya (PROKARYOTES) and Eukarya (EU-KARYOTES). The Prokarya contains a singlekingdom, the BACTERIA, which is dividedinto two subkingdoms, the ARCHAEA andthe EUBACTERIA. The Eukarya contains fourkingdoms, the ANIMALIA, PLANTAE, FUNGI,and PROTOCTISTA.

The Protoctista is not a natural phylo-genetic grouping, but contains those or-ganisms that do not fit into the other fourkingdoms, including numerous unicellulargroups (e.g. algae, oomycetes, and slimemolds). The recognition of a single super-kingdom, the Bacteria, is disputed, andmost current classification systems recog-nize three DOMAINS (taxa above the rank ofkingdom): the Bacteria, Archaea, and Eu-karya.

fixation The incorporation of inorganicmaterials such as carbon and nitrogen intoliving tissues by living organisms. An ex-ample is the incorporation of carbon fromcarbon dioxide into new organic com-pounds during PHOTOSYNTHESIS. See alsonitrogen fixation. Compare assimilation.

fixed dune See dune.

fixed quota A harvesting strategy in

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which a fixed number of individuals are re-moved from a population during a givenperiod of time. The balance between thefixed quota and the RECRUITMENT rate de-termines whether such harvesting is sus-tainable.

flagellate An organism that uses UN-DULIPODIA for locomotion. Several phyla ofthe PROTOCTISTA contain flagellates, espe-cially the Zoomastigota.

flammable Describing any material thatignites easily and burns rapidly.

flash point The lowest temperature atwhich a flammable liquid produces suffi-cient vapor to form an ignitable mixturewith air.

flavonoids A common group of plantcompounds and an important source ofnonphotosynthetic pigments in plants,which include the yellow and ivoryflavones and flavonols and the red, blue,and purple anthocyanins. They are widelyused in CHEMOTAXONOMY to distinguishbetween certain plant groups. Someflavonoids have a defensive role as PHY-TOALEXINS.

floc See coagulation.

flocculation 1. The aggregation of soilparticles into crumbs. Compacted struc-tureless clay soils can be flocculated by theaddition of neutral salts, particularly ofcalcium. The addition of lime to saturatedclay soils (liming) is a common agriculturalpractice, improving soil structure by en-couraging crumb formation and makingheavy soils more workable. See also soiltexture.2. See coagulation.

flocking In certain species of birds, so-cial attractions between individuals thatkeep groups together and may lead to arecognizable social structure. For example,many species of fruit- and seed-eaters formflocks outside the breeding season to for-age for food. Foraging flocks may containseveral different species, e.g. the mixed

winter flocks of tits (family Paridae) of thenorthern hemisphere, and the antbirds(family Formicariidae) and other speciesthat follow columns of army ants in SouthAmerican rainforests. Flocking at somestage in the life cycle is found in almost halfof all bird species.

flood-plain The flat or nearly flat partof a river valley that is covered by waterduring periodic floods. It consists mainlyof coarse unconsolidated sediment left be-hind as the river changes course and mi-grates across the flood-plain, and finersediment deposited when the river over-flows its banks.

flora All the plant species in a givenarea, e.g. the Cape flora. The term is alsoused with reference to the plant species in aparticular period of geological time, e.g.the Cretaceous flora.

floral province One of a number of re-gions into which the world is divided onthe basis of plant distribution. Eachprovince has one or more CENTERS OF DI-VERSITY and each contains unique familiesand genera of plants. The main provincesare the Boreal, which covers all of thenorth temperate zone; the Neotropical,covering tropical Central and South Amer-ica; the Paleotropical, covering tropicalAfrica and Asia; the Australian; the CapeProvince, covering a small area around theCape of Good Hope; and the Antarctic, in-cluding New Zealand and temperate SouthAmerica. Oceanic islands, particularly inthe Pacific, form minor provinces. Thereare floral affinities and connections be-tween the provinces, particularly near theirboundaries.

floristic Relating to all the plant speciespresent in a particular area, region, or com-munity.

floristic analysis The identification andlisting of all the plant species present in aparticular area. The resulting check listgives the floristic composition of the regionor community. See also phytosociology.

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flotation The separation of solids andliquids according to their specific gravities.Flotation is used to separate metal wastesin suspension from the liquid phase ofSEWAGE and industrial effluents. Metal hy-droxides and other suspended particles aresubjected to streams of gas bubbles, whichadhere to the particles, causing them tofloat to the surface, where they can beskimmed off.

flowering plants See Angiospermo-phyta.

flowmeter An instrument for measuringthe velocity of a moving fluid (liquid orgas).

flow web See food web.

flue gas The mixture of gases leaving achimney venting a combustion chamber. Itis a mixture of water vapor, oxides of car-bon, nitrogen and sulfur, and other POLLU-TANTS. See also fly ash.

fluidized Describing a mass of solid par-ticles that has been injected with water orgas so that it flows like a liquid.

fluidized bed incinerator An incinera-tor in which a hot granular material suchas sand is FLUIDIZED by the injection ofwaste, to which it transfers its heat. Suchincinerators are used to destroy municipalSEWAGE SLUDGE. See incineration.

flume 1. A channel that diverts water.2. An experimental channel used to studythe relationships between flow conditionsand the movements of sediments, for ex-ample to determine the erosive force of thewater.

fluoridation The addition of smallquantities of FLUORIDE compounds todrinking water in order to reduce the inci-dence of tooth decay in children. See alsofluoride.

fluoride A compound containing fluo-rine. Inorganic fluorides contain the fluo-ride ion, F–. Excessive amounts of fluorides

in food or drink can lead to fluorosis (mot-tling and softening of teeth). Some tooth-pastes contain fluoride to help reducetooth decay. See also fluoridation.

fluorosis See fluoride.

fluvial Relating to rivers or streams andtheir actions.

flux The rate of flow of mass, volume, orenergy expressed per unit cross-sectionperpendicular to the direction of flow.

fly ash The noncombustible particlescarried by flue gases, which settle out as afine ash.

fog A suspension of fine droplets of liq-uid in a gas, e.g. water in air, which reducesvisibility to below 1 km. Fog may form asa result of the cooling of a body of air dueto contact with cold ground or cold water,or where a body of warm air meets a coldfront. Fog formation is accelerated by thepresence of condensation nuclei such assmoke particles, which may lead to fog for-mation at humidities lower than the con-densation point.

fogging The application of a pesticide inthe form of a FOG generated by rapidlyheating the liquid chemical. Fogging iscommonly used to destroy such insects asmosquitoes and blackfly.

folivore An animal that feeds mainly onfoliage, for example, a tapir (Tapirus).

Food and Drug Administration (FDA)The branch of the US federal governmentresponsible for enforcing the Food, Drug,and Cosmetic Act and related consumersafety issues.

food begging Behavior that solicitsfood from a parent or other adult, or froma mate. For example, the bright orange oryellow lining of the beaks of some juvenilebirds, especially passerines, is exposedwhen the birds open their beaks wide tobeg, and the sight of this induces the adultto feed them.

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food begging

food chain The chain of organisms ex-isting in any natural COMMUNITY, throughwhich food energy is transferred. Each linkin the food chain obtains nutrients and en-ergy by eating the one preceding it and is inturn eaten by the organisms in the link fol-lowing it. At each transfer a large propor-tion (80–90%) of the potential energy islost as heat, and a smaller proportion asmechanical energy (e.g. for locomotion)and, at higher levels of the food chain, aselectrical energy for nerve transmission, forexample. For these reasons the number oflinks in a sequence is limited, usually tofour or five. The shorter the food chain, thegreater the available energy, so total energycan be increased by cutting out a step in thefood chain, for example if people were toconsume cereal grains instead of consum-ing animals that eat cereal grains.

Food chains are of two basic types: thegrazing or herbivore-based food chain,which goes from green plants to grazingherbivores and finally to carnivores; andthe detritus-based food chain, which goesfrom dead organic matter to microorgan-isms and then to detritus-feeding organ-isms. The food chains in a community areinterconnected, because most organismsconsume more than one type of food, andthe interlocking pattern is referred to as aFOOD WEB.

In most food chains and food webs, thegreatest diversity of PRODUCERS occurs

when population regulation by CONSUMERS

is at its most intense. This is termed atrophic cascade. For example, it occurs in alake in summer, when the pressures of highnumbers of predatory zooplankton lead tohigh diversity in the phytoplankton (seeplankton). Trophic cascades occur typi-cally where the producers are relativelyshort-lived and fast-growing, so they canrespond to the impact of the consumers.Such food webs show top-down control,where productivity is limited by the rate atwhich nutrients can be recycled, and henceby the number and activity of consumers.See also bottom-up control; ecological effi-ciency; trophic level; biomanipulation.

food web The feeding relationships bywhich energy and nutrients are transferredfrom one species to another (see foodchain). Food webs whose links emphasizefeeding relationships between organismsare sometimes called topological foodwebs or static food webs. They show whoeats what, but give no information aboutthe strength of the interaction – the amountof nutrients or energy taken in at eachTROPHIC LEVEL. Food webs that show theenergy flux between trophic levels arecalled energy flow webs, flow webs, orbioenergetic webs. A final category of foodweb, the functional food web or interac-tion food web, shows the importance ofeach population in the community in influ-

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top consumers(carnivores)

secondaryconsumers(carnivores)

primary consumers(herbivores)

producers(plants)

DEAD ORGANISMS

decomposers

Food chain: a simplified food web

encing the growth rates of the other popu-lations. Experimentation is needed to de-termine the structure of functional foodwebs.

Species in a food web that have a simi-lar trophic habit, i.e. have similar preda-tors and prey or other food, are termedtrophospecies. They form nodes in thefood web from which links (which repre-sent interactions) to other species radiate.The number of links between the basalspecies (the producers or, in the case of de-tritus-based food webs, the primary con-sumers) and the TOP PREDATOR is called thechain length. The average number of linksper species is called the linkage density ofthe food web. The total number of links inthe food web divided by the number ofpossible links is its connectance or con-nectedness.

foot-and-mouth disease A disease oflivestock caused by a wind-borne virus,causing fever and characteristic eruptionson the mucous membranes and skin, espe-cially in the mouth and in the clefts of thehoofs. In countries where it is endemic, it iscontrolled by vaccination, but in countriessubject to sporadic outbreaks it is con-trolled by slaughter of affected animals andadjacent herds.

foraging Behavior associated withsearching for, collecting or capturing, andconsuming food.

foraging strategy FORAGING behaviorthat has evolved to enable an organism toachieve a high rate of net energy intake,taking into account the energy costs of ob-taining that energy/food. The optimal for-aging theory proposes that foragingstrategy involves decisions that maximizethe net energy/food intake or some othernutritional goal such as the acquisition ofessential elements, and uses mathematicalmodels to attempt to predict foraging be-havior. It aims to explain why organismsdo not always make use of the full range oftheir diet, preferring to select certain itemsabove others. Such choices are likely to berelated to competition for resources in

their present environment, but may also bea result of NATURAL SELECTION in the past.

Foraminifera (forams) A class of PRO-TOCTISTA that contains small marine organ-isms from 10 mm to several centimeters indiameter enclosed in pore-studded shells.Free-swimming planktonic forms are im-portant components of marine FOOD

CHAINS. Many other foraminifera live onthe seabed in mud or sand or attached torocks, algae, or other organisms. Cytoplas-mic projections protrude through the poresof the shell, forming nets to trap food.Forams feed on small particles of dead orliving organic matter. Their shells sink tothe seabed at death, forming an importantcomponent of SEDIMENTARY ROCKS such asLIMESTONES. The delicately sculpted shellsare preserved as FOSSILS (microfossils), andare used to distinguish different rock strataof the same age, and are especially impor-tant in exploration for petroleum deposits.

forbs Herbaceous plants that are notgrasses (Poaceae), sedges (Cyperaceae), orrushes (Juncaceae).

forcing A horticultural practice thatcauses a plant to flower or fruit earlier thanit would under normal environmental con-ditions, for example, by growing it in agreenhouse or by subjecting it to an artifi-cial daylength.

foredune See dune.

foreshore The part of the shore that liesbetween the normal high- and low-watermarks.

forest 1. A plant COMMUNITY dominatedby trees whose crowns touch, forming acontinuous CANOPY. Compare woodland.2. A major plant community (biome) inwhich the dominant plants are trees. Seebiome; boreal forest; temperate deciduousforest; tropical forest.

forestry 1. The study of trees and timberproduction systems.2. The growing and managing of trees forcommercial timber production. This in-

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cludes the commercial management of ex-isting forests as well as the planting of na-tive or exotic species, and the selectivebreeding or other genetic modification oftrees for specific uses.

form The lowest taxonomic group,ranking below the variety, normally refer-ring to the recognizably different morphol-ogy found in several members within aplant species. See taxonomy.

formalin A mixture of about 40%formaldehyde, 8% methyl alcohol, and52% water (the methyl alcohol is presentto prevent polymerization of the formalde-hyde). Formalin is a powerful reducingagent and is used as a disinfectant, germi-cide, and fungicide and also as a generalpreserving solution, especially for biologi-cal specimens.

formation A unit of classification ofvegetation that is based on the form andstructure of the plant COMMUNITY ratherthan its species composition. Differentclassification schemes define formations indifferent ways. The term formation type isgenerally used to refer to a world vegeta-tion type (BIOME) of relatively uniform ap-pearance and life forms, e.g. tropicalrainforest or savanna. A formation type isequivalent to a CLIMATIC CLIMAX vegetationtype. See life form.

fossil The remains of, or impressions leftin rocks by, long dead animals and plants.The fossil assemblage in a particular stra-tum gives an indication of the environmen-tal conditions that prevailed at that time.

fossil fuel Fuel derived from ancient or-ganic remains, e.g. COAL, crude oil (see pe-troleum), NATURAL GAS, and PEAT. Thesefuels have been formed by the decomposi-tion or partial alteration of organic re-mains under pressure, usually in anaerobicconditions. Fossil fuels are a nonrenewableresource and burning such fuels releasescarbon dioxide that was fixed from the at-mosphere millions of years ago, contribut-ing to the GREENHOUSE EFFECT and GLOBAL

WARMING. They are, however, the mainsource of energy used in modern society.

fossil water Water contained in an un-derground AQUIFER that was stored there inthe distant past and is no longer being re-plenished from present-day PRECIPITATION.This water is a nonrenewable RESOURCE,and overexploitation will lead to the deple-tion of the aquifer and, in an unconfinedaquifer, to the retreat of the WATER TABLE

to greater depths, which may have a seri-ous effect on local agriculture and humansettlements.

fragmentation 1. See asexual reproduc-tion.2. The separation through human activityof once continuous habitats into fragmentsthat may or may not contain viable popu-lations of the organisms present. See habi-tat islands; island biogeography theory.

Framework Convention on ClimateChange (Global Warming Convention)An international treaty agreed at the EarthSummit in Rio de Janeiro, Brazil, in 1992,that requires signatory nations to reducetheir emissions of GREENHOUSE GASES, espe-cially carbon dioxide and methane, be-lieved to be responsible for GLOBAL

WARMING.

free radical See radical.

frequency The number of individuals ofa particular species in a given area. Seequadrat.

frequency distribution The pattern ofdistribution of the values of a VARIABLE, i.e.the number of individuals or events witheach value. This is usually plotted as aGRAPH or histogram, with number on oneaxis and value on the other.

frequency table A table showing thenumber of times a value appears in a dataset. The table may simply show the valuesand the number of occurrences, or it mayshow values and frequency and/or RELA-TIVE FREQUENCY. See frequency; tally chart;variable.

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fringing forest See riverine forest.

fringing reef A coral reef that formsalong a coast rather than at some distancefrom the shore (see corals).

frogs See Amphibia.

front A boundary between two distinctmasses of air. Often the air masses differ intemperature and/or moisture content, andare moving in different directions. Frontshave important effects on weather. Coldair is denser than warm air, so cold frontstend to push under warm fronts where thetwo meet, creating instability in the airmasses, often giving rise to thunderstormsand causing a change in wind direction.The warm air is cooled, resulting in cloudformation and PRECIPITATION.

frost The formation of ice crystals onsolid objects, such as grass or windowpanes, when the air temperature fallsbelow the freezing point of water (0°C)and its dew-point temperature is below0°C. Water vapor condenses out of the at-mosphere as ice. Hoar frost, by contrast,forms where wet surfaces, e.g. grass onwhich DEW has formed, is cooled by heatloss due to RADIATION, so the dew freezes.See condensation.

frost-free period For any particularspecies that is susceptible to certain lowtemperatures, the period between the lastFROST of summer and the first frost of au-tumn. Frost in this context is usually takento mean a temperature below –2°C, but theactual degree of frost required to preventgrowth varies with the species. The frost-free period represents the maximum lengthof the growing season. The term is alsoused in relation to the survival of insectpests, for many of which frost is lethal tothe active stage. In this context, the rele-vant degree of frost is that needed to killthe insect.

frost hardiness See cold tolerance.

frugivore An animal that feeds mainlyon fruit.

fruit See Angiospermophyta.

fruticose See lichens.

fucoids Brown ALGAE (phylum Phaeo-phyta) of the order Fucales, which includemany common medium to large brownseaweeds, such as the wracks (Fucus spp.).These are flattened, often branched, sea-weeds attached to rocks by means of hold-fasts at their base.

fuel cell A device in which an electro-chemical reaction occurs between a fuel(e.g. hydrogen) and oxygen to produceelectrical energy directly.

fugitive emission A gas, vapor, mist,fog, fume, liquid, or solid that escapes froma process, product, or piece of equipmentand is not trapped by a system designed tocapture potential POLLUTANTS.

fugitive species An opportunist SPECIES

that is capable of rapid colonization of anewly exposed area, but unable to survivecompetition from later arrivals. Suchspecies are typical of unstable environ-ments such as deserts and temporaryponds. They usually have short life cyclesand good long-distance dispersal mecha-nisms. See also ephemeral.

fumigant See fumigation.

fumigation The killing of pests usingfumes of volatile pesticides (fumigants),e.g. carbon disulphide and formaldehyde.This is an especially useful method ingreenhouses, buildings, soil, or compost.

functional response A change in therate of predation by an individual predatorin response to a change in the density of itsprey. Compare numerical response.

fundamental net reproductive rate Seereproductive rate.

fundamental niche The ecologicalNICHE that an organism could potentiallyoccupy if it were not limited by environ-

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fundamental niche

mental constraints or competition fromother species. See realized niche.

Fungi A kingdom of nonphotosyntheticmainly terrestrial organisms that lackCHLOROPHYLL and are quite distinct fromplants and animals. They are characterizedby having cell walls made chiefly of CHITIN,not the CELLULOSE of plant cell walls, andthey all develop directly from SPORES with-out an embryo stage. UNDULIPODIA arenever found in any stage of their life cycles.Fungi are generally saprophytic (seesaprobe) or parasitic (see parasitism). Theymay be unicellular or composed of fila-ments (termed hyphae) that together com-prise the fungal body or mycelium. Hyphaemay grow loosely or form a compactedmass, giving well-defined structures, as inmushrooms. The phylum includes mush-rooms, toadstools, yeasts, molds, andmildews.

Fungi are major DECOMPOSERS in soiland fresh water. Some form symbiotic as-sociations with algae or CYANOBACTERIA toform LICHENS, while others form extremelyimportant MYCORRHIZAE with the roots of

many plants, including most forest trees.Some parasitic fungi cause diseases inplants and animals, whereas others aresources of ANTIBIOTICS. Yeasts and otherfungi are used in commercial FERMENTA-TION processes such as brewing. Somefungi are pathogens; others stabilizesewage and digest composted waste. Seesymbiosis.

fungicide A chemical that inhibits fun-gal growth. Fungicides are used to preventfungal infection as well as to deal with ex-isting infections (chemicals called fungis-tats prevent fungi from growing). Thereare two main categories: inorganic chemi-cals such as copper-based fungicides andsulfur dust (which are surface treatments),and organic chemicals such as dithiocarba-mate, many of which are SYSTEMIC.

fungistat See fungicide.

fusion See nuclear energy.

fynbos See maquis.

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Gaia hypothesis The theory that theEarth functions as a single homeostatic sys-tem analogous to a single living organism.The activities of organisms in response tochanges in their physical and chemical en-vironment in turn modify the physical andgeochemical cycles, including climate, sothat the planet continues to support life.The Gaia hypothesis was proposed by theBritish scientist James LOVELOCK.

gallery forest See riverine forest.

gamete A cell capable of fusing with an-other cell to produce a ZYGOTE, from whicha new individual organism can develop.Gametes are usually HAPLOID, so fusion ofgametes results in the nucleus of the zygotehaving the DIPLOID number of chromo-somes. The typical male gamete is small,motile (by means of undulipodia), and pro-duced in large numbers. The typical femalegamete is large because of the food reservesit contains, and is produced in smallernumbers than the male gametes. In all ex-cept the most primitive organisms, the fe-male gamete is nonmotile. See also sexualreproduction.

game theory A method of analyzingmathematically social interactions and de-cision-making between individuals interms of gains and losses among opposingplayers. It involves calculating the proba-bilities and values of the outcomes of vari-ous choices made by the decision-makers.

gametophyte See alternation of genera-tions.

gamma diversity See species diversity.

gamma radiation A form of high-en-

ergy electromagnetic radiation emitted bychanges in the nuclei of atoms. Gammarays have very high frequency, about 10-10

to 10-14 m in wavelength, shorter than x-rays. Gamma rays penetrate tissues to agreater degree than other forms of radia-tion. See radioactive decay.

gap analysis A method of analyzingspecies distribution in order to detectwhich ecosystems are most in need of pro-tection. The ranges of rare and endangeredspecies are mapped; the maps are laid ontop of each other and overlaid by a map ofexisting reserves and protected areas. Thisreveals gaps that represent areas in need ofgreater protection.

garrigue A form of Mediterraneanscrub woodland that has developed as a re-sult of burning and grazing of the originalbroad-leaved evergreen forest of formertimes. It is found in limestone areas on thindry soils. It is dominated by aromatic herbsand prickly dwarf shrubs, including speciessuch as lavender (Lavandula vera), sage(Salvia officinalis), thyme (Thymus), andmints (Mentha). It is an impoverished floracompared with the MAQUIS that grows onbetter soils, which contains taller shrubsand scattered small trees such as olives(Olea europaea) and figs (Ficus).

gasification The generation of fuel gasfrom solid material. Examples are produc-ing gas from coal and the production of gascontaining carbon dioxide, hydrogen, andmethane from SEWAGE SLUDGE.

gasohol Ethyl alcohol (ethanol) used asa substitute for gasoline or as an additive togasoline. It can be produced by fermenta-

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G

tion of grain, potatoes, sugar-cane residue,etc.

gasoline See petroleum.

gauging station See water-flow meas-urements.

Geiger counter An instrument for de-tecting the presence and quantity of IONIZ-ING RADIATION. It is less sensitive than ascintillation counter.

gene A unit of hereditary material lo-cated on a chromosome that, by itself orwith other genes, determines a characteris-tic in an organism. It corresponds to a seg-ment of the genetic material, usually DNA

(the genes of some viruses consist of RNA).Genes may exist in a number of forms,termed ALLELES. For example, a gene con-trolling the characteristic height in peasmay have two alleles, one for ‘tall’ and an-other for ‘short’. In a normal diploid cellonly two alleles can be present together,one on each of a pair of HOMOLOGOUS

CHROMOSOMES: the alleles may both be ofthe same type, or they may be different.Genes can occasionally undergo changes,called MUTATIONS, to new allelic forms.

A gene may control the synthesis of asingle POLYPEPTIDE or a RNA moleculewhose GENETIC CODE is contained withinthe gene. There are three main types ofgenes: structural genes code for thepolypeptide that make up enzymes andother proteins, RNA genes code for riboso-mal RNA and transfer RNA used in the as-sembly of polypeptides, and regulatorgenes regulate the expression of othergenes. See mutation.

gene bank 1. A place where plant ma-terial is stored in a viable condition inorder to preserve plants that are in dangerof becoming extinct in the wild or cultivarsthat are being lost from cultivation. Thematerial also provides a source of genes forbreeding new varieties. Seeds are usuallystored at –20°C after being dehydrated toabout 4% of their normal water content.They may remain viable for up to 20 years,often much more, depending on the

species, after which they must be germi-nated and a new crop of seeds harvested.For species that do not store well, materialmay be kept in tissue culture. Pollen mayalso be stored, but has a shorter lifespan.2. See gene library.

gene center See center of diversity.

genecology The study of the genetics ofpopulations in relation to their environ-ment.

gene flow The movement of ALLELES

within and between populations of thesame species through interbreeding. Seealso GENETIC DRIFT.

gene-for-gene effects The evolution ofvirulence and RESISTANCE of pests andcrops, respectively, on a gene-by-genebasis. For example, the pest may have sev-eral genes involved in virulence for a par-ticular crop. The crop in turn may haveseveral different GENES involved in resis-tance, certain alleles of each conferring re-sistance against particular pest virulencealleles.

gene frequency The proportion of anALLELE in a population in relation to otheralleles of the same GENE.

gene library A collection of cloned DNA

fragments derived from the entire GENOME

of an organism. Such fragments are usuallystored in plasmids inside a suitable host,such as the bacterium Escherichia coli. Seealso genomic library.

gene pool The total number and varietyof GENES existing within a breeding popu-lation or species at a given point in time.

generalist A species with wide food orHABITAT preferences. Compare specialist.

generation (cohort generation time) Theaverage interval between the birth of par-ents and the birth of their offspring.

generation length (T) The average ageof parents of the current COHORT (i.e. new-

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borns). Generation length is related to theTURNOVER rate of breeding individuals in apopulation. Since offspring may give birthto their own offspring during the life oftheir own parents, generation length so de-fined is less than the cohort generationtime.

generation time 1. The mean time be-tween new generations of individuals.2. In unicellular organisms, the interval be-tween successive cell divisions.

gene sequencing Determination of theorder of BASES (def. 2) making up a GENE ona DNA molecule. Sequencing requires multi-ple cloned copies of the gene (see gene li-brary). Long DNA sequences are cut intomore manageable lengths using restrictionenzymes. These cleave DNA at specificbase sequences, and it is possible to recon-stitute the overall sequence once the con-stituent fragments have been analyzedindividually.

genet The organism developed from aZYGOTE, or a CLONE of it. Compare ramet.See also modular organism.

genetically modified crops (GM crops)Crops that have been modified by GENETIC

ENGINEERING.

genetic code The sequence of bases ineither DNA or RNA (especially messengerRNA) that conveys genetic instructions tothe cell. The basic unit of the code consistsof a group of three consecutive bases, thebase triplet or codon, which specifies in-structions for a particular AMINO ACID in aPOLYPEPTIDE, or acts as a start or stop signalfor translation of the message into apolypeptide assembly. For example, theDNA triplet CAA (which is transcribed asGUU in mRNA) codes for the amino acidvaline. There are 64 different triplet com-binations but only 20 amino acids; thusmany amino acids can be coded for by twoor more triplets. Certain triplets code notfor an amino acid, but for the beginningand end of a polypeptide chain.

genetic drift The random fluctuation of

ALLELE frequencies in a small populationdue entirely to chance. If the number ofmatings is small, then the gene frequenciesof the offspring may not be a perfect repre-sentation of the gene frequencies of theparental generation. See also bottleneck;fixation.

genetic engineering The direct intro-duction of foreign GENES (from other indi-viduals or artificially synthesized) into anorganism’s genetic material by microma-nipulation at the cell level. Genetic engi-neering techniques by-pass crossbreedingbarriers between species to enable genetransfer between widely differing organ-isms. The commonest method of geneticengineering is RECOMBINANT DNA TECHNOL-OGY.

Genes may be taken from living tissuesby isolating the messenger RNA producedwhen the gene is active, then recreating theDNA sequence of the gene using the enzymereverse transcriptase. Gene transfer isachieved by various methods, many ofwhich use a replicating infective agent,such as a VIRUS, PLASMID, phage (see bacte-riophage), cosmid, or YAC (yeast artificialchromosome). Other methods include mi-croinjection of DNA into cell nuclei and di-rect uptake of DNA through the cellmembrane.

Modified microorganisms are grown inlarge culture vessels and the gene productharvested from the culture medium. TRANS-GENIC mammals, including mice, sheep,and pigs, have been produced by microin-jection of genes into the early embryo, andit is also now possible to clone certainmammals from adult body cells (seeCLONING). Dicotyledonous plants, includ-ing tobacco and potato, have been trans-fected using the natural plasmid vector ofthe soil bacterium Agrobacterium tumefa-ciens. Genes have been introduced to cropplants for various reasons, for instance toreduce damage during harvest or to makethem resistant to the herbicides used incontrolling weeds. Genetically modifiedtomatoes and soya beans are now widelyavailable. Human proteins have been suc-cessfully transferred to bacteria, and awide range of therapeutic substances is

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produced commercially from geneticallyengineered bacteria. Pharming (the pro-duction of pharmaceuticals by geneticallyengineered animals) is increasing, withpharmaceuticals and products such as clot-ting factors being produced in the milk ofsheep or the eggs of chickens. There is hopethat in future many genetic diseases will betreatable by manipulating the faulty genesresponsible. However, genetic engineeringraises many legal and ethical issues, and theintroduction of genetically modified organ-isms into the environment requires strictcontrols and monitoring.

genetic erosion The loss of geneticVARIATION and shrinking of the GENE POOL

of wild plants or old CULTIVARS when new,improved cultivars are grown over wideareas. This can pose problems for the fu-ture, for instance if specific diseases arisethat target the new cultivars, or if climatechange requires drawing on a wider genepool to develop better-adapted cultivars. Itis particularly serious in CENTERS OF DIVER-SITY. See genetic resources.

genetic fingerprinting See DNA.

genetic resources The GENE POOL innatural and cultivated stocks of organisms.The term is used especially in relation todomesticated plants and animals and theirwild relatives. A large gene pool increasesthe opportunity for developing new vari-eties and the capacity to respond to envi-ronmental change and develop resistanceto pathogens. Attempts to preserve geneticresources include the establishment ofGENE BANKS and GENE LIBRARIES. Comparegenetic erosion.

genetics The study of inheritance andvariation and the factors controlling them.

genetic structure The nature of the ge-netic VARIATION among the individuals of aPOPULATION or COMMUNITY.

genome The set or sets of chromosomescarried by each cell of an organism. Hap-loid organisms have one set of chromo-somes, diploid organisms have two sets,

polyploid organisms have many sets some-times from the same ancestor (autopoly-ploids) and sometimes from differentancestors (allopolyploids).

genomic library A GENE LIBRARY con-taining entire genomes of particular or-ganisms. Such libraries are importantresources for comparing the nucleic acidsequences of different organisms, recogniz-ing the genes in newly sequenced organ-isms, and in studies of gene and proteinfunction.

genotype The genetic makeup of an or-ganism. The actual appearance of an indi-vidual (the PHENOTYPE) depends on thedominance relationships between alleles inthe genotype and the interaction betweengenotype and environment.

genus A taxonomic category involving acollection of similar SPECIES. Genera may besubdivided into subgenera and, especiallyin plant taxonomy, into sections, subsec-tions, series, and subseries. The scientificname of a species always includes the genusname (or its abbreviation) as the first wordof the binomial. Similar genera aregrouped into families. See also classifica-tion.

Geographical Information System(GIS) A system of computer hardwareand software that allows various kinds ofdata to be mapped to a standard surveymap showing geographic position, oftenoverlaying several layers of informationinto one image. For example, data may in-clude vegetation type, topography, geol-ogy, soil type, water quality, pollutionlevels, population density, land use, or landsubject to planning controls. Individual ob-servations can be spatially referenced in re-lation to each other. GIS is especially usefulto planners and conservationists and isused in the modeling of processes and forenvironmental risk assessment.

geographical isolation The separationof two populations of the same or closelyrelated species by a physical barrier such asa large river or mountain range. Random

genetic erosion

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GENETIC DRIFT and different SELECTION

PRESSURES in the two populations may leadto the gene pool diverging, perhaps to mor-phological differences and sexual incom-patibility. Compare ecological isolation;reproductive isolation.

geological time scale A system ofmeasuring the history of the Earth bystudying the rocks of the Earth’s crust.Since new rocks are generally deposited ontop of existing material, those lower downare oldest, although this is often disrupted,for example by volcanic activity, tectonicmovements (see plate tectonics), and ERO-SION. The strata of rock are classified ac-cording to their age, and a time scalecorresponding to this can be constructed.The main divisions (eras) are the Paleozoic,Mesozoic, and Cenozoic. These are furthersubdivided into periods and epochs.

geomagnetic poles See geomagnetism.

geomagnetism The force of the Earth’smagnetic field, which varies from the polesto the equator, and also through time. Thegeomagnetic poles do not coincide with thegeographic poles, but are currently locatedat 73°N 100°W and 68°S 143°E. The posi-tions of the geomagnetic poles fluctuateover both short periods of time and on ageological time scale (millions of years).The direction of the Earth’s magnetic fieldis reflected in the direction of the magnet-ism of minerals that crystallize frommolten lavas. Reversals of the field occurfrom time to time, and are used in deter-mining the age and sequence of magneticrocks, for example on either side of themid-oceanic ridges.

geometric growth Population growthin which breeding is confined to a particu-lar season, and the rate of increase in num-bers is proportional to the number ofindividuals in the population at the start ofthe breeding season. See also geometric se-ries.

geometric mean See mean.

geometric series A series of numbers in

which each number is obtained by multi-plying the previous one by a constant fac-tor. The numbers of individuals in apopulation of a pioneer or ephemeralspecies tend to represent a geometric seriesover time, as the more successful species re-strict other species to the remaining NICHES.

geomorphology The study of the land-forms of the Earth’s surface and theprocesses that have shaped them.

geophyte See Raunkiaer’s life-formclassification.

geosphere The inorganic part of theEarth, which does not contain any livingorganisms. It comprises the ATMOSPHERE,HYDROSPHERE, and LITHOSPHERE.

geothermal power Power generated byusing the heat of rocks in the Earth’s crustto heat water to drive steam turbines.

germination The first outward sign ofgrowth of a reproductive body, such as aspore or pollen grain. The term is mostcommonly applied to seeds, in which ger-mination involves the mobilization of foodreserves, followed by the emergence of thefirst root and shoot through the seed coat.

GIS See Geographical Information Sys-tem.

giving-up time The time after which aforaging animal leaves a patch of food toseek another one. According to OPTIMAL

FORAGING THEORY an animal attempts tomaximize its energy intake over the wholeperiod of foraging, rather than just whileactually feeding. The giving-up time de-pends on the quality of the food beingeaten and the distance between patches offood. As a patch becomes depleted, moreenergy is expended in feeding, which be-comes less cost-effective. Optimal foragingtheory suggests that a forager will leave allpatches at the same extraction rate (i.e.when the cost of extracting the energy fromthe food increases above a critical level), re-gardless of how rich they are.

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giving-up time

glacial drift Material deposited on theland surface by glaciers and ice sheets.

glacial period A period during whichice covers large areas of the Earth’s surfaceand glaciers advance or remain stationary.The term may be applied to the cold partsof an ice age, or to the entire ice age. Com-pare interglacial period.

glacial retreat The melting of glaciers,when they retreat higher up the valleys.

glaciation The formation of GLACIERS

and creeping ice sheets and their effects onthe land. The erosive power of glaciers, re-sulting mainly from the rocks and gravelthat become embedded in its base andsides, scours away the surrounding rocksas they move, creating U-shaped valleysand hanging valleys (tributary valleyswhose floors are well above the floor of themain valley, which has been scoureddeeper by the larger glacier it contains).The creep of large ice sheets, such as thoseof Antarctica, levels the underlying surface,forming (after the ice sheets retreat) a land-scape of rounded hills and plains, all at thesame level (a peneplain).

glacier A large mass of ice that movesslowly across the land, usually movingdown a valley or spreading outward overthe land from its source. It is formed bysnow in the surrounding area becomingcompacted, eventually forming ice. As theweight of ice accumulates, the pressure ofthe overlying ice causes the ice at the bot-tom to melt, lubricating the creep of theglacier downhill. This creep may be perma-nent or temporary.

glade An open space in a forest.

gley A waterlogged SOIL lacking in oxy-gen, in which raw HUMUS accumulates as aresult of lack of decomposition by bacteria.The defining feature is the gley horizonbelow the humus, blue-gray clay whosecolor is due to ferrous iron compounds thathave been reduced by microorganisms,flecked with localized areas of rust-coloredoxidized ferric compounds. The formation

of a gley is known as gleying. Gley soils aretypical of TUNDRA, meadows, and boggyareas.

global positioning system (GPS) Asatellite-based coordinate positioning tooland navigation system that can rapidly andaccurately determine the latitude, longi-tude, and altitude of a point on or abovethe Earth’s surface. It is based on a constel-lation of 24 satellites orbiting the Earth ata very high altitude and uses a form of tri-angulation based on the known positionsand distances of three satellites relative tothe surface of the Earth. First developed bythe US Department of Defense to providethe military with a state-of-the-art posi-tioning system, GPS receivers are nowsmall enough and economical enough to beused by the general public.

global stability The tendency for acommunity to return to its original stateeven after suffering a large perturbation.See also resistance. Compare local stabil-ity.

global warming A warming of theEarth’s surface temperature, which is re-sponsible for changes in global climate pat-terns. While the term may be used broadlyto encompass past warming events, such asinterglacials, it is more commonly used torefer to recent global warming due to theGREENHOUSE EFFECT and anthropogenic in-fluences. The quantities of many GREEN-HOUSE GASES within the atmosphere arerising, especially of carbon dioxide. Duringthe past 200 years it has been estimatedthat carbon dioxide levels in the atmos-phere have risen by 25–30% as a result pri-marily of changes in land use (e.g.deforestation) and the burning of fossilfuels (e.g. coal, oil, and natural gas). In ad-dition, levels of methane, another green-house gas, doubled in the 100 years up tothe year 2000. Since the late 19th centurythe globally averaged temperature of air atthe Earth’s surface has risen by 0.3–0.6°C(0.5–1°F).

Complex computer models have beendeveloped to try to predict the changes toclimate that may occur as a result of in-

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creased greenhouse gas emissions and an-thropogenic influences. One prediction isthat if greenhouse gas emissions continueon a business-as-usual basis, the estimatedaverage global temperature rise will be be-tween 1° and 3.5°C (2° and 6°F) by the endof the 21st century. Some of the conse-quences envisaged by such warming in-clude a rise of global sea levels of about0.50 m (1.6 ft) during the 21st century,boundary shifts in the world’s vegetationzones, extension of desertification, and areduction in extent of polar ice and glaci-ers.

Global Warming Convention SeeFramework Convention on ClimateChange.

global warming potential (GWP) Anindex that measures the relative potentialof various gases to contribute to green-house warming, and avoids the necessity todirectly calculate the changes in atmos-pheric concentrations. The reference gas iscarbon dioxide, and the global warmingpotential of other gases is given as a ratioof the radiative forcing that would resultfrom the emission of that gas over a certaintime period. It is calculated as the ratio ofthe radiative forcing that would resultfrom the emission of 1 kilogram of a green-house gas to that from emission of 1 kilo-gram of carbon dioxide over a fixed timeperiod (usually 100 years).

glyphosate A broad-spectrum herbi-cide, soluble in water.

GM crop Genetically modified crops.See genetic engineering.

Gondwanaland A former superconti-nent (very large landmass) formed about180 million years ago by the break-up ofan even larger supercontinent, Pangaea.Gondwana later broke up to form thesouthern land masses of South America,Australia, Antarctica, New Zealand, India,Africa, Madagascar, and some smallerislands. Many species that today havedisjunct distributions (see distribution)evolved on the single landmass of Gond-

wana, e.g. the monkey-puzzle family(Araucariaceae), which is found in SouthAmerica and Australia, and the Proteaceae,found in Australia and Africa. See also con-tinental drift; plate tectonics.

goodness of fit The degree to which ob-served data fit the values predicted by amodel. There are various statistical meth-ods for measuring goodness of fit, includ-ing the CHI-SQUARED TEST for frequencydata.

GPP See gross primary production.

GPS See Global Positioning System.

grab sample 1. A single sample of soil,air, or water taken at one particular placeat one time.2. A sample taken using a grab, i.e. a sam-pler with a scoop or bucket that is loweredto a stream bed. Grabs tend to disturb thesediments as they sample.

graft The transplantation of an organ ortissue in plants and animals. In plants,grafting is an important horticultural tech-nique in which part (the scion) of one indi-vidual is united with another of the same ora different species (the stock). Usually theshoot or bud of the scion is grafted ontothe lower part of the stock. Incompatibilitybetween species is much less common inplants than in animals, where ANTIBODY re-actions may cause rejection of the foreigntissue.

grain A dry indehiscent fruit, typical ofthe grass family (Poaceae), which includescereal crops such as wheat, corn, and bar-ley.

granivore An animal that feeds mainlyon grain and seeds.

granular activated carbon treatmentA filtering system in which water is passedthrough granules of activated carbon to re-move organic material. It is used to treatsmall water systems and in domestic waterand waste systems. It is also highly effectivein reducing levels of radon in water.

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granular activated carbon treatment

graph A pictorial representation of therelationship between the values of two ormore different variables. Usually there aretwo axes at right angles to each other, ahorizontal x-axis, and a vertical y-axis.The x-axis usually represents an indepen-dent variable, the y-axis the dependentvariable. The horizontal coordinate of apoint on a graph, as measured parallel tothe x-axis, is called the abscissa, whereasthe vertical coordinate, measured parallelto the y-axis, is the ordinate.

Data are usually plotted on the graph aspoints, which may or may not be joined bya line. A simple plotted curve, where pointsare joined by curving lines, is particularlysuitable where a variable shows a continu-ous distribution. Where the points of a fre-quency distribution are joined by straightlines, it is called a frequency polygon. If thegraph shows simply scattered points, it iscalled a scatter diagram. If the points takethe form of icons, it is called a pictograph.Alternatively, the points may be clusteredaround a line of best fit, a line that best rep-resents the distribution of the data on theassumption that a sample of infinite sizewould average a linear relationship. A con-tour diagram is a representation of thetopography of an area using the contoursto ‘build’ the composition of the landscape.

Sometimes a rectangular bar may bedrawn from the location centered aroundeach value of the variable on one axis to apoint parallel to the location of the corre-sponding variable on the other axis. Thelength of each bar represents the number ofsamples having that value. When bars aredrawn horizontally from the y-axis, this iscalled a bar chart; where the bars rise ver-tically from the x-axis, it is a histogram. Aline chart is a similar kind of graph, butwith the bars represent by simple singlelines.

When illustrating the relative abun-dance of certain values or properties in apopulation of individuals or samples, a cir-cular graph called a pie chart may be used,the relative proportions of individuals witheach property being shown as a sector ofproportionate size.

grassed waterway A belt of vegetation

planted where water flows to filter out un-wanted particles or to prevent RUNOFF watercausing erosion. See also bioremediation.

grassland A major BIOME dominated bygrasses (Poaceae). Grasslands occur inmany temperate regions where annualrainfall is 250–500 mm and in tropical re-gions with 750–1500 mm. They are wide-spread in the interiors of continents, wherethere is a distinct rainy season in spring andsummer. Many grasslands, especially in thewetter climates, are naturally maintainedby grazing mammals and, to some extent,fire. Grasslands have become much moreextensive as a result of the removal of for-est cover by humans in the last 5000 yearsand by deliberate burning and the grazingof domestic livestock. Temperate grass-lands contain other herbaceous plants inaddition to grasses, though trees are foundonly along streams and rivers. They in-clude the North American PRAIRIES, theSTEPPE of southwest Russia, the grasslandsof Mongolia, and the South American PAM-PAS. Tropical grassland is known as SA-VANNA and is found in large areas of SouthAmerica, East and South Africa, SoutheastAsia, and northern Australia. Grasslandssuch as the prairies, steppes, and savannasin times past supported vast herds of graz-ing mammals. Today these areas are alsoimportant for growing cereal grasses, suchas barley. See also chalk grassland.

gravimetric analysis The chemicalanalysis of substances by separating theirconstituents and weighing precipitates orresidues.

gravitational water See soil moisture.

grazer 1. An animal that feeds on herbs,especially grasses. The term also includesfish and aquatic invertebrates that suck orscrape algae from underwater surfaces.Grazers are larger than the plants or algaethey consume.2. A consumer that feeds on large prey, buttakes only a small part of each individualso that, although injured, the prey survives.For example, certain fish feed on the scalesof others.

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greenhouse effect A rise in temperatureof the Earth’s atmosphere due to the pres-ence of certain gases that allow light topenetrate the atmosphere, but do not letheat out, in much the same way as the glassof a greenhouse does. Solar (shortwave) ra-diation passes easily through the atmos-phere (or glass in a greenhouse) and isabsorbed by the Earth’s surface. It is re-emitted in the form of infrared (longwave)radiation, which is absorbed by watervapor, carbon dioxide, methane, chloroflu-orocarbons (CFCS), and other so-calledGREENHOUSE GASES in the atmosphere witha consequent increase in the atmospherictemperature. Some scientists believe thatincreasing atmospheric pollution by car-bon dioxide, mainly due to the burning of

FOSSIL FUELS, and many other gases is lead-ing to a rise in global temperatures (GLOBAL

WARMING), which will eventually affectother aspects of climate and have pro-foundly damaging effects on naturalecosystems and agriculture, as well as rais-ing sea level.

greenhouse gas A gas that is present inthe atmosphere and absorbs infrared radi-ation, causing the GREENHOUSE EFFECT.

green manure A crop that is grown tobe plowed in while still green to increasethe organic matter in the soil. Examples arelupines (Lupinus) and clovers (Trifolium),which increase the nitrogen content of thesoil. Such crops are usually grown toward

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green manure

energyproduction

5% NO2

5% O3 15%CFCs 20%

CH4

15%

40%

55%CO2

deforestation

Greenhouse effect: the extent to which various greenhouse gases contribute to global warming

infrared

ultraviolet light

O3

CO2

H2OCO2 + H2O

CH4

power stations

C + H2 +

photosynthesis deforestation combustion – fossil fuels respiration+excretion

CO2

CO2

H2O

O2 → CO2 + H2O6CO2 + 6H2O → C6H12O6 + 6O2

infrared

CO2 + CH4

3 2

OZONE LAYER

Greenhouse effect: the incident ultraviolet radiation is re-radiated as infrared, some of which isabsorbed by greenhouse gases such as carbon dioxide and methane

the end of the growing season, when thereis not enough time to grow crops that needto set seed.

Greenpeace An high-profile interna-tional conservation organization thatemploys direct confrontation with govern-ment agencies and polluting companies inan attempt to protect endangered species,prevent damage to the natural environ-ment, and increase public awareness of en-vironmental issues.

Green Revolution The widespread de-velopment and adoption of high-yieldstrains of wheat, corn, and rice during the1960s and early 1970s in an attempt to al-leviate world hunger (officially called theIndicative World Plan for Agricultural De-velopment). It followed the World FoodCongress, held by the Food and Agricul-tural Organization in 1963, and the subse-quent ‘Freedom from Hunger’ campaign.

grid lines A pattern of vertical and hor-izontal straight lines at right angles to eachother, running north to south and west toeast, that make a series of squares, or grid,on a map, chart, or aerial photograph. Agiven point on the map is denoted by refer-ence to its position in terms of both the ver-tical and horizontal axes.

grid north The location of north on amap containing grid lines, by convention inthe direction of the top edge of the map.

grike See limestone pavement.

Grime’s habitat classification A clas-sification scheme devised by the ecologist J. P. Grime, which classes habitats in termsof the concentration of resources and thelevel of disturbance or stability they offerplants, and the effects on plant life strate-gies. Habitats that offer reasonable re-sources and are seldom disturbed tend tobe occupied by competitors, large long-lived plants that take up considerable spaceboth above and below ground. Habitatsthat are cold, dry, or lacking in resourcesfavor stress tolerators, long-lived slow-growing plants that reproduce intermit-

tently when conditions are favorable.Highly disturbed habitats favor the growthof ruderals (weeds), fast-growing plants,often ephemerals, that produce large num-bers of seeds.

gross primary production (GPP) Thetotal fixation of energy by an autotroph(phototrophic or chemotrophic) or by theautotrophs in a population or community.It includes the fixation of energy that islater lost in respiration. See primary pro-duction. Compare net primary production.

gross production efficiency The per-centage of the food that is ingested by anorganism that is used in its growth and re-production. Compare net production effi-ciency.

ground cover 1. The proportion of theground occupied by plants of a givenspecies, as determined by a perpendicularprojection onto the ground of their aerialparts. It is usually expressed as a percent-age of the total area, and gives an estimateof plant abundance. See cover.2. Plants grown to prevent soil erosion.

ground layer The lowermost layer of aplant COMMUNITY, usually comprisingmosses, lichens, fungi, and low-growingherbs, especially rosette plants and creep-ing prostrate forms. Compare canopy; fieldlayer.

groundwater Water lying below theEarth’s surface, in the part of the rocks orsoil that is saturated with water. See fieldcapacity; water table.

growing season In an area with a sea-sonal climate, the season of rapid plantgrowth. The definition of the growing sea-son varies with geographical location. Inthe USA it is defined as the period betweenthe last killing frost (temperature lowenough to kill a plant or prevent its repro-duction) in spring and the first killing frostof the fall. In the UK it is loosely correlatedwith the period when the mean tempera-ture exceeds 6°C.

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growth An irreversible increase in sizeand/or dry weight. It excludes certain de-velopmental processes that involve no sizechange, such as cleavage and uptake ofwater by seeds (imbibition). Growth in-volves cell division and cell expansionthrough synthesis of new materials, and isclosely related to subsequent developmen-tal processes. If some measure of growth ofan organism, such as height or weight, isplotted throughout its life, a characteristicS-shaped (sigmoid) growth curve is ob-tained for most organisms. In some organ-isms, including many plants, growth neverstops entirely, though it may become ex-tremely slow (indeterminate or indefinitegrowth). Limits to growth are geneticallyset and vary with the species, but actualgrowth is highly dependent on environ-mental factors.

growth curve See growth.

growth inhibitor A substance thatslows the growth of a plant, for examplethe plant hormones abscisic acid and ETH-YLENE (ethene), which act at very low con-centrations. Compare growth retardant.

growth retardant A synthetic com-pound used to reduce stem elongation inplants, for example to produce more com-pact plants or to prevent the stems of cerealcrops becoming too long and thin. Com-pare growth inhibitor.

growth ring See annual rings.

guano The accumulated droppings ofbirds, bats, or seals, which are collectedand used as fertilizer. Guano is rich in nu-trients, especially calcium phosphate, andin places is mined on an industrial scale. Ittends to accumulate at the sites of densebreeding colonies, such as seabird colonieson offshore islands or bat colonies in caves.

guild A group of species that live in thesame environment and exploit the habitatand its resources in a similar way.

Gulf Stream A warm ocean current thatflows north from Florida to Newfound-

land, with an eastern extension, the NorthAtlantic Drift, which curves east to north-west Europe. Its temperature and salinityare relatively constant. The warming effectof the North Atlantic Drift on northwestEurope is considerable, ameliorating thewinters. There is concern that if GLOBAL

WARMING continues, increased meltwaterfrom the Arctic may disrupt the GulfStream, and especially the North AtlanticDrift, causing climate change.

gully erosion Severe erosion of land inwhich water flowing over slight depressionerodes them into trenches more than 30 cmdeep. It is typical of sloping land in areas ofhigh rainfall or sporadic fierce stormswhere the natural vegetation has been re-moved for agriculture or firewood or re-duced by overgrazing, e.g. many parts ofthe uplands of Haiti.

guttation Loss of water as liquid fromthe surface of a plant. Water is normallylost as vapor during TRANSPIRATION but, ifthe atmosphere is very humid, water mayalso be forced from the leaves through hy-dathodes (special secretory structures onleaves) as a result of root pressure (a formof hydrostatic pressure).

GWP See global warming potential.

gymnosperms A general term for anyseed plants except the angiosperms (seeAngiospermophyta), characterized bybearing naked seeds, i.e. seeds not enclosedin an ovary (fruit). Gymnosperms lack EN-DOSPERM and many also have advancedtracheids with a structure reminiscent ofangiosperm vessels, whereas in other gym-nosperms the XYLEM is composed solely ofmore primitive tracheids.

gyre A circular or spiral ocean current,like a giant eddy. There is a major gyre ineach of the main ocean basins about 30° oneither side of the Equator, caused mainlyby surface winds. Gyres flow clockwise inthe northern hemisphere and counterclock-wise in the southern hemisphere as a resultof the CORIOLIS FORCE.

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gyre

Haber process An important industrialprocess for NITROGEN FIXATION. It involvesreaction of atmospheric nitrogen with hy-drogen to produce ammonia. The reactiontakes place at high pressure using an ironcatalyst.

habit The typical form or shape of aplant, such as erect, prostrate, woody, etc.

habitat The place where a particular or-ganism lives, described in terms of its cli-matic, vegetative, topographic, and otherrelevant factors. Habitats vary in bothspace and time. For example, in a forest theconditions at ground level are very differ-ent from those in the leaf canopy. Condi-tions also vary between seasons. Howeverthe conditions found in a specific habitat ata given time are unique to that habitat eventhough they may resemble conditionsfound in other similar habitats. The termmicrohabitat describes a small area, per-haps only a few square millimeters or cen-timeters in size, e.g. the undersurface of astone.

habitat fragmentation The breakingup of a habitat into discrete patches sepa-rated by other types of habitat that maynot be suited to the species from the origi-nal habitat. For example, the clearing offorest for urban development.

habitat islands Isolated patches of aparticular habitat. See also island biogeog-raphy theory.

habitat patch 1. A fairly homogeneousarea of habitat that is capable of support-ing a population that is separated fromsimilar patches by areas of unsuitable habi-tat.

2. A part of a habitat containing a highabundance of prey or food. See foragingstrategy.

hadal Describing the part of the oceanbelow the general level of the deep-oceanfloor (600 m). It consists mainly of deeptrenches. Compare abyssal zone.

Hadean See Precambrian.

hail Precipitation in the form of balls orirregularly shaped pieces of ice. It is formedby the coalescence and freezing of super-cooled water droplets.

half-life The time required for theamount of a chemical or physical agent tobe reduced by one half. The term is mostcommonly use to indicate the time it takesfor half of the atoms of a radioactive iso-tope to decay (see radioactive decay). Intheory the time taken for all the parentatoms to decay is infinite, but the half-lifecan be measured.

When a radioactive substance enters thebody, its activity/concentration in the bodywill depend partly on the balance betweeningestion and excretion of the substance.Thus the effective half-life, teff, the time re-quired to reduce the activity in the body byhalf, depends in both the radioactive half-life, trad, and the biological half-life, tbiol:

teff = tradtbiol/(trad + tbiol)tbiol varies from species to species, and alsodepends on an individual’s metabolic rate,physical condition, age, and sex.

halons (bromofluorocarbons) Bromine-containing compounds derived frommethane or ethane and used in fire extin-guishers. They cause depletion of theOZONE LAYER when they break down in the

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H

stratosphere, and have long atmosphericlifetimes.

halophile An EXTREMOPHILE bacteriumof the domain ARCHAEA that lives in anextremely saline environment. See alsomethanogen; thermophile.

halophilic Describing an organism thatthrives in or grows best in a saline environ-ment.

halophyte A plant that grows in soilswith a high concentration of salt, as foundin salt marshes and other littoral habitats.Examples are species of Spartina. Halo-phytes are adapted to obtain water fromsoil water with a higher osmotic pressurethan normal soil water, so they need to beable to accumulate a high concentration ofsalts in their root cells. Some halophytes,such as species of Limonium, are also ableto excrete excess salt through specialglands on their leaves. Many have a succu-lent growth form with swollen stems orleaves.

halosere A series of successional stagesleading to a climax, originating in a salinearea, such as the edge of the sea or of asaline lake. The halosere extends from thepioneer plants at the edge of the bare mudto salt marshes as mud and silt are trappedby the plants and raise the land surface. Seealso sere; succession.

HAP See hazardous air pollutant.

haplobiontic See life cycle.

haploid Describing an organism, cell, ornucleus containing only one representativefrom each of the pairs of HOMOLOGOUS

CHROMOSOMES found in the normal DIPLOID

cell. Haploid chromosomes are thus un-paired and the haploid chromosome num-ber (n) is half the diploid number (2n).MEIOSIS, which usually precedes spore or,sometimes, gamete formation, halves thediploid chromosome number to producehaploid GAMETES. The diploid condition isrestored when the nuclei of two gametesfuse to give the ZYGOTE. Gametes may de-

velop without fertilization, or meiosis maysubstantially precede gamete formation,leading to the formation of haploid organ-isms, or haploid phases in the life cycles oforganisms. In a haploid cell all the genesare in the homozygous condition (only onekind of allele is present in each individual);heterozygosity (in which an individual maycontain more than one kind of allele for thesame gene) is impossible. See also alterna-tion of generations.

hardening See cold tolerance.

hardpan 1. A hardened SOIL horizon,usually in the middle or lower part of a soilprofile. A hardpan is typical of PODZOLS,where iron compounds leached from theupper layers accumulate in the B horizon,staining it red-brown and forming an im-permeable layer (iron pan) that can preventdrainage, resulting in the waterlogging ofhigher horizons.2. Also called calcrete, a hardened layer inor on a SOIL formed on calcareous sub-strata, formed in arid and semiarid regionsas a result of fluctuating climatic condi-tions by calcite that precipitates out of so-lution as water evaporates at the surface. Itmay be redissolved by rainwater, penetrat-ing the soil interstices and again precipitat-ing to form an impermeable crust. Seepodzol.

hard water Water containing dissolvedminerals, especially calcium and magne-sium salts, which make it difficult to getsoaps to form suds. Hard water depositscalcium carbonate, which accumulates inpipes, boilers, and kettles.

Hardy-Weinberg law In large popula-tions in which there is random mating, noselection pressures, no mutation, and noimmigration or emigration, the frequencyof alleles and genotypes remains constantdown the generations, i.e. there is no GE-NETIC DRIFT.

harvesting The removal of plants or an-imals from a population.

Hatch–Slack pathway See C4 plant.

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Hatch–Slack pathway

hazard indicator A quantitative meas-urement of a chemical or physical hazard.It may be based on various factors, such astoxicity, persistence in the environment, orpotential exposure. See also hazardousranking system.

hazardous air pollutant (HAP) A pol-lutant that poses a serious risk to health orthe environment and that is not covered byexisting legally enforceable ambient airquality standards, for example asbestosand mercury vapor.

hazardous chemical Any of a class ofchemicals for which the USA ENVIRONMEN-TAL PROTECTION AGENCY requires manufac-turers, importers, and employers toprovide labels and a detailed informationsheet (material safety date sheet or MSDS)covering such information as healtheffects, safety hazards, precautions andprotective equipment required, emergencyand first aid measures, procedures forclearing up spills, and recommended orstatutory exposure limits.

hazardous ranking system A screen-ing system used by the USA ENVIRONMEN-TAL PROTECTION AGENCY to evaluate therisks posed to public health and the envi-ronment by uncontrolled hazardous waste.Wastes are scored according to their po-tential for spreading out from the site in airor water, and the proximity and density ofhuman populations.

hazardous waste Solid waste that is apotential threat to public health, other liv-ing organisms, or the environment. Thereare usually national and international lawsregulating the handling and disposal proce-dures for such wastes. The USA ENVIRON-MENTAL PROTECTION AGENCY also has aspecific and detailed definition of suchwastes for legal purposes, which it uses todetermine which sites to clean up first.

haze Dust, condensed water vapor, andother material suspended in the atmos-phere that reduces visibility. A heat haze iscaused by warm air rising from hotground.

heat Energy being transferred between asystem and its surroundings as a result oftemperature differences. It is equal to thetotal kinetic energy (see energy) of theatoms or molecules of a system. Heat flowsspontaneously from hot matter to coldermatter.

heath 1. An area dominated by dwarfshrubs of the heath family (Ericaceae).Legumes (family Fabaceae) may also becommon components of heath floras, e.g.gorse (Ulex spp.). Heaths normally de-velop on poor sandy well-drained soils intemperate climates, and are usually subcli-max communities maintained by grazing,burning, and sometimes cutting. The termis also used, particularly in the UK, forheather-dominated communities on acidicPODZOLS, with heather (Calluna vulgaris)mixed with grasses and lichens, and forcommunities on chalk soils, with heatherand chalk grassland species. In Australiaheath communities are dominated by therelated family Epacridaceae. See also chalkgrassland; climax community.2. A dwarf shrub of the heath family (Eri-caeae).

heath forest (keranga) FOREST growingusually on sandy nutrient-deficient POD-ZOLS, dominated by short trees and shrubswith thick bark and leathery leaves, manyof them thorny. Heath forests are found insoutheast Asia, the coast of Gabon (centralAfrica), Guyana, and the Amazon basin.

heat island effect Elevated tempera-tures in urban areas caused by heat fluxesdue to the presence of buildings, roads, andso on, and to emissions of pollutants.

heavy metal A metallic element of highdensity, such as antimony, bismuth, cad-mium, copper, gold, lead, mercury, nickel,silver, tin, and zinc. These metals, whichare toxic even in low concentrations, per-sist in the environment and can accumulateto levels that stunt plant growth and inter-fere with animal life. See pollutant.

heavy metal tolerance Biochemicaland physiological adaptation to concentra-

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tions of heavy metals, such as copper, lead,and zinc, that would prevent the growth ofmost plant species or genotypes. Suchadaptation usually involves converting themetal to a harmless form; some species ex-clude the metal from the plant altogether,whereas others confine it to the roots, thusprotecting the more sensitive shoots. Cer-tain species or strains have evolved suchtolerance, and some, for example certainstrains of bent grass (Agrostis capillaris),benefit from the reduced competition insoil containing heavy metal ions. See alsobiomining; bioremediation; hyperaccumu-lator.

hectare (ha) A metric unit of area, de-noting an area of land 100 meters square(i.e. 10 000 sq m). 1 hectare = 2.47 acres.

heliophyte A plant typical of sunnyhabitats.

helophyte See Raunkiaer’s life-formclassification.

hemicryptophyte See Raunkiaer’s life-form classification.

hemiparasite See partial parasite.

Hepatophyta (liverworts) A phylum ofspore-bearing plants containing leafy andprostrate forms, commonly known as liv-erworts. See also alternation of genera-tions. Compare Bryophyta.

heptachlor A chlorinated hydrocarbonpesticide formerly used to treat seeds. Sus-pected of being a human carcinogen andknow to adversely affect the livers of live-stock that consume treated foodstuffs, itsuse has been banned in many countries.

herb 1. A small seed-producing plantthat does not produce persistent woody tis-sue, whose aerial parts die back at the endof the growing season. In some herbs thewhole plant dies. Compare annual; shrub;tree.2. A plant or plant part valued for its med-icinal, aromatic, or culinary properties.

herbaceous Lacking perennial stemsabove ground.

herbaceous perennial See perennial.

herbarium A collection of dried pressedplants, mounted and arranged systemati-cally together with collection data such asthe plant’s name, date and place of collec-tion, and collector’s name. Other notes ofuse in taxonomic studies or in future usesof the plant (for example, as a source ofpharmaceuticals) may also be kept.Herbaria provide a resource for bothanatomical and molecular reference.

herbicide (weedkiller) A chemical weed-killer that inhibits plant growth or kills theplant. There are two kinds: contact herbi-cides kill or damage the plant on contactwith its surface, whereas systemic herbi-cides are taken up by the plant and trans-ported through its tissues. Systemicherbicides are longer lasting than contactones, and usually more successful in elimi-nating the roots – important for speciessuch a dandelions, which have deep tap-roots and can regenerate from fragments ofroot. Selective systemic herbicides, such as2,4-D and 2,4,5-T, kill most broad-leavedplants, but leave cereals and grasses unaf-fected.

herbivore A plant-eating animal, espe-cially one of the herbivorous mammals,such as cattle, rabbits, etc. There may bevarious modifications associated with thisdiet, e.g. to the teeth and digestive system.Compare carnivore; omnivore.

herd immunity A level of immunization(which may be less than 100%) against aparticular disease in a particular popula-tion at or above which the disease is unableto persist in that population.

heredity The transmission of geneticallydetermined characteristics from one gener-ation to the next.

heritability The proportion of pheno-typic VARIATION due to genetic factors. Itcan be estimated from measurements of in-

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dividuals from different generations, and isused in plant and animal breeding to pre-dict how successful genetic selection will bein improving a particular trait. See pheno-type; genotype.

hermaphrodite 1. An animal possess-ing both male and female reproductive or-gans. The earthworm (Lumbricus) is acommon example.2. A plant bearing stamens and carpels inthe same flower. In many such plants, self-fertilization and inbreeding are preventedby specific self-incompatibility systems.

heterogeneous Nonuniform: made upof dissimilar components, or varying instructure or composition in different loca-tions.

heteromorphism The existence of morethan one form, used especially with refer-ence to life cycles in which the alternatinggenerations are markedly different mor-phologically, as in ferns and jellyfish. Seealternation of generations. Compare poly-morphism.

heterosis See hybrid vigor.

heterotroph An organism whose princi-pal source of carbon is organic, i.e. the or-ganism cannot make all its own organicrequirements from inorganic starting ma-terials. Most heterotrophic organisms arechemotrophic (see chemoheterotroph);these comprise all animals and fungi, mostbacteria, and parasitic plants. A few het-erotrophic organisms are phototrophic (seephotoheterotroph). The nonsulfur purplebacteria, for instance, require organic mol-ecules such as ethanol and acetate. Com-pare autotroph. See also chemotroph;phototroph.

heterotrophic succession See succes-sion.

heterozygous Having two different AL-LELES at a given gene locus. Usually onlyone of these, the dominant allele, is ex-pressed in the phenotype. Such an organ-ism is called a heterozygote. On selfing or

crossing heterozygotes some individualswith two recessive alleles (double reces-sives) may appear, giving viable offspring.Selfing heterozygotes halves the heterozy-gosity, and thus OUTBREEDING maintainsheterozygosity and produces a more adapt-able population. Compare homozygous.

HFC See hydrofluorocarbons.

hibernation A state of sleep and greatlyreduced metabolic rate that enables certainmammals to survive prolonged periods oflow temperature and food scarcity. Storedbody fat supplies enough energy for theirbodies to work slowly and maintain theirtemperatures just higher than their sur-roundings. Some, such as bats, wake andfeed on warm days.

Day length or food shortage may stim-ulate the hibernation mechanism, but intemperate and arctic animals the stimulusis cold. A ‘hibernation hormone’ has beensuggested but not isolated. Temperatureregulation is maintained but at a lowerlevel. When hibernation ends, body tem-perature rises spontaneously, starting atthe body core.

hierarchical classification A system ofCLASSIFICATION in which individuals aregrouped into a series of progressivelylarger and broader categories; lowergroups are always subordinate to and in-cluded in groups of higher ranking. Thegroupings are based on the affinities of theindividuals and groups. Hierarchical classi-fication is the basis of CLADISTICS. The IN-TERNATIONAL CODES OF NOMENCLATURE

define the sequence of ranks and theirnames in particular groups of organisms.For example, in plants the ranks are as fol-lows, in ascending order: form, variety,species, series, section, genus, tribe, family,order, class, phylum or division, and king-dom. Many taxonomists also recognize aneven higher rank, the domain.

hierarchy 1. A graded or ranked series,as in an EVOLUTIONARY TREE.2. (dominance hierarchy) A form of socialorganization in which certain individualsor groups of individuals in a population

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have different status or rank, which affectsaccess to food, mates, or other resources.See dominance.

high-level waste See radioactive waste.

histogram See graph.

Holarctic The circumpolar region thatencompasses North America, Europe, andAsia. It is a recognized BIOGEOGRAPHICAL

REGION, with significant similarities offlora and fauna, because these continentswere once joined in the supercontinentLaurasia. See also continental drift.

Holarctic floral province Alternativename for the Boreal floral region, whichconsists of northern Eurasia and NorthAmerica. See floral province.

holding pond A pond or reservoir thatstores polluted RUNOFF.

Holocene (Recent) The present epoch inthe geological time scale, being the secondepoch of the QUATERNARY period. It datesfrom the end of the last GLACIATION, about10 000 years ago, to the present day.

holophytic Describing the type of nutri-tion in which complex organic moleculesare synthesized from inorganic moleculesusing light energy. It is another term forphototrophic. See phototroph.

holozoic (heterotrophic) Describing or-ganisms that feed on other organisms orsolid organic matter, i.e. most animals andINSECTIVOROUS PLANTS.

homeostasis The maintenance of a con-stant internal environment by an organism.It enables cells to function more efficiently.Any deviation from this balance results inreflex activity of the nervous and hormonesystems, which tend to negate the effect.The degree to which homeostasis isachieved by a particular group, indepen-dent of the environment, is a measure ofevolutionary advancement.

homeothermy See homoiothermy.

homing The returning of an animal to asite it regularly uses for sleeping or breed-ing. The term is applied not only to sea-sonal migrations between feeding andbreeding grounds, but also to foraging trips.

hominid A member of the Primates,family Hominidae, which includes modernhumans (Homo sapiens) as well as extinctforms found in great number and variety offossils.

homogeneity In statistics, the propertyof samples or individuals from differentpopulations whereby they are similar withrespect to the variable being studied. Suchpopulations – and the resulting data – aresaid to be homogeneous.

homogeneous Uniform: made up ofsimilar components, or not varying instructure or composition in different loca-tions. See also homogeneity.

homoiothermy (homeothermy) Themaintenance of body temperature withinnarrow limits. Organisms that achieve thisby behavioral means, such as moving inand out of shade, are termed ectotherms,while those that have internal temperature-regulating mechanisms are termed en-dotherms. Compare poikilothermy.

homologous chromosomes CHROMO-SOMES that pair at MEIOSIS. Each carries thesame GENES as the other member of the pairbut not necessarily the same ALLELES for agiven gene. One member of each pair is ofmaternal origin, the other of paternal ori-gin. With the exception of the sex chromo-somes (X and Y) they are morphologicallysimilar. During the formation of the germcells only one member of each pair of ho-mologues is passed on to the gametes. Atfertilization each parent contributes onehomolog of each pair, thus restoring thediploid chromosome number in the ZY-GOTE. See also meiosis.

homologous structures Structures that,though in different species, are believed tohave the same origin in a common ances-tor. Thus the forelimbs and hindlimbs of

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all land vertebrates are said to be homolo-gous, being constructed on the same five-digit (pentadactyl) pattern.

homozygote See homozygous.

homozygous Having identical ALLELES

for any specified GENE or genes. A homozy-gote breeds true for the character in ques-tion if it is selfed or crossed with a similarhomozygote. An organism homozygous atevery locus produces offspring identical toitself on selfing or when crossed with agenetically identical organism. Homozy-gosity is obtained by inbreeding, and ho-mozygous populations may be welladapted to a certain environment, but slowto adapt to changing environments. Com-pare heterozygous.

horizon See soil.

host An organism used as a source ofnourishment by another organism, the par-asite, which lives in or on the body of thehost. In a definitive host the parasitereaches sexual maturity. In an intermediatehost the resting stage or young of the para-site is supported. See parasitism.

host specificity The degree to which aparasite, parasitoid, or symbiont is foundon more than one species of host. If it isfound on only one species, it is said to behighly specific.

hot desert See desert.

hot spring (geothermal spring) A springheated by the Earth’s crust, for example, byvolcanic activity or radioactivity in therocks below.

Human Genome Project An interna-tional research project, sponsored by theUS Department of Energy, the National In-stitutes of Health and the Wellcome Trust,started in 1990 to sequence the DNA of thehuman GENOME and map the location of allthe genes. The first working draft was pro-duced in June 2000, and the final draft(with an error rate less than 1 in 100,000bases, an accuracy of 99.9%) was com-

pleted in 2003: some 3.2 billion bases and31 000 genes. The project used automatedsequencing machines involving advancedrobotics, together with large amounts ofcomputing power, achieving sequencingrates of 12 000 bases a second. Resultswere published on the Internet as soon asthey were available, thus ensuring freepublic access to the genome.

humic acid A natural mixture of darkbrown acidic organic substances that canbe extracted from HUMUS with dilute alkaliand precipitated by acidification to pH1–2. See soil.

humidity The moisture content of theatmosphere. The absolute humidity is theamount of water vapor present in a unitvolume of air (usually 1 cubic meter of air).The relative humidity is the ratio of theamount of water vapor actually present inthe air to the greatest amount that the aircould hold at the same temperature, and isusually expressed as a percentage.

Various instruments are used to meas-ure humidity. In the wet-and-dry-bulbthermometer, a pair of thermometers isused, one – the wet-bulb thermometer –being kept moist by a thin muslin bagdipped in a container of distilled water.Provided the air is not saturated, evapora-tion from the cloth keeps the wet-bulbthermometer at a lower temperature thanthe dry-bulb thermometer, and the differ-ence between them is a measure of the sat-uration deficit, and hence the relativehumidity of the atmosphere. Such a pair ofthermometers may be enclosed in a framewith a handle (a whirling psychrometer orsling psychrometer), which can be whirledin the air like a football rattle, ensuringgood airflow around the bulbs. A hydro-graph, which uses specially treated hair thatchanges its length as the humidity changes,records changes in humidity on a revolvingdrum. See also hygrothermograph.

humification The process by whichHUMUS is formed from dead organic ma-terial. It involves the action of DECOM-POSERS, SAPROBES that feed on dead organicmaterial, breaking down complex organic

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molecules by oxidation reactions to formsimple inorganic molecules that can betaken up by plant roots. See soil.

hummock and hollow cycle A cyclicalchange in the vegetation of a developingraised peat bog. Sphagnum mosses colo-nize a pool of water on the peat, buildingup a low hummock. This allows plantssuch as ling (Calluna vulgaris) and cross-leaved heath (Erica tetralix) to move in.When they die, the hummock erodes,forming a water-filled hollow, so the cyclestarts again. The BOG consists of a mosaicof hummocks and hollows in variousstages of development.

humus The nonliving finely dividedorganic matter in SOIL derived from the de-composition of animal and plant sub-stances by soil bacteria (see humification).Humus consists of 60% carbon, 6% nitro-gen, and small amounts of phosphorus andsulfur, and is valued by horticulturalistsand farmers because it improves the fertil-ity, water-holding capacity, and work-ability of the soil. Humus has colloidalproperties (see colloid) that enable it to re-tain water, so it can improve the moisturecontent of sandy soils. It aids the formationof crumbs in the soil (see crumb structure),and is often added to clay soil to increasethe particle size, promoting drainage andaeration. Different types of humus are rec-ognized, according to the types of organ-isms involved in its decomposition, thevegetation from which it is derived, and thedegree of incorporation into the mineralsoil. Mull humus is found in deciduous andhardwood forests and grasslands in warmhumid climates. It is alkaline and bacteria,worms, and larger insects are abundant.Decay is rapid and layers are not distin-guishable. Mor or raw humus is usuallyacidic and characteristic of coniferous for-est areas. Few microorganisms or animalsexist in this type of humus, small arthro-pods and fungi being the most common.

hunter-gatherers Humans that live byhunting wild animals, fishing, and collect-ing wild fruits, berries, nuts, mushrooms,and other natural sources of food, but do

not cultivate the land or own domestic live-stock.

hurricane See depression.

Huxley’s line See Wallacea.

HWRP See Hydrology and Water Re-sources Program.

hybrid An organism derived from cross-ing genetically dissimilar parents. Thusmost individuals in an OUTBREEDING popu-lation could be called hybrids. However,the term is usually reserved for the productof a cross between individuals that aremarkedly different. If two different speciesare crossed the offspring is often sterile, forexample, the mule, which results from across between a horse and a donkey. Thesterility results from the nonpairing of thechromosomes necessary for gamete forma-tion. In plants this is sometimes overcomeby the doubling of the chromosome num-ber (see chromosome), giving a polyploid(see polyploidy), which may eventuallygive rise to a new species. By contrast, hy-brids derived from different varieties of thesame species are often more vigorous thantheir parents, and are selected and propa-gated by vegetative means by agriculturistsand horticulturists. See chimera; hybridvigor.

hybrid breakdown The situation inwhich a HYBRID is fertile, but its offspringare not.

hybridization Processes that lead to theformation of a HYBRID. Hybridization iscommon among angiosperm plant species,though often the offspring are partially orwholly sterile. In some the hybrids may beable to backcross with one or both parentalspecies, leading to HYBRID SWARMS. Artifi-cial hybridization of normally self-pollinat-ing species involves the transfer by hand ofpollen from one plant to another, oftenusing a paintbrush. Its success depends onthe presence or absence of incompatibilitysystems that prevent successful fertilizationbetween individuals of similar geneticmakeup. Such techniques have been impor-

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tant in the development of crop CULTIVARS

and ornamental plants. See speciation.

hybrid swarm A very variable series oforganisms resulting from the continualcrossing, recrossing, and backcrossing ofthe HYBRID generations of two originalspecies or varieties that may occur whenthe original barrier to reproduction breaksdown. See apomixis.

hybrid vigor (heterosis) The conditionin which the expression of a characteristicis greater in the heterozygous offspringthan in either of the homozygous parents.The effect arises from an accumulation ofdominant genes in the F1 (first filial) gener-ation. Thus, if height is controlled by twogenes, A and B, and tall and short formsare determined by dominant and recessivealleles respectively, then the crossing AAbb× aaBB would give an F1 AaBb, containingboth dominant genes for tallness. Usuallythe more unlike the parents are, the morehybrid vigor is released, but the effect di-minishes in subsequent generations asmore recessive homozygotes (see homozy-gous) reappear. For crop plants, hybridvigor is particularly important for charac-teristics such as growth rate, yield, and dis-ease resistance. Selfing the F1 generationreduces the amount of heterozygosity (seeheterozygous), so optimum vigor is bestmaintained by producing seed by crossingtwo different parental pure lines.

hybrid zone A region where HYBRIDS be-tween two geographical races occur, usu-ally at the boundary between two separatebut closely related populations. See hy-bridization; hybrid swarm.

hydration The taking up of or combin-ing of substances with water.

hydraulic lift The ability of plant rootsto take up water from deep soil layers andrelease it into shallower surface layers. Up-take from the deep moist soil layers is aidedby ROOT PRESSURE; water then diffuses outof roots in the drier surface soil. Hydrauliclift tends to happen particularly at night,when the transpiration streams ceases. It is

thought to benefit the plant by allowingnutrients in the upper soil layers to dissolveand be available for uptake. Mature treesmay move hundreds of liters of water everynight in this way, to the benefit of adjacentshrubs and herbs. See transpiration.

hydric Describing a habitat that is ex-tremely wet. Compare mesic; xeric.

hydrocarbons Organic compounds thatcontain carbon and hydrogen only, e.g.acetylene, butane. Many of the compo-nents of natural gas, petroleum, and coalare hydrocarbons.

hydroelectric power (hydropower) Arenewable source of energy derived byfalling or flowing water, which powers tur-bines that generate electricity. Some hydro-electric power plants are powered directlyby river water, while others are powered bywater stored in a reservoir behind a dam atsome height above the natural riverbed.Tidal power, in which the difference inheight between high and low tides form themain driving force, is a form of hydroelec-tric power.

hydrofluorocarbons (HFCs) Com-pounds that contain only hydrogen, fluo-rine, and carbon. They are used inmanufacturing, and are also by-productsof many industrial processes. HFCs wereintroduced as substitutes for CFCS becausethey do not significantly affect the OZONE

LAYER, but they are powerful GREENHOUSE

GASES with GLOBAL WARMING POTENTIALS

that range up to 11 700. See global warm-ing; ozone layer.

hydrogen An essential element in livingtissues. It enters plants, with oxygen, aswater and is used in building up complexreduced compounds such as carbohydratesand fats. Water itself is an importantmedium, making up 70–80% of the weightof organisms, in which chemical reactionsof the cell can take place.

hydrogen bond An electrostatic attrac-tion between a hydrogen atom and an elec-tronegative atom in another molecule.

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Hydrogen bonding is responsible for theproperties of water, including its relativelyhigh boiling point. It is important in manybiological systems for holding together thestructure of large molecules.

hydrogen economy An idealized econ-omy in which the only fuel is hydrogen,which is burned or oxidized in fuel cells togive electricity directly. The hydrogencould be produced by electrolysis of water,the electricity for this being generated bywind power, water power, or similar re-newable sources. Hydrogen fuel wouldcause no pollution and minimize the emis-sion of GREENHOUSE GASES.

hydrogen ion A positively charged hy-drogen atom, H+, i.e. a proton. Hydrogenions are produced by all ACIDS in water. Seeacid; pH.

hydrogen sulfide (H2S) A colorlesshighly poisonous, highly flammable gasthat smells like bad eggs. Some photosyn-thetic bacteria derive their electrons fromhydrogen sulfide instead of water:

2H2S + CO2 → H2O + CH2O + 2SSee autotroph.

hydrogeological maps Maps thatshow the distribution of AQUIFERS,AQUICLUDES, and bore holes, the depth tothe WATER TABLE, the groundwater quality(ions present), and the yield of groundwa-ter from bore holes.

hydrogeology The study of the chem-istry and movement of GROUNDWATER. Seewater cycle.

hydrograph See humidity.

hydroid See Cnidaria.

hydrological cycle See water cycle.

hydrology The study of the WATER CYCLE:the properties, distribution, and circulationof water in the ATMOSPHERE, BIOSPHERE,LITHOSPHERE, and HYDROSPHERE, and howthis changes with time. It encompasses ge-ology, meteorology, and oceanography.

Hydrology and Water Resources Pro-gram (HWRP) A program of the WorldMeteorological Association that aims tocollect and analyze hydrological data as abasis for assessing and managing freshwa-ter resources, e.g. for human consumption,sanitation, irrigation, hydroelectric powergeneration, and water transport, and forflood forecasting and the prediction ofdroughts.

hydrophilic Describing a molecule orsurface that has an affinity for water. Suchmolecules are usually polar, for exampleproteins. Compare hydrophobic.

hydrophily POLLINATION in which watercarries the pollen from anther to stigma.

hydrophobic Describing a molecule orsurface that has no affinity for water. Suchmolecules are nonpolar, e.g. benzene.Compare hydrophilic.

hydrophyte A plant that grows in wateror in extremely wet areas, for example ar-rowhead (Sagittaria) and water lilies. Hy-drophytes show certain adaptations tosuch habitats, notably development ofaerenchyma, reduction of cuticle, root sys-tem, and mechanical and vascular tissues,and divided leaves. Large intercellular airspaces in leaves, stems, and roots allowoxygen to diffuse through the plant. Hy-drophytes with floating leaves can ex-change gases with the atmosphere; manysubmerged hydrophytes, such as spikedwater milfoil (Myriophyllum spicata), haveno stomata, absorbing water and gasesover their entire surface. Compare meso-phyte; xerophyte. See also Raunkiaer’s life-form classification.

hydroponics (water culture) Thegrowth of plants in liquid culture solutionsrather than soil. The solutions contain thecorrect balance of all the essential mineralrequirements. The method is used commer-cially, especially for glasshouse crops, andalso in experimental work in determiningthe effects of mineral deficiencies.

hydropower See hydroelectric power.

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hydrosere Any plant community in aSUCCESSION that starts in fresh water. Grad-ual silting leads to mesophytic conditionsin the later seres. See sere.

hydrosphere All the water contained inthe atmosphere, land, sea, rivers, and lakes.

hydrostatic pressure The pressure ex-erted by a fluid as a result of its incom-pressibility. For example, water in a plantcell exerts a pressure on the cell wall; tur-gor is due to the hydrostatic pressurecaused by the uptake of water into cellswhose expansion is limited by the inelastic-ity of the cell wall.

hydrothermal activity The productionof hot springs and other bodies of waterheated by cooling magma or by radioactivedecay in rocks. See also geothermal power;hydrothermal vent.

hydrothermal vent (smoker) A site onthe seabed from which hot springs arisethat have been heated by contact withmolten rock, usually along a mid-oceanicridge (see plate tectonics). Temperaturesmay reach 300oC. These vents are oftenrich in minerals, especially sulfides, andsupport communities in which the primaryproducers are chemotrophic bacteria. Ventsrich in copper, iron, and manganese are par-ticularly hot, and the water is black; theyare known as black smokers. The coolerwhite smokers flow more slowly and usu-ally contain quantities of arsenic and zinc.

hygrometer An instrument used tomeasure atmospheric HUMIDITY.

hygroscopic water See soil moisture.

hygrothermograph An instrument thatmeasures temperature and relative humid-ity and records it on a scaled paper chartmounted on a cylinder that turns to give arecord over a period of time, usually aweek.

hyperaccumulator A plant capable ofaccumulating HEAVY METALS in its tissues,to the point where it can be ‘mined’ for the

metal. See also bioaccumulation; bioreme-diation.

hyperosmotic See hypertonic.

hyperthermophile An EXTREMOPHILE

that thrives in a habitat with high temper-atures, sometimes requiring a temperatureof at least 105°C. Some archaea (e.g. Py-rolobus) are able to survive temperaturesof 113°C, and may even fail to multiply attemperatures much below 90°C. See Ar-chaea.

hypertonic (hyperosmotic) Describing asolution with an osmotic pressure greaterthan that of a specified other solution, thelatter being HYPOTONIC. When separatedby a selectively permeable membrane (e.g.a cell membrane) water moves by OSMOSIS

into the hypertonic solution from the hy-potonic solution. Compare isotonic.

hypha (pl. hyphae) In fungi, a fine non-photosynthetic tubular filament thatspreads to form a loose network termed amycelium or aggregates into fruiting bod-ies (e.g. mushrooms).

hypolimnion The lower noncirculatingwater in a thermally stratified lake. If it liesbelow the COMPENSATION POINT, oxygenbecomes depleted, and reoxygenation oc-curs only when stratification breaks downin the fall. See stratification.

hypo-osmotic See hypotonic.

hypotonic (hypoosmotic) Describing asolution with an osmotic pressure less thanthat of a specified other solution, the latterbeing HYPERTONIC. When separated by aselectively permeable membrane (e.g. a cellmembrane) water is lost by osmosis fromthe hypotonic to the hypertonic solution.Compare isotonic.

hypoxic Deficient in oxygen.

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IBP See International Biological Pro-gram.

ICBN See International Codes of Nomen-clature.

Ice Age A period in the latter part of thePLEISTOCENE characterized by successivecoolings and warmings of the earth. In atleast four major GLACIATIONS (cold peri-ods), ice caps spread south from the Arcticand north from the Antarctic. Large areasof Britain, Europe, and North Americawere covered by ice from the North. Thecause of the Ice Ages is not known, but it isassumed that others will occur in the fu-ture. The term ice age (no capitalization) isused for similar climatic fluctuations thatoccurred prior to the Pleistocene.

ice core A cylindrical section of icetaken from a bore hole in a glacier or icesheet in order to study past climates by an-alyzing the gases in the air trapped betweenthe ice crystals.

ideal free distribution The distributionof individuals of a foraging species that re-sults if the consumers select the most prof-itable sites at a given time, assuming all arefree to move form one patch to anotherand are equally discriminating. As the mostfavorable sites fill up, later arrivals selecthabitats that have lower overall suitability,but which are less crowded.

IGBP See International Geosphere–Biosphere Program.

igneous rock Rock formed by crystal-lization from a molten magma, e.g. basaltand granite.

illuviation The accumulation of dis-solved or suspended materials in a soilhorizon as a result of LEACHING from an-other part of the soil profile. Comparehardpan.

immigration The flow of genes into apopulation when individuals from otherpopulations move in and interbreed withmembers of the population. See gene flow.Compare isolating mechanisms.

immunity The ability of plants and ani-mals to withstand harmful infective agentsand toxins. In most plants this is achievedby physical barriers preventing entry ofPATHOGENS and by physiological reactionsto isolate the pathogen and its effects. Inanimals it may be due partly to a numberof nonspecific mechanisms, such as inflam-mation and phagocytosis (cell eating) or an impervious skin (nonspecific immu-nity). In vertebrates it is largely the result ofa specific mechanism, whereby certain sub-stances (antibodies) or lymphocytes pre-sent in the body combine with anintroduced foreign substance (antigen) –specifically acquired immunity.

impact assessment An evaluation ofthe effect of a proposed project on thehuman and natural environment. As wellas taking into account its effects on naturalor cultivated habitats and landscapes,wildlife, and livestock, the assessment mayalso consider effects on the quality of life(noise, pollutants, visual impact, and soon), and on the local economy (numbers ofjobs created/lost, effects on local busi-nesses, etc.). It may also propose ways ofameliorating potential deleterious effects.

impermeable Describing a substance

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that does not allow another substance, es-pecially water, to penetrate or passthrough it.

inbreeding Breeding between closely re-lated individuals. The most extreme formof inbreeding is self-fertilization, which oc-curs in some plants. In animals, mating be-tween siblings or between parents andoffspring is generally the closest form of in-breeding. Inbreeding increases homozygos-ity (see homozygous) so that deleteriousrecessive genes are expressed more often inthe PHENOTYPE, and decreases heterozygos-ity (see heterozygous) and hence the poten-tial genetic variability of the population.There is also a general lowering of vigor ininbred stock (INBREEDING DEPRESSION),which is especially pronounced amongnormally outbreeding populations. Inhuman societies there are usually culturalrestraints on marriage between close rela-tives. See inbreeding depression. Compareoutbreeding.

inbreeding depression The tendencyfor species to suffer a decline in vigor on in-breeding, probably due to the increased oc-currence of individuals HOMOZYGOUS fordeleterious or lethal ALLELES. This is mostmarked in individuals resulting from self-fertilization, especially in plants that arenormally OUTBREEDING. It may be manifestat any stage of the life cycle. See inbreed-ing. Compare hybrid vigor.

incineration A WASTE treatment processthat involves controlled burning at hightemperatures. Hazardous components arereduced to carbon dioxide, water, and ashcontaining very little combustible material,which can then be stored. In addition to re-ducing toxicity, incineration also reducesthe volume of waste. The furnace used forcontrolled incineration is called an inciner-ator. In mass burn incineration, bulk wastevolume is reduced by about 90%, and itsmass reduced by about 70%. Unsortedwaste (except for large items such as refrig-erators and hazardous items such asaerosols) is fed in to the furnace, and thehot combustion gases are passed through a

boiler system to extract energy. See alsocombined heat and power plant.

incinerator See incineration.

inclusive fitness See fitness.

indefinite growth See growth.

independent assortment The law, for-mulated by Austrian geneticist GregorMendel, that GENES segregate indepen-dently at MEIOSIS so that any one combina-tion of ALLELES is as likely to appear in theoffspring as any other combination. Thework of T. H. Morgan later showed thatgenes are linked together on chromosomesand so tend to be inherited in groups. Thelaw of independent assortment thereforeapplies only to genes on different CHROMO-SOMES. The term can also be applied towhole chromosomes. See also linkage.

independent variable See variable.

indeterminate growth See growth.

indicator A substance used to test foracidity or alkalinity of a solution by a colorchange. Examples are litmus and phe-nolphthalein. A universal indicator showsa range of color changes over a wide rangefrom acid to alkaline, and can be used toestimate the pH.

indicator dyes See pH.

indicator species (ecological indicator)An organism that can be used to measurethe environmental conditions that exist in a locality. Lichen species are indicatorsof levels of pollution, because differentspecies are sensitive to different levels andtypes of pollutants. Certain floweringplants are indicators of acid (Sphagnummosses) or alkaline (Gypsophila species)conditions. Indicator species also provideinformation about past environments; forexample, certain species may be typical ofancient woodland or sites where wood-lands formerly stood. Tubifex worms indi-cate low levels of oxygen and stagnantwater.

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indigenous (native) Describing a spe-cies, race, or other taxon that occurs natu-rally in an area and has not beenintroduced by humans. Compare exotic.

indirect competition See competition.

industrial ecology The study of theflow of energy and materials from theirnatural sources through the manufacturingprocess and its products to their final dis-posal or recycling. It provides useful infor-mation for devising strategies to minimizepollution and reduce the consumption ofenergy, water, and other resources.

industrial ecosystem An industrial sys-tem that mimics a natural ecosystem in thatthe wastes from one industrial processform the raw materials for another.

industrial melanism An increase indark forms, for example in the moth Bistonbetularia, in industrial soot-polluted envi-ronments. Natural selection against nor-mal pale forms by predators results in darkoffspring being at a selective advantage insuch environments. This results in an in-crease in the numbers of the better camou-flaged dark forms. See also POLYMORPHISM.

industrial smog See smog.

infauna Animals that live under the sur-face of the seabed or other sediments, ei-ther by tunneling or by constructing tubes.

infiltration 1. The movement of waterthrough the SOIL surface. The properties ofdifferent soils, especially surface structureand texture, give them different capacitiesfor infiltration. See also percolation.2. The leakage of water from the soil intomanmade underground structures such aspipes through cracks and damaged sur-faces.3. The application of waste water to land,so that it penetrates the underlying soil.

influent 1. Water, waste water, or an-other liquid that flows into a stream, reser-voir, treatment plant, or industrial process.

2. A factor, such as an immigrating indi-vidual, that perturbs the balance of a com-munity.

ingestion 1. A form of heterotrophic nu-trition in which prey or pieces of prey ordetritus are taken in through the mouth.2. The taking into the body of toxic sub-stances through the mouth or lungs.

inheritance The receiving of character-istics by transmission of genetic materialfrom parent to offspring.

inhibition Resistance to a colonizingspecies by an already-established species,especially during a SUCCESSION.

inorganic Describing a substance that isnot derived from living organisms or theirremains, or which does not contain car-bon. However, simple carbon-containingcompounds such as oxides and sulfides ofcarbon are usually considered to be inor-ganic compounds.

Insecta The largest class of arthropods(see Arthropoda) and the largest class inthe animal kingdom. Most insects can fly.The body is characteristically divided intoa head, thorax, and abdomen. The headbears a pair of antennae, compound eyes,and simple eyes (ocelli). The mouthpartsare modified according to the diet. Thethorax bears three pairs of five-jointed legsand, typically, two pairs of wings. The ab-domen is usually limbless. Most insects areterrestrial and respiration is carried out bytracheae (branching tubes) with segmen-tally arranged spiracles (tiny holes openingto the outside). Usually the life cycle in-cludes complete METAMORPHOSIS, with alarval and pupal stage, but in some speciesmetamorphosis is incomplete – the larvae(nymphs) resemble the adult and there isno pupal stage. Many insects are benefi-cial, being pollinators of flowers andpredators of pests; others are harmful,being pests of crops, disease carriers, anddestroyers of clothes, furniture, and build-ings. See also biological control; pollina-tion.

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insecticide A chemical used to kill in-sects. It may be a natural substance, such as derris or pyrethrums, or a syntheticchemical such as organophosphates (e.g.malathion), carbamates, CHLORINATED HY-DROCARBONS, or dinitrophenols. Syntheticinsecticides tend to persist in the environ-ment and accumulate in food chains. Seealso bioaccumulation; botanical insecti-cide.

insectivore Any animal that feedsmainly on insects.

insectivorous plant (carnivorous plant)A plant adapted to supplementing photo-synthesis by obtaining nutrients, especiallynitrates, from small animals, especially in-sects. The plants have various means oftrapping and killing the insects, which arethen digested by enzymes secreted by theplant. Examples include: butterworts (Pin-guicula) with slippery, inrolling leaves;sundews (Drosera), with sticky glandularhairs that close over the trapped insects;Venus’s-flytrap (Dionaea muscipula), withhinged leaves that snap shut over thevictim, triggered by sensitive hairs; andpitcher plants (Nepenthaceae and Sarrace-niaceae), with leaves modified to form con-tainers into which insects and other smallanimals fall and drown.

in situ In its natural or original positionor place.

insolation The radiation received fromthe Sun, measured as the rate of delivery ofdirect solar radiation per unit of horizontalsurface. The radiation received at a givenpoint on the Earth’s surface depends on theposition of the Earth in its orbit, the incli-nation of the surface to the Sun’s rays, andthe thickness and transparency of the at-mosphere (again affected by latitude). Itvaries with the season and with latitude,being about 2.4 times greater at the equa-tor than at the poles.

integrated pest management (IPM)The use of both chemical and biologicalcontrol and good farming practice to re-duce pest numbers, i.e. the use of resistant

crop varieties and selective use of pesti-cides, so that natural predators are notharmed, as well as practices such as rota-tion of crops, which minimize the chancesof pest populations building up.

integrated pollution control (IPC) Asystem of emissions controls imposed onindustrial and other processes, usually im-posed by a licensing scheme, that encom-pass the effects of emissions on allenvironmental media: air, land, and water.Such controls take account of the potentialfor emissions to move from one medium toanother.

Integrated Risk Information System(IRIS) An electronic database of thehuman health effects that may result fromexposure to various substances found inthe environment. Maintained by theUnited States ENVIRONMENTAL PROTECTION

AGENCY, it is a valuable tool for use in riskassessment and other decision-making, andfor regulatory activity.

integrated waste management Awaste management strategy that encom-passes the avoidance or minimization ofwaste production as well as the maximiza-tion of recovery, reuse, and recycling of ap-propriate waste materials and theminimization of the volume of the finalwaste for disposal.

intensive farming Farming that aims toget the maximum output from a given areaof land by keeping animals in crowdedconditions or growing crops in quick suc-cession, often to the detriment of the envi-ronment. Growing several crops a yearrequires large amounts of fertilizer. If therotation of crops is abandoned, this willnecessitate more fertilizer and pesticide ap-plications. Keeping livestock indoors andfeeding them on concentrated foodstuffsrequires the use of more drugs to controldiseases associated with crowding.

interactional food web See food web.

interference coefficient A measure ofthe extent to which interference between

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CONSUMERS reduces individual rates of con-sumption as consumer density increases.

interglacial period A period of warmerclimate between two glacial periods,mainly associated with the PLEISTOCENE pe-riod. This can be detected by pollen analy-sis (see palynology), which reveals thevegetation succession as the climatechanges. See Ice Age. Compare interstadialperiod.

Intergovernmental Panel on ClimateChange (IPCC) A body of internationalscientific experts set up in 1988 to correlateand assess information on climate changeand propose suitable responses.

intermediate host See host.

intermittent growth See growth.

internal dose See absorbed dose.

International Biological Program(IBP) An international research program(1966–74) instigated by the InternationalUnion of Biological Sciences (IUBS), andlater taken over by the International Coun-cil of Scientific Unions (ICSU), whichaimed to study the biological productivityof the Earth’s terrestrial, marine, and fresh-water systems, human adaptability, and re-source use and management. It wasimportant in stimulating worldwide envi-ronmental awareness, and was a forerun-ner of the UNESCO’s Man and BiosphereProgram. Data were collected on ecosys-tems (biomes) such as deserts and tropicalrainforests, and mathematical models ofecosystem structure and function and theenvironmental factors affecting them weredeveloped. These provided a basis for de-veloping suitable management systems toimprove productivity and environmentalquality.

International Codes of NomenclatureA set of rules for the scientific CLASSIFICA-TION of taxa. The naming of families andlower taxa is based on the first valid namepublished, the starting-point being taken asthe publication of certain authoritative

books, for example for plants, Linnaeus’sSpecies Plantarum, published in 1753. Dif-ferent starting dates and authorities applyfor different groups of organisms. But if anorganism is later assigned to a differenttaxon or rank, a different name may begiven. Thus an organism may legitimatelyhave two names, according to which genusdifferent authors place it in. If several taxaare combined into one, that taxon takes onthe name of the former TAXON with theoldest name. There are several separatecodes for different groups of organisms:The International Code of BotanicalNomenclature (ICBN), which also includesalgae, fungi, and slime molds; the Interna-tional Code of Nomenclature of CultivatedPlants (ICNCP); the International Code ofBacteriological Nomenclature; and the In-ternational Code of Zoological Nomencla-ture (ICZN).

International Geosphere–BiosphereProgram (IGBP) A program of researchand study established in 1986 by the Inter-national Council of Scientific Unions to de-scribe and understand the interactivephysical processes that regulate the totalearth system, the unique environment thatit provides for life, the changes that are oc-curring in this system, and the manner inwhich they are influenced by human ac-tions. Resources

International Union for the Conserva-tion of Nature and Natural ResourcesSee World Conservation Union.

International Whaling Commission(IWC) An international organization es-tablished in 1946, following the signing ofthe International Whaling Convention bythe major whaling nations, to sponsor sci-entific studies of whale populations andrecommend restrictions on the commercialkilling of whales to ensure the sustainabil-ity of the whale populations and hence ofthe whaling industry.

interspecific Between species (e.g. COM-PETITION between species is interspecificcompetition). Compare intraspecific.

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interstadial period A period of warmerclimate within a glacial period, usuallyshorter and cooler than an interglacial. Be-cause of the relatively short timescale,some warmth-loving plants may not mi-grate back, so will not appear in the pollenrecord. See Ice Age. Compare interglacialperiod.

interstate waters Waters that flowacross or form part of state or internationalboundaries, the Great Lakes, and coastalwaters of the United States.

interstitial air The air trapped in thespaces between particles in sediments andsoils.

interstitial fauna See meiofauna.

intertidal The part of the LITTORAL zonebetween the lowest and highest tide levels.

interval scale See measurement scales.

in the field A term that refers to workcarried out in situ in a particular habitat.

intraspecific Within a species (e.g.within the gene pool or a population of aspecies). Compare interspecific.

intrinsic rate of increase (r) The percapita rate of increase of a population thathas reached a stable age structure, in whichfecundity and survivorship are stable andwhich is not crowded and is not experienc-ing any competitive or other pressures.

introduced species A species taken byhumans to a new location in which it didnot previously exist. Such species can havea devastating effect on the native flora andfauna: introduced predators have de-stroyed flightless ground-nesting birds inNew Zealand, and introduced diseases forwhich there are no natural BIOLOGICAL

CONTROLS can decimate domestic cropsand livestock.

intron See exon.

inversion An atmospheric condition in

which cold air is trapped by a layer ofwarm air above it. This may occur, for ex-ample, in still conditions when the air atground level cools rapidly by radiation, be-coming cooler than the air above. This canlead to frost pockets occurring in valleys. Italso traps pollutants forming SMOG.

invertebrates Animals without back-bones, e.g. insects, snails, starfish, andworms.

in vitro Literally ‘in glass’; describingexperiments or techniques performed inlaboratory apparatus rather than in a liv-ing organism. Cell tissue cultures and invitro fertilization (to produce test-tube ba-bies) are examples. Compare in vivo.

in vivo Literally ‘in life’; describingprocesses that occur or tests made withinthe living organism. Compare in vitro.

iodine A MICRONUTRIENT essential in ani-mal diets mainly as a constituent of thethyroid hormones. Iodine is not essential toplant growth although it is accumulated inlarge amounts by the brown algae.

ion An atom or molecule that carries anelectric charge as a result of the loss or gainof electrons.

ion exchange The reversible exchangeof ions of similar charge between an insol-uble solid and a solution that is in contactwith it. Ion exchange is used to purifychemicals, to soften or demineralize water,to separate substances from mixtures in so-lution, and to lower potassium levels in pa-tients suffering from kidney failure. Seealso cation exchange capacity.

ionization The formation of ions by theaddition or removal of electrons fromatoms, molecules, or radicals. It is a majorroute for the transfer of energy from elec-tromagnetic radiation to matter. See alsoelectromagnetic energy; ionizing radiation.

ionizing radiation Radiation thatcauses atoms or molecules to lose elec-trons, so forming ions. There are two main

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kinds: particulate (e.g. alpha and beta radi-ation, neutrons, positrons) and electro-magnetic (e.g. gamma radiation andx-rays). Alpha particles are made up of twoneutrons and two protons and carry a 2+

charge. They do not penetrate tissues well,but produce intense tissue ionization. Betaparticles are electrons or positrons, carry-ing only a single negative charge. They canpenetrate several millimeters into tissues,but induce only moderate ionization.Gamma rays and x-rays do not carry acharge, and have no mass, so can penetratedeeper into tissues, creating moderate ion-ization. Gamma rays have higher energythan x-rays. The deleterious effects of ion-izing radiation on living tissues is related tothe high water content of tissues. The radi-ation generates highly reactive H and OHradicals that can damage DNA and pro-teins. See also radical.

ionosphere The part of the Earth’s at-mosphere that extends from about 50 kmto the outer edge of the atmosphere ataround 500 km, in which ionization of at-mospheric gases is sufficient to affect thepropagation of radio waves. The amountof ionization varies with the time of day,seasons, and solar cycle.

IPC See integrated pollution control.

IPCC See Intergovernmental Panel onClimate Change.

IPM See integrated pest management.

IRIS See Integrated Risk InformationSystem.

iron (Fe) A metallic element that makesup some 5% of the Earth’s crust. It is an es-sential nutrient for animal and plantgrowth. Iron is a constituent of the redblood pigment hemoglobin and plays animportant part in many metabolic activi-ties.

iron pan See hardpan.

irradiation The exposure of an object,organism, or chemical to IONIZING RADIA-

TION such as high-energy particles orgamma rays or x-rays.

irrigation The supplying of water orwastewater to land by artificial means toenable plants to grow.

irruption A sudden unpredictable in-crease in the size of a population, leadingto the emigration of large numbers of indi-viduals.

island biogeography theory Thepremise that the distribution of plant andanimal species on islands or areas that aresimilarly isolated, such as small patches ofnatural vegetation surrounded by culti-vated land, shows a relationship betweenisland area and species numbers, represent-ing the equilibrium between IMMIGRATION

and EXTINCTION. See isolating mechanisms.

isocline A line linking points that havethe same rates of population increase for aparticular species. For example, points thatrepresent certain combinations of re-sources or species densities.

isoenzyme (isozyme) Any of multipleforms of the same enzyme, each having dif-ferent kinetic characteristics. Isoenzymesare coded for by different ALLELES, and areusually formed by different combinationsof the same subunits. They are readily sep-arated by ELECTROPHORESIS, and can beused to determine the genetic relationshipsbetween populations and provide evidencefor GENETIC DRIFT. For example, in forestry,they can confirm or refute the identity ofseeds purporting to come from differentlocal races of trees, each adapted to specificenvironmental conditions.

isolating mechanisms Structural, phy-siological, behavioral, genetic, geo-graphical, or other factors that restrict theinterbreeding of one population with an-other, hence restricting gene flow. Thedevelopment of isolating mechanisms pro-motes the formation of new varieties andspecies. See also ecological isolation; geo-graphical isolation; reproductive isolation.

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iso-osmotic See osmosis.

isopods Members of a large order ofcrustaceans (see Crustacea) whose bodiesare flat, elongated, and covered in a seriesof wide, armorlike plates. Isopods occur inthe sea, in fresh water, and on land. Someare internal or external parasites of fishand prawns. Isopods include pill bugs (Ar-madillo and Armadillidium spp.) and sowbugs (Oniscus spp.), both of which live inleaf litter, and gribbles (Limnoria spp.),which burrow into underwater timbers.

isotherm A line on a map or chart thatconnects points on the Earth’s surface thathave the same temperature at a given time,or the same mean temperature for a givenperiod.

isotonic Describing a solution with anosmotic pressure or concentration equal tothat of a specified other solution, usuallytaken to be within a cell. It therefore nei-ther gains nor loses water by osmosis.Compare hypertonic; hypotonic.

isotope One of two or more atoms ofthe same element that differ in atomicmass, having different numbers of neu-trons. For example 16O and 18O are iso-topes of oxygen, both with eight protons,but 16O has eight neutrons and 18O has tenneutrons. A natural sample of most el-ements consists of a mixture of isotopes.Many isotopes are radioactive and can beused for labeling purposes. The isotopes ofan element differ in their physical proper-ties and can therefore be separated by tech-niques such as fractional distillation,

diffusion, and electrolysis. See also isotopicdating.

isotopic dating (radioactive dating; ra-diometric dating) A method of determin-ing the age of materials by measuring theconcentrations of certain radioactive el-ements and their decay products. It is basedon the principle that radioactive elementsdecay to other elements (which may or maynot be radioactive) at a constant rate (thedecay constant). Using the ratio of theamount of the daughter element to theradioactive parent element, the age can becalculated. Such techniques can be used todate ancient rocks, using potassium–argondating (the decay of potassium-40 toargon-40 has a half-life of 11.8 × 109 years)or rubidium–strontium dating (half-life ofrubidium-87 to strontium-87 is 48.8 × 109

years).

isotropic Describing a substance whosephysical properties (e.g. magnetic suscepti-bility or ability to refract light) do not varywith direction.

isozyme See isoenzyme.

iteroparous Describing a plant that re-produces more than once in its lifetime, forexample most perennials. Some species,such as the century plant, Agave ameri-cana, live for many years, then flower andfruit only once and die – such species aredescribed as monocarpic.

IUCN See World Conservation Union.

IWC See International Whaling Com-mission.

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J See joule.

jet streams Concentrated high-speedhigh-altitude belts of air flowing broadlyeast to west. The polar-front jet stream oc-curs at 10–13 km altitude in middle to highlatitudes, associated with the boundary be-tween polar and tropical air and the relateddepressions. It averages 60 m/sec, and ismost persistent and intense in winter, whenthe temperature gradient between the twoair bodies is greatest. The subtropical jetstream flows at 12–15 km altitude, andchanges direction from easterly in summerto westerly in winter after the monsoon. Itis associated with the sinking air massesformed as warm air above the equator risesand disperses laterally, then cools in thesubtropics. This jet stream moves closer tothe equator in winter. In the northernhemisphere there is also a tropical easterlyjet stream not found in the southern hemi-sphere, because the lack of large land-masses leads to different airflow patterns.Jet streams are so powerful that they canexert a steering influence on depressionstraveling lower in the atmosphere.

joule (J) The SI unit of measurement ofwork, energy, and heat. 1 joule is the workdone when a force of 1 newton moves its

point of application 1 m in the direction ofthe force. It is equal to 0.2392 calorie.

jungle A form of dense tropical RAIN-FOREST or MONSOON FOREST, with tangledlianas and other climbers, palms, and bam-boo scrub. Jungle is not used as a scientificterm. It is a subclimax forest, found alongriverbanks and on the sites of former clear-ings. The dense undergrowth is a result ofgreater light penetration than in climaxrainforest. See climax community.

Jurassic The middle period of the Meso-zoic era, 215–145 million years ago. Dur-ing the Jurassic dinosaurs were becominglarge and abundant and bony fishes(teleosts) were also evolving rapidly. Fos-sils of one of the earliest known birds, Ar-chaeopteryx, and also of the first mammalsare found in the late Jurassic. The climatewas warm and humid, and there was wide-spread expansion of forests, which gaverise to thick deposits of COAL. The Jurassichas been called the ‘Age of Cycads’. Theseed-ferns declined, and there was a greatincrease in the number and diversity of cycads, palmlike GYMNOSPERMS, andconifers. Fossils of monocotyledonouspollen and stems of palms suggest that the angiosperms (see Angiospermophyta)evolved during this period.

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Kampfzone See elfin forest.

karst Any region underlain by LIME-STONE having a characteristic set of barrenlandforms with bare pavements, sink-holes, smaller holes, and deep fissures dueto solution weathering of well-jointedlimestone, and underground caves andstreams. The term was originally appliedonly to the landscape of the Karst in the Di-naric Alps bordering the Adriatic Sea.

kelp Large brown seaweeds that growbelow the low-tide mark in relatively shal-low water, anchored to the seabed or torocks. Kelp are harvested for fertilizer, ei-ther pulped into a liquid manure or burnedto provide ash. The ash is a rich source ofiodine, potash, and soda.

Kelvin scale See temperature.

keranga See heath forest.

kerosene See petroleum.

key A hierarchical list of characteristicsorganized so as to enable rapid identifica-tion of a specimen. Most keys consist of asuccession of contrasting choices, eachleading to further selections until byprocess of elimination the specimen is fi-nally identified. A key is not a natural clas-sification – it is not based on phylogeneticrelationships – but simply an aid to identi-fication. An error made at any stage, butparticularly near the top of the hierarchy,can lead to serious misidentification.

A simple key, which at each stage pre-sents the user with a pair of choices, iscalled a bracketed key. In an indented key,successive choices are indented below theimmediate parent feature. This shows

more clearly the characteristics of differenttaxa, making it easier to retrace your stepsif you have made an error. Alternativelythe choices may be organized as a flow-chart.

In a multi-access key, different alterna-tives for each character of the organism areoffered, and each is given a letter or num-ber code. Having made your selections forall the specimen’s characteristics, you thenlook up the combination of letters or num-bers to find the species.

key-factor analysis A statistical treat-ment of population data that includes in-formation on births, deaths, immigration,and emigration, and sometimes also vari-ous stages of the life cycle such as pupationor hibernation, that aims to identify thefactors most likely to cause changes in pop-ulation size.

keystone species An organism thatplays a pivotal role in maintaining the sta-tus quo of its ECOSYSTEM, and especially thecomposition of its local COMMUNITY. Forexample, the removal of a keystone preda-tor from an ecosystem causes a reductionin SPECIES DIVERSITY among its former prey.Monitoring a keystone species can givevaluable information about the health ofthe ecosystem and the degree to which itsresources are being exploited or overex-ploited. See also species diversity.

kieselguhr See diatoms.

kinetic rate coefficient A number thatdescribes the rate at which a water at-tribute such as BIOLOGICAL OXYGEN DE-MAND rises or falls.

kingdom Formerly, the highest ranking

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K

category in most classification systems.However, most taxonomists today recog-nize the rank of DOMAIN as higher thankingdom. The most widely accepted classi-fication system, the FIVE KINGDOMS CLASSI-FICATION, recognizes five kingdoms, ofwhich four (BACTERIA, FUNGI, PLANTAE, andANIMALIA) are natural on phylogeneticgrounds and the fifth (the PROTOCTISTA)contains unrelated organisms that do notfit into the other four kingdoms. Two su-perkingdoms are often recognized – theProkarya (see prokaryote) and Eukarya(see eukaryote), based on fundamentallydifferent cell structure, but increasinglythese groups are being replaced by thethree domains of Bacteria, ARCHAEA, andEukarya, based on more recent molecularevidence. See phylogeny. See also cladis-tics.

kin selection The concept that differ-ences in reproductive success among lin-eages of related individuals can beexplained in terms of the propagation ofgenes identical to their own – a kind of self-interest that is served by promoting the re-production of relations that share all or alarge proportion of their genes. For exam-ple, worker bees are all daughters of thesame queen, developed from unfertilizedeggs. They thus share all their genes. Bycaring for each other, and for their egg-laying mother, they promote the produc-tion of more workers that also carry iden-tical genes to their own. Were theythemselves to mate, their offspring wouldcarry only half their genes. The sametheory has been used to explain the sharingof care of the young between related ani-mals that are not the parents, and othercommunal behavior.

Köppen climate classification systemSee climate.

Kranze anatomy The specific arrange-ment of the photosynthetic tissues in theleaves of C4 PLANTS. See photosynthesis.

Krebs cycle See respiration.

krill Shrimplike marine crustaceans (seeCrustacea).

Krummholz See elfin forest.

K selection See life-history strategy.

K species A species that responds to Kselection (see life-history strategy) and haslow birth rates, a relatively long period ofdevelopment, and high survival rates. Seealso density dependence.

kurtosis The peakedness of a statisticaldistribution, as measured against a normaldistribution. Kurtosis may be positive ornegative. The larger the kurtosis, the morepeaked the distribution, the smaller thekurtosis, the flatter the distribution.

k-value The effectiveness of a factorcausing mortality, given by the differencein numbers in the population before andafter the factor acts, expressed as log10 (be-fore) – log10 (after). This may be given forthe population as a whole or for a particu-lar stage in the life cycle.

Kyoto Protocol An international treatysigned in 1997 by which consenting gov-ernments agreed to reduce their emissionsof greenhouse gases to less than their 1990levels by the year 2010. Compare Mon-treal Protocol.

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lagoon 1. A body of shallow coastalwater with restricted access to the sea,from which it is usually separated by a reef,spit, or sand bar, or by a series of islands.2. A shallow pond used as temporary stor-age for WASTEWATER, SEWAGE SLUDGE, liq-uid waste, or spent nuclear fuel rods.3. An artificial treatment pond where thenatural decomposition of biological wasteis accelerated by the action of sunlight,bacteria, and oxygen.

lag phase The period of relatively slowgrowth shown by microbial populationswhen they are first introduced to newmedia.

Lagrangian measurements See water-flow measurements.

Lagrangian models A system of model-ing based on a moving reference point.Such models may be use to track movingbodies of air or gaseous POLLUTANTS. Sta-tistical Lagrangian models use long-termaverages of concentration and depositionfields of pollutants and of precipitation, aswell as climate data and annual frequenciesof wind direction and speed, and also takeinto account the different species of prod-ucts resulting from breakdown of the pol-lutants. Alternatively the model may bebased around a comparative study of a suc-cession of air mass trajectories arriving at areceptor at relatively short intervals (e.g.every 6 h). Lagrangian models are in starkcontrast to the more complex Eulerianmodels, in which the governing equationsare solved for each grid point on the trajec-tory at each time interval.

LAI See leaf area index.

lake A large body of water surroundedby land. It is an aquatic BIOME.

Lamarckism A largely discreditedtheory of evolution postulated in 1809 byFrench biologist Jean-Baptiste de Lamarck(1744–1829) that acquired characteristicscan be inherited, so resulting in permanentchanges in populations.

land bridge (isthmus) A piece of landconnecting two large land masses, whichpermits the migration of plants and ani-mals between the land masses. For exam-ple, the Isthmus of Panama, which wassubmerged by the sea for most of the TER-TIARY, allowing North and South Ameri-can species to evolve separately, was raisedup to become land again during thePLIOCENE, allowing mixing of the species. Itis thought that humans may have firstmoved into North America from Siberiaacross a land bridge where the Bering Seanow lies. See also geographical isolation.

land farming See land spreading.

landfill A land-based disposal facilityfor solid waste. The waste is deposited inan excavated pit sealed to prevent theleaching of contaminants into the sur-rounding soil, rocks, or water, compactedor leveled, then buried under soil. The aretwo main types: sanitary landfills, whichtake municipal waste, and hazardouswaste landfills to contain hazardous waste.

landfill gas Gas generated by the anaer-obic decomposition of solid waste in land-fill sites. It is a mixture of gases, includingodorous organic sulfur compounds, andcontains significant amounts of methane. Ifthe METHANE concentration exceeds a con-

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centration in air of 5%, it poses a risk ofexplosion, so landfill gas is usually activelyor passively vented, or siphoned off for usein COMBINED HEAT AND POWER PLANTS. Seedecomposition.

Landsat satellite See remote sensing.

landscape architecture The creation ofnew outdoor landscapes, such as gardensand parks, and other green open spaces, inorder to create a natural setting for urbanlife and to enhance the environment forhuman use and enjoyment.

landscape ecology The study of land-scapes taking into account the distributionpatterns of communities and ecosystemsand the ecological processes that affectthem over time. See ecology.

land spreading 1. The spreading of ma-nure, mulch, or fertilizer over the land.2. (land farming) The spreading of HAZ-ARDOUS WASTE onto the soil (sometimescovering it with a further layer of soil) toallow it to be naturally decomposed by mi-crobes.

land-use capability (agricultural poten-tial) The suitability of land for agri-culture. This takes into account soil type,climate, and topography, and the presenceof problems such as industrial waste sites.Land is ranked into classes according to theanticipated sustainability of crop produc-tion on the land, the degree to which is canbe cultivated, and the effort needed tobring it into cultivation. Maps showingland-use capability rankings are used byplanners. Environmental potential mapsare similar, but are intended for assessingmultiple-use locations, so other factorssuch as potential subsidence, rock founda-tions, mineral resources, and susceptibilityto flooding are also taken into account.

Langelier index (LI; saturation index;water balance index) A formula used todetermine the overall tendency of a sampleof water to be either scale-forming or cor-rosive. It takes into account pH, water tem-perature, calcium hardness, and total

alkalinity. If all these factors are in balance,the water will be neither scale-forming norcorrosive.

La Niña An irregularly occurring cli-matic perturbation characterized bybelow-normal sea surface temperatures inthe eastern and central Pacific, and higher-than-normal temperatures in the westernPacific. It causes upwelling of cold wateralong the western coast of South Americathat disrupts local and global weather pat-terns in the opposite fashion to EL NIÑO. LaNiña often follows an El Niño event.

lapse rate The rate at which tempera-ture decreases with altitude. In the TROPOS-PHERE the average lapse rate is about 6.5°Cper 1000 meters.

larva See Insecta.

latency The interval between first expo-sure to a toxic chemical and the appear-ance of an effect.

lateral Situated at or on a side, or relat-ing to a side.

laterite A product of rock decay that isred in color due to its high content of ox-ides of iron and aluminum. It may form acrust on the surface of the SOIL in tropicalregions with alternating wet and dry sea-sons. During the wet season soluble min-eral salts are washed down into the lowersoil horizons, but in the dry season evapo-ration from the surface draws the solutionback up by capillarity, and iron and alu-minum oxides accumulate, forming thecrust, a process called laterization. Such asoil is called a latosol. See red-and-yellowforest soil. Compare hardpan.

laterization See laterite.

Latin square An experimental designgrid in which the number of treatmentsequals the number of replications, so eachtreatment occurs once in every column androw. In field trials this design eliminates theeffects of differences in environmental fac-tors such as soil nutrients and water con-

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tent, which may vary up, down, and acrossthe experimental area. However, a Latinsquare is not suitable for large numbers oftreatments, because it would require im-practicably large numbers of replications.

latitude The angular distance north orsouth of the EQUATOR (latitude 0°) of apoint on the Earth’s circumference, whichis equal to the angle subtended at the cen-ter of the globe by the meridian betweenthe Equator and the point in question. To-gether with the LONGITUDE this gives thepoint’s exact location.

latosol See laterite.

Laurasia The northern continent pro-duced by the splitting of the supercontinentPANGAEA along the line of the northern At-lantic Ocean and the Tethys Sea. It in-cluded the land masses that eventuallyseparated to become North America,Greenland, Europe, Asia, and the Pacificislands east of Sulawesi.

leachate See leaching.

leaching The removal of HUMUS and SOIL

nutrients in solution by water movingdown the soil profile. It makes soils moreacid because cations (e.g. potassium andmagnesium ions) are replaced by hydrogenions. Leaching leads to the formation ofpodsolized and lateritic soils. Leaching ofnutrients, especially nitrates, from agricul-tural land can lead to serious pollution ofwater supplies. The term is also applied towater percolating through solid waste andextracting dissolved or suspended materi-als from it. In both cases the water con-taining the leached materials is called theleachate. See also laterite; podzol.

lead (Pb) A dense gray soft metallic el-ement that occurs in small quantities in awide variety of minerals, especially galena.It oxidizes readily when cut, forming a sur-face layer of lead monoxide, which pro-tects the metal from further corrosion.Lead is hazardous to health if breathed orswallowed, and most countries have legis-lation to restrict its use in paints, gasoline,

and pipework, and its use in pesticidesprays is being reduced. Emissions of leadtetraethyl from vehicle fuel is thought to betoxic, especially to children. Lead poison-ing affects particularly the nervous system,gastrointestinal tract, and blood-formingtissues, and it is believed to affect brain de-velopment in children.

lead tetraethyl See lead.

leaf A flattened appendage of a plantstem that arises as a superficial outgrowthfrom the apical meristem. Leaves arearranged in a definite pattern, have buds inthe angle between leaf and stem, and showlimited growth. It is the main photosynthe-sizing organ of a plant.

leaf area index (LAI) The ratio of thetotal surface area of a plant’s leaves to thearea of ground available to that plant:

LAI = leaf area/ground area.The LAI is a useful guide when assessingthe density of plants that can be success-fully cultivated on a given area of land.

leaf fall The shedding of leaves as a re-sult of the formation of an abscission zoneat the base of the leaf stalk (see abscission).In both evergreen and deciduous trees oldleaves are continually shed throughout thegrowing season, but in deciduous speciesthe remaining leaves are also shed at theonset of winter or a dry season.

Leakey, Richard Erskine (1944– )Kenyan anthropologist and conservation-ist. Richard Leakey was born in Nairobi,the son of the famous anthropologistsLouis and Mary Leakey. Having left schoolat sixteen, he first worked as a hunter andanimal collector before turning in 1964 tothe search for fossil man. In the 1960s and1970s, Leakey did significant anthropolog-ical research at Lake Turkana in northernKenya. In the 1980s, following a period ofill-health, he changed his career and de-voted much of his time to conservation. Inparticular, Leakey has been a leading fig-ure in the fight to preserve the African el-ephant by banning the trade in ivory.

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lean burn A combustion engine technol-ogy that reduces unwanted exhaust emis-sions by burning unusually weak mixturesof gasoline and air. It achieves this by hav-ing a pool of enriched mixture close to thesparking plugs, which enables the plugs toignite the fuel; ignition then spreads to theweaker remainder.

least significant difference In analysisof variance, a statistical test of the degreeto which the means of two samples or pop-ulations differ, used once the analysis ofvariance has already shown that they dodiffer. A level of significance is selected, forexample 5%. In this case, the least signifi-cant difference at the 5% level is the differ-ence between two samples or means thatwould be exceeded only 1 in 20 timesunder random sampling conditions. Seealso t-test.

leghemoglobin A protein found in theROOT NODULES of leguminous plants(Fabaceae) that contain the nitrogen-fixingbacterium Rhizobium. It is coded for by aplant gene, but is synthesized only in thepresence of the bacteria. It is thought totransport oxygen to the bacteria in a waythat avoids destruction of the nitrogenase(nitrogen-fixing) enzyme, which is sensitiveto oxygen. See nitrogen fixation.

Legionella A genus of rod-shaped bac-teria (bacilli) that include L. pneumophila,the causative organism of Legionnaires’disease. This is a pneumonia-like infectionthat may prove fatal to vulnerable peoplesuch as the elderly. It appears to spread byinhaling contaminated droplets of air fromair conditioning systems, the bacteriummultiplying in the condensing towers of thecooling system.

legume Any plant of the familyFabaceae, e.g. peas, beans, etc., alsoknown as pulses.

lek A territory held by males of certainanimal species during the breeding seasonfor the purpose of displaying to the femalesand attracting them to mate. It is defendedagainst rival males. In some species the lo-

cation of the territory may be almost as im-portant as the display itself in attracting theattention of a potential mate, dominantmales holding leks in the center of thedisplay ground. Examples of such speciesinclude sage grouse (Centrocercus uro-phasianus), dugongs (Dugong dugon), anddung flies (family Scatophagidae). See ter-ritoriality.

leptokurtic A distribution of values thatis symmetrical, similar in shape to a NOR-MAL DISTRIBUTION, but which has a muchhigher central peak and consequently flat-ter tails on either side.

lethal dose The dose of a toxic sub-stance that will kill 50% of test organismswithin a designated period of time.

leukemia A cancer of the blood-formingorgans in humans and other warm-blooded animals, characterized by a largerise in the number of white blood cells inthe tissues, and often also in the blood. Thedisease is due to damage to the genetic ma-terial. Some incidents can be related to highdoses of radiation.

Lewis–Leslie matrix A simple matrixmodel used to predict the structure of aPOPULATION through time. It consists of amatrix of values of fecundity and survivor-ship for individuals of each age.

LI See Langelier index.

liana A long-stemmed woody climbingplant that grows from ground level to theCANOPY of trees, and often hangs downfreely from branches, e.g. curare (Strych-nos toxifera). Lianas may have stems up to100 m long, and may climb over thecanopy to gain maximum sunlight. Manyflower only when they reach the top of thecanopy. Up to a quarter of all the woodygrowth in some RAINFORESTS may be lianas.They often bind the forest trees together,and if one tree falls or is felled, it is held upby the others until it rots away. A temper-ate example is old man’s beard (Clematisvitalba).

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lichens Symbiotic associations betweenan alga or cyanobacterium (the photo-biont) and a fungus (the mycobiont). Theyare slow-growing but can colonize areastoo inhospitable for other plants.

There are three main forms of lichens:crustose lichens have relatively simple, usu-ally unbranched thalli and grow closely at-tached to the substrate; foliose lichens areoften only loosely attached, and have lobed‘leafy’ thalli; fruticose lichens are attachedat only one point, and may be upright andbushy or dangling and tassel-shaped. Somelichens have an intermediate form in whicha crustlike thallus gives rise to uprightbranched structures.

Certain lichens are important pioneersof newly exposed areas, and can tolerateextremes of desiccation. Their metabolicproducts contribute to rock disintegration.Many lichens grow extremely slowly,maybe only 1 mm a year, so large lichensmay be hundreds or even thousands ofyears old. Some species are tolerant or in-tolerant of POLLUTION, and are valuable IN-DICATOR SPECIES.

Liebig’s law of the minimum Theconcept, first proposed in 1840 by Justusvon Liebig, that the health and growth rateof a plant and the size it reaches depend onthe availability of the scarcest of its essen-tial nutrients. The modern version of this isthe concept of LIMITING FACTORS.

life cycle The sequence of developmen-tal changes making up the span of an or-ganism’s life from the fertilization ofgametes to the same stage in the subse-quent generation. The cycle may involveonly one form of the organism, as in higheranimals and plants, involving a simple pro-gression from the formation of the ZYGOTE

to the release of GAMETES by the same in-dividual (haplobiontic). In most plants,many algae, and some animals there is asuccession of individuals in the life cycle,linked by sexual or asexual reproduction,usually an alternation between a HAPLOID

and a DIPLOID generation, as in ferns. Thistype of life cycle is called diplobiontic. Indifferent species the haploid or the diploidstage may the dominant form. Haploid and

diploid generations may or may not differmorphologically. See alternation of gener-ations. See also polymorphism.

life form The overall morphology,habits, and lifestyle of an organism. A de-scription of the vegetation of an area usu-ally includes reference to life forms – trees,shrubs, succulents, annuals, etc. Differentlife forms are associated with differenttypes of environment. See Raunkiaer’s life-form classification.

life history All the changes an organismgoes through from fertilization until death,including its pattern of growth and repro-duction.

life-history strategy The idea that SE-LECTION PRESSURE acts on an organism’s lifehistory – in particular the number of youngit produces, their rate of development,amount of parental care, and survival rates– to adapt it for survival in either a stablepopulation near the carrying capacity of itsenvironment or an unstable population inan unpredictable or short-lived environ-ment. There are two contrasting strategies– in K-selection, individuals produce smallnumbers of young that require consider-able parental care or investment and take arelatively long time to develop but have ahigh survival rate, while in r-selection largenumbers of young are produced, which aresmaller at birth/hatching, and which de-velop rapidly to sexual maturity but havelow survival rates. r-selection provides forrapid expansion of the population underfavorable conditions, while K-selectionmakes it less likely that a population willoutstrip its resources under relatively sta-ble conditions.

life table A summary of the survivalrates of individuals in a population at dif-ferent ages or at different stages in the lifecycle. In a cohort life table or dynamic lifetable a group of individuals born withinthe same short time interval (i.e. a COHORT)is followed from birth to death of the lastsurvivor. This type of life table is suitablefor ANNUAL species in which there is littleor no overlap of generations. It is also suit-

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able for sessile animals and for organismsin a restricted area such as an island, wheredispersal is limited and it is possible tosample marked individuals throughouttheir lifetimes.

For more mobile animals and popula-tions containing several generations, a lifetable can be constructed from knowledgeof the survival of individuals of known ageover a single time interval. Such a time-specific, vertical, or static life table requiresa knowledge of the ages of individuals, andalso makes the assumption that the sur-vivorship rates and the numbers in each co-hort remain the same from year to year,and that birth rates, death rates, and popu-lation size are static. In a dynamic-compos-ite life table, instead of a true cohort, acomposite number of the animals markedover a period of years is used and the dataare pooled. This method again makes thesame assumption as the static life table.

life zone A zone in which the flora andfauna are typical of a particular latitude orrange of altitude. These zones reflect envi-ronmental gradients. The term is mostcommonly used in relation to relativelylocal changes such as the zonation of com-munities on a mountainside. Comparebiome.

light Electromagnetic radiation in thevisible spectrum, i.e. with wavelengthsranging from 400 nm (extreme violet) to770 nm (extreme red). The wavelength oflight is the distance from one peak of theelectromagnetic wave to the next. Thespectrum of light is the span of wave-lengths visible to the human eye. Whitelight contains light of all wavelengths. Ob-jects appear colored because they reflectcertain wavelengths of light, but not oth-ers. While light may be considered aswaves of electromagnetic energy, it mayalso be viewed as discrete packets of energythat carry the electromagnetic field, calledphotons. A photon has no charge or mass.Its energy depends on the wavelength ofthe light, shorter wavelengths havinghigher energy. All photons travel at thespeed of light, about 299,492 kps per sec-ond. When light is reflected or refracted by

certain substances, it is separated into twoor more components, such that the waveaction of each is concentrated in a differentplane. This is called polarized light. Seeelectromagnetic energy.

light attenuation instrument (transmis-someter) A device that measures the at-tenuation of light in water due tosuspended particles. The degree of attenu-ation of light is related to the concentrationof suspended materials, but not to thenumber of suspended particles: a singlelarge particle might produce the same at-tentuation as hundreds of smaller ones.

light bottle See dark bottle.

light compensation point See compen-sation point.

light meter An instrument for measur-ing the intensity of incident or reflectedlight, consisting of a photosensitive celland a milliammeter or millivoltmeter cali-brated in light levels.

lightning A discharge of electricity be-tween clouds and the earth, which pro-duces a brilliant flash and the sound ofthunder. It occurs when electrostaticcharges build up on the undersurface ofclouds. See nitrogen cycle.

light pollution The emission of lightinto the night sky by street lights and otherartificial lighting, which produces a glowin the sky and reduces the visibility of thestars.

light reaction See photosynthesis.

light saturation point See compensa-tion point.

lignin See wood.

limestone A SEDIMENTARY ROCK con-taining at least 50% by weight of calciumcarbonate, and sometimes also magnesiumcarbonate. It may be organic, having beenformed from the calcareous skeletal re-mains of living organisms, such as corals or

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algae; formed from the accumulated frag-ments of earlier limestones; or derived bysolution and precipitation of preexistinglimestones.

limestone pavement A large area oflimestone that has been weathered intoblocks (clints) by solution processes thathave carved out a series of deep, smooth-sided clefts (grikes). The grikes have a char-acteristic flora that often includes rarespecies.

limestone scrubbing The use of LIME-STONE to remove sulfur and other pollu-tants from waste gases.

lime-sulfur spray An orange liquidmade from elemental sulfur combined withcalcium hydroxide and water, which isused to kill various mites and to controlpeach leaf curl, scab, and powderymildews.

liming See flocculation.

limiting factor Any factor in the envi-ronment that governs the behavior ormetabolic activity of an organism or sys-tem by being above or below a certainlevel. In general, a number of different fac-tors (e.g. temperature, light intensity, car-bon dioxide concentration, availability ofprey) may be limiting, but at any one timethe factor that is closest to its critical mini-mum is the limiting factor. For instance,the rate of photosynthesis rises with in-creasing light intensity so long as there issufficient carbon dioxide available, but athigh light intensities carbon dioxide maybecome the limiting factor instead.

limiting resource A resource present ininsufficient quantity to the extent thatgrowth or reproduction of individuals in apopulation is restricted. The term is usuallygiven to the RESOURCE that is having thegreatest restrictive effect at any one time.Compare limiting factor.

limnetic Relating to deep fresh water.

limnology The scientific study of fresh-

water and its flora and fauna, and thechemical and physical properties of inlandwaters.

lindane An organochlorine PESTICIDE

used as an INSECTICIDE and wood preserva-tive. Its use is banned in many countries,because it is harmful to bees and to fresh-water and marine aquatic life, and is alsotoxic to humans. See organochlorines.

line of best fit See graph.

link See food web.

linkage The occurrence of GENES to-gether on the same CHROMOSOME so thatthey tend to be inherited together and notindependently. Groups of linked genes aretermed linkage groups and the number oflinkage groups of a particular organism isequal to its HAPLOID chromosome number.Linkage groups can be broken up by cross-ing over at MEIOSIS to give new combina-tions of genes. Two genes close together ona chromosome are more strongly linked,i.e. there is less chance of a crossover be-tween them, than two genes farther aparton the chromosome. Linkage is indicatedwhen the associated inheritance of two ormore non-allelic genes is greater thanwould be expected from INDEPENDENT AS-SORTMENT. The genes on a single chromo-some form one linkage map. See allele;gene; independent assortment.

linkage density See food web.

lipid Any of various fatty or oily sub-stances found in plants and animals. Lipidsare classified into simple lipids and com-pound lipids. The simple lipids includeglycerides (acylglycerols), and the waxes.Compound lipids include the phospho-lipids (phosphoglycerides or phospha-tides).

Simple lipids include plant pigmentssuch as carotene and xanthophylls, andalso more complex substances such as nat-ural rubber. Glycerides are important stor-age lipids in seeds, especially in suchspecies as the castor oil bean (Ricinis com-munis). Lipids are also important energy

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storage compounds in animals, forming‘fat’, which may also help insulate againstthe cold. Waxes coat plant surfaces, help-ing to provide a waterproof layer and me-chanical barrier.

liter A unit of volume equal to 10 cubicdecimeters or 0.264172 US gallon.

lithology The characteristics of a rockformation: its mineral composition, grainsize and distribution, and bedding. Theterm is usually applied to visible features.

lithosere A succession of plant commu-nities that start on a bare rock surface. Seesere.

lithosphere The solid outer part of theEarth, including its crustal plates (see crust;plate tectonics) and the brittle uppermostpart of the mantle. It thickness varies fromabout 1–2 km at the crests of the MID-OCEANIC RIDGES to 140 km under olderparts of the oceanic crust, and up to 300km in parts of the continental crust.

lithotroph An organism that derives itsenergy from the oxidation of inorganiccompounds or elements.

litmus A dye obtained from certainlichens (Lecanora tartarea and Roccellatinctorum). In acids blue litmus turns redand in alkalis red litmus turns blue. Litmuspaper is prepared by soaking absorbentpaper in litmus solution and then drying it.Litmus solution changes over a pH range4.5–8.3.

litter Dead plant remains on the soil sur-face.

Little Ice Age The period betweenabout 1550 and 1860 during which therewas a worldwide expansion of glaciers andthe climate in mid-latitudes became tem-porarily colder.

littoral 1. The zone of the seashore be-tween the high and low tide mark. Theterm is also applied to organisms living inthis zone. Since tidal ranges vary continu-

ally, the zone is often defined in terms ofthe upper and lower limits of certainspecies of organism. Compare benthiczone; sublittoral.2. The zone between the water’s edge anda depth of about 6 m in a pond or lake,where light reaches the bottom sediments.Rooted HYDROPHYTES, both emergent andsubmergent, are found in this zone. Com-pare profundal; sublittoral.

liverworts See Hepatophyta.

livestock manure See manure.

living fossil A modern organism withanatomical or physiological features foundelsewhere only in extinct species. Examplesinclude the deepsea fish the coelacanth(Latimeria chalumnae), and the maiden-hair tree (Ginkgo biloba), discovered inJapan in the 17th century, and later inChina, but found only in cultivation.

llanos SAVANNA GRASSLANDS of theOrinoco river basin in Venezuela andnortheast Colombia. It is a mainly treelessarea covered in swamp grasses and sedgeswith just a few scrub oaks and dwarfpalms, trees being concentrated mainlyalong the rivers and in areas of higher alti-tude. Cattle ranching is an important localindustry, together with oil extraction.

loam A medium-textured SOIL contain-ing a mixture of large and small mineralparticles. Loams are easy soils to work andcombine the good properties of sandy andclay soils.

local diversity See species diversity.

local guild See guild.

local mate competition Competitionbetween males for mates that takes placeclose to their place of birth, resulting in fre-quent mating with close relatives.

local population See subpopulation.

local stability The tendency for a COM-

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MUNITY to return to its original state whensubjected to a small perturbation.

locus (pl. loci) The position of a GENE ona CHROMOSOME. ALLELES of the same geneoccupy the equivalent locus on HOMOLO-GOUS CHROMOSOMES. See linkage.

loess A fine-textured uniform SOIL com-prising mainly quartz particles about0.015–0.005 mm in diameter. Widespreadin central Europe, southern Russia, China,the central USA, and Argentina, it is de-rived from wind-blown clay and silt parti-cles originally deposited at the edge of theice sheets at the end of the last Ice Age.Loess is a fertile soil, often calcareous, andis widely used for growing wheat and othercrops.

logging The felling of trees, usually forcommercial purposes.

logistic equation (logistic model) Amathematical expression of the S-shapedgrowth curve of a single-species populationin which there are no discrete generations,which shows the response of populationgrowth to competition in a confined spacewith limited resources:

dN/dt = rN(N – K)/Kwhere N is the number of individuals in thepopulation, t is time, r is the BIOTIC POTEN-TIAL of the organism concerned, and K isthe carrying capacity (saturation value) forthat organism in the environment.

logistic model See logistic equation.

lognormal distribution See distribu-tion.

LOI See loss on ignition.

long-day plant See photoperiodism.

longitude The angular distance of apoint east or west of the Greenwich merid-ian, an imaginary line running from theNorth Pole to the South Pole at right anglesto the Equator that passes through theLondon suburb of Greenwich in the UK,which represents 0° of longitude. It is ex-

pressed as the number of degrees east orwest of this meridian, which is equal to theangle subtended at the center of the globebetween the Greenwich Meridian and thepoint in question. Together with the LATI-TUDE this gives the point’s exact location.

longshore drift The lateral movementof sand and shingle along a shoreline dueto wave action.

Longworth trap A trap used to catchsmall mammals without injuring them. Itconsists of a metal box in which foodand/or nesting material are placed, ap-proached by a narrower tunnel. On enter-ing, the mammal trips a door that dropsbehind it.

loss on ignition (LOI) A technique formeasuring the organic content of soil andsediment samples based on the loss inweight of the sample as the organic matteris completely burned off.

Lotka–Volterra model A model of thedynamics of predator–prey or herbivore–plant systems. Fluctuations in the preypopulation are described by

dN/dt = rN – aPNwhere N is the number of prey, r a con-stant, P the number of predators, t is time,and a the searching efficiency or attackrate. Fluctuations in the predator popula-tion are described by

dP/dt = fa PN – qPf being the predator’s efficiency at turningfood into predator offspring and q anotherconstant. The model shows a tendency forpredator–prey populations to undergo os-cillations, in which increasing predatorpopulation leads to a decrease in prey pop-ulation. This leads to a decrease in preda-tor numbers, allowing prey to build upagain, and so on.

Lovelock, James Ephraim (1919– )British scientist. Lovelock had developed asensitive electron-capture detector and, inthe summer of 1966, working as an inde-pendent scientist, he used it to monitor thesupposedly clean Atlantic air blowing ontothe west coast of Ireland. Here he detected

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the presence of chlorofluorocarbons(CFCS). Although unable to pursue the mat-ter further at the time through lack of fund-ing, Lovelock did further work in theAntarctic in 1971, where, again, he foundatmospheric CFCs. It was partly as a con-sequence of this work that ROWLAND beganto ponder their role in the atmosphere. It is,however, as the author of the GAIA HY-POTHESIS, first presented in his Gaia (Lon-don, 1979), and developed further inseveral sequels, that Lovelock is bestknown.

low-emissivity windows Windows con-structed to inhibit transmission of radiantheat while allowing sunlight to passthrough, thus reducing energy loss frombuildings.

lowest achievable emission rate Underthe CLEAN AIR ACT, an emission rate that re-flects either the most stringent emissionlimitation contained in the implementationplan of any state for such source (unless the

owner or operator of the proposed sourcedemonstrates such limitations are notachievable) or the most stringent emissionslimitation achieved in practice, whicheveris the more stringent. In any case, applica-tion of this term does not permit a pro-posed new or modified source to emitpollutants in excess of existing new sourcestandards.

low-level radioactive waste See radio-active waste.

low-NOx burner A combustion tech-nology that reduces emissions of nitrogenoxides.

Luca See origin of life.

luxury consumption A means of ex-ploiting short-term abundance of a nutri-ent by consuming and storing more than isneeded for growth.

Lydekker’s line See Wallacea.

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Lydekker’s line

macchia See maquis.

MacArthur, Robert Helmer (1930–72) American ecologist. MacArthur madea number of contributions to ecology. Hispaper Population Ecology of some War-blers of Northeastern Coniferous Forests(1958) described his investigations intohow five closely related species of warblercould spend the spring in the spruce forestsof New England apparently feeding on thesame diet and occupying the same niche.MacArthur managed to establish that thewarblers tended to occupy and hunt in dif-ferent parts of the tree, so preserving thecurrent dogma – that in equilibrium com-munities no two species occupy the sameniche. At a more theoretical levelMacArthur produced the broken stickmodel (see random niche model), which heused to predict the relative abundance ofspecies in a particular ecosystem. With Ed-ward Wilson he published Theory of Is-land Biogeography (1967). MacArthurand Wilson were struck by the apparentstability of the number of species found onislands. They assumed immigration andextinction to be the major forces operatingand further assumed that these twoprocesses are in equilibrium. This allowedthem to predict that smaller islands shouldhave fewer species than large ones, as toowould distant islands over those nearer tothe mainland. In the same 1967 volumeMacArthur and Wilson clearly formulateda distinction that was emerging in a varietyof forms in the writings of ecologists, thatof r- and K-strategists. r-strategists are op-portunistic species living in a variable envi-ronment; they typically have highreproductive rates, heavy mortality, shortlives, and rapid development. K-strategistson the other hand are larger, develop more

slowly, and are in general more stable. In1962 MacArthur succeeded in showingthat the basic theorems of natural selectionapply to both r- and K-species.

macrobiota The larger soil organisms,especially tree roots and vertebrates suchas moles. Compare macrofauna; meio-fauna; mesobiota; mesofauna; microfauna.

macroevolution EVOLUTION on a scaleabove the species level. It includes the evo-lution of new genera, families, orders,classes, and phyla. Compare microevolu-tion.

macrofauna The larger animals of a soilcommunity, usually invertebrates of bodywidth approximately 2–20 mm, but ex-cluding larger animals such as earthworms.Compare macrobiota; meiofauna; meso-biota; mesofauna; microfauna.

macroflora The larger plant structuresin the soil, such as tree roots. Compare mi-croflora.

macromolecule A very large molecule,usually a polymer, having a very high mo-lecular weight (with 10,000 or moreatoms). Proteins and nucleic acids are nat-urally occurring examples.

macronutrient A nutrient required inmore than trace amounts by an organism.It may be an organic or inorganic com-pound. Compare micronutrient.

macroparasite See parasitism.

MACT See maximum available controltechnology.

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magma See plate tectonics.

magnesium An element essential forplant and animal growth. It is contained inthe CHLOROPHYLL molecule and is thus es-sential for photosynthesis. Deficiency ofmagnesium may result in CHLOROSIS,stunted growth, and in some specieswhitening and puckering of the leaf edges.It may be remedied by adding magnesiumsulfate or magnesium oxide to the soils. Inanimals magnesium is found in bones andteeth. As magnesium carbonate it is foundin large quantities in the skeletons of cer-tain marine organisms, and is present insmaller quantities in the muscles andnerves of higher animals.

magnesium sulfate A colorless crys-talline solid that occurs naturally in manysalt deposits in combination with otherminerals. It is used as a source of magne-sium in fertilizers.

magnetic north The direction in whicha compass needle points, as opposed totrue north (the North Pole). It is the north-ernmost pole of the Earth’s magnetic field,the location of which may vary over geo-logical time. Compare true north.

mainland-island metapopulation Seemetapopulation.

maintenance threshold (maintenancerate) The level or rate of consumptionbelow which a CONSUMER cannot survive.

malathion An ORGANOPHOSPHATE insec-ticide used especially to kill aphids, mites,flies, and mosquitoes.

male sterility 1. A condition in plants inwhich pollen production is prevented bythe mutation of one or more of the genesinvolved. Male sterility is used by plantbreeders to ensure cross-pollination takesplace. It is also used in GENETIC ENGINEER-ING to ensure that genetically modified va-rieties do not release pollen containingmodified genes.2. The release of artificially sterilized maleanimals into a wild population with the

aim of reducing the breeding rate of thepopulation as a form of BIOLOGICAL CON-TROL, for example in the control of mos-quitoes.

mallee See maquis.

Malthusian theory The theory, pro-posed by Thomas Malthus (1766–1834),that population growth will always con-tinue to the limit of subsistence, where re-sources become exhausted, and that theonly way to avoid famine, war, and illhealth is to impose strict reproductive con-trols.

Mammalia The class of vertebrates thatcontains the most successful tetrapods.They are homoiothermic, with an insulat-ing body covering of hair and usually withsweat and sebaceous glands in the skin.The socketed teeth are differentiated intoincisors, canines, and grinding premolarsand molars. Mammals have a relativelylarge brain and an external ear (pinna), andthree auditory ossicles in the middle ear.Oxygenated and deoxygenated blood areseparated in the four-chambered heart anda diaphragm assists in respiratory move-ments. Typically, the young are born aliveand are suckled on milk secreted by themammary glands. A bony secondary palateallows the retention of food in the mouthwhile breathing. Mammals evolved fromactive carnivorous reptiles in the Triassic.There are two subclasses: Prototheria,which comprises a single order (Mono-tremata) containing all the egg-layingmammals; and Theria, which contains allthe mammals that bear live young. Sub-class Theria is divided into two infra-classes: Metatheria, which comprises themarsupials (pouched mammals); and Eu-theria, which contains the placental mam-mals.

managed ecosystem An ecosystem inwhich regular human intervention is usedto achieve sustainable or high yields ofcrops or game.

Mandibulata See Arthropoda.

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manganese A metallic trace element re-quired by plants and animals. Manganeseis involved in certain metabolic processes.In plants, deficiency of manganese maycause dwarfing, mottling of the upperleaves, CHLOROSIS between the leaf veins,blight of sugar cane, and gray speck ofoats. In animals, it is needed for bone for-mation.

mangrove Any of a range of trees andshrubs that form forests and dense thicketsalong muddy coast and estuaries and in saltmarshes, some of which have prop roots(see aerial root). Adventitious roots archdownward to the mud and send up newtrunks. The prop roots trap sediment,building up the land and aiding SUCCES-SION. Many mangrove species have pneu-matophores, or breathing knees, that riseup out of the mud. They have abundantlenticels, through which air diffuses to sup-ply the waterlogged roots below the mud.Mangrove seeds often germinate while stillin the fruit attached to the parent plant,putting down a long root toward the mud.When the seed drops, it spears its way intothe mud. Mangroves thus act as pioneerspecies, colonizing bare mud. The wood ofsome species is exploited locally for itsbark, which can be used for tanning, and asa source of wood chips. In some parts ofthe world, valuable mangrove forest barri-ers that protect the coast are being felledfor timber. See also halophyte; succession.

man-made mineral fibers (MMMF)Manufactured inorganic fibers such as as-bestos, glass wool, or fused rock productsused in thermal and acoustic insulation,and certain ceramic fibers. Some of thesefibers, such as asbestos, pose human healthrisks because they are fine enough to lodgeand persist in the lungs. See asbestos.

mantle 1. The part of the Earth that liesbetween the CRUST and the central CORE. Itoccupies some 84% of the planet’s volumeand accounts for 68% of its mass. It rangesfrom 40 to 2900 km in thickness, with anaverage of about 2300 km. The outer partof the mantle consists of solid rock, but therock becomes more plastic and mobile as

pressure and temperature increase withdepth.2. See Mollusca.

manure An organic FERTILIZER based onanimal excreta, often mixed with straw.The term is also used to mean fertilizers ingeneral. The term livestock manure issometimes used to refer to manure derivedfrom domestic animals. Compare compost.

map A graphical representation of theEarth’s surface on a plane surface. It mayinclude location and distance guides suchas projections, grids, and scales. Informa-tion is often displayed in the form of sym-bols. The commonest maps show thesurface TOPOGRAPHY, elevations beingshown as bands or shades of color, CON-TOUR LINES, or spot heights.

Maps that depict the geology of an areacome in two forms: solid editions show theunderlying rock types supposing the layerof superficial drift, such as glacial deposits,alluvium or peat, have been stripped away.Drift editions show predominantly the su-perficial deposits, the bedrock being shownonly where it lies uncovered at the surface.Soil survey maps show the distribution ofsoil types according to various classifica-tions, and are usually on a similar scale tomaps of the local geology. Land-use capa-bility maps (agricultural potential maps)show the suitability of the soils for agricul-ture (see land use capability). For multi-usepurposes environmental potential mapsmay be useful.

map measurer A device for measuringdistances to scale on a map, which involvesrunning a knurled wheel along the pro-posed route or unknown distance.

maquis A broad-leaved evergreen forestcharacteristic of a Mediterranean-type cli-mate consisting of scrub and found onpoor soils in regions with a pronounceddry season. It is made up of thickets ofevergreen shrubs with leathery leaves orspiny bushes and shrubs up to about 3 mtall, e.g. gorse (Ulex), broom (Cytisus,Genista), laurels (Laurus), and members ofthe Ericaceae (heath family). These plants

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are interspersed with aromatic herbs andsmaller shrubs, such as thyme (Thymus),Cistus, and myrtle (Myrtus), and scatteredsmall trees such as olive trees (Olea eu-ropaea) and fig trees (Ficus). In many partsof the Mediterranean it is a plagioclimax,the result of burning and grazing of theoriginal evergreen Mediterranean forest.Similar vegetation in California is calledchaparral. In Italy, it is called maccia, inSpain, mattoral, in Australia, mallee, andin the Cape Province of South Africa, fyn-bos. See also garrigue.

mariculture The cultivation of marineorganisms, such as prawns and oysters, intheir natural habitats.

marine Relating to or living in the sea.

maritime climate A CLIMATE modifiedby proximity to the sea. Because waterwarms and cools slower than land, prox-imity to the sea or prevailing onshorewinds has a moderating influence on cli-mate, bringing warmer winters and coolersummers, as well as increased precipitationowing to the moisture picked up as windstravel over the oceans.

mark-recapture method A techniquefor estimating the population size of mo-bile or elusive animals based on the recap-ture rate of marked individuals. Samples ofthe population are trapped and marked atintervals, then released. From the assump-tion the marked individuals become ran-domly distributed among the wildpopulation, and that subsequent trappingis random, subsequent samples represent aspecific proportion of the total marked tounmarked individuals, and from this thetotal size of the population can be esti-mated:

S1 × S2/S2Mwhere S1 and S2 are the numbers of indi-viduals in the first and second samples re-spectively, and S2M is the number ofmarked individuals in the second sample.

marl A soil containing a high proportionof calcium carbonate, as in limestoneareas. It is usually an alluvial deposit.

marram grass A grass (Ammophila are-naria) planted on sand dunes to stabilizethem. It is capable of rapid growth to com-pensate for burial by sand, and its rhizomeshelp bind the loose sand.

marsh An area of more or less perma-nently wet ground with no surface accu-mulation of peat, usually dominated byherbaceous vegetation, especially rushes,reeds, and sedges. It may be freshwater,saltwater, tidal, or nontidal.

marsh gas The gas METHANE producedby the anaerobic bacterial decompositionof organic matter under water.

marsupial See Mammalia.

mass burn incineration See incinera-tion.

mass extinction See extinction.

masting The production by trees andshrubs of especially large quantities ofseeds only in certain years, which are calledmast years.

mast years See masting.

mathematical model A formulation ofa quantitative hypothesis that enables pre-diction of the behavior of a system and canbe tested by experiment.

mating strain See mating type.

mating system See breeding system.

mating type (mating strain) The equiv-alent of gender in lower organisms, espe-cially microorganisms and fungi, wherethere are no obvious morphological differ-ences between the sexes. Mating types areidentified by their behavior (one donatesmaterial to the other) or physiology, andare often identified as plus (+) and minus(–) strains. Only individuals of differentmating type will mate together.

matric potential (ym) The componentof water potential (see osmosis) that is due

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to capillary and imbibitional forces. Matricpotential can be significant in cell walls andintercellular spaces, in dry seeds or in dryclay soils, and decreases the movement ofwater.

matrix (pl. matrices) Any medium inwhich something is embedded, e.g. thestroma in a CHLOROPLAST, and the pecticsubstance in which cellulose microfibrilsare embedded in the plant cell wall.

mattoral See maquis.

maturation The process of becomingmature or fully developed. For example, ananimal developing full adult characteristicsor a fruit becoming ready to be dispersed.

maximum and minimum thermome-ter A thermometer that records themaximum and minimum temperatures ob-tained in a given time period. It consists ofa U-shaped tube, closed at one end, con-taining an indicator liquid, with a tem-perature scale behind each arm. Astemperature falls, the air in the closed sideof the tube contracts, and liquid rises upthat arm, pushing up a small metal indexbar. Expansion of the air when the temper-ature rises forces the liquid down on thatside and up on the free side, pushing up an-other index bar. The two index bars indi-cate the extremes of temperature. Thethermometer is reset by drawing the indexbars down to the liquid using a small mag-net.

maximum available control technol-ogy (MACT) The best available controltechnology for reducing polluting emis-sions, taking into account both technicalfeasibility and cost. Under the UnitedStates CLEAN AIR ACT, this must achieve notless than the average emission levelachieved by controls on the best perform-ing 12% of existing sources of similar pol-lution.

maximum sustainable yield (MSY)The maximum yield (number of individu-als that can be harvested) that can be re-moved repeatedly from a population

without reducing its numbers and drivingit toward extinction, i.e. the maximumyield at which removal can be balanced byrecruitment.

May, Robert McCredie (1936– )Australian–American theoretical ecologist.Whereas an earlier generation of theoreti-cal physicists had become interested in bi-ology through their desire to understandthe molecules of life, May and his genera-tion were more attracted to the problem ofunderstanding the abundance and distribu-tion of species. Greatly influenced by thepioneering work of Robert MACARTHUR,they could see tempting analogies betweenthe flow of energy in a physical system andthe structure and growth of an ecosystem.May has worked on a number of detailedproblems on the population dynamics ofvarious species and is noted for his influen-tial Theoretical Ecology (1976).

meadow An area of moist GRASSLAND

representing a subclimax maintained by mowing, traditionally for hay in mid-summer. See climax community.

mean A measure of the average value ofa set of numbers. The arithmetic mean isthe sum of a set of values divided by thenumber of values. For example, the arith-metic mean of the three values 3, 4, and 5is (3 + 4 + 5)/3 = 4. The geometric mean isthe nth root of the product of a set of nnumbers. For example, the geometric meanof 16 and 4 is the square root of 16 × 4 =8. The geometric mean of 2, 4, and 27 isthe cube root of 2 × 4 × 27 = 6.

mean sea level See sea level.

measurement scales There are manydifferent scales for measuring variables.For qualitative data, a nominal scale classi-fies objects into categories based solely ondescriptive features, for example, animal,vegetable, and mineral. There is no logicalorder. An ordinal scale classifies by rank,assigning a numerical value to a descriptivefeature, for example, cold, cool, warm, orhot might be marked 1 to 4 on an ordinaltemperature scale. For quantitative vari-

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ables, an interval scale is used, the numbersrelating to values that differ by a fixed,equal value, for example the degrees on ascale of temperature. Such scales may re-late values to an arbitrary zero point, e.g.the Celsius or Fahrenheit scales. Where thezero point represents an absence of the fea-ture being measured (i.e. it is an absolutezero), such a scale is called a ratio scale,e.g. the Kelvin scale of temperature.

Mediterranean climate A CLIMATE ofmiddle latitudes characterized by warm,dry summers and cool, wet winters.

megafauna Animals large enough to beseen with the naked eye. Compare micro-fauna.

meiofauna (interstitial fauna) The inter-mediate-sized animals in a SOIL or sedimentcommunity that live within the soil, sedi-ment, or rock fissures. They are usuallygreater than 40 µm and less than 2 mm inlength, e.g. certain mites and springtails(Collembola). Most are detritus feeders.

meiosis The process of cell divisionleading to the production of daughter nu-clei with half the genetic complement of theparent cell (i.e. HAPLOID nuclei). Cellsformed by meiosis give rise to GAMETES,and FERTILIZATION restores the correctchromosome complement.

Meiosis consists of two divisions duringwhich the chromosomes replicate onlyonce. As in mitosis, the stages prophase,metaphase, and anaphase occur, but dur-ing prophase HOMOLOGOUS CHROMOSOMES

attract each other and become pairedforming bivalents. At the end of prophasegenetic material may be exchanged be-tween the CHROMATIDS of homologouschromosomes, an important source ofVARIATION between daughter cells. Meiosisalso differs from mitosis in that afteranaphase, instead of nuclear membranesforming, there is a second division, whichmay be divided into metaphase II andanaphase II. The second division ends withthe formation of four haploid nuclei,which develop into gametes. See homolo-

gous chromosomes. See also alternation ofgenerations. Compare mitosis.

melanin See melanism.

melanism 1. The possession of a darkappearance due to the presence of a darkbrown pigment, melanin.2. The occurrence in a population of indi-viduals that have a more than usually darkskin, feathers, or fur.

melatonin See photoperiodism.

Mendelism The theory of inheritancefounded on the work of the Austrian monkGregor Mendel (1822–84), according towhich characteristics are determined byparticulate ‘factors’ that are transmitted bythe germ cells. It is the basis of classical ge-netics. Mendel formulated two laws to ex-plain the pattern of inheritance heobserved in plant crosses. The first law, theLaw of Segregation, states that any charac-ter exists as two factors, both of which arefound in the somatic (body) cells but onlyone of which is passed on to any one ga-mete. The second law, the Law of Inde-pendent Assortment, states that thedistribution of such factors to the gametesis random; if a number of pairs of factors isconsidered, each pair segregates indepen-dently.

Today Mendel’s ‘characters’ are termedGENES and their different forms (factors)are called ALLELES. It is known that aDIPLOID cell contains two alleles of a givengene, each of which is located on one of apair of HOMOLOGOUS CHROMOSOMES. Onlyone homology of each pair is passed on toa gamete. Thus the Law of Segregation stillholds true. Mendel envisaged his factors asdiscrete particles but it is now known thatthey are grouped together on chromo-somes. The Law of Independent Assort-ment therefore applies only to unlinkedgenes. See independent assortment; link-age.

mercury (Hg) A silvery metallic elementthat is liquid at room temperature. It oc-curs naturally as cinnabar (mercurous sul-fide) and as elemental mercury in cinnabar

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and some volcanic rocks. The vapor ishighly poisonous. Mercury is used in ther-mometers and other scientific apparatusand mercury compounds are used in fungi-cides and timber preservatives.

meristem A distinct region of actively orpotentially actively dividing cells in a plant,primarily concerned with growth.

meromictic Describing a lake in whichvertical mixing of the water never occurs.

mesic Describing a habitat that is nei-ther wet (hydric) nor dry (xeric), but hasintermediate moisture content.

mesobiota Soil or sediment organismsof intermediate size, e.g. larger insects,earthworms, and nonmicroscopic fungi.

mesoclimate The climate of a relativelysmall area such as a valley or town.

mesofauna Soil or sediment animals ofintermediate size, such as nematodes,earthworms, arthropods, and mollusks.

mesophilic Describing microorganisms(mesophiles) that grow best at moderatetemperatures, usually taken to be20–45°C. Compare psychrophilic; ther-mophilic.

mesophilic digestion See digestion(def. 2).

mesophyte A plant that is adapted togrow under adequate conditions of watersupply and has no particular adaptationsto withstand environmental extremes. Indrought conditions wilting is soon appar-ent because the plants have no specialmechanisms to conserve water. Most an-giosperms are mesophytes. Compare hy-drophyte; xerophyte.

mesoplankton See plankton.

mesothelioma See asbestos.

mesotrophic Describing water with lev-els of plant nutrients intermediate between

those of OLIGOTROPHIC and EUTROPHIC

water.

Mesozoic The middle era in the most re-cent (Phanerozoic) eon of the GEOLOGICAL

TIME SCALE, dating from about 230–66 mil-lion years ago, during which time the su-percontinent of PANGAEA broke up intoseparate continents that began to driftacross the globe. Known as the Age of Rep-tiles, it is divided into three main periods:the TRIASSIC, JURASSIC, and CRETACEOUS.Vertebrate life diversified during this era,and the first mammals, lizards, turtles,crocodiles, and birds appeared. At the endof the Triassic period a mass EXTINCTION

wiped out some 35% of existing animalgroups. Later diversification culminated inthe domination of land, air, and sea by di-nosaurs, pterosaurs, and plesiosaurs. Asthe climate warmed during the Triassic pe-riod the dominant vegetation of seed-fernsgave way to cycads, and the Mesozoic issometimes also called The Age of Cycads.In the Jurassic period, tropical and temper-ate regions were dominated by forests ofpalmlike cycads and conifers, and ginkgoeswere also widespread. The first an-giosperms arose in the Cretaceous periodand rapidly diversified, becoming the dom-inant vegetation by the end of the Meso-zoic as the climate cooled and dried. Theera ended with another mass extinction,and the demise of the dinosaurs.

metabolic pathway See metabolism.

metabolic rate The volume of oxygenconsumed by an organism per unit time perunit body mass. This is the basal or restingmetabolic rate (BMR or RMR), the lowerlevel of energy use under normal condi-tions in which the organism is resting andis not consuming or digesting food. Ankind of activity (‘work’) or stress, or an in-creased need to maintain body heat (per-haps due to temperature fall), will lead toan increase metabolic rate.

metabolism The chemical reactionsthat take place in cells. The molecules tak-ing part in these reactions are termedmetabolites. Some metabolites are synthe-

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sized within the organism, whereas othershave to be taken in as food. It is metabolicreactions, particularly those producing en-ergy, that keep cells alive. Metabolic reac-tions characteristically occur in small steps,comprising a metabolic pathway, in whichthe products of one reaction form the start-ing materials for the next, and are regu-lated by various FEEDBACK MECHANISMS.Metabolic reactions involve the breakingdown of molecules to provide energy (CA-TABOLISM) and the building up of morecomplex molecules and structures fromsimpler molecules (ANABOLISM).

metabolite See metabolism.

metalimnion (thermocline) The middlelayer of water in a thermally stratified lake,between the EPILIMNION and the HY-POLIMNION, which shows a rapid decreasein temperature with depth.

metal tolerance The ability of certainplants to tolerate high concentrations ofmetals, especially heavy metals (e.g. lead,copper, and zinc) in the soil. The plantsmay contain modified enzymes capable ofworking in the presence of the metal; theymay have specific ion transport mecha-nisms that prevent metal uptake by theroots; or they may be able to transfermetals to vacuoles, where they will not in-terfere with metabolism. See also bioaccu-mulation; biomining; bioremediation;hyperaccumulator.

metamerically segmented Having abody divided along its longitudinal axisinto a series of similar units (called seg-ments or metameres), at least in its embry-onic stage. Metamerically segmentedanimals include annelid worms (see Annel-ida) and chordates (see Chordata).

metamorphic rock Rock that has beenchanged as a result of the pressure of otherrocks, high temperatures, percolating solu-tions, etc., for example, marble.

metamorphosis A phase in the life his-tory of many animals during which there isa rapid transformation from the larval to

the adult form. Metamorphosis is wide-spread among invertebrates, especially ma-rine organisms and arthropods, and istypical of the amphibians.

metapopulation A population dividedinto a series of interacting subpopulationslinked by migration, for example wheresmall patches of suitable habitat are sepa-rated by unsuitable habitat. For example,in a mainland-island metapopulation dif-ferent local populations are in differenthabitat patches or islands near a largerpatch, the mainland, from which individu-als can migrate to the other patches.

Each metapopulation will have its owncharacteristic spatial distribution and de-mographics, which together make up themetapopulation structure. The degree ofinteraction between the subpopulationsmay vary. A spatially explicit metapopula-tion model relates the extent of migrationbetween local populations to the distancebetween them. A spatially implicitmetapopulation model simply assumesequal opportunities for migration betweenlocal populations. A metapopulation inwhich some local populations (sources)show a positive growth rate at low densi-ties, while others (sinks) show a negativegrowth rate when there is no immigration,is called a source-sink metapopulation.

metazoans In some older classifica-tions, a subkingdom (the Metazoa) of mul-ticellular animals whose bodies arecomposed of specialized cells grouped to-gether to form tissues and that possess acoordinating nervous system. This sub-kingdom included all animals except sin-gle-celled animals and sponges. CompareAnimalia.

meteorology The study of the Earth’satmosphere and its influence on weatherand climate.

meter 1. The SI unit of length, specifi-cally the distance traveled by light in a vac-uum in 1/299,792,458 of a second. 1 m isequal to 39.37 inches.

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2. An instrument used for counting ormeasuring, for example an electricity meteror pH meter.

methane A colorless gas produced natu-rally by the anaerobic bacterial decomposi-tion of organic matter in marshes,coalmines, and the intestines of termitesand RUMINANTS such as cattle; it also makesup some 99% of NATURAL GAS. Methane isan important GREENHOUSE GAS. It can beburned for use in heating or power genera-tion.

methanogen An EXTREMOPHILE bac-terium of the domain ARCHAEA that pro-duces methane gas as a by-product of itsmetabolism. See also halophile; thermo-phile.

methanol See ethanol.

methoxychlor A CHLORINATED HYDRO-CARBON used as a contact insecticide. It ishighly poisonous, affecting nerve transmis-sion, and accumulates in the fatty tissues ofanimals and is also toxic to freshwater andmarine aquatic life. See biomagnification.

methyl bromide A colorless volatile liq-uid used as a pesticide, particularly to fu-migate soil and agricultural products.When released into the atmosphere, it con-tributes to depletion of the OZONE LAYER.See ozone hole.

methylmercury compounds A class oforganic compounds in which mercury iscombined with a methyl group, which areused as fungicides in seed dressings andtimber preservatives. Birds and other ani-mals that eat the seeds may be poisoned bythe mercury.

micelle A complex between clay miner-als and humus that bears a net electroneg-ative charge on its surface, thus attractingand retaining cations and contributing tothe long-term fertility of the SOIL.

microalgae Algae that are too small tosee with the naked eye.

microbial film A thin layer of microbes.

microbial loop A nutrient cycle involv-ing only microbes, such as the nutrient cy-cling between phytoplankton and theirconsumers in seas and lakes, and aroundsome mica minerals (silicates) in the soil.Such cycles are almost independent of thenutrient cycles involving larger producersand consumers.

microbiology The study of microscopicorganisms (e.g. bacteria and viruses), in-cluding their interactions with other organ-isms and with the environment. Microbialbiochemistry and genetics are importantbranches of science because of the use ofmicroorganisms in BIOTECHNOLOGY andGENETIC ENGINEERING.

microclimate The climate of a smallarea, which may range in size from anurban area to the immediate area around ashrub or under a log.

microconsumer An organism, such as abacterium or fungus, that feeds by break-ing down dead organic matter, releasingsimpler organic and inorganic substancesto the environment – in other words, a DE-COMPOSER. See saprobe.

microcosm A small-scale, simplifiedecosystem designed for experimental pur-poses, which is intended to represent theprocesses that go on in a larger, analogousecosystem. For example a sample of pondwater in a jar, maintained by artificial lightand bubbled air.

microenvironment See habitat.

microevolution EVOLUTION on a scalewithin a species, which results from the ac-tion of natural selection on the geneticVARIATION between individuals of a POPU-LATION. Compare macroevolution.

microfauna Soil animals so small thatthey can be seen only with a microscope.Compare macrofauna; megafauna; meio-fauna.

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microflora Plantlike soil organisms sosmall that they can be seen only with amicroscope. The term is often used to in-clude bacteria, fungi, and algae.

microfossil See fossil.

microhabitat See habitat.

micrometer (µm) A unit of length equalto 10–6 m (one-millionth of a meter), for-merly called a micron.

micron See micrometer.

micronutrient (trace element) A nutri-ent required in trace amounts by an organ-ism. For example, a plant can obtainsufficient of the essential trace elementmanganese from a solution containing 0.5parts per million of manganese. Many actas enzyme cofactors or are components ofpigments. They include boron, cobalt, cop-per, manganese, molybdenum, and zinc.See deficiency disease. Compare macronu-trient.

microorganism A microscopic organ-ism: one so small that it can be seen onlywith the aid of a microscope. The term isusually restricted to bacteria, viruses, pro-tozoans, and microscopic fungi and algae,excluding such organisms as rotifers.

microparasite See parasitism.

microplankton See plankton.

microsere See sere.

mid-oceanic ridge See plate tectonics.

migration An instinctive regular two-way movement of part or all of an animalpopulation to and from a given area, usu-ally along well-defined routes. It is closelylinked to the cycle of the seasons and istriggered off by seasonal factors such as in-creasing and decreasing daylengths inspring and autumn. Many birds, hoofedmammals, bats, whales, fish, and insectsmigrate, often covering immense distances.For example, the Arctic tern breeds on the

northernmost coasts of Eurasia and Amer-ica and winters around the Antarctic pack-ice 11 000 miles to the south. Migratorymammals such as the wildebeest live inhabitats with fluctuating climatic condi-tions and migrate in order to find an ad-equate food supply.

mildew A fungal disease of plants inwhich the mycelium is visible on the sur-face of the plants as white or grayish pow-dery patches.

millibar (mb) A unit of pressure equal toone-thousandth of a bar. One bar is 105

pascals (105 newtons per m2) or 14 lb/in2,or approximately one atmosphere. Thestandard pressure of the atmosphere at sealevel is 1013 millibars.

millimolar Describing the concentra-tion of a solution expressed as the numberof millimoles (one-thousandths of a MOLE)of dissolved substance per dm3 (liter) of so-lution, e.g. a 10-millimolar solution con-tains 10 millimoles of dissolved substanceper dm3. See mole.

mimic An animal (the mimic) that re-sembles another animal (the model), andby so doing gains an advantage from its re-semblance to the model. See also mimicry.

mimicry The resemblance of one animalto another by which the mimic gains ad-vantage from its resemblance to the model.For example, in Batesian mimicry certainedible insects mimic the warning col-oration of noxious insects and so areavoided by their predators. Natural selec-tion produces more accurate mimicry asonly those individuals closely resemblingthe model will be mistaken for it and leftalone. In Müllerian mimicry a group ofpoisonous animals resemble each other, forexample, bees, wasps, and hornets, in-creasing the likelihood that potentialpredators will learn to avoid them.

mineralization 1. The process by whichthe organic constituents of an organism arereplaced by inorganic materials for form afossil.

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mineralization

2. The release of inorganic material fromorganic matter during aerobic or anaerobicdecomposition.3. Any process by which minerals are in-troduced into a rock.

minimum viable population See pop-ulation.

Miocene An epoch of the Tertiary, 25–7million years ago. The climate becamedrier and the grasses evolved and spreadrapidly, this perhaps explaining the re-placement of early mammals by more mod-ern forms. About half to three-quarters ofexisting mammalian families are repre-sented in rocks of the Miocene.

mist A suspension of water droplets inair that reduces the visibility to not lessthan 1 kilometer. Compare fog.

mitigation Action taken to minimize theeffects of environmental damage or POLLU-TION. These include avoidance of the harm-ful agent, means of reducing orcompensating for its effects, and therestoration or replacement of damagedecosystems.

mitochondrion (pl. mitochondria) Anorganelle of all plant and animal cellschiefly associated with aerobic RESPIRA-TION. It is surrounded by two membranesseparated by an intermembrane space; theinner membrane forms finger-like pro-cesses called cristae, which project into thegel-like matrix. The diameter is alwaysabout 0.5–1.0 µm, and the length averages2 µm. Mitochondria are most numerous inactive cells (up to several thousand percell). The matrix is also involved in aminoacid metabolism and in fatty acid oxida-tion.

It is believed that mitochondria andCHLOROPLASTS may be the descendants ofonce independent organisms that early inevolution invaded EUKARYOTE cells, leadingto an extreme form of SYMBIOSIS. See en-dosymbiont theory.

mitosis The ordered process by whichthe cell nucleus divides in two during the

division of body (i.e. somatic or non-germline) cells. The CHROMOSOMES repli-cate prior to mitosis to form two sisterCHROMATIDS, which are then separatedduring mitosis in such a way that eachdaughter cell inherits a genetic complementidentical to that of the parent cell. Mitosismay be followed by cytokinesis (cell divi-sion). Compare meiosis.

mixed deciduous forest See temperatedeciduous forest.

mixed feeder An organism that feeds onboth plants and animals.

mixing height Where there is an inver-sion in the atmosphere, in which a mass ofstable air caps an unstable layer below, theheight to which pollutants can be mixed iscalled the mixing height. This extends tothe level of the INVERSION.

MMMF See man-made mineral fibers.

model A mathematical expression of theway a system (for example, an ecosystemor the atmospheric circulation) works thatcan be subjected to testing by experimenta-tion, observation, or analysis of existingdata. A successful model allows for predic-tion of the behavior of the system undervarious conditions. An analytical model at-tempts to explain existing data, while asimulation model attempts to predict thebehavior of the system if one or more vari-ables are perturbed. For example a simula-tion of the effects of increasing habitat losson a population may give an idea of thetimescale in which conservation effortsmust be concentrated.

modular organism An organism thatgrows by repeatedly producing similarparts, e.g. the leaves, shoots, and branchesof plants, or the polyps of corals. Non-modular organisms are called unitary or-ganisms.

molality The concentration of a sub-stance expressed in moles per unit weight.The molal concentration is the number of

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moles of solute in one kilogram of solution.Compare molarity.

molarity A measure of the concentra-tion of a solution based upon the numberof molecules or ions present, rather than onthe mass of solute, in any particular vol-ume of solution. The molarity (M), ormolar concentration, is the number ofmoles of solute in one cubic decimeter(liter). Thus a 0.5M solution of hydrochlo-ric acid contains 0.5 x (1 + 35.5)g HCl perdm3 of solution. Compare molality.

mole (mol; M) The SI base unit ofamount of substance, defined as theamount of substance that contains as manyelementary entities as there are atoms in0.012 kilogram of 12C. The elementary en-tities may be atoms, molecules, ions, elec-trons, photons, etc., and they must bespecified. The amount of substance is pro-portional to the number of entities. Onemole contains 6.022 045 × 1023 entities.

molecule A particle formed by the com-bination of atoms in a whole-number ratio.A molecule of an element (combiningatoms are the same, e.g. O2) or of a com-pound (different combining atoms, e.g.HCl) retains the properties of that elementor compound. Thus, any quantity of acompound is a collection of many identicalmolecules. Molecular sizes are characteris-tically 10–10 to 10–9 m.

Molina, Mario José (1943– ) Mexi-can physical chemist who worked withROWLAND on the chlorofluorocarbons(CFCs) used as the propellant in mostaerosol cans and on their affect on theozone layer.

Mollusca (mollusks) A phylum of bilat-erally symmetrical unsegmented inverte-brates, including the aquatic bivalves,mussels, octopuses, squids, etc., and theterrestrial slugs and snails. The body is di-vided into a head, a ventral muscular loco-motory organ (foot), and a dorsal visceralhump that houses most of the body organsand is covered by a tissue layer (mantle),which typically secretes a calcareous shell

into which the head and foot can retract.The mantle extends into folds forming acavity containing the gills. The raspingradula is used for feeding. In aquatic formsdevelopment usually occurs via a swim-ming larva.

mollusk See Mollusca.

molybdenum A micronutrient (trace el-ement) needed for plant growth. Defi-ciency of molybdenum may lead toCHLOROSIS between the leaf veins.

monitoring Periodic or continuous ob-servation or measurement of variables ororganisms to evaluate performance orprogress of a reaction or procedure; for ex-ample the testing of water to ensure itssafety for human consumption, or therecording of the growth of plants followinga specific treatment.

monocarpic See iteroparous.

monoclimax See climax community.

monocotyledons See Angiospermophyta.

monoculture The growing of a singlespecies or variety of crop over a large area.The economic reasons for this are the abil-ity to grow crops of uniform height for har-vesting, and economies of scale in theharvesting process. The major disadvan-tage is increased susceptibility to pests andpathogens, especially if a single variety isgrown. Compare center of diversity.

monogamy The exclusive pairing of amale and a female for the purpose of mat-ing, in which each partner remains com-pletely faithful to the other. Such a pairbond may last for a single breeding season,or for life, depending on the species.

monomictic Describing a lake in whichvertical mixing of the water takes placeonly once a year. Compare dimictic;meromictic.

monosaccharide See carbohydrates.

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monosaccharide

monotreme See Mammalia.

monsoon A weather pattern in the sub-tropics in which the prevailing winds blowin opposite directions at different times ofthe year. It is due to the different heating ofland and sea, which causes seasonalchanges in atmospheric pressure systemsthat are further influenced by shifts inupper wind patterns and jet streams. It isespecially well developed over southernand eastern Asia, where the wet summermonsoon from the southwest dominatesthe climate.

monsoon forest A TROPICAL FOREST

that occurs in regions that experience a fewmonths of drought, such as India, Myan-mar (Burma), and Indo-China. Species di-versity is lower than in RAINFOREST, andmonsoon forests tend to have more opencanopies and very dense undergrowth.Many of the trees are deciduous, sheddingtheir leaves in the dry season.

Montreal Protocol An internationalagreement between some 30 countries thatcame into force on January 1, 1989 withthe aim of protecting the ozone layer bycontrolling emissions of CFCS, HALONS, andother ozone-depleting substances. The goalwas to completely phase out the produc-tion and use of CFCs by 1996, and to re-strict halon consumption to 1988 levels.More than 265 countries have now ratifiedthe agreement. See also Clean Air Act.

monuron A persistent herbicide used es-pecially to control broad-leaved weeds.

moorland An area of acidic land domi-nated by low-growing ericaceous shrubssuch as heather (Calluna vulgaris) and bil-berry (Vaccinium myrtillus), mixed withgrasses, sedges, and rushes. The term ismost commonly used for upland areas, butin some parts of the UK it is also used forlow-lying land on deep peat.

mor See humus.

morbidity The state of being diseased.The morbidity rate is the number of indi-

viduals suffering from a particular diseaseper 100 thousand of the population.

morph A local population of a speciesthat is phenotypically distinct from otherpopulations, yet remains capable of inter-breeding with them.

morphogenesis The development ofform and structure.

morphology The study of the form oforganisms. The term may be used synony-mously with anatomy although generallythe study of external form is termed mor-phology whereas the study of internalstructures is termed anatomy.

morphometry The measurement of ex-ternal form. The term may be applied tolandscape features such as drainage sys-tems as well as to organisms.

mortality rate (m) The number ofdeaths per capita of the population, or of aspecific age group, calculated for a givenperiod of time, often one year.

mosquitofish A small live-bearing fishnative to rivers in the southeastern UnitedStates. It feeds on mosquito larvae, and hasbeen introduced to many parts of the worldto help control mosquitoes.

mosses See Bryophyta.

motile Capable of independent locomo-tion.

mountain coniferous forest See borealforest.

MSY See maximum sustainable yield.

mucilage Any substance that swells inwater to form a slimy solution. Mucilagesare often complex substances, and maycontain proteins, cellulose, and derivativesof uronic acids. In plants, mucilages areoften used to retain water, e.g. mucilages inthe center of succulent desert plants and onthe surface of many seeds, helping the seedto take up water during germination. They

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may also form part of the cell wall matrix.Most bacteria have an outer layer of mu-cilage, and many seaweeds contain mu-cilage in their tissues, which helps retainmoisture when exposed at low tide, andalso on their surfaces, helping reduce resis-tance to wave action and thus minimizedamage.

mud flat A large flat area of mud, usu-ally in an estuary or around a coast, that isperiodically inundated by the sea.

mulch 1. A protective covering of com-post, paper, bark chippings, cocoa shells,etc. spread over the ground to reduce evap-oration, control weeds, enrich the soil, andprotect against temperature fluctuations.Straw is used to keep soft fruits such asstrawberries (Fragaria ananassa) clean anddry.2. In agriculture, the process of leavingcrop residues on the soil surface or buryingthem to add to the organic matter in thesoil and reduce evaporation. Crops areplanted by machinery that penetrates themulch. In humid areas mulch does notbreak up readily, and may deprive the cropof nitrogen, so fertilizer is often addedbelow the mulch. Intercropping (the plant-ing of a second crop between the rows ofthe main crop) may be used to provideplants for mulching after the main crop hasbeen harvested.

mull See humus.

multicropping The practice of growingmore than one crop on the same piece ofland in the same year.

multifactorial experiment An experi-ment in which more than one factor is var-ied at time or more than one treatmentgiven to the subject or system under inves-tigation.

multifactorial inheritance See quanti-tative variation.

multiple regression techniques See re-gression.

Munsell soil color chart system Astandard scheme for naming the color of aSOIL, a feature important in the identifica-tion and differentiation of soil horizons.

muskeg A type of peat bog characteris-tic of the BOREAL FOREST of North America.Muskegs are usually dominated by blackspruce (Picea mariana), and INSECTIVOROUS

PLANTS such as sundews (Drosera) andpitcher plants (Sarracenia) are common.See peat.

mutagen Any physical or chemicalagent that induces mutation or increasesthe rate of spontaneous mutation. Physicalmutagens include ultraviolet light, x-rays,and gamma rays.

mutant See mutation.

mutation A change in one or more ofthe bases in DNA, which results in the for-mation of an abnormal protein. An organ-ism that has undergone a mutation is calleda mutant. Mutations are the ultimatesource of all genetic VARIATION. They areinherited only if they occur in the cells thatgive rise to the gametes; mutations in other(somatic) cells may give rise to CHIMERAS

and cancers. Gene mutations alter only asingle gene, resulting in new ALLELES of thegene and hence new variations upon whichnatural selection can act. Chromosomemutations result in the gain, loss, or re-arrangement of whole segments of chro-mosomes and are usually, but not always,lethal. Most mutations are deleterious butare often retained in the population be-cause they also tend to be recessive and canthus be carried in the genotype without af-fecting the viability of the organism. Thenatural rate of mutation is low, and varieswith different gene loci. The mutation fre-quency can be increased by MUTAGENS. Seealso polyploidy.

mutual antagonism The situation inwhich two species have reciprocal negativeeffects on each other, for example, theymay both compete for the same resources,or they may prey on each other.

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mutual antagonism

mutual interference Interference be-tween PREDATORS or CONSUMERS that leadsto a reduction in the consumption rate ofindividual predators/consumers that in-creases with predator/consumer density,even when resources are not limiting, forexample, where aggressive behavior (oftenterritorial) reduces the time and energyavailable for predation or foraging.

mutualism The close relationship be-tween two or more species of organisms inwhich all benefit from the association.There are two types of mutualism: obliga-tory mutualism, in which one cannot sur-vive without the other, for example thealgal–fungal partnership found in lichens;and facultative mutualism (protocoopera-tion), in which one or both species can sur-vive independently, for example, marinecrabs and their associated invertebratefauna of sponges, cnidarians, etc., that at-tach to the crab shell and act as camou-flage. In the association between thebacterium Rhizobium and members of thefamily Fabaceae, which leads to the forma-tion of root nodules in which nitrogen fix-ation occurs, the bacterium can also liveindependently of the plant. In lichens,often the alga can live independently of thefungus. In many mutualistic associations,the relationship can change from aparasitism to a mutualism or vice versa.Compare amensalism; commensalism;symbiosis.

mya Abbreviation for million years ago.

mycelium See Fungi.

mycobiont See lichens.

mycorrhiza (pl. mycorrhizae) The asso-ciation between the hyphae of a fungus andthe roots of a higher plant. Two main typesof mycorrhiza exist, ectomycorrhizae (ec-

totrophic mycorrhizae), in which the fun-gus forms a mantle around the smallerroots, as in many temperate and borealtrees, and endomycorrhizae (endotrophicmycorrhizae), in which the fungus growsaround and within the cortex cells of theroots, as in many herbaceous plants. Or-chids and heathers have further specializedtypes of mycorrhizae, in which there isboth a mantle and cell penetration.

The fungus benefits by obtaining carbo-hydrates and possibly B-group vitaminsfrom the roots. The tree benefits in thatmycorrhizal roots absorb nutrients moreefficiently than uninfected roots, and it iscommon forestry practice to insure the ap-propriate fungus is applied when plantingseedling trees.

The mycorrhizal relationship is some-what flexible. In seedling trees and orchidsand sometimes as adults, the plant is in ef-fect parasitic on the fungus, but the rela-tionship changes, and in the fungal fruitingseason the fungus takes a lot from theplant, which is transporting nutrients to itsroots. Between times the relationship is ef-fectively mutualistic.

mycotrophic Describing an associationbetween an organism and a fungus inwhich the organism derives nutrients fromthe fungus (see mycorrhiza). The term isalso used in a more restricted sense to referonly to associations where the fungus ex-tends into the aerial parts of a plant, as insome heathers (Ericaceae) and orchids (Or-chidaceae). See mutualism; symbiosis.

myxomatosis See myxoma virus.

myxoma virus The causative agent ofmyxomatosis, an often fatal disease of rab-bits that is sometimes used to control theirnumbers. Some individuals have genetic re-sistance to the virus.

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NAD See nucleotide; respiration.

nanoplankton See plankton.

natality rate (b; birth rate) The numberof births per year in a population, usuallyshown as the number of births per 1000 in-dividuals.

native See indigenous.

natural gas Gas containing 50–90%methane found trapped in certain types ofrock formations. It is classified as a CLEAN

FUEL, widely used for domestic heating andin industry. It can be compressed for use invehicles. Liquefied petroleum gas (LPG) isproduced during natural gas productionand petroleum refining. It gives a similarperformance to gasoline. In the UnitedStates reformulated gasoline is being devel-oped to emit lower levels of pollutants thantraditional gasoline.

natural resource See resource.

natural selection The process, whichDarwin called the ‘struggle for survival’, bywhich organisms less adapted to their envi-ronment tend to perish, and better-adaptedorganisms tend to survive to reproduce andpass on their genes to the next generation.It may be summed up as ‘the survival of thefittest’, fitness in this context being a meas-ure of an organism’s ability to contributeto the next generation (see fitness). Therehas been much debate about the level atwhich natural selection acts: traditionallyit is regarded as the action of the environ-ment on individual organisms that deter-mines which genotypes survive andreproduce, but there are strong arguments

for natural selection acting at the level ofthe gene (the selfish gene hypothesis) andsome authors argue that it happens in thelong term at higher levels such as popula-tion or species. In the end the less success-ful types will die out. According toDARWINISM, natural selection acting on avaried population results in evolution. Seeevolution.

nature and nurture The interaction be-tween inherited and environmental factors(nature and nurture, respectively) in deter-mining the observed characteristics of anorganism. It is often applied in a discussionof behavioral characteristics, such as intel-ligence, in which the relative importance ofinherited and environmental factors, in-cluding such factors as social background,are a matter of great controversy. The termHERITABILITY is sometimes used as an alter-native, meaning the proportion of the totalvariation caused by genetic influencesalone.

nature conservation The managementof the Earth’s natural resources and envi-ronment to ensure they are used wisely andnot depleted or degraded.

nature reserve An area set aside pri-marily for the preservation and protectionof certain species of animals or plants orcertain habitats.

Nearctic One of the six zoogeographi-cal regions, including North America fromthe Central Mexican Plateau in the southto the Aleutian islands and Greenland inthe north. The fauna include mountaingoats, pronghorn antelopes, caribou, andmuskrat.

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N

nearest-neighbor distance The aver-age distance between individuals in a pop-ulation. Assuming that individuals aredistributed at random with respect to oneanother, the expected distance betweenneighbors is given by:

√(π/n)/2 m, where n is the density of in-dividuals per square meter. Ratios of lessthan 1 indicate clumping. Ratios greaterthan 1 indicate relatively even spacing.

nectarivore An animal that feedsmainly on nectar.

negative feedback See feedback mecha-nism.

neighborhood size The number of in-dividuals in a population that are withinthe dispersal distance of a single individual.

nekton Animals of the pelagic zone of asea or lake that are free-swimming and in-dependent of tides, currents, and waves,such as fish, whales, squid, crabs, andshrimps. Nekton are limited in distributionby temperature and nutrient supply, anddecrease with increasing depth.

Nematoda A large phylum of marine,freshwater, and terrestrial invertebrates,the roundworms. Most are free-living, e.g.Anguillula (vinegar eel), but many are par-asites, e.g. Heterodera (eelworm of pota-toes) and Ascaris (found in pigs’ andhuman intestines). Some cause serious dis-eases in humans, e.g. Wuchereria (causingelephantiasis). Nematodes are bilaterallysymmetrical with an unsegmented smoothcylindrical body pointed at both ends andcovered with a tough cuticle. The bodycavity is not a true coelom and there are noblood or respiratory systems. The muscu-lar and excretory systems and embryonicdevelopment are unusual. Nematodes arenot closely related to any other phylum.

neoDarwinism Darwin’s theory of evo-lution through NATURAL SELECTION, modi-fied and expanded by genetic studiesarising from the work of Mendel and hissuccessors. This fusion of Darwin’s theoryof natural selection with Mendel’s genetics

was called the neoDarwinian synthesis inthe 1930s. The inclusion of genetics in evo-lutionary studies answered many questionsthat Darwin’s theory raised but could notadequately explain because of lack ofknowledge at the time it was formulated.Notably, genetics has revealed the sourceof variation on which natural selection op-erates, namely MUTATIONS of GENES andCHROMOSOMES, and provided mathemati-cal models of how ALLELES fluctuate in nat-ural populations, thereby quantifying theprocess of evolution. More recent discover-ies in molecular biology have added to theunderstanding of the causes of variationand the nature of evolution at the molecu-lar level.

neon See atmosphere.

Neotropical One of the six main zoo-geographical regions of the earth. It in-cludes South and Central America, theWest Indies, and the Mexican lowlands.The characteristic fauna includes sloths,armadillos, anteaters, cavies, vampire bats,llama, alpaca, peccary, rhea, toucan, curas-sows, and certain hummingbirds.

NEP See net ecosystem production.

nephelometry A method of measuringturbidity in water by passing light throughthe sample and measuring the amount oflight that is deflected by the suspended par-ticles.

neritic The marine environment fromlow water level to a depth of about 200 m,a zone that in many areas corresponds tothe extent of the continental shelf. It makesup less than 1% of the marine environ-ment. Nutrients are relatively abundant inthis zone and it is penetrated by sunlight.

net assimilation rate A measure ofplant productivity based on net assimila-tion by leaves. The net assimilation rate isthe rate of increase in dry weight dividedby the area of the leaf.

net ecosystem production (NEP) Thedifference between the gross primary pro-

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duction of all the primary producers in anecosystem (plants, algae, and bacteria) andthe loss of nutrients and/or energy throughtheir respiration; in other words, the NET

PRIMARY PRODUCTION of the ecosystem. Seeproducer.

net photosynthesis The rate of carbondioxide uptake by a plant minus the rate ofcarbon dioxide evolution by respiration.

net plankton See plankton.

net primary production (NPP) TheGROSS PRIMARY PRODUCTION of an organ-ism, population, or community (i.e. thetotal energy or nutrients accumulated)minus the loss of energy or nutrients by res-piration.

net production efficiency The percent-age of the food that is assimilated by an or-ganism that is used in its growth andreproduction. Compare gross productionefficiency.

net reproductive rate (R0) The numberof offspring a female may be expected toproduce during her lifetime.

neutral theory The theory that mostevolutionary changes are caused by ran-dom GENETIC DRIFT (random changes in thefrequencies of alleles in a population)rather than by NATURAL SELECTION. GeneticMUTATIONS arise at random and clearlysome will have increased or decreased fit-ness and be subject to selection. But many,perhaps a majority, are neutral – they dono affect the fitness or survival of the car-rier. They therefore survive in the GENOME

and changes in their frequencies are due tothe BREEDING SYSTEM and chance ratherthan to natural selection. See evolution.

neutral variation Genetic diversity thatconfers no obvious selective advantage. Seenatural selection; variation.

niche (ecological niche) The functionalrole of an organism, population, or speciesin a community. It includes the habitat inwhich the organism lives, the resources it

uses, and the periods of time in which it isactive there and its interactions with otherorganisms. The range of conditions toler-ated and resources used by an individual,population, or species is called its nichebreadth. A statistical measure of nichebreadth is niche width: the STANDARD DEVI-ATION of the distribution of resource used.

If two species occupy the same niche,competition may occur until either one hasreplaced the other or the two divide theniche between them to some extent. Thuscoexisting species tend to differ, if onlyslightly, in their niche requirements, a phe-nomenon known as niche differentiation.If coexisting species occur in a similar posi-tion along one niche dimension (e.g. an al-titude or vegetation gradient), they willdiffer along another (e.g. food prefer-ences), a phenomenon known as nichecomplementarity. Thus coexisting specieswill tend to specialize to occupy betweenthem all the available ‘space’ within a givenset of niche parameters; this is known asniche packing. The overlap in resourcesused or conditions tolerated by two ormore different species is their niche over-lap. During ecological SUCCESSION, coloniz-ing species tend to monopolize aproportion of the available resources, leav-ing less choice and a narrower potentialniche breadth for the next species in thesuccession, a behavior termed niche pre-emption.

A similar niche may be occupied by dif-ferent species in different areas, for exam-ple the giant anteater (Myrmecophagatridactyla) of South America occupies thesame type of niche as the aardvark (Oryc-teropus afer) of southern Africa. Con-versely one type of organism may evolve byADAPTIVE RADIATION to fill several differentniches, as in the finches of the GalápagosIslands.

If a niche is considered to be a spacewith n dimensions, with the different axesrepresenting resources and environmentalfactors, then the niche space occupied by acommunity is the ‘volume’ into which theniches of all the species fit. Thus the num-ber of species in the community depends onthe total available niche space and the av-erage size and dimensions of each species’

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niche

niche. The size of niche that a given specieswould occupy in the absence of competi-tion is termed its fundamental niche,whereas the minimum niche space in whichit can persist, even in the presence of com-petitors and predators, is called its realizedniche.

nicotine A poisonous alkaloid that is themain active component of tobacco and isalso used as an insecticide.

nitrate A salt of nitric acid. Nitrates arepresent in the atmosphere and in the soil;they are highly soluble and when present insignificant concentrations in drinkingwater can harm humans (especially babies)and livestock. High levels of nitrates thatenter water supplies from the overuse offertilizers and from livestock waste, sewagesystems, and garbage dumps cause healthproblems and EUTROPHICATION.

nitrification The oxidation of ammoniacompounds to nitrites or nitrites to ni-trates, carried out by certain nitrifying bac-teria in the soil. The chemosynthetic soilbacteria Nitrosomonas and Nitrobacter

carry out the first and second stages, re-spectively, of this conversion. The processis important in the NITROGEN CYCLE be-cause many plants assimilate nitrates astheir source of nitrogen. Compare denitri-fication.

nitrogen An essential element found inall amino acids and therefore in all pro-teins, and in various other important or-ganic compounds, e.g. nucleic acids.Gaseous nitrogen (N2) forms about 78%by volume of the ATMOSPHERE but is un-available in this form except to a fewnitrogen-fixing bacteria (see nitrogen fixa-tion). Nitrogen is incorporated into plantsas the nitrate ion NO3

– or, especially inacid soils, the ammonium ion NH4

+, ab-sorbed in solution from the soil by roots.Plants suffering from nitrogen deficiencytend to develop chlorosis and become etio-lated, the effects being seen first in the old-est parts. In animals, the nitrogen com-pounds urea and uric acid form the mainexcretory products. See also nitrogen cycle.

nitrogen cycle The circulation of nitro-gen between organisms and the environ-

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ANIMALS(protein)

feeding

PLANTS(protein)

decay

N2free nitrogen inatmosphere

bacteria

leguminousnodules

assimilation

nitrification

NHammoniumcompounds

denitrificationindustrialfertilizers(Haber process)

NO3–

nitrates

decay

denitrificationfixationbylightning

+4

Nitrogen cycle

ment. Atmospheric gaseous nitrogen canbe used directly only by certain nitrogen-fixing bacteria (e.g. Clostridium, Nostoc,Rhizobium). They convert nitrogen to am-monia, nitrites, and nitrates, which are re-leased into the soil by excretion and decay.Some are free-living, whereas others formsymbiotic associations with plants (see ni-trogen fixation). Another method by whichatmospheric nitrogen is fixed is by light-ning, which causes nitrogen and oxygen tocombine. The oxides so produced dissolvein rain to form nitrous and nitric acids; inthe soil these acids combine with mineralsalts to form nitrites and nitrates. This is an insignificant process when comparedwith microbial NITROGEN FIXATION. Whenplants and animals die, the organic nitro-gen they contain is converted back into ni-trate in the process termed NITRIFICATION.Apart from uptake by plants, nitrate mayalso be lost from the soil by DENITRIFICA-TION and by LEACHING. The use of nitrogenfertilizers in agriculture and the emission ofnitrogen oxides in car exhaust fumes haveinfluenced the nitrogen cycle and con-tributed to urban air pollution (see photo-chemical smog) and acid precipitation.

nitrogen fixation The formation of ni-trogenous compounds from atmosphericnitrogen. In nature this may be achieved byelectric discharge in the atmosphere or bythe activities of certain microorganisms.For example, symbiotic bacteria of thegenus Rhizobium are associated with legu-minous plants, causing the root cortex toform nodules, which house the bacteria.These bacteria contain the nitrogenase en-zyme that catalyzes the fixation of molecu-lar nitrogen to ammonium ions, which theplant can assimilate. In return the legumesupplies the bacteria with carbohydrate.Many other nitrogen-fixing symbioses areknown, e.g. the nitrogen-fixing root nod-ules of bog myrtle (Myrica gale) and alder(Alnus glutinosa) appear to contain unicel-lular protoctists called plasmodiophorans,while water ferns (Azolla) have CYANOBAC-TERIA in their roots. Free-living bacteriathat can fix nitrogen include members ofthe genera Azotobacter, Klebsiella, andClostridium, some sulfur bacteria (e.g.

Chlorobium), and most cyanobacteria (e.g.Anabaena). Some yeast fungi have alsobeen shown to fix nitrogen. Industrially,nitrogen fixation is mainly achieved usingthe HABER PROCESS. See nitrogen cycle; sym-biosis.

nitrogenous wastes Organic wastesthat contain significant amounts of nitro-gen.

noise pollution Harmful or unwantedsounds in the environment.

nomenclature An internationally rec-ognized system of naming of living organ-isms and fossils that forms the basis of theirscientific classification. See classification;International Codes of Nomenclature; tax-onomy.

nominal scale See measurement scales.

nonequilibrium models Models ofPOPULATION DYNAMICS that take into ac-count spatial and temporal variation to ex-plain the behavior of populations that arenot in a state of equilibrium.

nonequilibrium theory See equilib-rium theory.

nonionizing electromagnetic radiationRadiation that has no effect on the struc-ture of atoms, but which heats tissues andmay have harmful effects. Examples in-clude radio waves, microwaves, and low-frequency electromagnetic fields fromhigh-voltage electric power cables.

nonpoint source A source of POLLU-TION that cannot be recognized at a singlelocation, for example, photochemicalsmog caused by vehicle exhausts.

nonpotable Undrinkable. The term mayrefer to water that is unsafe to drink due tocontamination with bacteria or pollutants,or unpalatable water.

nonrandom sampling See sampling.

nonrenewable resources RESOURCES that

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have been built up over a long period oftime (on a geological time scale) that can-not be renewed within a human lifespan,either because they take too long to accu-mulate or because the conditions that fa-vored their accumulation no longer exist.Examples include FOSSIL FUELS, water inAQUIFERS that is not being replenished fastenough (see fossil water), and many min-eral resources. See sustainable develop-ment.

normal distribution A symmetricalbell-shaped curve produced on a GRAPH orhistogram of distribution. A normal distri-bution is used to model the variation in aset of experimental results and allows com-parisons with other sets of results. For thewhole range of experimental values itshows the number of results that have eachvalue. There is an even distribution of val-ues on either side of the mean. See standarddeviation.

North Atlantic Deep Water A body ofcold saline water formed by winter coolingas the Norwegian Current flows out fromthe Norwegian Sea over the rim of theridge that stretches from Scotland and Ice-land to Greenland, and plunges down tothe of the Atlantic Ocean, where it spreadsslowly south along the western edge of theNorth Atlantic. It is joined by water fromthe Labrador Sea, and from highly salinewater formed by evaporation in theMediterranean Sea, which flows into theNorth Atlantic through the Strait ofGibraltar. It takes about 200 years forwater in the NADW to reach the AntarcticCircumpolar Current and spread into theIndian and Pacific Oceans.

northern coniferous forest See borealforest.

NPK Nitrogen, phosphorus, and potas-sium, used together in a fertilizer.

nuclear energy Energy released byprocesses that disrupt the nuclei of atoms,especially the breakdown (nuclear fission)in specially designed reactors of naturallyunstable elements, resulting in the alter-

ation of atomic nuclei and thus the forma-tion of different elements. The alternativeterm atomic energy is not accurate, as itimplies the rearrangement of atoms ratherthan of nuclear particles. In nuclear fissionthe nucleus of an atom breaks up into twolighter nuclei either naturally or when ex-cited by particles such as alpha particles,deuterons, neutrons, or protons. The reac-tion released more neutrons, which stimu-late further reactions, setting up a chainreaction that releases vast amounts of en-ergy. In a nuclear reactor such chain reac-tions are controlled and the heat evolvedused to raise steam to drive turbines forgenerating electricity. The main element in-volved in the production of nuclear energyby fission is uranium, which splits to formtwo lighter elements (fission fragments).Other by-products of the fission of ura-nium and plutonium include strontium-90,iodine-131, and cesium-137. The persis-tence of radioactive products of fission ismeasured as the half-life – the time takenfor half of a given mass of an element to bechanged into another isotope or element byradioactive decay. The early products offission are unstable and rapidly decay,emitting beta particles (electrons). Laterproducts are more stable and have longerhalf-lives. Cesium-137, for example, has ahalf-life of 30 years. These radioactive fis-sion products are dangerous by-productsof the generation of nuclear energy, andtheir storage and containment imposeshigh costs on the nuclear energy industry.When not contained, they can cause seri-ous contamination of the environment, es-pecially when released into the atmosphereor the sea. Another drawback is the possi-bility of catastrophic accidents such asthose at Chernobyl (see Chernobyl nuclearaccident) and THREE-MILE ISLAND. Support-ers of nuclear energy point to the fact thatit does not contribute to global warming.

Another form of nuclear energy thathas not yet been harnessed for industrialuse is nuclear fusion, in which light el-ements react together to form heavier ones.Nuclear fusion reactions are the mainsource of energy for the Sun, involving the‘burning’ of hydrogen to form helium. Theheat from nuclear fission or fusion reac-

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tions can be used to produce steam or otherhot gases to drive electricity-generatingturbines. Energy released during radio-active decay can be stored in semiconduc-tor materials to produce batteries for heartpacemakers, remote-controlled explora-tion instruments, and other applications.See high-level radioactive waste; low-levelradioactive waste; radioactive decay.

nuclear fallout See radioactive fallout.

nuclear fission See nuclear energy.

nuclear fusion See nuclear energy.

nuclear membrane See nucleus.

nuclear pore See nucleus.

nuclear power Electricity produced bypower plants in which NUCLEAR ENERGY isconverted into heat, which is used to pro-duce steam to drive turbines, which powerelectricity generators.

nuclear reaction A reaction that in-volves a change in the nucleus of an atom,for example radioactive decay, nuclear fis-sion or fusion, or neutron capture. It differsfrom chemical reactions, which are limitedto changes in the number and/or distribu-tion of electrons surrounding the nucleus.See nuclear energy; radioactive decay.

nuclear reactor A device that initiatesand controls nuclear fission reactions toproduce heat. See nuclear energy.

nuclear winter The significant coolingof the global climate suggested to be a pos-sible consequence of nuclear war, due tothe resulting high-altitude dust cloudscaused by the nuclear explosions and theburning of forests, fossil fuels, and plasticsset alight by firestorms triggered by ex-ploding nuclear warheads. Such a large ac-cumulation of dust would block sunlightfor weeks afterward, perhaps reducing sur-face temperatures by as much as 22°C(40°F). The low temperatures, frosts, anddim light, as well as the radioactive fall-out, would destroy plant and animals life.

nucleoplasm See nucleus.

nucleotide A compound formed by con-densation of a nitrogenous base (a purine,pyrimidine, or pyridine) with a sugar (ri-bose or deoxyribose) and phosphoric acid.ATP is a mononucleotide (consisting of asingle nucleotide), the coenzymes NADand FAD are dinucleotides (consisting oftwo linked nucleotides), and the nucleicacids (see DNA: RNA) are polynucleotides(consisting of chains of many linked nu-cleotides).

nucleus (pl. nuclei) 1. An organelle ofEUKARYOTE cells containing the genetic in-formation (DNA) and hence controlling thecell’s activities. It is the largest organelle,typically spherical and consisting of nu-cleoplasm bounded by a double mem-brane, the nuclear membrane, which isperforated by many pores (nuclear pores)that allow exchange of materials with thecytoplasm. The outer nuclear membrane isan extension of the endoplasmic reticulum,a network of membrane compartments dis-tributed throughout the cytoplasm, inwhich glycoproteins and other complexcompounds are synthesized. In the nondi-viding (interphase) nucleus the genetic ma-terial is irregularly dispersed as chromatin;during nuclear division (MITOSIS or MEIOSIS)this condenses into densely staining CHRO-MOSOMES, and the nuclear envelope.2. The central positively charged ‘core’ ofan atom.

NUE See nutrient use efficiency.

null hypothesis A statistical hypothesisthat can be tested experimentally. It pro-poses that any observed difference (for ex-ample, between the means of two samples)is due to chance alone and not to the effectof an internal or external factor. To testhow significant the difference between theexpected result and that proposed by thenull hypothesis, a statistical test such asStudent’s T-TEST may be used.

numerical response A change in thesize of a predator population as a result of

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a change in the density of its prey. Com-pare functional response.

numerical taxonomy (taxometrics) Theassessment of similarities between organ-isms by mathematical procedures. It in-volves statistical analysis of manymeasurable characteristics and uses phe-netic rather than phyletic evidence. The re-sults are usually expressed as one or moreDENDROGRAMS.

nutrient A chemical substance that anorganism must obtain from its environ-ment in order to sustain life and grow. Nu-trients are used to build up the body’s cells,and to release energy for vital processes.

nutrient cycle See biogeochemical cycle.

nutrient enrichment The addition ofnutrients to natural waters through run-offfrom land treated with fertilizers, effluentfrom sewage treatment works and live-stock manure, urban drainage systems andindustrial sources, or deforestation. See eu-trophication.

nutrient foraging The regulation of nu-trient uptake. Nutrient foraging strategiesinclude changes to root architecture (pat-

terns of branching and spacing of roots) inresponse to soil conditions, and to stemgrowth. Roots or stems may become thin-ner and longer in poor nutrient conditions.Symbiotic associations with microbes orother plants (e.g. mycorrhizae and rootnodules) may also be affected, and alsoprocesses that change the availability ofions in the soil, such as the exudation ofmucilages from roots.

nutrient recycling See biogeochemicalcycle.

nutrient use efficiency (NUE) The ratioof productivity to the uptake of a specificessential nutrient.

nutrification The addition of nutrientsto an ecosystem that increases the produc-tion of BIOMASS. This may lead to a de-crease in the oxygen concentration in soilor water.

nutrition The way in which an organ-ism assimilates and utilizes NUTRIENTS. Or-ganisms may be classified on the basis ofthe way in which they derive nutrients andenergy. See autotroph; chemoautotroph;chemoheterotroph; heterotroph; litho-troph.

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obligate Describing an organism that re-quires specific environmental conditionsfor its survival and cannot adopt an alter-native mode of living. For example, an ob-ligate aerobe is an organism that can growonly under aerobic conditions and cannotsurvive in anaerobic conditions. An oblig-ate parasite cannot live in the absence of itshost. Compare facultative.

occupational exposure standards (OES)Limits for exposure to chemical, biologi-cal, or physical hazards, beyond whichthere will be a risk to health. Such limitsmay be defined by manufacturers of prod-ucts such as chemicals and equipment, orby industry regulators.

occupational health An area of legalresponsibility for employers and employ-ees, concerning protection from physicalhazards, stress, and occupational diseasesdue to poor ventilation and lighting, risksrelated to equipment, machinery, or build-ing design, shift patterns, and chemicals inthe workplace. It involves planning to pre-vent or minimize such risks, safety train-ing, and monitoring safety and emergencyprocedures.

ocean The continuous body of more orless salty water that occupies some 71% ofthe Earth’s surface, excluding lakes and in-land seas.

ocean current A net movement ofocean water along a definable path due togravity, wind friction, or gradients in waterdensity.

oceanic The marine environment be-yond the continental shelf, which is usuallydeeper than 200 m. It makes up about 99%

of the total marine environment. Comparesublittoral.

oceanic crust See crust.

oceanography The study of the oceans,their distribution and mapping, thephysics, chemistry, biology and ecology oftheir waters, and the exploitation of theirresources.

odor threshold The lowest concentra-tion of a substance in air or water that hasa detectable odor.

OECD See Organization for EconomicCooperation and Development.

OES See occupational exposure stan-dards.

oil 1. A triacylglycerol that is liquid at ornear room temperature. The commonestfatty acids in oils are oleic and linolenicacids, which are unsaturated. Storage oils,synthesized in the endoplasmic reticulum,make up as much as 60% of the dry weightof certain seeds, such as the castor bean(Ricinus communis). After oil is extractedfrom seeds, the residual ‘cake’ is used tofeed livestock or, if poisonous (as in castorbean), as fertilizer.

Essential oils are volatile oils secretedby aromatic plants; they are the source ofcharacteristic odors and flavors. Some es-sential oils repel insects or grazing animals,while others (allelochemicals) deter en-croaching neighboring plants. Resinousoils help prevent loss of sap and protectagainst the entry of pathogens and para-sites. Essential oils are common in XERO-PHYTES, where in hot conditions theevaporating oils increase the density of the

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O

boundary layer at the leaf surfaces, so re-ducing TRANSPIRATION.2. See petroleum.

oil field An area rich in petroleum de-posits, especially one that is already beingexploited.

oil fingerprinting A detailed chemicalanalysis that distinguishes between oilsfrom different sources. It is used to tracethe sources of oil spills.

oil refinery A processing unit that sepa-rates crude petroleum or petroleum mix-tures into their component parts,producing fractions suitable for differentpurposes such as fuel or lubrication.

oil shale A brown or black SEDIMENTARY

ROCK that yields liquids or gaseous hydro-carbons on heating.

oil slick A layer of mineral oil on thesurface of water. It has the effect of makingthe surface of the water smooth. The oilprevents oxygen and light penetrating intothe water below, so may suffocate marinelife. Spillages from oil tankers are a majorpollutant of coastline.

okta A unit of cloud cover, defined ascloud covering one-eighth of the sky area.It is used especially in relation to aviation.

old-growth forest In North America, aforest that has remained undisturbed sincebefore European settlement. It is thereforeat a late stage of succession and probablyresembles the climax vegetation (see cli-max community).

Oligocene An epoch of the TERTIARY,38–25 mya. It is characterized by the grad-ual disappearance of earlier mammalgroups, including primitive insectivoresand primates, and their replacement bymore modern forms. The world’s climateranged from temperate to subtropical, andthe epoch was marked by an expansion ofgrasslands at the expense of forests, achange that promoted the evolution oflarge herbivores. Around the great Tethys

Sea there were tropical swamps, whichgave rise to extensive deposits of lignite inGermany and neighboring countries. Theangiosperms were evolving and diversify-ing rapidly, and overtook the gym-nosperms in their abundance. Flowers andinsects were evolving together, and com-plex COEVOLUTION was taking place as pol-lination mechanisms diversified. See alsogeological time scale.

Oligochaeta See Annelida.

oligotrophic Describing lakes and pondsthat are low in nutrients and consequentlylow in productivity. Compare eutrophic.

omnivore An animal that eats bothplant and animal material, e.g. somespecies of bears. Compare carnivore; herbi-vore.

ontogeny The course of development ofan organism from fertilized egg to sexualmaturity with the production of gametesand the next generation.

OP See osmosis.

opacity The degree to which light is ableto penetrate a substance. For example,clear glass has an opacity of zero; a brickwall has an opacity of 100%. Opacity isused to indicate the degree of particulatepollution in air or water.

open community See community struc-ture.

open-pit mining The extraction of min-erals or metal ores that lie near the surfaceby removing the overlying rocks and soil toexpose the ore, which is then broken upand removed. Such mining techniques canproduce unsightly scars on the landscape.

operational sex ratio See sex ratio.

operon A genetic unit found in prokary-otes and comprising a group of closelylinked genes acting together and coding forthe various enzymes of a particular bio-chemical pathway. They function as a unit,

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since either all are transcribed or none.Transcription is controlled by regulatorgenes and proteins, and inducer molecules.Such systems are usually controlled by neg-ative feedback mechanisms. An example isthe lac operon in the bacterium Escherichiacoli, which is involved in the metabolism oflactose. In the absence of an inducer mol-ecule (the substrate lactose or one of itsderivatives), no transcription occurs. But if lactose binds to the regulator protein,transcription can proceed and enzyme syn-thesis is initiated. See also feedback mech-anism.

opportunist species See fugitive species.

optimal foraging theory The theorythat organisms attempt to balance the costsand benefits of feeding by adopting strate-gies that maximize their food intake perunit of time or minimize the time requiredto acquire sufficient food. Other factors,such as predator avoidance, may also betaken into account when simulating suchstrategies.

optimum similarity The degree of sim-ilarity between competing species at whichboth species show optimum FITNESS. Anychange would result in the lowering of fit-ness of individuals from at least one of thespecies. See also competitive exclusionprinciple.

order A TAXON that consists of a collec-tion of similar families. Plant orders gener-ally end in ales (e.g. Liliales) but animalorders do not have any particular ending.Orders may be divided into suborders.Similar orders constitute a class.

ordinal scale See measurement scales.

ordinate See graph.

Ordovician The second oldest period ofthe PALEOZOIC era, some 510–440 millionyears ago. It is characterized by an abun-dance of marine invertebrates (e.g. bra-chiopods and echinoderms) but an almosttotal absence of vertebrates apart fromsome jawless fish. Many of the inverte-

brates were primitive forms of life thathave no living representatives. See also ge-ological time scale.

ore A mineral that is mined and workedto extract a valuable constituent, such as ametal.

organ A part of an organism that ismade up of a number of different tissuesspecialized to carry out a particular func-tion. Examples include the lung of an ani-mal and the stem of a plant.

organelle See cell.

organic 1. Derived from or relating toliving organisms.2. (Chemistry) Describing a compoundthat contains carbon.

organic farming Farming that does notinvolve the use of chemical pesticides or ar-tificial (inorganic) fertilizers.

organic loading The amount of organicmatter in water. This arises naturally bythe decomposition of plants and animal re-mains, and from organic matter washedinto rivers and lakes from the surroundingland, but it may be greatly increased byrunoff and seepage of livestock manure,sewage, and industrial effluents into water-ways and groundwater. Deforestationcauses a significant increase in organic loaddue to the decomposition of the felled tim-ber and other vegetation and to the soilerosion that often follows. High organicloads may lead to EUTROPHICATION.

organic matter The organic componentof an ecosystem, especially the organic ma-terial present in soils.

Organization for Economic Coopera-tion and Development (OECD) An in-ternational organization founded in 1961to promote world trade, economic growthand employment, and an increase in livingstandards in member countries. It attemptsto boost this by liberalizing internationaltrade and facilitating the movement of cap-ital, including development aid, between

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Organization for Economic Cooperation and Development

countries. It also researches and compileseconomic data relating to natural re-sources, pollution and environmental deg-radation, farming, capital markets andtaxation, scientific research, and educa-tion.

organochlorines Organic compoundscontaining chlorine used in pesticides.Organochlorines are not readily brokendown and persist in animal tissues, affect-ing breeding success and causing otherproblems. They are banned from use inmost countries, but are still used in some,especially where insect-borne diseases suchas malaria are prevalent.

organomercury compounds See methyl-mercury compounds.

organophosphates Organic phosphate-containing chemicals, such as malathionand parathion, used to control pests suchas aphids and mites. Many are systemic in-secticides, being taken up by the roots ofplants and affecting any insect that bitesinto or sucks sap from them. These insecti-cides are nonselective and highly toxic, de-stroying natural predators of pest speciesand affecting animals even farther up theFOOD CHAIN. Insects also rapidly evolveresistance to them. Organophosphates,which are chemically similar to some nervegases, are also toxic to humans, affectingthe nervous system. Strict precautionsagainst skin contact and inhalation mustbe taken. Some organophosphates persistin the soil.

Oriental One of the six zoogeographicalregions of the earth. It includes the south-ern Asian countries of India, SoutheastAsia, and the western Malay archipelago.The characteristic fauna include the Indianelephant, rhinoceros, macaque, gibbon,orang-utan, jungle fowl, and peacock. Theboundary between this region and the Aus-tralasian region has been the subject ofcontention in the past.

origin of life Geological evidencestrongly suggests that life originated onEarth about 4600 million years ago. The

basic components of organic matter –water, methane, ammonia, and relatedcompounds – were abundant in the atmos-phere, which had much greater reducingproperties than today’s atmosphere. Untilrecently, it was widely accepted that lifeevolved in warm lagoons or hot springs,where energy from the sun (cosmic rays)and lightning storms caused simple mol-ecules to recombine into increasingly com-plex organic molecules that eventuallyshowed the characteristics of living organ-isms. However, recent discoveries suggestthat life may have evolved deep under-ground in fissures in hot rocks – a habitatin which today vast numbers of THER-MOPHILE bacteria live. Life probably had itsorigins in organic self-replicating mol-ecules that consumed the chemicals aroundthem to duplicate themselves. These mol-ecules were probably a form of RNA, nick-named the ur-gene – a combined replicatorand catalyst. RNA is a relatively unstablemolecule, and eventually the more stableDNA evolved, together with a system formaking RNA copies of it – a kind of prim-itive RIBOSOME. This was the Last UniversalCommon Ancestor (Luca).

oscillation A regular cyclical change inclimate, especially in global climate. SeeSouthern Oscillation; Pacific Decadal Os-cillation.

osmometer An instrument for measur-ing osmotic pressure (see osmosis).

osmoregulation The process by whichanimals regulate their internal osmoticpressure by controlling the amount ofwater and the concentration of salts intheir bodies, thus counteracting the ten-dency of water to pass in or out by osmo-sis.

osmosis The movement of solvent froma dilute solution to a more concentrated so-lution through a selectively permeablemembrane (or semipermeable membrane),i.e. one that allows the passage of somekinds of molecule and not others. For ex-ample, if a concentrated sugar solution (inwater) is separated from a dilute sugar so-

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lution by a selectively permeable mem-brane, water molecules can pass throughfrom the dilute solution into the concen-trated one by DIFFUSION but sugar mol-ecules will not cross the membrane.

Osmosis between two solutions willcontinue until they have the same concen-tration. If a certain solution is separatedfrom pure water by a membrane, osmosisalso occurs. The pressure necessary to stopthis osmosis is called the osmotic pressure(OP or π) of the solution. The more con-centrated a solution, the higher its osmoticpressure. Osmosis is a very important fea-ture of both plant and animal biology. Cellmembranes act as differentially permeablemembranes and osmosis can occur into orout of the cell. It is necessary for an animalto have a mechanism of OSMOREGULATION

to stop the cells bursting or shrinking. Inthe case of plants, the cell walls are slightlyelastic – the concentration in the cell can behigher than that of the surroundings, andosmosis is prevented by the pressure ex-erted by the cell walls (wall pressure).

Where the solvent is water, physiolo-gists now describe the tendency for waterto move in and out of cells in terms ofwater potential, which is the difference be-tween the energy of the system and that ofpure water. A solution that contains thesame concentration of osmotically activesolutes as the cell, so there is no net move-ment of water between the solution and thecell, is said to be iso-osmotic. A solutionthat has a higher concentration of osmoti-cally active solutes than the cell is termedhyperosmotic, and water moves out of thecell. Conversely, a solution with a lowerconcentration than the cell is hypo-osmotic, and water moves from it into thecell.

osmotic desert See physiological drought.

osmotic pressure See osmosis.

outbreeding Breeding between individ-uals that are not closely related. In plantsthe term is often used to mean cross-fertil-ization, and various methods exist to pro-mote it naturally, e.g. stamens or ovariesmaturing before pistils. In animals behav-

ioral mechanisms often promote outbreed-ing. The most extreme form – crossing be-tween species – usually results in sterileoffspring and there are various mecha-nisms to discourage it. Outbreeding in-creases heterozygosity (see heterozygous),giving more adaptable and more vigorouspopulations. Compare inbreeding.

outbreeding depression In species thatshow ecotypic variation over relativelysmall distances, OUTBREEDING may reduceFITNESS because the resulting individualwill not be so closely adapted to local envi-ronmental conditions as either of its par-ents.

outfall The point at which effluent isdischarged from a conduit, drain, or sewerinto receiving waters.

overcompensation An apparent in-crease in the growth, FITNESS, or reproduc-tive success of plants resulting fromincreased herbivory. The effects are oftendue to changes in plant architecture causedby grazing; for example, where removal ofa flowering stalk results in the productionof several more. Such effects are not neces-sarily advantageous, for example the seedsmay be smaller or there may be fewer pro-duced per flower.

overdispersion See dispersion.

overexploitation The excessive use ofnatural resources, resulting in environmen-tal degradation and long-term deleteriousecological effects.

overpopulation A population densitythat exceeds the capacity of the environ-ment to support it in a healthy state.

overturn See fall bloom.

oxidant A substance containing oxygenthat reacts chemically with other materialsto produce new substances. Oxidants areimportant contributors to PHOTOCHEMICAL

SMOG.

oxidation An atom, an ion, or a mol-

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ecule is said to undergo oxidation or to beoxidized when it loses electrons. Theprocess may be effected chemically, i.e. byreaction with an oxidizing agent, or electri-cally, in which case oxidation occurs at theanode. For example,

2Na + Cl2 → 2Na+ + 2Cl–

where chlorine is the oxidizing agent andsodium is oxidized, and

4CN– + 2Cu2+ → C2N2 + 2CuCNwhere Cu2+ is the oxidizing agent and CN–

is oxidized.

oxidation ditch (oxidation pond) Anartificial body of water in which organicwastes are decomposed by bacteria, e.g. asewage lagoon.

oxygen An element essential to livingorganisms both as a constituent of carbo-hydrates, fats, proteins, and their deriva-tives, and in aerobic RESPIRATION. It entersplants in both carbon dioxide and water,the oxygen from water being released ingaseous form as a by-product of photosyn-

thesis. Plants are the main, if not the only,source of gaseous oxygen and as such areessential in maintaining oxygen levels inthe air for aerobic organisms.

oxygen demand See biological oxygendemand; chemical oxygen demand.

oxygen method See dark bottle.

ozone (trioxygen; O3) A pale blue gaswith a pungent odor, and a powerful oxi-dizing agent. It is unstable and decomposesto oxygen on warming. It can seriously im-pair the respiratory system. See ozonelayer; photochemical smog.

ozone depletion A reduction over timeof the concentration of ozone in the OZONE

LAYER. See Farman; Molina; Rowlands.

ozone hole See ozone layer.

ozone layer (ozonosphere) A layer con-sisting of OZONE (O3) molecules scattered

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CFC

Earth

Sun

O2

O 3

O 2

O3

+ CI

O

CIO + O2

O

Cl + O 2

O 2

O3

O2

O +

O +O2

+ CIO+

+

+

+O

+C

l

UV

Ozone layer: chlorine from CFCs combine with ozone to form chlorine monoxide (ClO), whichreact with atomic oxygen (O) to form O2 and chlorine atoms. The presence of the chlorine atoms

has a significant effect on the amount of ozone present.

through the stratosphere roughly 15–50km above the Earth’s surface. The ozone isformed by the action of ultraviolet solar ra-diation on oxygen molecules. The ozonelayer absorbs about 99% of the harmfulULTRAVIOLET radiation entering the Earth’sATMOSPHERE, and hence provides a shieldfor living organisms without which life onEarth would be impossible. This absorp-tion of ultraviolet radiation has two conse-quences: it warms the stratosphere, helpingmaintain the temperature of the Earth’ssurface; and it stabilizes the stratosphere,preventing it from mixing with other lay-ers, so reducing winds in the lower atmos-phere.

In the early 1980s it was discoveredthat the ozone layer was becoming thinner,with seasonal ‘holes’ (ozone holes) appear-ing over Antarctica and elsewhere. This de-

pletion is believed to be caused by increas-ing atmospheric concentrations of chlorinecompounds, especially CFCS – very stablechemicals used as refrigerants and aerosolpropellants – which disrupt the delicatebalance between ozone production andbreakdown in the atmosphere. A singleatom of chlorine or fluorine can destroyabout 105 molecules of ozone. Otherozone-destroying chemicals include nitro-gen oxides from aircraft engine exhaustsand HALONS – gases widely used in industryand in certain kinds of fire extinguishers.Such findings led to the introduction in1987 of restrictions on the use of CFCs,with a complete ban proposed to take ef-fect by 2000. However, even with a ban inforce it may take many years before theozone layer is stabilized. See Montreal Pro-tocol.

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Pacific Decadal Oscillation (PDO) AnEL NIÑO-style shift in the temperature of thePacific Ocean that has two components, a15- to 20-year oscillation and a second os-cillation of about 70 years. It affects seasurface temperature, sea-level pressure,and surface wind stress. Although the tem-perature difference is only 1 to 2°C, it hasa significant effect on the climate of NorthAmerica, altering the path of the JET

STREAM, and hence of storms. Duringwarm phases there is increased PRODUCTIV-ITY in marine coastal ecosystems in Alaskaand decreased productivity off the remain-der of the west coast of the USA. The PDOis found in both north and south Pacific,but not in equatorial regions.

Palearctic One of the six zoogeographi-cal regions, including Europe, Russia andthe former Soviet states, northern Arabia,and the Mediterranean coastal strip ofAfrica. The fauna include the hedgehog,wild boar, and fallow and roe deer.

paleobotany See paleontology.

Paleocene The oldest epoch of the TER-TIARY PERIOD, 65–55 million years ago. Itwas a period of warm humid climate withlarge areas of temperate and subtropicalforests, but only limited grasslands, be-cause the grasses were of very recent origin.The dinosaurs had become extinct, andthere were various primitive mammals, in-cluding the first insectivores, rodents, andprimates. During the Paleocene there was agreat deal of COEVOLUTION of plants andanimals, the plants evolving adaptations toanimal pollination and fruit and seed dis-persal. In the latter part of the Paleocene,large angiosperm fruits and seeds evolvedas fruit- and seed-eating mammals also

evolved and diversified. See also geologicaltime scale.

paleoecology See paleontology.

paleontology The study of extinct or-ganisms, including their fossil remains andimpressions left by them. Sometimes thesubject is divided into paleobotany, thestudy of fossil plants, paleozoology, thestudy of fossil animals, and paleoecology,the study of past life and environments andthe interactions between them.

Paleozoic The first and oldest era inwhich multicellular life became abundant,about 590–230 million years ago. It is di-vided into six main periods: the Cambrian,Ordovician, Silurian, Devonian, Carbonif-erous, and Permian. Beginning withaquatic invertebrates and algae, the eraended with the invasion of land by treeferns and reptiles. See also geological timescale.

paleozoology See paleontology.

palynology (pollen analysis) The studyof living and fossil pollen. It is mainly usedas a means of obtaining information aboutthe composition and extent of past floras,and may give reliable quantitative informa-tion on the vegetative cover many thou-sands of years go. Study of living and fossilpollen can also be used as a character to de-termine plant PHYLOGENY and the natureand adaptation of pollen grains.

pampas Temperate GRASSLAND in SouthAmerica, found especially in Argentina.Originally dominated by tussock grasses,much of the wetter northeastern pampas is

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cultivated, while the drier areas remainunder short grasses and xerophytic shrubs.

Pangaea A vast landmass or superconti-nent, which once contained much of theEarth’s land. It broke up during the Meso-zoic era into two smaller landmasses,GONDWANALAND in the south and LAURASIA

in the north. See continental drift; plate tec-tonics.

parallel evolution (parallelism) The de-velopment of similar features in closely re-lated organisms as a result of strongselection in the same direction (see direc-tional selection). This may occur betweenspecies of the same genus that are widelyseparated geographically but which live insimilar environments. There are few exam-ples of this phenomenon and some author-ities deny its existence. A commonlyquoted example is the similarity betweenAustralian marsupials and placental mam-mals, which share a common ancestor inthe distant past. Both groups have speciesthat look and behave like wolves, cats,mice, moles, and anteaters. Compare con-vergent evolution.

paralytic shellfish poisoning See redtide.

parapatric speciation See speciation.

paraquat A nonselective contact HERBI-CIDE that causes severe and often irrevers-ible damage to lungs, liver, and kidneys ifingested. Paraquat becomes inert on con-tact with the soil.

parasite See parasitism.

parasitism An association between twoorganisms in which one, the parasite, ben-efits at the expense of the other, the host.The tolerance of the host varies from beingalmost unaffected to serious illness andoften death. An obligate parasite such aspotato blight (Phytophthora infestans) canlive only in association with the host,whereas a facultative parasite such as thedamping off fungus (Pythium) can exist inother ways, for example as a saprophyte.

Some parasites – ectoparasites – such asfleas (Order Siphonaptera) live on the out-side of their hosts, whereas others – endo-parasites – such as the pig tapeworm(Taenia solium) are internal parasites. Inbrood parasitism, e.g. the Europeancuckoo (Cuculus canorus) and SouthAmerican brown-headed cowbird (Molo-thrusater), the female birds lay their eggs inthe nests of other species, leaving the otherspecies to rear their young.

Parasites can also be classified into mi-croparasites (microscopic organisms suchas bacteria, fungi, viruses, and proto-zoans), and macroparasites (larger animalssuch as arthropods and worms).

parasitoid An animal (usually a wasp ora fly) that spends part of its life as a para-site, and part as a predator. For example,parasitic braconid wasps (family Bra-conidae) undergo their larval developmentinside the body of another insect, eventu-ally killing and consuming it, but are free-living as adults. Such wasps are importantagents of BIOLOGICAL CONTROL, used forkilling aphids and similar pests. See alsoparasitism.

parathion See organophosphates.

parental investment The energy andresources that a parent organism puts intocaring for its offspring, providing it withresources, so enhancing the survivalchances of the offspring.

parthenocarpy The development offruit in unfertilized flowers, resulting inseedless fruits. It may occur naturally, as inthe banana (Musa), which is triploid. Itmay also be induced artificially by the ap-plication of the plant hormone auxin, as incommercial tomato growing.

parthenogenesis The development ofunfertilized eggs to form new individuals.It occurs regularly in certain plants, e.g.dandelion (Taraxacum), in which meiosisis defective, and in some animals (e.g.aphids). Animals produced by partheno-genesis are always female and, if diploid,

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look exactly like the parent. See alsoapomixis.

partial parasite (hemiparasite) A para-sitic plant that has chlorophyll and canphotosynthesize, but depends on anotherplant for its water and mineral salts. An ex-ample is mistletoe.

particle size distribution The propor-tion of particles of different sizes in asample, usually expressed as weight per-centages. There are standard terms for par-ticles of different sizes. However, the sameterms may be assigned to different sizeranges by soil scientists and geologists.One of the commoner scales in use is theUdden–Wentworth particle size scale.

particulates 1. Substances made up ofseparate particles.2. Particles of pollutants. Two types arerecognized: primary particulates, whichoccur in smoke and industrial emissions;and secondary particulates, which areformed in the air by chemical reactionssuch as those involved in producing PHO-TOCHEMICAL SMOG. See also PM10.

pascal (Pa) The SI unit of pressure. 1pascal is a pressure of 1 newton per squaremeter. (A newton is the force needed togive an acceleration of one meter per sec-ond to one kilogram of mass.)

passerine A songbird with a perchinghabit. More than half of all living species ofbirds are passerines.

pasture Land covered in grass or othersmall plants, used as grazing by livestock.

patch dynamics The POPULATION DY-NAMICS associated with migration betweendiscrete patches of habitat. It provides thebasis for some models of competition.

patchiness 1. The degree to which aHABITAT shows spatial variation in its suit-ability for a particular species, or the de-gree to which the distribution of apopulation shows spatial variation withinthe habitat.

2. The distribution of different types ofHABITAT within a landscape. Habitat qual-ity may be affected by the presence of dif-ferent habitat types nearby, providing suchresources as nesting sites or pollinators.Landscape patchiness affects populationdynamics and the success and survival ofparticular species.

patch-use model An optimal foragingmodel concerned with how foraging ani-mals choose between patches containingprey.

pathogen Any organism that is capableof causing disease or a toxic response in an-other organism.

PDO See Pacific Decadal Oscillation.

peat Partially decomposed plant ma-terial that accumulates in waterloggedanaerobic conditions in temperate humidclimates, often forming a layer several me-ters deep. Peat varies from a light spongymaterial (sphagnum moss) to a densebrown humidified material in the lowerlayers. If mineral salts are present in thewaterlogged vegetation, neutral or alkalinefen peat is formed (the salts neutralize theacid produced by decomposition). If thereare no mineral salts in the water (as inrain), acid bog peat is formed. Peat is usedas a fuel and is the first step in COAL for-mation. It is also used to improve soil andas a component of potting compost in hor-ticulture. These uses have led to concernabout the destruction of natural peat bogs.See also bog; fen.

peck order See dominance.

ped See crumb structure.

pedogenesis The process of SOIL forma-tion from unconsolidated parent material.

pedology The study of the formation,structure, distribution, and classification ofSOILS.

pelagic Inhabiting the open upper wa-ters rather than the bed of a sea or ocean.

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Compare benthic zone. See also photiczone; plankton.

Pennsylvanian The US name for theUpper CARBONIFEROUS period.

peppered moth See industrial melanism.

peptide See protein.

per capita Per unit or individual of thepopulation or per person.

percolation The trickling of a liquidthrough a substance composed of solidparticles; for example the trickling ofsewage through a filter bed, or of rainwaterthrough soil. See also infiltration.

perennating organ In BIENNIAL orPERENNIAL plants, a storage organ that en-ables the plant to survive an adverse sea-son, such as winter or a dry season. Mostsuch organs are underground, e.g. rhi-zomes, corms, bulbs, swollen taproots, andtubers. Perennating organs are also used bymany plants in asexual reproduction (VEG-ETATIVE PROPAGATION).

perennial A plant that may live for sev-eral years. Perennials may reproduce intheir first growing season or may have toattain a certain age before seed productioncommences. Herbaceous perennials dieback each year and survive until the nextgrowing season as tubers (e.g. Dahlia),bulbs (e.g. daffodil), rhizomes (e.g. Iris),etc. Woody perennials, such as trees andshrubs, persist above ground throughoutthe year but may show adaptations (e.g.leaf fall) to survive unfavorable seasons. Afew plants exhibit semelparity, living forseveral years without flowering and thenflowering once and dying after settingfruit, e.g. the century plants, Agave (com-pare iteroparity). See also deciduous; ever-green; perennating organ.

period See geological time scale.

periphyton A community of micro-scopic organisms, such as diatoms, algae,

and cyanobacteria, that live on underwatersurfaces.

permafrost See tundra.

permanent wilting point The point atwhich soil has dried to the extent thatplants can no longer remove the remainingwater held in the soil particles, and begin towilt and will not recover unless more wateris added to the soil. At the permanent wilt-ing point the water potential (see osmosis)of the soil is equal to or lower than thewater potential of the plant. See also fieldcapacity.

permeability The ease with which asubstance (e.g. rock or soil) will allow a liq-uid or gas to pass through it.

Permian The most recent period of thePALEOZOIC era, some 280–250 millionyears ago. Life became dominated by a fewtypes of reptiles, while amphibians weregreatly reduced in number and size. Insectgroups appeared. As the climate warmedand dried during the Permian, the ferns,seed ferns, and clubmosses that dominatedPermian marshes and swamps were joinedby the evolving conifers and some othergymnosperms, whose evolution was prob-ably driven by the need to cope with drierenvironments. These groups graduallybegan to replace the older floras domi-nated by nonseed-bearing plants. See alsogeological time scale.

persistence The length of time an intro-duced substance stays in the environmentbefore it is decomposed, degraded, orwashed away. Persistence has a big effecton the potential toxicity of POLLUTANTS:less toxic pollutants that are persistent,such as organochlorines, may cause moreharm than highly toxic pollutants, such ascyanides, that have low persistence.

persistent organic pollutants (POPs)Introduced organic compounds that areharmful to the environment or to humanhealth and that remain for along time inthe environment without being brokendown – longer than one growing season or

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one year after introduction. See persis-tence.

Peru Current A cold-water ocean cur-rent that flows north along the coast ofSouth America in the southeast PacificOcean, especially along Chile, Peru, andEcuador, causing fog in coastal areas, butkeeping the rainfall very low. The flow isreinforced by upwelling of deep water dueto the effects of surface winds and the CORI-OLIS FORCE, making the waters rich in nu-trients, supporting a rich fishing industry.During EL NIÑO episodes, the current isforced farther south, leading to a suddendecline in plankton, fish, and bird popula-tions and heavy storms.

pest Any species considered undesirablein its present location, especially one thatcompetes with humans for food, fiber, orshelter, feeds on people, contaminates foodand goods, transmits pathogens, and gen-erally threatens human health or comfort.

pest control Limiting the numbers ofpest populations by killing them, prevent-ing them from breeding, or preventingthem from attacking people, livestock orcrops, or from any other undesirable activ-ities. See biological control.

pesticide A chemical that kills PESTS. Seealso botanical insecticide; contact pesti-cide; contact pesticide; fungicide; insecti-cide; pesticide tolerance; systemic pesti-cide.

pesticide tolerance 1. The developmentof tolerance of (resistance to) PESTICIDES.During successive generations individualsmay arise whose enzymes or other meta-bolic processes can tolerate the presence ofpesticides or break them down in the bodyto harmless substances. Such resistant indi-viduals are more likely to survive in thepopulation and pass on their genes for re-sistance, so resistance will gradually spreadthrough the population. For example,some rats have become resistant to war-farin. One way to overcome this is to ad-minister more than one poison or pesticideat once, because very few individuals will

have resistance to more than one type. Seealso pesticide treadmill.2. A legal limit, enforced by the UnitedStates Environmental Protection Agency,for the amount of a chemical residue that ispermitted to remain in or on a harvestedfood or feed crop as a result of the applica-tion of a chemical for pest control pur-poses. Such limits are designed to be wellbelow the point at which the chemicalwould pose a health risk to consumers.

pesticide treadmill The cycle of eventsin which the application of pesticides leadsto pests evolving resistance and secondarypests arising that were originally preyed onor outcompeted by organisms affected bythe pesticides, requiring the application ofmore and different pesticides, leading to re-sistance to these new chemicals and moresecondary pests, and so on. See also pesti-cide tolerance.

pest pressure hypothesis The idea thatconsumers readily locate and feed on abun-dant species, so reducing their densities, al-lowing other, less common species to growand multiply, so that herbivores can actu-ally promote high diversity in plants.

petrochemical A chemical manufac-tured from the products of refining crudeoil and coal carbonization, or produced asa fuel oil residue.

petrol See petroleum.

petroleum A mixture of hydrocarbonsformed originally from marine animals andplants, found beneath the ground trappedbetween layers of rock. It is obtained bydrilling (also called crude oil). Differentoilfields produce petroleum with differingcompositions. The mixture is separatedinto fractions by fractional distillation in avertical column. The main fractions are:Diesel oil (gas oil) in the range 220–350°C,consisting mainly of C13–C25 hydrocar-bons. It is used in diesel engines.Kerosene (paraffin) in the range 160–250°C, consisting mainly of C11 and C12hydrocarbons. It is a fuel both for domes-tic heating and jet engines.

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Gasoline (petrol) in the range 40–180°C,consisting mainly of C5–C10 hydrocar-bons. It is used as motor fuel and as a rawmaterial for making other chemicals.Refinery gas, consisting of C1–C4 gaseoushydrocarbons.

In addition lubricating oils and paraffinwax are obtained from the residue. Theblack material left is bitumen tar.

petroleum gas See natural gas.

pH A measure of the acidity or alkalinityof a solution on a scale 0–14. A neutral so-lution (such as pure water) has a pH of 7.Acid solutions have a pH below 7; alkalinesolutions have a pH above 7. The lower thepH value, the higher the concentration ofH+. The pH is the logarithm to base 10 ofthe reciprocal of the hydrogen-ion concen-tration of a solution; i.e. log10 1/[H+].

phage See bacteriophage.

phagoplankton See plankton.

phanerophyte See Raunkiaer’s life-form classification.

Phanerozoic See Precambrian.

phenology The study of biological phe-nomena in relation to the seasons (e.g. mi-gration and seasonal breeding).

phenotype The observable characteris-tics of an organism, which are determinedby the interaction of the GENOTYPE with theenvironment. Many genes present in thegenotype do not show their effects in thephenotype because they are masked bydominant alleles (see dominant). Genotyp-ically identical organisms may have verydifferent phenotypes in different environ-ments, an effect particularly noticeable inplants grown in various habitats.

phenotypic variance (Vp) A statisticalmeasure of the variation in value of a phe-notypic character between individuals of a population. Compare phenotypic varia-tion.

phenotypic variation Variation in theappearance of individuals in a population.This is due to the effects of both the GENO-TYPE and the environment. Compare phe-notypic variance.

phloem Plant vascular tissue in whichfood is transported from areas where it ismade to where it is needed or stored. Itconsists of sieve tubes (sieve elements),which are columns of living cells (thoughwithout nuclei) with perforated end wallsthat allow passage of substances from onecell to the next; and companion cells, fullof mitochondria, which provide energy forthe transport. Phloem also contains fibers(sclerenchyma) and parenchyma.

phosphates See phosphorus cycle.

phosphorescence 1. The absorption ofenergy by atoms followed by emission ofelectromagnetic radiation. Phosphores-cence is a type of luminescence, and is dis-tinguished from fluorescence by the factthat the emitted radiation continues forsome time after the source of excitation hasbeen removed. In phosphorescence the ex-cited atoms have relatively long lifetimesbefore they make transitions to lower en-ergy states. However, there is no definedtime distinguishing phosphorescence fromfluorescence.2. In general usage the term is applied tothe emission of ‘cold light’ – light producedwithout a high temperature. The lightcomes from excited atoms produced di-rectly in the reaction – not from the heatproduced. It is thus an example of chemi-luminescence. In biological systems, thelight emitted is often light that has beenpreviously absorbed, but which is re-emit-ted at a different wavelength. Phosphores-cent seas are due to the bioluminescence ofDINOFLAGELLATES, in particular Noctilucaspecies. Mechanical disturbance of thewater causes them to emit flashes of blue light, which is produced by enzyme-catalyzed reactions in special vesicles calledscintillons. Phosphorescence may some-times be seen on rotting wood from certainfungi (known as ‘fox fire’). See also bio-luminescence.

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phosphorus One of the essential el-ements (macronutrient) in living organ-isms. Phosphorus has an important role inthe metabolic activities in plants and ani-mals and is essential for plant growth andskeleton formation in vertebrates. Phos-phorus may be added to soils as inorganicbasic slag or as phosphate fertilizers, or inorganic fertilizers such as GUANO andbonemeal.

phosphorus cycle The circulation ofphosphorus between organisms and the en-vironment. Phosphorus occurs in the envi-ronment as (ortho)phosphates (PO4

3–)derived from the WEATHERING of rocks, es-pecially of the mineral apatite, as organicphosphates released during the DECOMPOSI-TION of dead organisms, and as colloidalphosphate, a form not available to organ-isms. High levels of aluminum and ironmay cause phosphates to precipitate out ofthe soil, becoming unavailable to living or-ganisms. Phosphorus occurs in natural wa-ters in the form of soluble phosphatesderived from runoff and water percolatingthrough the soil and into lakes and water-ways. Its concentration is often boosted byrunoff from land treated with fertilizersand from sewage effluents. Rivers carryphosphates into the oceans. Phosphates areassimilated from the soil by soil organismsand plant roots, and from water by bac-teria and phytoplankton. In lakes the cy-cling of phosphorus is affected bySTRATIFICATION and seasonal TURNOVER.See also biogeochemical cycle; eutrophica-tion.

photic zone In lakes, seas, and oceans,the zone of surface water penetrated bylight. It is divided into two further zones bythe compensation depth (the depth atwhich the rate of carbon fixation equalsthe rate of carbon loss by respiration). Thezone above this is termed the euphotic zoneand the zone below the dysphotic zone.The depth of the euphotic zone depends onthe turbidity of the water. In the dysphoticzone the light penetration or temperature isso low that oxygen consumption by respi-ration exceeds oxygen production by pho-tosynthesis. Compare aphotic zone.

photoautotroph See autotroph.

photobiont See lichens.

photochemical air pollution Seeozone layer; pollution.

photochemical smog A type of SMOG

that occurs most commonly in urban areasand is caused by the photochemical reac-tions of the exhaust emissions of vehicleswith sunlight. These reactions produce thehighly toxic gas OZONE, as well as nitrogendioxide. Photochemical smog has a brown-ish tinge, and causes reduced visibility, ag-gravation of the respiratory tract, irritationof the eyes, and damage to plants.

photoheterotroph A phototrophic or-ganism that uses organic compounds as itsmain source of carbon. See phototroph.

photoinhibition 1. The reversible inhi-bition of PHOTOSYNTHESIS in certain leavesby light of a particular quality.2. The stopping or slowing of a process bylight, for example, germination of someseeds is inhibited by light.

photoperiodism The response of an or-ganism to changes in day length (photope-riod). In plants, leaf fall and flowering arecommon responses to seasonal changes inday length, as are migration, reproduction,molting, and winter-coat development inanimals. Many animals, especially birds,breed in response to an increasing springphotoperiod, a long-day response. Someanimals (e.g. sheep, goats, and deer) breedin autumn in response to short days so thatoffspring are born the following spring.The substance melatonin, produced by thepineal gland, is thought to play a role inregulating such changes. Diapause and sea-sonal changes in form, as in aphids, arephotoperiodically induced in insects. Suchresponses are especially common in speciesof high and mid-latitudes, where daylength is variable and provides a good indi-cator of the time of year. The length of daythat triggers a photoperiodic response iscalled the critical day length.

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Plants are classified as short-day plants(SDPs), e.g. cocklebur, or long-day plants(LDPs), e.g. cucumber and barley, accord-ing to whether they flower in response toshort or long days. Day-neutral plants, e.g.pea and tomato, have no photoperiodic re-quirement. However, the critical factor isnot the length of the day, but the length ofthe dark period, because flowering of SDPsis inhibited by even a brief flash of red lightin the dark period, and an artificial cycle oflong days and long nights inhibits flower-ing in LDPs. The response is mediated by apigment called phytochrome, a proteinthat can absorb red light, which converts itto a form (PFR) that absorbs far-red light.Absorption of far-red light causes the phy-tochrome to revert to the red light-absorb-ing form (PR). This change is linked tosequences of metabolic reactions that trig-ger the physiological or morphological re-sponse of the plant. Compare circadianrhythm.

photophosphorylation See photosyn-thesis.

photorespiration A light-dependent meta-bolic process of most green plants that re-sembles true (or ‘dark’) RESPIRATION only inthat it uses oxygen and produces carbondioxide. Carbon dioxide production dur-ing photorespiration may be up to fivetimes greater than in dark respiration. Pho-torespiration wastes carbon dioxide andenergy, using more ATP than it produces(see C3 plant). Photorespiration rates in C4PLANTS are negligible. Attempts are beingmade to introduce the C4 photosyntheticsystem into important C3 crop plants bymeans of genetic engineering. This may bean advantage for crops in the tropics, be-cause the rate of photorespiration is muchgreater at higher temperatures.

photosynthesis The synthesis of or-ganic compounds using light energy ab-sorbed by the green pigment CHLOROPHYLL.With the exception of a small group of bac-teria, organisms photosynthesize frominorganic materials. All green plants pho-tosynthesize, as well as algae and certainbacteria. In green plants, photosynthesis

takes place in CHLOROPLASTS, mainly inleaves and stems. Directly or indirectly,photosynthesis is the source of carbon andenergy for all except chemoautotrophic or-ganisms. The mechanism is complex andinvolves two sets of stages: light reactionsfollowed by dark reactions. The overall re-action in green plants can be summarizedby the equation:

CO2 + 4H2O → ?[CH2O?] + 3H2O + O2

The light reactions of photosynthesisinvolve reaction centers in the chloroplastsand involves chlorophyll a molecules. Thewhole unit is called a photosystem. Thereare two kinds of photosystems in greenplants, Photosystems I and II, which useslightly different forms of chlorophyll a.Both are usually involved in the light reac-tions of photosynthesis. Light energy ispassed to the reaction center chlorophyllmolecules, where it excites electrons toleave the photosystems and pass along anelectron transport chain. The electrons arereplaced by electrons from the photolysisof water, releasing molecular oxygen andprotons:

2H2O → O2 + 4H+ + 4e-

These reactions are collectively calledthe light reactions.

Energy released as the electrons flowalong the electron transport chain is usedto generate ATP (adenosine triphosphate),a process called photophosphorylation,and reducing agents (NADPH). ATP andNADPH are used in the stroma (non-mem-brane portion) of the chloroplast to reducecarbon dioxide to carbohydrate and forother syntheses – the so-called dark reac-tions. Carbon dioxide is ‘fixed’ by combi-nation with the 5-carbon sugar ribulosebisphosphate (RuBP), forming two mol-ecules of phosphoglyceric acid (PGA). Thisreaction is catalyzed by the enzyme ribu-lose bisphosphate carboxylase (rubisco). Ina series of reactions using NADPH andATP from the light reactions, PGA is con-verted to a succession of 3-, 4-, 5-, 6- and7-carbon sugar phosphates in a series of re-actions collectively termed the Calvin cycleor Calvin–Benson cycle. These productsare then used in the synthesis of carbohy-

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drates, fats, proteins, and other com-pounds, and RuBP is regenerated.

In determining photosynthesis rates in awhole plant, a distinction is made betweengross photosynthesis (GP, the total carbondioxide uptake or total oxygen released inthe light) and net photosynthesis (NP,which is GP minus the carbon dioxide re-leased or oxygen consumed by respira-tion): NP = GP – R. Net photosynthesis isthe actual net carbon dioxide uptake oroxygen evolution that is measured in a con-trolled experiment.

photosynthetic bacteria A group ofbacteria able to photosynthesize throughpossession of chlorophyll pigments. Theyinclude the CYANOBACTERIA, green sulfurbacteria (Chlorobia), green nonsulfur bac-teria (Chloroflexa), purple sulfur bacteria,and purple nonsulfur bacteria (both in thephylum Proteobacteria). The photosyn-thetic bacteria fix carbon using the Calvincycle, but use a variety of hydrogensources. The Cyanobacteria derive hydro-gen from water, the sulfur bacteria usehydrogen sulfide or reduced sulfur com-pounds, and the nonsulfur bacteria obtainhydrogen from organic sources, such asethanol, lactate, or pyruvate.

phototroph An organism that uses lightenergy to synthesize its organic require-ments. Compare chemotroph.

phycobiont See lichens.

phylogeny The evolutionary history ofgroups of organisms, in particular the rela-tionships between groups of organismsbased on their past evolutionary history. Itprovides a basis for classification.

phylum (pl. phyla) One of the majorgroups into which a kingdom of organismsis classified. Phyla may be divided into sub-phyla. In some plant classifications (espe-cially older ones) the term ‘division’ is usedinstead of phylum.

physiognomy The study of the struc-ture of natural communities.

physiographic factors Environmentalfactors other than climatic, biotic, andedaphic (soil) factors, which affect the pre-vailing conditions in a habitat and the dis-tribution of plants and animals. Theyinclude factors relating to topography,such as altitude, slope, drainage, and de-gree of erosion of the land.

physiological drought A situation inwhich plants are unable to take up suffi-cient water even though there is water inthe soil. Physiological drought occurs insalt marshes and other brackish coastalhabitats, and in deserts where there arehigh concentrations of salts in the upperlayers of the soil. It also occurs in coldweather because the permeability of the en-dodermal cells decreases rapidly below5°C, so the root resists the entry of water.See osmosis.

physiological ecology The study of thephysiological processes in organisms in re-lation to their environments.

physiology The way in which organ-isms or parts of organisms function. Com-pare MORPHOLOGY.

phytoalexin A chemical produced by aplant that inhibits the growth of a patho-genic fungus.

phytochrome See photoperiodism.

phytogeography (plant geography) Thestudy of the geographical distribution ofplant species.

phytophagous Describing an organismthat feeds on plants.

phytoplankton See plankton.

phytoremediation See bioremediation.

phytosociology The classification ofplant COMMUNITIES based on their speciescomposition and distribution. Modernphytosociology uses quantitative methodsbased on computer analysis of data. It en-compasses not only the geographical distri-

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bution and classification of plant com-munities, but also their organization, de-velopment, and interdependence.

phytostimulation See rhizodegradation.

phytotoxic Describing a substance thatis toxic to plants.

phytotron A large chamber or green-house with a precisely controlled environ-ment in which plants may be grown forexperimentation. Such factors as tempera-ture, light, humidity, and photoperiod canbe varied or held constant as desired.

picoplankton See plankton.

piezometer A device for measuring thehydraulic head of groundwater at a partic-ular level. It consists of a narrow observa-tion well that allows groundwater to enterat a particular depth.

pine forest See boreal forest.

pioneer species A species that colonizesa new physical environment, for example,land exposed by retreating glaciers, fellingof forest, or bare mud on coasts or estuar-ies. Pioneer species are the first stage inplant SUCCESSION. Most are fast-growingand fairly short-lived, with small seeds andgood powers of seed dispersal. They maymodify the environment so that newspecies can move in and perhaps replacethem.

Pisces A term that includes the twoclasses of fish – Osteichthyes (bony fish)and Chondrichthyes (cartilaginous fish).Fish are cold-blooded aquatic vertebrateswith a streamlined body, a powerful mus-cular finned tail for propulsion, and pairedpectoral and pelvic fins for stability andsteering. There is usually a body coveringof scales. They breathe by means of inter-nal gills, which open to the outside throughgill slits or pores. The bony fish have a skel-eton of bone, only one external gill openingeach side, which is covered by a flap calledan operculum, and a body covering ofoverlapping scales. The cartilaginous fish,

which include the sharks, rays, and skates,are mostly predatory fish with a cartilagi-nous skeleton, separate gill openings notcovered by an operculum, and a skin cov-ering of small scales that are modified inthe mouth as rows of teeth.

pitfall trap A device for the live captureof ground-dwelling invertebrates, whichconsists of a jar containing bait, which isburied in the ground with its mouth atground level. A raised cover protects the jarfrom rain and also prevents larger animalsfrom exploring it. Small invertebratescrawl under the cover and fall into the jar.

plagioclimax See climax community.

plankton A varied collection of aquaticorganisms that drift freely, not being at-tached to any substrate and not possessingany organs for locomotion. The most im-portant components of the plant plankton(phytoplankton) are the DIATOMS uponwhich the planktonic animals (zooplank-ton) (e.g. crustaceans) feed. Other mem-bers of the phytoplankton includemicroscopic algae and CYANOBACTERIA.The larvae of many species (e.g. cod) makeup a large part of the plankton, especiallyin early summer. The plankton form thebasis of the FOOD CHAIN in the sea. Plank-ton may be classified according to the sizeof the organisms: picoplankton are lessthan 2 µm in diameter, nanoplankton be-tween 2 and 20 µm, microplankton be-tween 20 and 200 µm, and mesoplanktonover 200 µm in diameter. Plankton largerthan 25 µm in diameter are sometimescalled net plankton, because they are largeenough to be caught in a plankton net.Compare nekton.

Plantae The kingdom that containsplants. It contains organisms that can (usu-ally) make their own food by taking in sim-ple inorganic substances and building theseinto complex molecules by PHOTOSYNTHE-SIS. This process uses light energy, ab-sorbed by the green pigment CHLOROPHYLL,which is found in all plants but no animals.There are a few exceptions, in the form ofcertain parasitic plants. Most plants have

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cellulose cell walls, and have starch (seecarbohydrates) as a storage polysaccha-ride, whereas animals have no cell walls,store glycogen, and do not have plastids orchlorophyll; and fungi usually lack cellu-lose and also do not form plastids. Plantslack motility, with the exception of the ga-metes of many species. One major charac-teristic that distinguishes plants from otherplantlike organisms, such as algae or fungi,is the possession of an embryo that is re-tained and nourished by maternal tissue.Fungi and algae lack embryos and developfrom spores. Plants also differ from fungiin having a regular alternation of diploidand haploid generations. Compare algae;Animalia; Fungi; Protoctista.

plant breeding The selection and im-provement of plants for economic or aes-thetic purposes. At its simplest, it involveschoosing individuals with desirable charac-teristics, controlling their mating, and se-lecting suitable individuals from theirprogeny for further breeding. It may justconsist of choosing the best seeds to plantfor the next year’s crop. Breeding can becontrolled by hand-pollination, develop-ment of hybrids (and induction of POLY-PLOIDY to render the hybrid progenyfertile), and vegetative propagation to getidentical progeny. Additional variationmay be introduced by chemicals or radia-tion that induce mutations. Genetic engi-neering now enables scientists to producehighly specific combinations of geneticallydetermined characteristics, and tissue cul-ture allows the rapid propagation of se-lected varieties and individuals (clones).See hybridization; vegetative propagation.

plant-derived pesticide See botanicalinsecticide.

plasma membrane See cell.

plasmid A genetic element found withinbacterial cells that is not part of the bacte-rial DNA and replicates independently ofit. Plasmids are widely used as cloning vec-tors (agents that transfers pieces of ‘for-eign’ DNA) in GENETIC ENGINEERING.

plasmolysis Loss of water from awalled cell (e.g. of a plant or bacterium)due to OSMOSIS to the point at which theprotoplast shrinks away from the cell wall.

plasticity The ability of an organism tochange its form in response to changing en-vironmental conditions. For example,some plants develop hairier leaves if movedto a windy site; leaves growing in shadehave thinner broader laminae and lessdensely packed mesophyll than leavesgrowing in full sunlight. These changes canoccur only in parts that develop and growafter the environmental change. They arephenotypic changes that reflect changes inthe activation of genes, but not in the genesthemselves.

plate tectonics A theory formulated inthe 1960s to explain the phenomenon ofCONTINENTAL DRIFT and the formation ofmajor features on the Earth’s surface. Itsees the Earth’s crust as a mosaic of rigidmajor and minor plates up to 100 kmthick, which move relative to each other asa result of convection currents in the man-tle below. The plates bearing the conti-nents are thicker and less dense than thosebearing the ocean floor.

Where two plates are moving apart,molten rock (magma) from the mantlewells up in the space between them, hard-ening to new crust, forming an underwaterridge of mountains called a mid-oceanicridge (e.g. the mid-Atlantic ridge, on whichIceland sits). As new rock wells up in themiddle of the ridge it pushes the mountainflanks apart, causing the sea floor to spread(sea-floor spreading). On land, such up-wellings form rift valleys, deep gorges withmountains on either side, such as the EastAfrican Rift Valley.

Where a continental and an oceanicplate moving toward each other meet, as ishappening along the west coast of theNorth America, a subduction zone isformed, where the denser plate is forcedunder the lighter one. The descending platemelts to form magma, which may then sur-face through cracks and faults, formingvolcanoes. Where two plates of continentalcrust meet, the result is a crumpling of the

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rocks to form mountains such as the Hi-malayas and the European Alps.

Where two plates slide past each other,friction may build up, and sudden move-ments releasing it cause earthquakes, e.g.along the San Andreas Fault in California.

Platyhelminthes A phylum of primitivewormlike invertebrates, the flatworms, in-cluding the classes Turbellaria (aquaticfree-living planarians), and the parasiticTrematoda (flukes) and Cestoda (tape-worms). Flatworms are unsegmented ani-mals with no coelom or blood system. Theflat body provides a large surface area forgaseous exchange. The gut, when present,has only one opening (the mouth) and asucking pharynx. Tapeworms, which livein the guts of other animals, have no gut,but simply absorb dissolved nutrients allover their body surface.

pleiomorphism The occurrence of dif-ferent morphological stages during the lifeof an organism. Examples are the larval,pupal, and adult forms of an insect. Com-pare polymorphism.

Pleistocene The first epoch of the QUA-TERNARY PERIOD, which started with aGLACIATION about 2 million years ago andended with the last glaciation about10 000 years ago. Several Ice Ages drovemany organisms toward the Equator whileothers (e.g. mammoth) became extinct. Forexample, TUNDRA covered temperate partsof the USA and Central Europe wheretoday deciduous forests are the naturalvegetation. Many present-day mammals ofSouth America and Africa resemble pre-IceAge mammals of Europe. Modern humans(Homo sapiens) evolved during this period.See geological time scale; Ice Age.

Pliocene The epoch of the TERTIARY PE-RIOD, about 7–2 million years ago, whichfollowed the MIOCENE. In the Pliocene thehominids, such as Australopithecus andHomo, became clearly distinguishablefrom the apes. Grasses became more abun-dant, perhaps because the climate was be-coming drier. Modern conifers werespreading, and leptosporangiate ferns un-

derwent adaptive radiation. See also geo-logical time scale.

plutonium (Pu) A radioactive silverymetallic element found only in minutequantities in uranium ores but readily ob-tained, as 239Pu, by neutron bombardmentof natural uranium. The readily fissionable239Pu is a major nuclear fuel and nuclearexplosive. Plutonium is highly toxic be-cause of its radioactivity; in the body it ac-cumulates in bone.

PM10 The concentration (in mg m-3) ofparticles in the atmosphere that have a di-ameter equal to or less than 10 µm. It ispollutant PARTICULATE level standard, usedto measure compliance with the CLEAN AIR

ACT. Such particulates are believed to bethe most damaging to human health.

podzol (podsol) The type of SOIL foundunder heathland and coniferous forests intemperate and boreal climates where thereis heavy rainfall and long cool winters,such as the BOREAL FORESTS of northernNorth America and Eurasia. It is stronglyacid and often deficient in nutrients, espe-cially iron compounds and lime, as a resultof leaching by the heavy rain or snow-meltin spring. Beneath the humus layer lies ableached horizon (A horizon) composedmainly of quartz sand. In the clay-rich Bhorizon below the leached layer iron com-pounds accumulate, staining the layerbrown and forming an impermeable HARD-PAN that prevents drainage. Where thislayer is sufficiently impermeable to main-tain the A horizon in a waterlogged condi-tion, the soil is termed a GLEY podsol. Thetype of vegetation that grows on podsolstends to produce highly acidic leaf litter,which further increases the acidity of thesoil.

pogonophores Tube worms that growup to 3 m long, found in large numbers inthe communities that live around HY-DROTHERMAL VENTS on the ocean floor.They contain symbiotic bacteria that fixcarbon using energy from the oxidation ofhydrogen sulfide, forming the basis of theFOOD CHAIN.

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poikilothermy The condition of havinga body temperature that varies approxi-mately with that of the environment. Mostanimals other than birds and mammals arepoikilothermic (‘cold-blooded’). Comparehomoiothermy.

point quadrat See cover.

point source A single identifiablesource from which pollutants are dis-charged, e.g. an outfall pipe or chimney.

Poisson distribution A kind of datadistribution in which the variance is equalto the area. It occurs when very large num-bers of samples or measurements aretaken, so that even when an event or valuehas a low probability of occurring, it willoccur eventually. For example, it is typicalof the measurement of plant DENSITY usingQUADRATS.

polar vortex A region of extremely coldair that forms over Antarctica in winter,which is surrounded by strong westerlywinds that isolate the cold air from sur-rounding air masses in lower latitudes.Temperatures in the vortex may fall as lowas –200°C in the lower stratosphere. Theseparticular conditions are ideal for chemicalreactions between sulfuric acid aerosols,chlorine, and nitrogen oxides, causing se-vere depletion of the OZONE LAYER.

polder An area of flat low-lying land,drained and reclaimed from the sea andprotected from inundation by embank-ments, dams, or levees. Polders are oftenbelow sea level. They are particularly com-mon in the Netherlands and Belgium.

pollarding A method of pruning that in-volves cutting off the branches to leaveabout 2 m of trunk above ground. Thisproduces new bushy growth out of reach ofgrazing animals. It is commonly seen inriverside willows, in street trees to provideshade, and in certain woodland species toprovide a source of small poles for fencingor firewood out of reach of grazing mam-mals. See also coppice.

pollen The male spores of seed plants,produced in large numbers in the pollensacs, or stamens. See palynology.

pollination The transfer of pollen fromthe anther to the stigma in angiosperms,and from the sporangiophores to the mi-cropyle in gymnosperms. Plants may beself-pollinating, e.g. barley (Hordeum vul-gare), thus ensuring that seed will be set.Cross pollination (the transfer of pollenfrom one individual to another), also takesplace and is brought about by wind, water,insects, or other animals.

pollutant A contaminant in a concen-tration that adversely changes the environ-ment. A pollutant may be chemical (e.g.CFCs), of biological origin (e.g. sewage),physical (e.g. RADIOACTIVE WASTE, dustfrom soil erosion, heat emissions, noise), orpathogenic in nature (e.g. Cryptosporid-ium bacteria in swimming pools). Primarypollutants are those that are emitted intothe environment from a specific source,e.g. sulfur dioxide from power plants. Sec-ondary pollutants are formed in the envi-ronment itself, e.g. photochemical smog.

pollutant standard index (PSI) An in-dicator of one or more pollutants used towarn people about the current potentialhealth risks, for example, an indicator ofthe sulfur dioxide concentration in anurban area.

polluter-pays principle The principlethat those who cause pollution shouldmeet the costs of clearing it up. This issometimes extended to include the costs ofmaking good the damage it causes.

pollution Any damaging or unpleasantchange in the environment that resultsfrom the physical, chemical, or biologicalside effects of human activities. Pollutioncan affect the atmosphere, rivers, seas, andthe soil.

Air pollution is caused by the domesticand industrial burning of carbonaceousfuels, by industrial processes, and by vehi-cle exhausts. Industrial emissions of sulfurdioxide cause ACID RAIN, and the release

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into the atmosphere of CFCS, used in re-frigeration, aerosols, etc., leads to thedepletion of the OZONE LAYER in the strato-sphere. Carbon dioxide, produced byburning fuel and by motor vehicle ex-hausts, is slowly building up in the atmos-phere, and is one of the factors leading toan overall increase in the temperature ofthe atmosphere (see greenhouse effect). Ve-hicle exhausts also contain carbon monox-ide, nitrogen oxides, and other hazardoussubstances, such as fine particulate dusts.Lead was formerly a major vehicle pollu-tant, but the widespread introduction oflead-free gasoline has eliminated this prob-lem in most countries. PHOTOCHEMICAL

SMOG, caused by the action of sunlight onhydrocarbons and nitrogen oxides fromvehicle exhausts, is a problem in manymajor cities, and may contain significantamounts of ozone. In plants, gaseous pol-lutants may inhibit stomatal action, dam-age leaf surfaces, or inhibit enzymes. Sulfurdioxide causes chlorosis between the leafveins, and the death of leaves in foresttrees. Oxides of nitrogen can cause blacknecrotic lesions on leaves. Acid rain causedby sulfur dioxide and nitrogen oxides leadsto the release of toxic aluminum in soils,and inhibition of nutrient uptake. Fine par-ticulate dust blocks stomatal pores andprevents light from reaching the leaf cells.Not all air pollution results from humanactivity. For example, volcanic activity dis-charges vast quantities of ash and toxicgases into the atmosphere. Also, naturalsandstorms and vegetation fires contributedust and smoke, but the incidence of theseevents may be exacerbated by DEFORESTA-TION and DESERTIFICATION brought aboutby humans.

Water pollution and soil pollution iscaused by dissolved chemicals, suspendedparticulates, and floating substances, suchas oil. Such pollutants include substancesthat are biodegradable, such as sewage ef-fluent and nitrates leached from agricul-tural land, which if allowed to enter watercourses can lead to EUTROPHICATION andalgal BLOOMS. Nonbiodegradable pollu-tants, such as certain chlorinated hydrocar-bon pesticides (e.g. DDT) and HEAVY

METALS, such as lead, copper, and zinc in

some industrial effluents, accumulate inthe environment. Heavy metals, even whenpresent at low concentrations, are toxic toplants, inhibiting water or nutrient uptake,damaging cell membranes, and inhibitingenzymes. Soil contaminated with fuel ashmay contain increased levels of boron,which at high concentrations causeschlorosis and necrosis to tissues. Soil cont-aminated with mining waste may havehigh levels of copper, which damages rootcell membranes and inhibits growth. Anycontamination leading to high concentra-tions of magnesium or manganese impedescalcium uptake.

Overcultivation of marginal arid re-gions and the use of irrigation water inareas where evaporation exceeds precipita-tion can lead to SALINATION of soils. Defor-estation and intensive agriculture can alsocause lowering of the water table, againleading to accumulation of salts near thesoil surface. This has several effects onplants. Sodium competes with potassiumfor uptake and osmoregulation, affectingthe osmotic balance of the soil, and upset-ting stomatal regulation. Chloride also hasosmotic effects and it competes with otheranions, preventing their uptake and lead-ing to deficiency. Other forms of pollutionare noise from airplanes, traffic, and indus-try and the disposal of RADIOACTIVE WASTE.

pollution credit See Clean Air Act.

polyandry See polygamy.

polycentric distribution The occur-rence of a population, species or other tax-onomic group in several widely separatedplaces.

Polychaeta See Annelida.

polychlorinated biphenyl (PCB) Atype of compound based on biphenyl(C6H5C6H5), in which some of the hy-drogen atoms have been replaced by chlo-rine atoms. They are used in certainpolymers used for electrical insulators.PCBs are highly toxic and concern has beencaused by the fact that they can accumulatein the food chain.

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polyclimax See climax community.

polygamy A mating system in which amale pairs with more than one female at atime (polygyny) or a female pairs withmore than one male at a time (polyandry).Compare monogamy.

polygenic inheritance See quantitativevariation.

polygyny See polygamy.

polymorphism A distinct form of VARI-ATION in which significant proportions ofdifferent types of individuals exist within apopulation of a species at the same timeand in the same place, such that the fre-quency of the rarest form cannot be ex-plained on the basis of recurring mutation.The term is generally used where the fre-quency of the least common morph ex-ceeds 1 in 20, less frequent occurrencesbeing usually due to rare mutation or re-combination events.

If the differences persist over many gen-erations there is a balanced polymorphism.This may happen where the heterozygotes(see heterozygous) have an advantage notpossessed by either of the homozygotes(see homozygous), or where differentforms are successful in different MICROHAB-ITATS or in different years. For example,carriers of the recessive allele for sickle-cellanemia (i.e. those who are heterozygousfor the allele) enjoy greater protection frommalaria than noncarriers, so the incidenceof sickle-cell anemia (due to the possessionof two recessive alleles) is more common inareas where malaria is prevalent. An exam-ple of success in different habitats are thewater striders, freshwater insects of thegenus Gerris, which hunt on the water sur-face film. Species of large permanent lakeshave short wings or none at all, and do nottravel from lake to lake. Those of tempo-rary ponds have long functional wings anddisperse regularly as ponds dry up. Speciesof more persistent small ponds that never-theless may dry in summer often have bothlong- and short-winged forms, a situationknown as alary polymorphism.

Polymorphism usually results from theoccurrence of different alleles of a gene.The caste system in social insects results, insome cases, from differences in nutritionrather than genotype and is thus an envi-ronmental polymorphism rather than a ge-netic polymorphism.

polyp See Cnidaria; corals.

polypeptide A compound that containsmany amino acids linked together bybonds formed between the carboxylate(–COOH) group on one acid and theamino (–NH2) group on another. The re-sulting –CO.O.NH– linkages are known aspeptide bonds. They are the linkages pre-sent in proteins.

polyploidy The condition in which acell or organism contains three or moretimes the HAPLOID number of CHROMO-SOMES. It occurs when chromosomes fail toseparate during MEIOSIS, giving rise to ga-metes that are DIPLOID instead of haploid.Fertilization results in triploid or tetraploidindividuals. Polyploidy is far more com-mon in plants than in animals and veryhigh chromosome numbers may be found;for example in octaploids and decaploids(containing eight and ten times the haploidchromosome number). Some 60% ofmonocotyledons and 40% of dicotyledonsare polyploid.

Polyploids are often larger and morevigorous than their diploid counterpartsand the phenomenon is therefore exploitedin plant breeding, in which the chemicalcolchicine can be used to induce poly-ploidy.

Polyploids may contain multiples of thechromosomes of one species (autopoly-ploids) or combine the chromosomes oftwo or more species (allopolyploids).Chromosome pairing problems duringmeiosis may prevent hybrid individualsfrom reproducing sexually. However, asubsequent doubling of the chromosomenumber restores the ability to pair andthese new polyploids may be fertile. Onceformed, a polyploid individual may be in-capable of reproducing with its parents,and immediately constitutes a new species.

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Polyploidy thus contributes to evolution.Polyploids have a greater store of geneticvariability than nonpolyploids, and harm-ful recessive alleles are more easily maskedby normal dominant alleles, so furtheradding to their evolutionary potential.Polyploidy is rare in animals because thesex-determining mechanism is disturbed.

polysaccharides See carbohydrates.

pond A body of still fresh water in asmall surface depression. A pond is smallerthan a lake, but larger than a pool.

ponding A condition in a filter bed usedto treat SEWAGE in which, as a result of ex-cessive fungal growth, the surface of the fil-ter becomes covered in settled sewage,reducing air flow, so that the bed becomesanaerobic and treatment stops.

pool A body of still fresh water in asmall surface depression, such as the sur-face of a marsh. A pool is smaller than aPOND.

pooter A device used to collect small an-imals. It consists of a jar sealed with a bungthrough which pass two tubes pointing inopposite directions. The operator sucks onone tube, which ends inside the jar, whereits opening is covered in muslin. Thisdraws up the small animals through theother tube and into the jar.

POP See persistent organic pollutants.

population A group of interbreedingorganisms of the same species (or othergroups within which individuals may ex-change genetic information) occupying aparticular space. A population is continu-ally modified by increases (birth and immi-gration) and losses (death and emigration),and is limited by the food supply and theeffects of environmental factors such asdisease. The concept is easier to apply toanimals than to plants. Many plants can-not be divided strictly into populations be-cause they may reproduce vegetatively,with the result that one individual can oc-cupy a large area and parts can detach.

Some other species are almost entirely self-fertilizing or reproduce by agamospermy,so do not interbreed with any other indi-vidual. The number of individuals in a pop-ulation per unit area (or sometimes per unitvolume or other suitable parameter) is thepopulation density. In some species orunder some conditions the population sizemay fluctuate between extremes over afairly regular period – this is known as apopulation cycle. The minimum viablepopulation (MVP) is the smallest popula-tion that can persist for a long time, usuallytaken to be 1000 years.

The sum of the characteristics of a pop-ulation – its density, dispersion, the move-ment and age classes of its members,genetic VARIATION, and size and distribu-tion pattern of its habitats – is known asthe population structure.

population dynamics The study of thefactors influencing the fluctuations in num-bers in a population or of its GENE POOL.These factors include those affecting birthand mortality rates, immigration and emi-gration, and reproductive potential (for ex-ample, seed size, dispersal, and dormancy).

population ecology The study of thevariations in the size and density of popu-lations in space and time and their interac-tions with their environment.

population equilibrium A state inwhich numbers of individuals in a popula-tion remain stable because the number ofdeaths equal the number of births.

population genetics The study of in-herited VARIATIONS in populations of or-ganisms, and their distribution in time andspace. It involves the quantification ofGENE FREQUENCIES, and the effects of GE-NETIC DRIFT, immigration and emigration,NATURAL SELECTION, and MUTATION ontheir distribution. See gene pool.

population matrix See Lewis–Lesliematrix.

population pyramid A diagram repre-senting the age structure of a population, in

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which the youngest age class is placed atthe bottom and successive age classes arestacked above it. It is usually shown as a se-ries of horizontal bars, the length of whichrepresents the numbers of individuals ofeach age.

population regulation The tendencyfor a population to regulate its own num-bers at a fairly constant level. For example,factors such as abundant food resourcesmay cause density to increase when it islow, and other factors, perhaps grazing ex-ceeding the CARRYING CAPACITY of the habi-tat, cause it to decrease when it is high.

population vulnerability analysis(PVA; population viability analysis) Ananalysis of the populations of a species, acommunity, or ecosystem to assess thechances that it may become extinct. It en-compasses study of ecological, economic,and political issues and other factors relat-ing to its conservation.

Porifera A phylum of primitive multicel-lular animals, the sponges, that probablyevolved a multicellular structure indepen-dently of the other multicellular animals.All are sessile and almost all are marine.The body of a sponge is a loose aggregationof cells, with minimal coordination be-tween them, forming a vase-like structure.Flagellated cells (choanocytes) line thevase, and cause water currents to flow inthrough apertures (ostia) in the body walland out through one or more openings (os-cula) at the top. Sponges have an internalskeleton of chalk or silica spicules or pro-tein fibers (as in the bath sponge).

positive feedback See feedback mecha-nism.

potable water Treated or untreatedwater that is considered safe to drink.

potassium (K) One of the essential el-ements in plants and animals. It is ab-sorbed by plant roots as the potassium ion,K+, and in plants is the most abundantcation in the cell sap. Potassium is impor-tant in OSMOREGULATION and other meta-

bolic activities in animals and plants. A de-ficiency of potassium in plants leads to acharacteristic purple coloration of theleaves, and poor root growth, flower andfruit formation. Potassium is usually addedto soils in compound fertilizers.

potassium–argon dating See isotopicdating.

prairie An extensive area of relativelylevel GRASSLAND in North America, withvery few trees. The plant communities con-sist of various dominant grass speciesmixed with a variety of perennial herbs.Different communities occur on differentsoils, drainage, and topography. There aretwo main types – tall-grass prairie (long-grass prairie) in the east, where the grassvarieties are mainly tall, and short-grassprairie in the west. Large tracts of formerprairie are now urbanized or under wheatcultivation.

preadaptation An adaptation thatevolved in one habitat but happens to beadvantageous to survival in an adjacenthabitat, so allowing the organism to radi-ate into it. See adaptation; adaptive radia-tion.

Precambrian The time in the Earth’s ge-ological history that precedes the CAM-BRIAN, i.e. from the origin of the Earth,nearly 5 billion years ago, to the start of theCambrian, around 570 million years ago.The term ‘Precambrian’ is now usedmainly descriptively, and has been largelydiscarded as a geological term in the lightof greater knowledge of the early evolutionof life. Precambrian time is now dividedinto three eons: Hadean, from the Earth’sorigin to about 3900 mya; Archean, 3900–2390 mya; and Proterozoic, 2390–570mya (the Cambrian marks the start of thePhanerozoic eon, which extends to the pre-sent day).

The oldest fossils discovered so far areremains of bacterialike organisms, datingfrom about 3500 mya. Indeed, there isabundant evidence of flourishing coloniesof CYANOBACTERIA and other bacteriathroughout the Archean and Proterozoic

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eons. This takes the form of stromatolites,rock structures representing the remains ofsediment trapped or precipitated bycyanobacteria communities. Stromatolitesare still found in a few special locationstoday. The earliest remains of single-celledEUKARYOTES are much later, dating fromabout 1400 mya, while the first appear-ance of multicellular animals is in the so-called Ediacara fauna, in rocks dated to thelast 100 million years of Precambrian time.

precautionary principle A strategythat if there is a strong suspicion that cer-tain activities that cause changes to the en-vironment may prove to be harmful orundesirable, action should be taken to pre-vent or ameliorate those effects, even if thescientific basis for such suspicions is notabsolutely certain. The Rio Declaration(see Earth Summit) states that where thereare threats of serious or irreversible dam-age, lack of full scientific certainty shall notbe used as a reason for postponing cost-effective measures to prevent environmen-tal degradation.

precipitation 1. The formation of an in-soluble solid as a result of a chemical reac-tion that occurs in solution. Precipitation isused to separate suspended material fromsewage by adding chemicals to cause it toprecipitate.2. Moisture that falls onto the Earth’s sur-face from clouds, for example rain, hail,sleet, or snow. There are two main meth-ods of measuring precipitation. A non-recording gauge is a simple cylinder thatcollects rain through a funnel of standard-ized diameter. The volume of water col-lected over a given period is measured.Snow or hail is melted before measuring. Arecording gauge is linked to a rotatingdrum that provides detailed data on thetime and intensity of the precipitation,which is recorded as mm h–1.

precipitation enhancement The in-creasing of precipitation that results fromchanges in the colloidal stability of clouds.This may be due to intentional seeding ofclouds with condensation nuclei, or the ef-fect of pollution.

precocity The occurrence of reproduc-tion at an early stage of the life cycle of anorganism relative to other individuals ofthe same or related species.

predation The capturing and killing bya PREDATOR of another animal (the PREY) inorder to eat it.

predation curve A graphical represen-tation of the interaction between the num-bers and behavior of PREDATOR and PREY

populations. It may show, for instance, ifpredator or prey behavior changes withprey population density, and indicatewhether prey switching is going on, orwhether there are external factors limitingthe predator population.

predator An animal that captures andkills another animal (the PREY) in order toeat it. A predator that is at the top of thefood chain is often called a top predator.Top predators are often KEYSTONE SPECIES:if their populations are healthy, it is a goodindication that the ecosystem as a whole isin a healthy state, because the rest of theFOOD CHAIN must be in a healthy enoughstate to support it.

predator–prey ratio The ratio of thenumber of species of predators to the num-ber of prey species in a given community.When this is modeled assuming that thereis competition between predators for themost prey-rich areas and competition be-tween prey for the areas with the fewerpredators, the ratio remains fairly con-stant. In practice, feeding relationships areextremely complex with interactions be-tween many different levels in the FOOD

WEB. Ratios are also complicated by omni-vores, where the same species may becounted as both predator and prey. Seealso Lotka–Volterra model.

predator–prey relationships The func-tional and numerical interactions betweenpredator and prey populations. Such stud-ies may include the relationship between apredator’s consumption rate and the den-sity of its prey – a functional response. Orthey may explore changes in the size of a

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predator population in response to changesin density of its prey population – a nu-merical response. A predator may opt tohunt mainly the most common species ofprey, even though other species are readilyavailable – this is called apostatic prey se-lection. See also predator–prey ratio; pre-dation curve.

prescribed burning Deliberate con-trolled setting of surface fires in forests orgrasslands to remove dry dead vegetationand prevent more destructive fires, or tokill off unwanted species that compete fornutrients with commercial species.

preservationism A conservation move-ment that seeks to preserve natural envi-ronments in their pristine state, notallowing any kind of exploitation, fromhunting and fishing to logging or tourism.

press experiment An experiment inwhich variables in a the system are changed(e.g. one or more environmental factors arechanged or individuals are added to or re-moved from a population) continuously,so that the system is in a state of constantperturbation. Compare pulse experiment.

prevalence The proportion of poten-tially inhabitable sites in which a particularspecies is present. It is usually expressed asa percentage.

prey An organism that is likely to bekilled and eaten by a PREDATOR.

prey switching The switching of apredator’s preference from one preyspecies to another, often in response tochanges in prey density or accessibility, orto competition from other predators.

primary data Direct observations orvalues of variables obtained in the field orlaboratory, before any data processing hastaken place.

primary drinking water regulation Aregulation that applies to public water sys-tems, specifying the maximum permitted

levels of particular contaminants deemednot to have adverse effects on health.

primary forest Forest that is believednot to have been affected by any human ac-tivity throughout its history. Compare sec-ondary forest.

primary production The total assimi-lation (see gross primary production) oraccumulation (see net primary production)of nutrients or energy by plants and otherautotrophs.

primary productivity The rate of pro-duction of BIOMASS per unit area by plantsand other autotrophs. Compare secondaryproductivity.

primary sex ratio See sex ratio.

primary succession See succession.

Primates The order of mammals thatcontains the monkeys, great apes, and hu-mans. Most primates are relatively unspe-cialized arboreal mammals with a veryhighly developed brain, quick reactions,and large forward-facing eyes allowingbinocular vision. The opposable thumband (usually) big toe are used for graspingand the digits have nails. The young un-dergo a long period of growth and devel-opment, during which they learn fromtheir parents. The New World monkeyshave prehensile tails; the more advancedOld World monkeys lack prehensile tails,and great apes are larger tailless primatesthat typically swing from trees by theirlong arms.

prisere See sere.

probability The chance that a givenevent will occur, or that over a series of ob-servations a particular kind of observationwill occur regularly as a given proportionof the total number of observations. Statis-tical probability is usually based on aninfinite number of observations. For exam-ple, in genetics, if a heterozygous plant isselfed, the probability of finding the doublerecessive is 1 in 4, or a 25% chance. The

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greater the number of offspring the betterthe chance that this actual percentage willbe achieved.

producer The first TROPHIC LEVEL in aFOOD CHAIN. Producers are those organ-isms that can build up foods from inor-ganic materials, i.e. green plants, algae,and photosynthetic and chemosyntheticbacteria. Producers are eaten by herbi-vores, which are primary CONSUMERS.

production efficiency The percentageof the energy assimilated by an organismthat becomes incorporated into new BIO-MASS.

productivity The rate of production ofbiomass per unit area by a specific organ-ism or group of organisms.

profundal The deepwater zone of a lakebeyond a depth of 10 m. Little light pene-trates this zone and thus the inhabitants areall heterotrophic, depending on the littoraland sublittoral organisms for basic foodmaterials. Commonly found inhabitantsinclude bacteria, fungi, mollusks, and in-sect larvae. Species found in the profundalzone are adapted to withstand low oxygenconcentration, low temperatures, and lowpH. Compare littoral; sublittoral; photiczone.

prokaryote (procaryote) An organismwhose genetic material (DNA) is not en-closed by membranes to form a nucleus butlies free in the cytoplasm. Organisms canbe divided into prokaryotes and EUKARY-OTES, the latter having a true nucleus. Thisis a fundamental division because it is as-sociated with other major differences.Prokaryotes constitute the domain BAC-TERIA, with two kingdoms, or even do-mains, the ARCHAEA and the EUBACTERIA.All other organisms are eukaryotes.Prokaryote cells evolved first and gave riseto eukaryote cells. See cell; endosymbionttheory.

protein A large, complex moleculemade up of one or more polypeptidechains, i.e. it consists of AMINO ACID mol-

ecules joined together by peptide links. Themolecular weight of proteins may varyfrom a few thousand to several million.About 20 amino acids are present in pro-teins. Simple proteins contain only aminoacids. In conjugated proteins, the aminoacids are joined to other groups. Proteinsare vital compounds found in all living or-ganisms.

Proterozoic See Precambrian.

Protista (protists) In some classifica-tions, a kingdom of simple single-celled eu-karyotic organisms including the algae andprotozoans. It was introduced to overcomethe difficulties of assigning such organisms,which may show both animal and plantlikecharacteristics, to the kingdoms Animaliaor Plantae. Today the grouping is consid-ered artificial, and protists are included inthe kingdom PROTOCTISTA.

Protoctista (protoctists) A kingdom ofsimple eukaryotic organisms that includesthe algae, diatoms, slime molds, fungus-like oomycetes, and the organisms tradi-tionally classified as protozoa, such asflagellates, dinoflagellates, ciliates, andsporozoans (see eukaryote). The membersof this kingdom do not share clear phylo-genetic links, but are grouped together sim-ply because they do not belong in any ofthe other four kingdoms (see Five King-doms classification). Most are aerobic,some are capable of photosynthesis, andmost possess UNDULIPODIA at some stage oftheir life cycle. Protoctists are typicallymicroscopic single-celled organisms, suchas the amebas, but the group also has largemulticellular members, for example theseaweeds and other large algae, some ofwhich are sometimes classified as plants.Protoctists show a wide range of nutri-tional habits, including photoautotrophs,heterotrophs, phagotrophs, and mixo-trophs.

protoplasm The living contents of acell, comprising the cytoplasm plus nucleo-plasm.

protoplast The protoplasm and plasma

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protoplast

membrane of a cell after removal of the cellwall, where present.

protozoa An old name for animal-likeProtoctista that also included some photo-synthetic organisms such as Euglena andVolvox.

pruning The removal of part of a plant.This may be done to remove injured or dis-eased parts, or to train the plant into a par-ticular shape. See also coppice; pollarding.

psammosere A SUCCESSION that devel-ops on sand dunes.

PSI See pollutant standard index.

psu See salinity.

psychrometer An instrument for meas-uring HUMIDITY.

psychrophilic Describing microorgan-isms that can live at temperatures below20°C. Compare mesophilic; thermophilic.See also extremophile.

pteridophyte A general term, nowlargely obsolete, that includes any vascularnonseed-bearing plant. Pteridophytes in-clude the clubmosses (phylum Lycophyta),horsetails (Sphenophyta), ferns (Filicino-phyta), and whisk ferns (Psilophyta or in-cluded in Filicinophyta). See alternation ofgenerations.

pulse experiment An experiment inwhich one or more variables are changed(e.g. one or more environmental factors arealtered or individuals are added to or re-moved from a population or community)at one single moment, and the progress ofthe experiment or the population dynamicsare monitored before and after the pertur-bation. Compare press experiment.

purine A nitrogenous organic moleculewith a double ring structure. Members ofthe purine group include adenine and gua-nine, which are constituents of the nucleicacids, and certain plant alkaloids, e.g. caf-feine and theobromine.

PVA See population vulnerability analy-sis.

pyramid of biomass A type of ecologi-cal pyramid based on the total amount ofliving material at each TROPHIC LEVEL in thecommunity, which is normally measuredby total dry weight or calorific value perunit area or volume, and shown diagram-matically. The biomass depends on theamount of carbon fixed by green plantsand other producers. The pyramid of bio-mass usually has a more gentle slope thanthe PYRAMID OF NUMBERS because organ-isms at successively higher levels in thepyramid tend to be larger than thosebelow.

pyramid of energy A type of ecologicalpyramid in which the energy contained inthe organism at each stage of a food chainis depicted diagrammatically. It shows thatthe energy flux decreases at progressivelyhigher trophic levels, energy being lost be-

tween levels due to inefficiencies of diges-tion and assimilation, respiration, heatloss, energy of movement, nervous trans-mission, and so on.

pyramid of numbers A type of ecolog-ical pyramid in which the number of indi-vidual organisms at each stage in the FOOD

CHAIN of the ecosystem is depicted dia-grammatically. The producer level formsthe base, and successive TROPHIC LEVELS thetiers. The shape of the pyramid of numbersdepends upon the community considered;generally, the organism forming the base of

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producer(green plant)energy

lostenergy

lostenery available

primaryconsumer(herbivore)

secondaryconsumer(carnivore)

tertiaryconsumer(top carnivore)

Pyramid of energy

a food chain is numerically very abundant,and each succeeding level is represented byfewer individual organisms, culminating

with the final large predator. The pyramidof numbers may be partly inverted (basesmaller than one or more of the upper tiers)if the producing organisms are large.

Theoretically the higher the level in thepyramid, the fewer organisms it can sup-port. This has important implications re-garding human food supplies because itillustrates that more people can be sup-ported in a given area if their diet is mainlyvegetarian – more land is needed per headif meat forms a large part of the diet.

pyrethrum See botanical insecticide.

pyrimidine A nitrogenous organic mol-ecule whose ring structure is contained inthe pyrimidine bases cytosine, thymine,and uracil, which are constituents of thenucleic acids, and in thiamine (vitamin B1).

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pyrimidine

herbivore(e.g.aphids)

carnivore III(e.g. hawks)

carnivore II(e.g.warblers)

carnivore I(e.g. beetles)si

ze/c

lass

of

anim

al

numbers of individuals

Pyramid of numbers

Q10 A coefficient that shows by whatfactor the rate of a chemical reaction in-creases for every 10°C rise in temperature.For most metabolic reactions Q10 = 2, i.e.the reaction rate doubles for every 10°Crise in temperature.

quadrat A square area (standard size isone meter square) taken at random, withinwhich the composition of organisms isnoted. The quadrat sampling technique ismostly used in plant ecology to study plantcommunities but quadrats are also used asa sampling unit to count and weigh ani-mals for an estimate of density or to dis-cover animal distribution in a selectedarea. Permanent quadrats can be estab-lished that are examined at given intervalsas a means of assessing changes in speciescomposition in an area over a period oftime.

qualitative variation (discontinuousvariation) A form of variation in which acharacter has two or more distinct forms.Examples are human blood groups andMendel’s pea characters. It generally oc-curs when there are two or more allelicforms of a major gene in a population.

quantitative variation (continuous vari-ation) A form of variation in which acharacter shows a continuously varyingrange of values, for example height in hu-mans or grain yield in cereal crops. It oc-curs where traits are controlled by manydifferent genes (polygenes). The pattern ofinheritance of such traits is called poly-genic inheritance or multifactorial inheri-tance.

quartile The 25th, 50th, or 75th per-centile (data value equal to 25, 50, or 75%of the highest value) of a set of data or adistribution.

quartz (SiO2) A mineral composed ofcrystalline silica that is widespread in rocksof all kinds, the second most abundantmineral in the Earth’s crust.

Quaternary The most recent period ofthe CENOZOIC era from about 2 millionyears ago to the present day and composedof PLEISTOCENE and HOLOCENE epochs. Lit-erally, the ‘fourth age,’ it is characterizedby the emergence of humans. See also IceAge.

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race In classification, a TAXON below therank of species, sometimes being placed be-tween subspecies and variety. The term issometimes used instead of ecotype forgroups of individuals that have uniformecological preferences or physiological re-quirements. See ecotype.

radiation The dissemination of energyfrom a source. In the absence of absorp-tion, the energy decreases as the inversesquare of the distance from the source.Types of radiation include electromagneticradiation (see electromagnetic energy) andthe emission of particles such as alpha andbeta radiation and gamma rays (see radio-active decay). See also ionizing radiation.

radiation sickness The collection ofsymptoms shown by persons sufferingfrom radiation injury resulting from exces-sive exposure of part or all the body to ION-IZING RADIATION. These usually includefatigue, nausea, vomiting, and diarrhea,and later hemorrhage, inflammation of themouth and throat, and hair loss.

radiation standards Regulations im-posing maximum exposure limits forradioactive material in order to protect thehealth of workers and the general public.

radical A group of atoms in a molecule,for example, the carbonate radical is CO3

–.Where the radical has a single unpairedelectron, it is highly reactive, and is calleda free radical. They are often formed inlight-induced reactions, as in the photo-chemical reactions with CFCS that causedamage to the OZONE LAYER.

radioactive dating See isotopic dating.

radioactive decay The spontaneousbreakdown of a radioactive isotope. Suchisotopes break down with the emission ofalpha particles, beta particles, or gammawaves, forming other isotopes or even dif-ferent elements. A useful measure of therate of decay is the half-life – the time re-quired for half of the atomic nuclei in thesample to decay. See also ionizing radia-tion; isotope.

radioactive fallout (nuclear fallout) Theradioactive material that falls to theground or into the Earth’s atmosphere orwater at some distance from the site of anuclear explosion.

radioactive isotope (radioisotope; ra-dionuclide) Any of several differentforms of the same element that have differ-ent masses and whose nuclei are unstable,spontaneously breaking down with theemission of particles, e.g. alpha, beta, orgamma radiation. See also isotope; radio-active decay.

radioactive waste (nuclear waste) Anywaste material that contains radioactivematerial. Radioactive waste can come fromthe mining, extraction, and processing ofradioactive ores, the normal operation ofnuclear reactors, decommissioned nuclearreactors, the manufacture of nuclearweapons, and from radioactive materialsused in hospitals, laboratories, and indus-try.

Since radioactive waste can be veryharmful to all living matter and since it cancontain radionuclides that have half-livesof many thousands of years the problem ofhow to dispose of radioactive waste is veryimportant. Different levels of radioactivityhave to be dealt with. High-level waste has

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to be stored until its radioactivity has beenreduced to a level at which it can beprocessed. Intermediate-level waste can becontained by burial in deep mines or belowthe seabed. Low-level waste can be storedin steel drums at special sites. Spent nuclearfuel is an example of high-level waste. Re-actor components and processing plantsludge are examples of intermediate levelwaste. Solids or liquids slightly contami-nated by radioactive substances are exam-ples of low-level waste.

radioisotope See radioactive isotope.

radiometric dating See isotopic dating.

radionuclide See radioactive isotope.

radon See atmosphere.

rainfall Water formed by condensationwhen moist air is cooled below its dewpoint. The droplets form clouds, and coa-lesce until they become heavy enough toovercome the frictional resistance of the airand fall to the ground. Carbon dioxide inthe atmosphere dissolves in rainwater toform a very weak solution of carbonic acid(H2CO3), sufficient to cause slow weather-ing of rocks and building stone. See alsoacid rain.

rainforest (equatorial forest; tropical rain-forest) A TROPICAL FOREST in climateswith regular heavy rain, which supportsprolific plant growth. Tropical rainforestsalso require constantly high temperatures.Rainforest is a species-diverse broad-leavedforest, with tall canopy trees forming aclosed canopy penetrated occasionally byeven taller trees (EMERGENTS). There is alower layer of smaller trees, and dense un-dergrowth, as well as climbing vines andlianas, strangling figs, and many epiphytes.With the constant climate, flowering andfruiting take place all year round, allowingfor the evolution of specialist herbivores.Most rainforest trees are evergreen. Tem-perate rainforest (see temperate deciduousforest) forms in moist coastal regions, suchas the northwest coast of North Americaand Tasmania. Growth here, while lush, is

not so prolific as in the tropics, but there isa dense growth of mosses and ferns.

rain gauge See precipitation.

rain shadow A geographical area thatreceives less rain than neighboring areasbecause higher land in the direction of theprevailing wind causes clouds to rise andcool, thus shedding their rain before reach-ing the lower land on the far side of thehills. Examples include the deserts ofNevada and eastern California, which arein the rain shadow of the Sierra Nevadaand the Coast Range.

rain-wash The movement of materialover the land surface and down slopes as aresult of rainfall. There are usually twoprocesses involved: rain-wash, the loosen-ing and detachment of small soil particlesby the impact of raindrops; and soil-wash,the downhill movement of material by sur-face water flow (runoff). See also solifluc-tion. Compare creep.

ramet An offshoot or other moduleformed by vegetative growth that is capa-ble of an independent existence, for exam-ple, the runners of strawberry or the polypsof colonial hydroids (see Cnidaria). Ram-ets are clones of their parents, and theirproduction is a form of ASEXUAL REPRO-DUCTION.

Ramsar Convention See Conventionon Wetlands of International Importance.

random error A deviation of an ob-served value from a true value that occursas if selected at random from a probabilitydistribution of such errors.

randomization A form of experimentaldesign in which experimental units are al-located to random positions in the experi-ment, usually by means of random numbertables. Randomization is used when simi-lar units are being observed under con-trolled conditions, such as pure-breedingplants being grown in a phytotron. It in-creases the number of degrees of freedom,so reducing the error variance and making

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it easier to detect significant differences be-tween treatments. Compare Latin square.

random niche model (broken stickmodel) A model of SPECIES DIVERSITY inwhich species abundance is determined bya random partitioning of resources be-tween different species. It was first con-ceived by imagining a stick that wasbroken at random points along its length.The length of the different pieces repre-sented the proportion of the resources uti-lized by one species. See also dispersion.

random sample A sample in whicheach individual organism or QUADRAT sam-pled is independent of all other individualsand also independent of any obvious phys-ical features of the environment. In an eco-logical survey, this is usually achieved bylaying out a grid over the sample area, andusing random number tables to select coor-dinates for points at which to placequadrats. See also randomization.

range 1. The difference between thelargest and smallest data values in a sam-ple. The size of the range is determined bythe DISPERSION of the data about the mean.2. (geographic distribution) The geo-graphic area over which a species occurs.3. A large area of open grassland used forgrazing or browsing.

range management The managementof livestock production or the grazing ofmammals on large areas of open grassland.It involves controlling stocking density sothat the CARRYING CAPACITY of the land isnot exceeded, and ensuring the populationdensity and other aspects of the grazingmanagement do not lead to environmentaldegradation.

rank 1. The hierarchical status of aTAXON in a classification scheme. For ex-ample, the taxon Annelida has the rank ofphylum, while the taxon Oligochaeta hasthe rank of class.2. The position of a data value when all thedata are arranged in ascending order ofvalue.

rarefaction A method of determiningthe relationship between species diversityand sample size by progressively reducingthe sample size by randomly deleting indi-viduals.

ratio scale See measurement scales.

Raunkiaer’s life-form classification Aclassification of plant growth forms basedon the persistence of the shoots and the po-sition of the resting buds. The system sim-plifies assessment of the percentages ofdifferent plant forms in any given type ofvegetation, especially in temperate commu-nities. A cryptophyte is a plant in which theresting buds are below the soil surface or inwater. Within this group, plants are cat-egorized according to their habitat as landplants (geophytes), marsh plants whoseoverwintering buds are under water (helo-phytes), and aquatic plants (hydrophytes).A hemicryptophyte is a perennial plant,usually nonwoody, with its overwinteringbuds at or just below soil level, and arecommon in cold, moist climates, and in-clude many rosette plants. Chamaephytesinclude small bushes and herbaceousperennials with buds at or near the soil sur-face and are commonly found in cold orsemiarid climates. Phanerophytes areperennial plants with persistent shoots andbuds on upright stems well above soil level,e.g. herbs, shrubs, trees, and climbingplants. Therophytes survive part of theyear as seeds and complete their life cyclesduring the remainder of the year.

raw sewage Untreated domestic or in-dustrial waste water.

reafforestation See reforestation.

realized niche The portion of its poten-tial NICHE (see fundamental niche) occu-pied by a species. This will be less than thefull potential niche if predators or competi-tors are present.

Recent See Holocene.

recessive Describing an allele that is ex-pressed in the PHENOTYPE only when it is

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recessive

present in the homozygous condition (i.e.both alleles are recessive, a condition calleddouble recessive). Compare dominant.

recharge area An area of land in whichthere is a net annual movement of waterfrom the surface to the groundwater. It de-termines the maximum sustainable rate atwhich an aquifer can be exploited.

reciprocal predation A situation inwhich two species or individuals prey oneach other, a kind of competitive interac-tion.

reciprocal transplant experiment Anexperiment in which individuals from dif-ferent environments are moved to the otherenvironment to compare their genotype–environment interactions.

reclamation 1. The conversion of landthat is periodically inundated by the sea orwhich is waterlogged for other reasons, itsdrainage and management for farming,urban, or industrial development. See alsopolder.2. The cleaning up of contaminated land tomake it suitable for farming, urban, or in-dustrial development.3. The transformation of substances foundin waste into useful materials.

recombinant DNA technology Thetechnique by which foreign DNA, whetherfrom another organism or genetically engi-neered, is inserted into another DNA frag-ment or molecule. The product –recombinant DNA – is fundamental tomany aspects of GENETIC ENGINEERING, par-ticularly the introduction of foreign genesto cells or organisms. There are now manytechniques for creating recombinant DNA,depending on the nature of the host cell ororganism receiving the foreign DNA.

recombination The regrouping ofgenes that regularly occurs during MEIOSIS

as a result of the INDEPENDENT ASSORTMENT

of chromosomes into new sets, and the ex-change of pieces of chromosomes (crossingover). Recombination results in offspringthat differ both phenotypically and geno-

typically from both parents and is the mostimportant means of producing VARIATION

in sexually reproducing organisms. Seecrossing over; independent assortment;meiosis.

recruitment Additions to a populationas a result of births or immigration. Net re-cruitment is the difference between theseadditions and the losses due to deaths andemigration.

recycling The processing of waste ma-terial so that it can be used again. For ex-ample, the making of new paper and glassfrom waste paper and glass, or the extrac-tion of metals from waste cans. This savesraw materials and some of the energyneeded to make these products from rawmaterials, as well as saving on landfillspace.

red-and-yellow forest soil An acidSOIL formed in wet subtropical regions. Theheavy rain causes LEACHING and accumula-tion of salts in the B horizon. Iron oxides inthe A horizon give the soil its red color, butif the soil is sandy, it is usually yellow. Suchsoils have a low nutrient value, contain lit-tle humus, and are easily eroded.

Red Data Book A set of informationon rare, threatened, and endangeredspecies worldwide, compiled by the WorldConservation Monitoring Center, andpublished at regular intervals by theSpecies Survival Commission of the WorldConservation Union. The Red List ofThreatened Plants is incorporated with theRed Data Book enabling continuous up-dating and monitoring of data concerningthese species.

red desert soil A type of coarse SOIL

formed in hot deserts, which is rich in saltsand lime but poor in humus. Such soilsmay be cultivated if irrigated.

Red List of Threatened Plants SeeRed Data Book.

Red Queen effect A theory of evolutionthat considers that most of a lineage’s evo-

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lution consists of keeping up with environ-mental changes rather than adapting tonew environments. The name comes fromthe Red Queen in Lewis Carroll’s Throughthe Looking Glass, who had to keep run-ning as fast as she could just to stay in thesame place.

red tide A BLOOM of marine planktonthat is toxic and often fatal to fish and hu-mans who consume the shellfish (paralyticshellfish poisoning). Shellfish concentratethese toxins when they feed on the plank-ton. The water may be colored red, green,or brown by the organisms. Red-coloredtides are especially toxic, and are due toblooms of DINOFLAGELLATES. Such bloomsmay be natural, or they may be caused bynutrient enhancement of the water, for ex-ample by phosphates and nitrates from fer-tilizer runoff or wastewater, or untreatedsewage discharged into the sea fromcoastal resorts.

reduction The gain of electrons by suchspecies as atoms, molecules, or ions. Itoften involves the loss of oxygen from acompound, or addition of hydrogen. Re-duction can be effected chemically, i.e. by the use of reducing agents (electrondonors), or electrically, in which case thereduction process occurs at the cathode.For example,

2Fe3+ + Cu → 2Fe2+ + Cu2+

where Cu is the reducing agent and Fe3+ isreduced, and

2H2O + SO2 + 2Cu2+ →4H+ + SO42– + 2Cu+

where SO2 is the reducing agent and Cu2+

is reduced.

reforestation (reafforestation) Theplanting of trees on land from which treeshave previously been removed.

refugia (sing. refugium) Isolated areas inwhich plant and animal species were ableto survive a period of substantial climaticchange that made the surrounding areasunsuitable for them to live in, for examplean ice-free area during a glaciation, or acool mountain top during a period ofwarming. Such refugia serve as centers of

DISPERSAL if the climate becomes more fa-vorable again. See also geographical isola-tion.

regolith A general term for loose uncon-solidated material such as rock fragments,mineral grains, soil, and other deposits thatrest on solid bedrock. Much is derivedfrom weathering of the rock below, orfrom nearby areas.

regression 1. A return to a former state,or to a more primitive state, for examplewhen cultivated land reverts to naturalvegetation.2. A retreat of the sea from a coast as a re-sult of a fall in sea level.3. A statistical method for investigating theinterdependence of variables. Regressionanalysis attempts to find a mathematicalmodel to explain how changes in the inde-pendent variable(s) affect the response ofthe dependent variable(s). Such modelsmay be used to predict values of the re-sponses. Where there is more than one de-pendent or independent variable, this iscalled multiple regression.

regular distribution See dispersion.

rehabilitation ecology (restoration ecol-ogy) Where an area has been contami-nated or degraded, and it is impossible torestore it to its original wild condition, theestablishment there of as similar a commu-nity as possible. Compare remediation.

relative frequency In probabilitytheory, the proportion of events in which avariable has a particular value in a sampleof infinite size is its relative frequency. Forexample, in tossing a coin, with a largeenough sample size the proportion ofheads will approach 50%, a relative fre-quency of one-half. See also probability.

relict population A population with avery restricted range that is all that remainsof a former larger and more widely distrib-uted population.

relief The shape of the land – the differ-ence in height above or below sea level

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across the landscape. A relief map usuallyhas contours or colors showing differentelevations.

relief map See relief.

remediation The cleaning up of a dam-aged or degraded site and restoration of theenvironment to its former state, so far as ispossible, or to remove, contain, or in otherways make safe pollutants, toxic spills, orother contamination. Compare rehabilita-tion ecology.

remote sensing The detection, identifi-cation, location and/or analysis of land,water, or living organisms by the use of dis-tant sensors/recording devices. Examplesinclude satellite imaging, aerial photogra-phy, radar, sonar, and radio-tracking.

rendzina A type of brown earth SOIL

typical of humid to semiarid grasslandsover calcareous rocks. The rendzina is richin lime and has an upper humus-rich hori-zon. See brown earth.

renewable energy Energy from asource that is not in limited supply, for ex-ample, geothermal energy, sunlight, wind,tides, and the burning of waste.

renewable resource A resource that isnot in limited supply, or whose supply rateis likely to exceed any proposed rate ofconsumption, for example an aquifer in anarea of high rainfall.

replication The mechanism by whichexact copies of the genetic material areformed. Replicas of DNA are made whenthe double helix unwinds as a result of he-licase enzyme action and the separatedstrands serve as templates along whichcomplementary NUCLEOTIDES are assem-bled through the action of the enzymeDNA polymerase. The result is two newmolecules of DNA each containing onestrand of the original molecule, and theprocess is termed semiconservative replica-tion.

reproduction The production of new

individuals by existing individuals. Seeasexual reproduction; sexual reproduction.

reproductive barrier Any factor thatprevents two species or populations fromproducing viable fertile hybrids. They maylive in different habitats or locations andseldom meet, or use different attraction sig-nals during courtship or have flowers de-signed to attract different pollinators; theymay breed at different times; their repro-ductive organs may be incompatible (e.g.different lengths of stigma and stamens inprimroses Primula vulgaris); the gametesmay be chemically incompatible, or geneticincompatibility (for example, between dif-ferent polyploids) which may prevent thezygote from dividing successfully or pre-vent the resulting hybrid forming viablegametes. See also ecological isolation; geo-graphical isolation; reproductive isolation.

reproductive isolation The preventionof gene flow between members of a popu-lation due to REPRODUCTIVE BARRIERS thatprevent them interbreeding, thus separat-ing distinct breeding groups. Reproductiveisolation may also be a consequence ofgradual genetic divergence due to otherisolating factors. It may eventually lead toSPECIATION, as in the monkeyflowersMimulus cardinalis, which is pollinated byhummingbirds, and M. lewisii, which isbee-pollinated. The differences in flowercolor and structure involve very few genes,but the flowers attract different pollina-tors, so do not normally interbreed in thewild. Compare ecological isolation; geo-graphical isolation.

reproductive rate The number of off-spring produced by an individual per unittime or over a particular period of time.The average reproductive rate is the num-ber of young produced per mother or perbreeding pair averaged over the popula-tion. The net reproductive rate is the aver-age number of offspring an individualwould produce in its lifetime. It depends onage-specific mortality rates. The averagenumber of offspring that each individual ina population gives rise to in one unit oftime is the fundamental net reproductive

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rate (R), a factor expressing the relation-ship between the size of a population to itssize one time unit later. If R=0 the popula-tion is stable, if R>1 it is growing, and ifR<1 it is declining.

Reptilia The class of vertebrates thatcontains the first wholly terrestrialtetrapods, which are adapted to life onland by the possession of a dry skin withhorny scales, which prevents water loss byevaporation. Fertilization is internal andthere is no larval stage. The young developdirectly from an amniote egg that has aleathery shell and is laid on land, i.e. it iscleidoic. Respiration is by lungs only andthe heart has four chambers, although oxy-genated and deoxygenated blood usuallymix. Other advanced features are theclawed digits and the metanephric kidney.Like amphibians, but unlike birds andmammals, reptiles are poikilothermic.

Reptiles, notably the dinosaurs, werethe dominant tetrapods in the Mesozoicperiod. Modern forms include the predom-inantly terrestrial lizards and snakes (orderSquamata), as well as the aquatic croco-diles and turtles. Reptiles evolved fromprimitive Amphibia and Mesozoic reptilesincluded aerial members (e.g. Pteranodon)and aquatic forms (e.g. Ichthyosaurus), aswell as the terrestrial dinosaurs (e.g.Tyrannosaurus). Some groups gave rise tobirds and mammals. Primitive reptiles hada pineal eye but this is lost in most modernforms.

reservoir 1. An artificial or natural lakeor pond managed as a store of water,which is released in a controlled manner.2. An underground rock with sufficient in-terstitial space to store water, oil, or nat-ural gas.3. A store of a nutrient in a BIOGEOCHEMI-CAL CYCLE. Abiotic reservoirs include rock,sediments, soil, water, and the atmosphere.Biotic reservoirs include nutrients con-tained in the bodies of living organisms.Reservoirs are important when consideringnutrient and biogeochemical cycles.

resistance The ability of a population to

avoid displacement from its present stateby a disturbance.

resource Any item in the environmentthat can be used by a living organism, forexample water or nutrients. Items that maybe exploited by humans, such as timber,coal, and minerals, are termed natural re-sources. See also essential resource; limit-ing resource.

resource partitioning See differentialresource utilization.

respiration The oxidation of organicmolecules to provide energy in plants andanimals. In animals, food molecules arerespired, but autotrophic plants respiremolecules that they have synthesized byphotosynthesis. Respiration occurs in allcells. The energy from respiration is used toattach a high-energy phosphate group toADP (adenine diphosphate) to form theshort-term energy carrier ATP (adeninetriphosphate), which can then be used topower energy-requiring processes withinthe cell. ATP is not transported betweencells, but is made in the cell where it is re-quired.

The chemical reactions of respirationnormally require oxygen from the environ-ment (aerobic respiration). Some organ-isms are able to respire, at least for a shortperiod, without the use of oxygen (anaero-bic respiration), although this processproduces far less energy than aerobic respi-ration. A few bacteria can survive indefi-nitely in anaerobic conditions.

The complex reactions of cell respira-tion fall into two stages, glycolysis and theKrebs cycle. The first stage, glycolysis, oc-curs within the CYTOPLASM, but the Krebscycle enzymes are localized within the MI-TOCHONDRIA of EUKARYOTES; cells withhigh rates of respiration (e.g. insect flightmuscles) have many mitochondria. Glycol-ysis results in partial oxidation of therespiratory substrate to the 3-carbon com-pound pyruvate. It does not require oxy-gen, so can occur in anaerobic conditions,when the pyruvate is converted to ethanolin plants and most bacteria or to lactic acidin animals and certain bacteria. The yield

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of glycolysis is two molecules of ATP plustwo molecules of the reduced coenzymeNADH2 for each molecule of glucoserespired. In the presence of oxygen, thepyruvate is further oxidized in the Krebscycle.

In the Krebs cycle, which requires freeoxygen, pyruvate is converted into the 2-carbon acetyl group, which becomes at-tached to a coenzyme forming acetyl coen-zyme A. This then enters a cyclic series ofreactions during which carbon dioxide isevolved and hydrogen atoms are trans-ferred to the coenzymes NAD and FAD.The energy released by the Krebs cycle istransferred via the reduced coenzymesNADH2 and FADH2 to an electron trans-port chain embedded in the inner mito-chondrial membrane, which drives theformation of more ATP. Overall, 38 mol-ecules of ATP are generated for each mol-ecule of glucose oxidized during aerobicrespiration, compared with only two mol-ecules of ATP during anaerobic respira-tion. At the end of the electron transportchain the electrons react with protons andoxygen to give water. In aerobic respira-tion, therefore, pyruvate is completely oxi-dized to carbon dioxide and water.

response A change in an organism or inpart of an organism that is produced as areaction to a stimulus.

restoration ecology See rehabilitationecology.

retrogression A reversion to an earlierstage of a SUCCESSION, with less species di-versity and complexity, due to a distur-bance, often the effects of a pollutant.

Revelle, Roger (1909–91) Americanoceanographer. Revelle was responsiblefor directing much of the work at theScripps Institute of Oceanography, LaJolla, California, that eventually led to thediscovery of sea-floor spreading and mag-netic reversals. He also turned in the 1950sto what was then the far from fashionabletopic of GLOBAL WARMING. The issue hadfirst been raised by ARRHENIUS in 1895.Revelle lobbied for real measurements and

as a result gas recorders were set up in1957 at Mauna Loa, Hawaii, and at theSouth Pole. By 1990 the carbon dioxideconcentration had risen to 350 parts permillion, an increase of 11%.

reverse osmosis A method for purifyingwater in which the water is forced underpressure through a membrane that is notpermeable to the impurities it is desired toremove.

rhizodegradation (phytostimulation)The breakdown of contaminants in the soilby microbial activity that is enhanced inthe presence of the rhizosphere (the area ofsoil around the plant roots). Both plantsand microorganisms may be involved. Forexample, hydrocarbon pollutants are bro-ken down into derivatives that are lesstoxic and less persistent in the environ-ment. See also bioremediation; rhizofiltra-tion.

rhizofiltration The decontamination ofpolluted water by passing it through theroots of certain plants that extract the tox-ins. The pollutants become adsorbed orprecipitated onto the plant roots and theplants are harvested and disposed of whenthe roots become saturated with contami-nants. Compare bioremediation; rhizo-degradation.

rhizosphere See rhizodegradation.

ribonucleic acid See RNA.

ribosome A small organelle that is thesite of protein synthesis in the CELL.

Richter scale See earthquake.

rift valley See plate tectonics.

Rio Conference See Earth Summit.

risk factor Any factor that is associatedwith an increased risk to the wellbeing ofan individual or environment. For exam-ple, prey animals have to compromise be-tween gathering food and keeping alookout for predators. Risk factors such as

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duration and intensity of exposure to pes-ticides can be quantified and managed.Analysis of risk factors is an early step inplanning for the conservation of species.

risk management The process of tak-ing and acting on decisions about the levelof risk that constitutes a threat to publichealth and the best method of controllingor minimizing that risk.

river continuum The concept of a riveras a single continuous ECOSYSTEM, in whichthere is a gradient of environmental factorsand of trophic relationships. While thereare predators throughout the system, in theupper reaches of a river, where consider-able amounts of organic matter enter thesystem from the river banks and overhang-ing vegetation, there are many organismsthat break it down, supporting communi-ties of detritus-feeders farther down theriver. As the river becomes wider and morelight enters it, there are more autotrophicorganisms, so more primary productionand an increase in grazers. When much ofthe river becomes too deep for aquaticplants, heterotrophic feeders again pre-dominate.

riverine forest (fringing forest; galleryforest) A narrow strip of forest similar toa RAINFOREST that occurs along river banksin drier regions than those usually colo-nized by rainforests.

RNA (ribonucleic acid) A nucleic acidcomprising a single polynucleotide chainsimilar in composition to a single strand ofDNA except that the sugar ribose replacesdeoxyribose and the pyrimidine base uracilreplaces thymine. RNA is synthesized inthe nucleus, using DNA as template, andexists in three main forms: messenger RNA(mRNA) is synthesized during transcrip-tion from template DNA in the nucleus,and passed out through the nuclear enve-lope to the ribosomes in the cytoplasm,which decode it in the process of transla-tion to synthesize peptides; transfer RNA(tRNA), which when activated carries spe-cific amino acids to the ribosome to bejoined together; and ribosomal RNA

(rRNA), which is the main component ofthe ribosome itself. In certain viruses, RNAis the genetic material. See transcription;translation.

rock Any mineral matter making up partof the Earth’s CRUST. Ecologists tend to usethe term to mean mainly consolidated solidmaterial, while geologists include in it un-consolidated material such as sand, mud,clay, peat, and gravel (see regolith). Bio-genic rocks are derived from the structuresor activities of living organisms, e.g. lime-stones that represent fossilized coral reefs.Chemical rocks are formed by the precipi-tation of chemicals or the mineralization oforganic remains. Fragmental rocks (clasticrocks) are formed by the consolidation ofrock fragments that have been weatheredor eroded from pre-existing rocks. See alsoigneous rock; metamorphic rock; sedimen-tary rock.

rock cycle The cycle of formation,degradation, and disintegration of ROCKS.Rocks are uplifted, then weathered anderoded, transported by rivers or ice, de-posited somewhere else, consolidated intorocks and possibly metamorphosed, thenuplifted again to start a new cycle.

Rodentia The largest and most success-ful order of mammals, including Rattus(rat), Mus (mouse), Sciurus (squirrel), andCastor (beaver). Rodents are herbivorousor omnivorous mammmals with one pairof chisel-like incisor teeth projecting fromeach jaw at the front of the mouth and spe-cialized for continuous gnawing. Rodentsare found universally and are mostly noc-turnal and terrestrial. They are noted fortheir rapid breeding.

root The organ that anchors a plant tothe ground and that is responsible for theuptake of water and mineral nutrientsfrom the soil. See mycorrhiza.

root nodule See nitrogen fixation.

root pressure The pressure that maybuild up in a plant root system due to theosmotic potential of the root cells, which is

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thought to help force water upward in theXYLEM vessels. Root pressure tends to buildup at night, when the rate of transpirationis low, and is a cause of GUTTATION in somespecies, especially grasses. See osmosis.

root–shoot ratio The ratio of theweights of the roots to the weight of theshoots of a plant. It indicates the ability ofthe plant’s roots to compete for nutrientsand water (a high ratio indicates competi-tive success), or the ability of the shoots tocompete for light for photosynthesis (a lowratio indicates success). A high ratio ischaracteristic of plants in the early stagesof a succession, whereas success in compet-ing for light becomes more important inthe later stages as the vegetation becomestaller and denser.

rosette plant Any plant whose leavesradiate outward from a short stem at soillevel, e.g. daisy (Bellis perennis). Thisgrowth form helps plants withstand tram-pling or grazing and in exposed habitatsavoids exposure to strong winds.

rotation of crops The planting of dif-ferent crops on the same land in successiveyears to ensure that species-specific pestsdo not get a hold, and that the land is notdepleted of the same combination ofminerals. Leguminous crops are frequentlyincluded in crop rotation for their nitro-gen-fixing abilities, and plowed back in tothe soil to replenish the supply of nitrates(see nitrogen fixation).

roundworms See Nematoda.

Rowland, Frank Sherwood (1927– )American chemist. In 1973, Mario MOLINA

took to Rowland, his postdoctoral adviser,some calculations suggesting that CFCS

(chlorofluorocarbons), which were widelyused in aerosol propellants, rise to theupper atmosphere and destroy the ozonelayer, located 8 to 30 miles above theEarth. As the layer protects us from harm-ful ultraviolet rays, its destruction couldhave disturbing consequences. Rowlandand Molina published their preliminary re-sults in June 1974. Rowland discovered

that 400.000 tons of CFCs had been pro-duced in the United States in 1973, andthat the bulk of this was being dischargedinto the atmosphere. He calculated that atthe then current production rate therewould be a long-term steady-state ozonedepletion of 7–13%. The National Acad-emy of Sciences published a report in Sep-tember 1976 supporting the work ofRowland and Molina, and in October1978 CFC use in aerosols was banned inthe United States. Final confirmation camewhen Joe FARMAN discovered in late 1984 a40% ozone loss over Antarctica. For hiswork on CFCs Rowland shared the 1995Nobel Prize for chemistry with MarioMolina and Paul CRUTZEN.

ruderal Describing a plant that growson wasteland, agricultural land, or aroundor on human dwellings. Ruderals includemany weeds that cannot tolerate muchcompetition and have relatively high nutri-ent demands.

ruminants Hoofed herbivorous mam-mals (order Artiodactyla) that chew thecud, e.g. cattle, deer, goats, and sheep. Aruminant’s stomach has a series of cham-bers that house symbiotic microorganismscapable of digesting CELLULOSE, a majorconstituent of plant cell walls that mostmammals have difficulty digesting. Food isregurgitated from the first chamber (therumen) and chewed again while the animalis resting before being swallowed again forcomplete digestion.

Ruminants have their own internal ni-trogen cycle. Microbial fermentation ofprotein in their food releases ammonia,which is converted into urea in the liver,some of which is excreted into the saliva.When the saliva passes with the food intothe rumen, the microbes convert the ureainto protein. This conserves water, becauseless is needed to dilute the urine. Desert ru-minants such as camels drink rarely com-pared to nonruminants such as donkeys.

runoff The flow of rainwater orsnowmelt from the land surface intostreams and rivers.

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Sagan, Carl Edward (1934–96) Amer-ican astronomer. Sagan’s main area of re-search was the solar system, particularlythe physics and chemistry of planetary at-mospheres and surfaces. In 1984 Sagancoauthored, with R. Turco, O. Toon,T. Ackerman and J. Pollock, an influentialpaper, Nuclear Winter: Global Conse-quences of Multiple Nuclear Explosions,referred to since as the TTAPS paper. Theauthors argued that even a relatively small-scale nuclear bomb of 5000 megatonswould create enough atmospheric smoke(300 million tons) and dust (15 milliontons) to produce a temperature drop of20–40°C, which would persist for manymonths. This prolonged NUCLEAR WINTER

would destroy much of the world’s agricul-ture and industry. The impact of the paperon politicians and the public was dramaticalthough the paper and the nuclear-winterargument itself was heavily criticized bymany other influential scientists.

salination (salinization) The accumula-tion in water or soil of salts of magnesium,potassium, and sodium. Salination of soilsoccurs in arid and semiarid regions, wherewater is drawn to the surface by CAPILLAR-ITY due to high rates of evaporation. As thewater evaporates, salts accumulate in theupper horizons of the soil. Plants cannotgrow well in such soils and thus salinationcontributes to DESERTIFICATION. It can alsooccur where salty GROUNDWATER seeps intoland adjacent to the coast, often when ex-cess water has been drawn off for irriga-tion.

salinity The concentration of dissolvedsalts or ions in a given volume of water,measured when all the organic matter hasbeen completely oxidized and all bromide

and iodide converted to chloride. It is usu-ally measured as parts per thousand byweight: 1 part per thousand (ppt or 0/00) isequal to 1 practical salinity unit (psu).

Fresh water has zero salinity, while theaverage salinity of sea water is about 35psu, and that of estuarine water variesfrom 0 to 35 psu. Inland lakes and seassuch as the Red Sea may have higher salin-ities. Increased salinity raises the relativedensity, lowers the freezing point, andgreatly influences marine ecosystems inthat the flora and fauna have to adapt tosaline conditions.

salinization See salination.

Salisbury, Sir Edward James (1886–1978) British botanist. Salisbury’s firstsubstantial work was his Plant Form andFunction (1938) written in collaborationwith Felix Fritsch, a widely used textbook.However Salisbury was primarily a plantecologist and did much work both on theeffects of soil conditions and on the seed-producing capacity of British species. Thiswas presented in his The Reproductive Ca-pacity of Plants (1942). He also carried outa long-term study of sand-dune ecology,the results of which were published in hisDowns and Dunes (1952).

salt Chemically, a compound formedwhen an acid is neutralized by a base. Mostsalts are crystalline ionic compounds, andmost minerals are composed of salts. Ineveryday language, the term salt usuallymeans common salt, sodium chloride(NaCl).

saltation An important method oftransport of sediment, such as sand grainsor river sediments, in which the particles

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are lifted steeply off the land or river bedand swept along and then, as the windspeed or current slackens, gently descendagain.

salt dome (salt plug) An intrusion ofsalt (halite) that pushes up existing sedi-ments into a dome below which naturalgas, oil, or water may accumulate inporous rocks or in the porous salt deposits.

salt gland A specialized secretory struc-ture involved in the removal of excess saltfrom the leaves of certain HALOPHYTES,such as sea lavenders (Limonium). The saltis exuded in solution, and washed awayfrom the plant by rain or by the next tide.Certain marine birds and reptiles thatdrink salt water have a nasal gland justabove each eye (birds) or between the eyeand nostril (marine reptiles, e.g. sea tur-tles), which remove salt from the blood andexcrete it as brine.

salt marsh Vegetation growing in salineor brackish marshy places, such as river es-tuaries and sheltered muddy coasts. Saltmarshes consist mainly of HALOPHYTES.They occur on land regularly inundated bythe tides, and represent an early stage invegetation succession (see halosere). Thedominant species include salt-tolerantgrasses such as Puccinellia spp. and some-times sea lavenders (Limonium). In wetterplaces, pioneer species such as glasswort(Salicornia), cordgrass (Spartina), sea aster(Aster tripolium), and dwarf shrubs suchas seablite (Suaeda) are found. Saltmarshes can be productive grazing land,and serve as important barriers against in-undation of the hinterland by the sea. Soilaccumulates steadily and a salt marsh caneventually rise above the level of inunda-tion and lose its salinity.

salt pan A basin or pool in a semiarid re-gion in which salts are precipitated andsaline water evaporates, e.g. Death Valley,California. Such salts are called evaporites.

salt plug See salt dome.

salvage The reuse of waste materials.

sampling There are many differentmethods of sampling. Random samplingcollects information in an unbiased way. Itinvolves the use of random numbers to se-lect individuals or locations to be sampled.For very large populations systematic ran-dom sampling is used: the total number ofindividuals is divided by the number of in-dividuals to be collected in each sample,giving a sampling interval. A random num-ber is used to select the first individuals tobe sampled, after which the sampling inter-val is added to that number to locate thenext sample, and so on, so that samples aretaken at regular intervals.

Various methods are used for samplingpopulations of living organisms and theirhabitats. In point sampling, sample pointsare selected by random numbers. This mayinvolve using a QUADRAT of selected size.Alternatively, a transect may be taken – aline from two specific points, often acrossan environmental gradient or slope – andan area of defined width sampled on eitherside of it or on one particular side.

sampling estimate A method of esti-mating the size of an animal population bycounting in sample plots. It assumes thatall members of the population have anequal chance of being sampled, and thatthere is negligible recruitment and mortal-ity during the sampling period.

sand Mineral particles consisting mainlyof quartz, feldspar, and mica, and measur-ing between 2.00 and 0.05 mm in diame-ter. Sandy soils contain at least 85% sandand not more than 10% clay. They arelight and drain well. However, retention ofnutrients and water is poor. See also soilstructure; soil texture.

sand dune See dune; xerophyte; xe-rosere.

sand filter A container of sand that fil-ters suspended solids from SEWAGE. Sandparticles are relatively large, and air canpass between them, aiding bacterial de-composition.

sand sea See erg.

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sandstone A sedimentary ROCK consist-ing of sand particles bound by cementingminerals such as silica or calcium carbon-ate.

sandy soil See sand.

saprobe (saprophyte) An organism thatderives its nourishment by digesting andthen absorbing the products or remains ofother organisms. Many fungi and bacteriaare saprobes, and are important DECOM-POSERS in food chains, returning nutrientsto the soil by putrefaction and decay.

saprophyte See saprobe.

saprotrophic The mode of nutrition ofa SAPROBE. See decomposer.

sapwood See wood.

saturation index See Langelier index.

saturation point The level of light in-tensity above which PHOTOSYNTHESIS nolonger increases with increasing light inten-sity.

savanna Tropical GRASSLAND with tallcoarse tufted grasses and often scatteredtrees or tall bushes. In Africa, baobab(Adansonia digitata) and Acacia and Eu-phorbia species are common. The climatealternates between cool dry winters andhot summers with heavy rains. Some sa-vannas have many grazing animals that af-fect the vegetation structure; these andother savannas have periodic or irregularfires in the dry season. Savannas are foundin large areas of South America (campos),East and South Africa, South-east Asia,and northern Australia. See climax com-munity.

scatter diagram See graph.

scavenger See detritivore.

Schimper, Andreas Franz Wilhelm(1856–1901) German plant ecologist. Dur-ing travels to the West Indies in 1881 and1882–83, Brazil (1886), Ceylon (Sri

Lanka) and Java (1889–90), and the Ca-nary Islands, Cameroons, East Africa, Sey-chelles, and Sumatra (1898–99) with theValdivia deep-sea expedition, he made eco-logical studies of tropical vegetation. Hisresults led to publication of important pa-pers on the morphology and biology of epi-phytes and littoral vegetation, culminatingwith his masterpiece, Pflanzengeographieauf physiologischer Grundlage (1898;Plant Geography Upon a PhysiologicalBasis), which relates the physiologicalstructure of plants to their type of environ-ment.

sclerophyllous Describing vegetationdominated by trees or shrubs with small orleathery leaves resistant to drought. Seegarrigue; maquis.

screening The removal of coarse float-ing and suspended solids from SEWAGE bythe filtering through racks or screens(sieves).

sea-floor spreading See plate tectonics.

sea grasses Any of various angiospermsthat live in the intertidal or immediate sub-tidal zone, e.g. eelgrass (Zostera noltii),tapeweeds (Posidonia). The term is some-times extended to include seaweed com-munities of green, brown, and redmacroscopic algae.

sea level The level of the ocean surface,usually taken to be the mean level halfwaybetween low and high tide averaged overall stages of the tide over a long period oftime (mean sea level). Where this is esti-mated on a global scale it is called theglobal mean sea level. Sea level is used as abaseline for measuring the elevation ofland or the depth of the seabed.

season A subdivision of the year consist-ing of a period of supposedly uniform orsimilar climatic conditions. In tropicalareas, temperatures are uniform through-out the year and a distinction is made be-tween wet and dry seasons only. Intemperate latitudes, the seasons are basedon the equinoxes and solstices. In the

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northern hemisphere the four seasons arespring (March, April, May); summer (June,July, August); fall (September, October,November); and winter (December, Janu-ary, February). In polar regions there is atwo- season year: summer and winter.

seaweed Any large marine alga (macro-alga). Seaweeds grow on rocky coasts fromhigh-water mark to well below low-watermark, where some species (e.g. kelps) mayform underwater forests. A few species arefree-floating, e.g. Sargassum weed, whichmay form large floating masses of seaweed,especially in the Sargasso Sea of the NorthAtlantic, supporting an entire ecosystem ofhighly adapted organisms.

secondary forest A forest growing onthe site of a former forest that has beencleared. It consists mainly of PIONEER

SPECIES that will be replaced as successionproceeds.

secondary productivity The rate of ac-cumulation of BIOMASS per unit area byHETEROTROPHS. Compare primary produc-tivity.

sediment Solid particles derived fromexisting rocks and deposited by wind,water, or ice or precipitated out of solutiononto land or onto the beds of rivers, lakes,and seas.

sedimentary rock A rock formed byconsolidation and compression of sedi-ments laid down by wind or water.

sedimentation A process used in watertreatment to remove solid particles beforeFILTRATION. It takes place in a sedimenta-tion (settling) tank.

seed See Angiospermophyta; gymno-sperms.

seed bank A collection of seeds kept forresearch purposes or for the conservationof rare, threatened, or endangered species.

seed dressing A chemical applied to

seeds to protect them against attack byfungi or insects. See fungicide.

seed plants See spermatophyte.

segregation The separation of the twoALLELES of a gene into different gametes,brought about by the separation of ho-mologous chromosomes at anaphase 1 ofMEIOSIS. See independent assortment.

seismic wave See earthquake.

seismograph See earthquake.

seismology The study of EARTHQUAKES.

seismometer See earthquake.

selection differential (S) The differencebetween the mean value of a phenotypiccharacter of a selected group of individualsand that of the population from which theycame.

selection pressure The degree to whichthe environment tends to change the bal-ance of ALLELES in a population or the fre-quency of a particular allele in thepopulation. It is not quantifiable. Compar-ison of survival rates of individuals withdifferent alleles gives a measure of the FIT-NESS of a genotype relative to others.Strong selection pressures result in evolu-tionary change. See evolution; natural se-lection.

selective breeding See artificial selec-tion.

selectively permeable membrane Seeosmosis.

selective pesticide A PESTICIDE that tar-gets only selected groups of pests, leavingother species unharmed.

selenium (Se) A highly toxic micronutri-ent that occurs in meat, nuts, oranges, andin bread made from wheat grown in soilscontaining selenium. Deficiency of sele-nium has been linked to disorders such asprostate cancer. In higher amounts it is

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toxic, and livestock grazing on selenium-rich soils, as in South Dakota, may sufferselenium toxicity.

self-limitation A situation where aspopulation density increases, intraspecificcompetition causes the reproduction rateor survival rate to decrease.

self-purification The reducing of thebiological oxygen demand (BOD) in astream following an inflow of organic ef-fluent that has raised BOD. As microbesdecompose the organic matter and there istherefore less of it, microbial populationsand BOD decline, and increased growth ofalgae and plants, together with natural tur-bulence, raise the oxygen concentration.

self-thinning The reduction in numbersof plants as density increases, due to COM-PETITION for resources and other plant in-teractions.

semelparity See perennial.

senescence The phase of the agingprocess of an organism or part of an or-ganism between maturity and naturaldeath. It is usually characterized by a re-duction in capacity for self-maintenanceand repair of cells, and hence by deteriora-tion.

sensor A device that detects the value ofor change in a physical entity such as pres-sure, temperature, pH, light, or sound, andconverts it into a signal from which mean-ingful data can be derived.

septic tank An underground storagetank for domestic waste in an area not con-nected to sewers. The waste is partially orwholly broken down by anaerobic mi-croorganisms, and the final effluent may beallowed to soak away, or the tank may beemptied at regular intervals.

sequestration See chelating agent.

seral stage See sere.

sere Any plant COMMUNITY in a SUCCES-

SION in which each community itself effectschanges in the HABITAT that determine thenature of the following stage. The succes-sive stages are known as seral stages. Seresresult eventually in a CLIMAX COMMUNITY.Stages in a secondary succession, which ap-pear when the biotic components of a pri-mary sere are destroyed, e.g. by fire, arecalled subseres; seres in microhabitats arecalled microseres. The initial (pioneer)community in a succession is termed a pris-ere. See also halosere; hydrosere; xerosere.

set-aside Land that is taken out of culti-vation or other usage and left to revert toits natural state, often incurring a govern-ment subsidy, so as to prevent overproduc-tion or to protect the environment fromoverexploitation or soil degradation inareas of poor soils. The term is also usedfor land that is set aside in national parks,nature reserves, and other protected areas.

sewage Liquid-borne waste that con-tains organic matter in solution or suspen-sion, especially that produced by domesticand commercial premises.

sewage farm An area of land on whichsewage or other wastewater is spread asmanure or to purify it by oxidation and mi-crobial DECOMPOSITION.

sewage fungus A slime or woollygrowth in sewage and on objects in waterspolluted by sewage, which is formed bycommunities of filamentous bacteria,fungi, and protozoa.

sewage sludge (sludge) A slimy semi-solid substance produced by the sedimen-tation and precipitation of solid material insedimentation tanks during sewage treat-ment.

sex The quality of being either male orfemale, with the ability to produce one oftwo types of gamete – sperm in the maleand eggs (ova) in the female. An individu-al’s sexuality may be determined by geneticor environmental factors or both, accord-ing to the species, or by whether or not anegg has been fertilized.

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sex ratio The ratio or males to females(or vice versa) in a population. A distinc-tion is made between the primary sex ratio– the ratio of males to females among zy-gotes – and the operational sex ratio, theratio of males to female at the time of mat-ing, because in many species the sexes havedifferent mortality rates.

sexual dimorphism A difference be-tween the phenotypes of males and femalesof the same species, e.g. a difference in size,color, or courtship display.

sexual reproduction The formation ofnew individuals by fusion of two HAPLOID

nuclei or sex cells (gametes) to form adiploid zygote. In unicellular organismswhole individuals may unite but in mostmulticellular organisms only the gametescombine. In organisms showing sexuality,the gametes are of two types: male and fe-male (e.g. in animals, spermatozoa andova). They may be produced in special or-gans (e.g. testis and ovary in animals, an-ther and carpel in angiosperms, antheridiaand archegonia in lower plants and somealgae). The gametes may be derived fromthe same parent (autogamy) or from twodifferent parents (allogamy). Individualsproducing both male and female gametesare termed hermaphrodite. If a flower pro-duces both male and female gametes, theplant is said to be monoecious if there areseparate male and female flowers; those in which male and female gametes areborne on different individuals are termeddioecious. See alternation of generations;natural selection; recombination.

sexual selection The selection by onesex of particular characteristics in the otherwhen choosing a mate, usually mediatedby courtship behavior.

shade plant A plant that can tolerateand thrive in low light intensity. Some, butnot all, shade plants are sensitive to verybright light and cannot live in open habi-tats. Shade plants are found where trees orshrubs form a canopy that cuts out muchof the light, especially red light, leaving a

higher proportion of far-red to red lightthan in sunlight. Compare sun plant.

Shannon-Weaver index (H) A loga-rithmic measure of SPECIES DIVERSITY that isweighted by the abundance of each species.

shelter belt A strip of trees planted up-wind of a site to shelter the land or build-ings from the prevailing wind.

shifting cultivation See slash-and-burn.

shoot The aerial photosynthetic portionof a plant that generally consists of a stemupon which leaves, buds, and flowers areborne.

short-day plant See photoperiodism.

shrub A woody PERENNIAL plant smallerthan a tree that branches very close to theground so that there is no obvious maintrunk.

shrub layer In the layered structure of aforest, a layer of low-growing woodyplants that includes shrubs and saplings.

sievert (Sv) A unit of radiation dose: 1Sv is a dose equivalent to the gamma radi-ation received from 1 mg of radium en-closed in platinum 0.5 mm thick deliveredfor 1 hour at a distance of 1 cm, or 100rems. For most purposes a millisievert(1/1000th of a sievert, mSv) is more useful.

sieve tube See phloem.

sigmoid curve (S-shaped curve) See ex-ponential growth.

silage Cattle feed produced by theanaerobic microbial decomposition (FER-MENTATION) of green plant matter (such asfreshly harvested grass) mixed with dilutedmolasses and sometimes chemical additivesin a pit or storage tower (silo).

silicon A micronutrient found in manyanimals and plants, especially grasses,where it can be a useful taxonomic feature.

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It forms the skeleton of certain marine ani-mals, e.g. the siliceous sponges.

silt Mineral particles between 0.05 and0.002 mm in diameter. A silt SOIL has morethan 80% silt and less than 12% clay, andhas a smooth soapy texture. Comparesand.

Silurian The period, some 440–405 mil-lion years ago, between the Ordovicianand the Devonian periods of the Paleozoic.It was a relatively warm period in theEarth’s history, during which the first landplants appeared. It is characterized by sim-ple plants such as liverworts; fossils ofplants similar to vascular plants, e.g.Cooksonia, in the later part of the period,some of which had simple strands of tra-cheids; primitive jawless fish; and many in-vertebrates. See also geological time scale.

silviculture The management of forestor woodland for the production of timberand/or other wood crops.

Simpson’s index (D) A measure ofspecies diversity weighted by the relativeabundance (see abundance) of each spe-cies.

sink 1. A natural reservoir that can takein and store energy or materials without it-self undergoing change, e.g. the deepoceans are sinks for carbon dioxide2. An ecosystem, habitat, community, orpopulation that receives an input of ma-terials or individual organisms.

SI units (Système International d’Unités)The internationally adopted system ofunits used for scientific purposes. It hasseven base units (the meter, kilogram, sec-ond, kelvin, ampere, mole, and candela)and two supplementary units (the radianand steradian). Derived units are formedby multiplication and/or division of baseunits; a number have special names. Stan-dard prefixes are used for multiples andsubmultiples of SI units. See Appendix.

slash-and-burn (shifting cultivation) Asmall-scale agricultural system typical of

semi-nomadic people, in which a smallarea of natural vegetation is cleared by cut-ting down and burning, cultivated for up to5 years, then abandoned as the soil be-comes less fertile and yields fall. The aban-doned area is soon colonized by PIONEER

SPECIES from the surrounding natural vege-tation, and undergoes succession back toclimax forest. Slash-and-burn is now prac-ticed mainly in tropical rainforests wherethere is no significant population pressureon the land.

sling psychrometer See humidity.

sludge See sewage sludge.

smog Naturally occurring fog contami-nated with smoke and/or invisible pollu-tants. See photochemical smog.

smoker See hydrothermal vent.

social dominance See dominance.

society A minor plant COMMUNITY

within a larger community, characterizedby a specific dominant species. For exam-ple, a Trillium-dominated society in a com-munity such as an oak woodland.

sociobiology The study of social behav-ior, based on the assumption that all be-havior has evolved as an adaptation to theenvironment.

sodium An element essential in animaltissues, and often found in all terrestrialplants, although it is believed not to be es-sential in most, with the exception of somesalt-tolerant C4 PLANTS. It is implicated inCRASSULACEAN ACID METABOLISM. Sodiumoccurs widely in nature as sodium chloride(NaCl) in seawater and as deposits of halitefrom dried-up seas.

sodium chloride See salt.

soft detergents BIODEGRADABLE deter-gents.

soft water Water that does not contain

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appreciable amounts of dissolved mineralssuch as calcium and magnesium salts.

softwood See wood.

soil The accumulation of mineral parti-cles and organic matter that forms a super-ficial layer over large parts of the Earth’ssurface. It provides support and nutrientsfor plants and is inhabited by numerousand various microorganisms and animals.A section down through the soil is termeda soil profile and this can characteristicallybe divided into three main layers or hori-zons. Horizon A, the topsoil, is darker thanthe lower layers due to the accumulation oforganic matter as humus. It is the most fer-tile layer and contains most of the soil pop-ulation and a high proportion of plantroots. Horizon B, the subsoil, contains ma-terials washed down from above and maybe mottled with various colors dependingon the iron compounds present. Horizon Cis relatively unweathered parent materialfrom which the mineral components of thelayers above are derived. The depth andcontent of the horizons are used to classifysoils into various types, e.g. podsols andbrown earths. The texture, structure, andporosity of soil depends largely on the sizesof the mineral particles it contains and onthe amount of organic material present.Soils also vary depending on environmen-tal conditions, notably rainfall.

soil color Color is an important featureof different soil types and horizons. For ex-ample, it can give an indication of theamount of humus in a horizon, or the de-gree of oxidation of iron minerals. See alsoMunsell soil color chart system.

soil conservation The management ofsoil to prevent or reduce soil erosion bywind and water, and to avoid overuse,leading to mineral depletion and loss of soilstructure, i.e. to ensure the sustainable useof soil.

soil erosion The loosening and removalof topsoil or soil from upper horizons bythe action of wind, excessive runoff orother moving water, and excessive down-

hill movement of loosened material underthe influence of gravity. Erosion is ofteninitiated or exacerbated by human activity,such as overgrazing and plowing acrosscontours instead of along them on steepslopes, so that water funnels down the fur-rows, removing natural vegetation fromsteep slopes in areas of heavy rainfall. Thecultivation of grassland has led to muchsoil erosion – the loss of the close mesh ofgrass roots following plowing renders soilssusceptible to erosion if it is a time of highwinds or heavy rainfall. See also DustBowl.

soil moisture Water stored in soils. Ithas three components. Hygroscopic waterclings to the surface of soil particles by mo-lecular attraction sufficiently powerful thatit is unavailable to plants. Capillary waterforms thicker films and fills small spacesbetween soil particles. It is rich in nutrientsand available to plants. Gravitationalwater is soil water that is neither hygro-scopic nor capillary water, but free to flowunder the influence of gravity. See field ca-pacity.

soil pH The acidity or alkalinity of thewater in soil (soil solution). This dependson the soil’s parent rock, the rate of weath-ering, the nature of any solutions percolat-ing through it, the amount of rainfall, thestructure of the soil, and the nature of theoverlying vegetation. pH has a major effecton the availability of nutrients to plants.An increase in acidity can interfere withroot osmoregulation, affect the root’s abil-ity to exchange gases, increase the concen-tration of heavy metals, and may affect thebalance of nutrient ions in the soil. In alka-line soils iron, phosphate, and trace el-ements such as manganese may be trappedin insoluble compounds.

soil profile See soil.

soil skeleton The physical structure ofthe soil – its mineral particles. See soilstructure; soil texture.

soil structure The arrangement of themineral particles in the soil – whether they

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are free or bound into aggregates by de-composing organic matter

soil survey The systematic investigationand mapping of soils in the field.

soil texture The sizes of the differentmineral particles in a soil, which also influ-ence its structure, drainage and aeration,and the retention of water by CAPILLARITY.The size of the particles depends in part onthe parent rock and how it weathers. Themain size classes of mineral particles aregravel and stones (greater than 2 mm in di-ameter), sand (0.05 to 2 mm), silt (0.002 to0.05 mm), and clay (smaller than 0.002mm). Clay soils, which have small parti-cles, tend to be wet and sticky, with poordrainage and aeration. However, they tendto be electrostatically charged and attracthumus, forming a clay–humus complexthat can attract and retain minerals. Thehumus sticks the clay particles together,forming larger aggregates and improvingdrainage and aeration. Clay soils can befurther improved by adding lime (liming),which promotes clumping of particles.Sandy soils have large particles, and there-fore larger spaces between them fordrainage and aeration. They may need ad-dition of HUMUS or MULCHES to improvewater and mineral retention and preventleaching.

soil type The most widely used classifi-cation system for soils is that of the UnitedStates Soil Survey, which classifies soilsinto 11 orders on the basis of their soil pro-files, acidity, and moisture content.

solar energy (solar radiation) Energytransmitted from the Sun as electromag-netic radiation (see electromagnetic energy).It covers the spectrum from ultraviolet ra-diation through visible light to infrared ra-diation (heat). As well as reaching thesurface of land and sea directly as radia-tion, it may also be transformed into otherforms of energy, driving the atmosphericcirculation, setting up thermal gradients inoceans and lakes, and becoming incorpo-rated into biomass (chemical energy).

solar power The use of SOLAR ENERGY

to generate electricity.

solar radiation See solar energy.

solid edition See map.

solid waste Any solid material dis-carded as WASTE, e.g. garbage, sludge fromsewage treatment plants, commercial anddomestic refuse, and mining slag or spoil. Itmay include HAZARDOUS WASTE, such asrefuse from hospitals or industrial plants.

solifluction The slow downhill creep ofsoil and other loose material typical ofareas where the ground is subjected to al-ternate freezing and thawing. Freezing ex-pands the water between the particles,raising them slightly. When the ice thaws,the particles come to rest a little fartherdown the slope.

solonchak A soil with a high solublesalt concentration in the top 30 cm and noobvious horizons except for deposits ofsalts like gypsum and carbonates. It isformed from saline parent rock in areas ofwarm to hot climate with high evaporationrates and a pronounced dry season, such asparts of the Mediterranean and subtropics.Solonchaks require irrigation if they are tobe cultivated. See soil.

solstice Either of the two moments inthe year when the Sun in its orbit reachesits greatest distance north or south of theequator. It occurs on or about June 22(summer solstice in the northern hemi-sphere and winter solstice in the southernhemisphere) and on or about December22. The day of the summer solstice has thelongest period of daylight and the shortestnight of the year, and vice versa for thewinter solstice.

somatic polymorphism The presenceon the same individual of two or moreforms of a particular organ, for example,leaves of different shape. See modular or-ganism.

sonar See echo sounder.

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soot Carbon dust contaminated withoily compounds derived from the incom-plete combustion of wood or fossil fuels.

sorbent See sorption.

sorption The uptake of a gas or liquidby a solid by either ADSORPTION or ABSORP-TION. The substance that takes up the gasor liquid is called the sorbent.

source–sink metapopulation See meta-population.

Southern Oscillation (SO) A fluctua-tion of the intertropical atmospheric circu-lation in which air moves between thesoutheastern Pacific subtropical high andthe Indonesian equatorial low, driven bythe temperature difference between thetwo areas. When air is blown toward thewestern Pacific, it causes warm surfacewater to accumulate there, but promotesupwelling along the western coast of SouthAmerica, bringing colder nutrient-rich sur-face water. At the opposite stage of the SO,warm western Pacific water is carried far-ther east, reducing the nutrient content ofthe surface waters and affecting marineecosystems. These oscillations are also as-sociated with periods of intense instabilityand storms. See also El Niño.

specialist A species with narrow food orHABITAT preferences. Compare generalist.

speciation The formation of one ormore new SPECIES from an existing species.Speciation occurs when a POPULATION sep-arates into isolated subpopulations that de-velop distinctive characteristics as a resultof NATURAL SELECTION or random GENETIC

DRIFT, and cannot then reproduce with therest of the population, even if there are nogeographical or other physical reasons toprevent them from doing so. Speciationthat occurs as a result of such factors as ge-netic mutations and genetic drift, andchanges in behavior, with no external bar-riers to reproduction is called sympatricspeciation. Often there are more obviousbarriers to interbreeding, such as geo-graphical changes (e.g. in river courses) –

this is called allopatric speciation; habitatsbecoming uninhabitable (e.g. following ur-banization), as changes in timing of flow-ering, or when populations at the marginsof the main population experience differ-ent selection pressures as they colonizedifferent environments (parapatric specia-tion). Another cause of rapid speciation inplants is POLYPLOIDY. See also adaptive ra-diation; reproductive barrier.

species A TAXON comprising one ormore POPULATIONS, all the members ofwhich are able to breed among themselvesand produce fertile offspring. They arenormally isolated reproductively from allother organisms, i.e. they cannot breedwith any other organisms. Some speciescan interbreed with other related species,but the HYBRIDS are often wholly or par-tially sterile. For some plant species theremay be an almost complete continuum be-tween closely related species, as in certainorchids. This may also happen in disturbedareas or in regions where the range of twoor more species or even genera overlap,producing hybrid swarms (see apomixis).Where species form apomictic clones thatare very difficult to distinguish from eachother, they are termed aggregate species,e.g. bramble (Rubus fruticosus). In general,species are regarded as distinct if they re-main reproductively isolated for most oftheir geographical range and have recog-nizably distinct morphological characteris-tics. Within a species, there may besubgroups with distinct morphological or(especially in microorganisms) physiologi-cal characteristics; these groups are termedsubspecies or races. Groups of similarspecies are classified together in genera. Seealso binomial system of nomenclature;genus; population; speciation.

species diversity The number of SPECIES

in an area or community and their relativeABUNDANCE. There are various scales of di-versity. Alpha (local) diversity is the num-ber of species in a small area of fairlyuniform habitat. Gamma (regional) diver-sity is the number of species in all the habi-tats in a region with which there are nosignificant barriers to dispersal. It is related

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to the local diversity and the number of dif-ferent habitats in the region. The differencein species diversity between two habitats istermed the beta diversity, and is related tothe whether the species are generalists orspecialists and their degree of habitat spe-cialization. The greater the habitat special-ization and the greater the proportion ofspecialists the bigger the difference inspecies between habitats, and the greaterthe beta diversity. If all the species in bothhabitats are generalists, the two habitatsare effectively one, and beta diversity = 1.

species pool The biota of a large areafrom which species may migrate to sur-rounding areas. For example, an islandpopulation may be augmented by migra-tion from the nearby mainland. See alsoequilibrium theory.

species richness The number of speciesin a community compared with the num-ber of individuals in the community.

species saturation A community thatcontains the maximum diversity of speciespossible under the prevailing conditions. Ina stable environment, species diversity islimited by local interactions betweenspecies.

species turnover The change in speciescomposition of a community or area thatresults from COLONIZATION, EXTINCTION,and recolonization events. Species turnoverrates will be high in disturbed or newlyavailable habitats (e.g. in the early stages ofSUCCESSION), and low in long-establishedcommunities in stable environments.

spermatophyte Any seed-bearing plant.

Sphagnum A genus of mosses, oftencalled peat mosses or bog mosses. A Sphag-num moss can hold over 20 times its owndry weight of water and these mosses areimportant BOG-formers, producing PEAT

that can be harvested for fuel or, more re-cently, for horticultural use, a trade thathas seriously reduced some bogs. In areasof high rainfall where rocks are fairly im-permeable and soils are acidic, Sphagnum

bogs can cover larger areas, e.g. parts ofnorthern Scotland and the Great DismalSwamp in North Carolina and Virginia inthe United States, which once covered5700 sq km but is now reduced to 1940 sqkm. See also hummock and hollow cycle.

spiders See Arthropoda.

sponges See Porifera.

sporangium (pl. sporangia) A repro-ductive body in which asexual SPORES areformed.

spore See sporangium.

sporophyte See alternation of genera-tions.

spring A place where GROUNDWATER

flows naturally out of the ground wherethe WATER TABLE intersects the ground sur-face.

spring overturn Mixing of the water oflakes in spring due to wind action, result-ing in nutrients from the bottom sedimentsbeing mixed into the water column andoxygen from the surface being carried intodeep water.

spring tide See tide.

stability 1. A situation in which air thatis forced to rise tends to return to its previ-ous level once the factor causing the rise isremoved. This occurs if the rate at whichthe rising air cools with height due solely toits vertical movement (see adiabatic cool-ing and heating) is greater than the rate atwhich the surrounding air cools withheight. The rising air becomes cooler anddenser than the surrounding air, so it sinksdown again. See atmosphere.2. (stable equilibrium) The situation wherethe number of individuals in a populationor the level of a resource quickly returns toits original value following a disturbance.

stabilization pond A large shallowpond in which WASTEWATER is allowed todecompose naturally by the action of mi-

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croorganisms, algae, and sunlight, the wastebeing mixed and oxygenated by the wind.

stabilizing selection Selection thatserves to maintain the genetic structure of apopulation as it is. It will normally be thedominant form of selection in a populationin a stable environment with little competi-tion. The range of variation in the popula-tion may also decrease as the mostcommon forms are favored and the lesswell adapted forms decline in number witheach generation, resulting in a restrictedrange of phenotypes. See natural selection.

stable equilibrium See stability.

stagnation Lack of movement in a massof air or water. Stagnation leads to a build-up of POLLUTANTS, where present. Stagnantwater tends to become deoxygenated, be-cause there is no turbulence to mix air fromthe surface into deeper water.

standard deviation (s) In statistics, ameasure of the dispersion of values assum-ing that they form approximately a normaldistribution. It is calculated by squaring allthe deviations from the mean, calculatingtheir mean, then finding the square root.This gives a value s, which is the point ofmaximum slope either side of the center ofthe distribution curve.

standard error (SE) The estimatedSTANDARD DEVIATION of an estimate of aparameter. The standard error of the meanof a sample from a population with a nor-mal distribution is given by:

SE = s/√nwhere s is the sample standard deviation.The standard error indicates the reliabilityof the sample mean as an estimate of thetrue mean of the population and n is thesample size.

standard index of association Ameasure of the degree to which two speciesare associated with each other in their dis-tribution:

dik = (Oik – Eik) / sikwhere dik is the difference between the ob-served (Oik) and the expected (Eik) number

of locations in which species i and k wouldbe expected to occur together by chance,and sik is the standard deviation of the ex-pected number. See also association.

standing crop The total weight of allthe living organisms present in an ECOSYS-TEM at a given moment, usually expressedas dry weight per unit area.

starch See carbohydrates.

stationary source See point source.

steady state A state in which the inputto a system (energy, resources, individuals)is at the same level as the output. In termsof a living COMMUNITY, the input comesfrom the energy and raw materials used inphotosynthesis or other forms of au-totrophism, and the output from respira-tion and decomposition. If input andoutput are in balance, there is no net loss orgain of energy or materials by a communityor ecosystem. A CLIMAX COMMUNITY inequilibrium with the climate is consideredto be in a steady state.

stem See shoot.

stenohaline Describing an organismthat is extremely sensitive to changes insalinity, and is unable to tolerate muchvariation in osmotic pressure.

steppe Temperate GRASSLAND domi-nated by drought-resistant species ofperennial grasses, found in regions of LOESS

soil and extreme temperature range, oftentypically on CHERNOZEM soils, in a zonefrom Hungary eastward through theUkraine and southern Russia to CentralAsia and China. There are pronounced sea-sons, with hot summers and cold winters,with greater temperature extremes andlower rainfall in the east. The species com-position varies with the climate. Typicalgrasses include feathergrasses (Stipa) andsheep’s fescue (Festuca ovina), togetherwith bunchgrass (Schizachyrim) and blue-grass (Poa spp.). Many of the herbs, suchas Tulipa, Allium, and the grass Poa bul-bosa, have underground PERENNATING OR-

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GANS and die back at the start of summer.There are also many EPHEMERAL speciesthat germinate as soon as rain falls, beforethe grasses take over. Large areas of formersteppe are under cultivation for grain pro-duction.

sterile 1. Describing an organism that isunable to reproduce.2. Describing a pure culture of microor-ganisms that is not contaminated by otherspecies.

sterilization 1. The removal or destruc-tion of all microorganisms, includingspores, from an object.2. The process of making an organism un-able to reproduce.

Stevenson screen A shelter for housingthermometers for use in meteorologicalmeasurements in conditions in which theygive standard readings. It consists of a ven-tilated box with an air space between aninner an outer roof. It contains wet-and-dry-bulb thermometers for measuringhumidity and temperature, which arescreened from the sun’s radiation,

stilt root An enlarged form of prop root,seen in some mangroves and a few palmsand other trees, which helps support plantsin unstable soils. See aerial root.

stomata (sing. stoma) One of a largenumber of pores in the epidermis of plantsthrough which gaseous exchange occurs.See stomatal rhythm; transpiration.

stomatal rhythm A daily rhythm ofstomatal opening and closing, usually gov-erned by a biological clock, and fine-tunedby environmental signals. The rhythm ismaintained for some time in constant con-ditions, such as darkness, but eventuallyfades. The stomata of most plant speciesopen in the morning and close in theevening to conserve moisture when condi-tions are no longer suitable for PHOTOSYN-THESIS. Some species living in hot dryclimates open in the morning, then closefor some hours around midday, openingagain later in the afternoon, thus avoiding

water loss from TRANSPIRATION at a timewhen evaporation rates are at their highest.In plants with CRASSULACEAN ACID METABO-LISM, the stomata remain closed by day andopen at night, when carbon dioxide is fixedinto organic acids.

storm sewer A conduit that collectsrun-off from heavy rain and snowmelt, in-cluding street wash, and transports it backto the groundwater. It is not connected tothe sewerage system that carries waste-water, although in some cases (e.g. somecity runoff) it may direct the runoff into atreatment plant.

stormwater Excess runoff due to stormsand rapid snowmelt. See storm sewer.

strain Any group of similar or identicalindividuals, such as a clone, mating strain,physiological race, or pure line. See clone;race.

stratification 1. A seed treatment thatenables seeds that require VERNALIZATION

to germinate the following spring: theseeds are placed between layers of moistsand or peat and exposed to low tempera-tures, usually by leaving them outsidethrough the winter.2. The existence of layers of water of dif-ferent density in a LAKE or other body ofwater. The differences in density may bedue to temperature or to salinity. In manylakes in summer there is a warm low-density layer (epilimnion) lying above acolder denser layer (the hypolimnion), thezone of rapid temperature change betweenthe two layers being called the thermocline.In autumn, strong winds create turbulence,and the layers become mixed. While strati-fication persists, nutrients are continuallylost from the epilimnion as planktonic andlarger organisms die and their bodies sinkbelow the thermocline. With the autumn‘turnover’, nutrients are returned to theupper layers, ready to promote growth inspring (see fall bloom; spring overturn).See also dimictic.

stratigraphy The study of sediments –their formation, composition, sequences,

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and consolidation into sedimentary rock. Itincludes deducing the history of a sedimen-tary rock from a study of the structure, or-ganization, and relationships of thedifferent sediment layers of which it iscomposed. See geological time scale.

stratopause See atmosphere.

stratosphere See atmosphere; ozone layer.

stratospheric cooling A cooling of thestratosphere (see atmosphere) due toGREENHOUSE GASES that cause warming inthe troposphere but prevent infrared radia-tion from the Earth from reaching thestratosphere. See greenhouse effect.

stratum (pl. strata) A horizontal or in-clined layer of rock of similar composition,especially one that is part of a series of par-allel layers arranged on top of each other.

stream A relatively small body of waterflowing in a channel.

street canyon An urban area enclosedby buildings (e.g. a street) in which the highdensity of traffic leads to carbon monoxideand other pollutants reaching high concen-trations.

stress tolerator A plant or other organ-ism that copes with a stressful habitat (e.g.dry, cold climate, or infertile soil) by grow-ing only slowly, thus making low demandson its environment, living for a long timeand reproducing only when it has sufficientresources.

strip cropping (strip farming) On slop-ing land, the planting of long strips of landparallel to the contours with differentcrops to reduce soil erosion.

strip farming See strip cropping.

strip mining See open-pit mining.

stromatolite A structure formed by mi-crobial mats of CYANOBACTERIA that secretelayers of calcium carbonate, forming largecushion-like masses. They occur only in

certain protected warm lagoons today.Fossilized stromatolites have been found inrocks dating back at least 3000 millionyears.

structural diversity Within the area oc-cupied by a COMMUNITY, the range of phys-ical structures that may provide suitablehabitats for species.

subclimax See climax community.

subduction zone See plate tectonics.

sublittoral 1. The marine zone extend-ing from low tide to a depth of about 200m, usually to the edge of the continentalshelf. Light penetrates to the seabed, andthe water is well oxygenated. Large algae(e.g. kelps) are found in shallower waterswhereas certain red algae (Rhodophyta)may be found in deeper water. Comparebenthic zone; littoral.2. The zone in a lake or pond between thelittoral and profundal zones, extendingfrom the edge of the area occupied byrooted plants to a depth of about 6 to 10m, where the water temperature declines.Its depth is limited by the compensationlevel – the depth at which the rate of pho-tosynthesis is equaled by the rate of respi-ration, and below which plants cannot live(but some phytoplankton can). Comparelittoral; profundal.

subpopulation (local population) Theindividuals of a particular species that livein the same habitat patch. Comparemetapopulation.

subsere See sere.

subspecies See species.

substitutable resource A RESOURCE

that is capable of being interchanged withanother resource to satisfy the requirementof the consumer.

subtropical Relating to the zone on ei-ther side of the Equator, between latitudes23.5° and 34.0° in either hemisphere.

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succession A progressive series ofchanges in vegetation and animal life of anarea over time from initial colonization tothe final stage, or climax. The climax is adynamic equilibrium because, although thesuccession can progress no further underthe environmental factors present at thetime, the populations present change, e.g.trees die, creating gaps for other species tocolonize. In addition, the climate is seldomcompletely stable – at best it is cyclical,with variations from year to year (see cyclicclimax; sere). In some successions, such asthose from wetlands to terrestrial commu-nities, the early occupants change the envi-ronment, making it possible for laterspecies to move in. In others, modificationof the environment by early colonists haslittle or no effect on the subsequent perfor-mance of later species in the succession –this is called successional tolerance.

Where succession is driven only byprocess operating within the community, itis termed autogenic; where it is driven byexternal influences that alter environmen-tal factors, it is a termed allogenic. A suc-cession that takes place from a newlycolonized habitat, starting with PIONEER

SPECIES, is called a primary succession. Onethat follows a disturbance such as a fire,which destroys the original community, iscalled a secondary succession. An arrestedsuccession is one that has been preventedfrom reaching its natural climax, for exam-ple by grazing or other disturbance. Seealso facilitation.

Where a succession of species involvesmainly plants, it is termed an autotrophicsuccession; if involving mainly animalspecies, it is a heterotrophic succession.

successional tolerance See succession.

succulent A plant with swollen stems orleaves that have large parenchyma cellsthat store water. They normally have awater-resistant waxy cuticle. Succulentsare characteristic of dry habitats such asdeserts and many of these succulents arearmed with spines or similar structures,e.g. cacti. They also occur in saline habitatssuch as salt marshes.

sugar See carbohydrates.

sulfur An essential element in living tis-sues, being contained in nearly all proteins.Plants take up sulfur from the soil as thesulfate ion SO4

2-. The sulfides released bydecay of organic matter are oxidized to sul-fur by sulfur bacteria of the genera Chro-matium and Chlorobium, and furtheroxidized to sulfates by bacteria of thegenus Thiobacillus. There is thus a cyclingof sulfur in nature.

sulfur bacteria Filamentous autotrophicchemosynthetic bacteria that derive energyby oxidizing sulfides to elemental sulfurand build up carbohydrates from carbondioxide. They use sulfides instead of wateras a source of electrons in photosynthesis,releasing sulfur instead of oxygen. An ex-ample is Beggiatoa. They are found mainlyin sulfur-rich muds and springs, includingHYDROTHERMAL VENTS. A few archaea, e.g.Sulfolobus, can oxidize elemental sulfur.As well as sulfides, some bacteria oxidizethiosulfates, polythionates, and sulfites.Sulfur bacteria play an important role inthe cycling of sulfur in the ecosystem.

Sulfur-respiring anaerobic bacteria,such as Desulfovibrio, transfer electronsfrom carbon compounds to sulfur com-pounds. They are responsible for the re-lease of hydrogen sulfide into anaerobicsediments. The mineral pyrite (iron sulfide)is thought to have been formed by the re-action of this hydrogen sulfide with ironcompounds present in mud. See also pho-tosynthetic bacteria.

sulfur cycle See sulfur; sulfur bacteria.

sulfur dioxide (SO2) A colorless chok-ing gas, readily soluble to form an unstablesolution of sulfurous acid (H2SO3). Sulfurdioxide is released from many industrialprocesses and from the burning of FOSSIL

FUELS. It is a serious air pollutant, causingrespiratory irritation and triggering asthmain susceptible people. It is a major con-stituent of ACID RAIN.

sulfur dust A powdered form of sulfur

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used to kill fungal pathogens on grapes andother crops.

Sun The star around which the Earthand other members of the Solar Systemorbit. Its average density is relatively low,because it is made up of at least 90% hy-drogen and helium, but the density of its core is six times that of Earth’s core, and the temperature here reaches 15.000.000K. The intense pressure and density in the core make possible reactionsin which hydrogen atoms are convertedinto helium, generating the energy radiatedby the Sun. Most of this energy is initiallyin the form of gamma rays, which are ab-sorbed and re-emitted as heat and light bymaterial farther from the core. Hot ionizedparticles stream out of the Sun as the solarwind, a spiraling stream of about 400 kmper second, dragging magnetic fields withit and affecting the Earth’s upper ATMOS-PHERE.

sun plant A plant that can tolerate andthrive in high light intensities. Such plantshave thicker leaves than SHADE PLANTS,have numerous stomata on the undersideof the leaf, where transpiration rates arelower, and small air spaces so the leaf doesnot lose water too readily. Compare shadeplant.

supercontinent See Gondwanaland; Pan-gaea.

superkingdom A taxonomic rankabove the rank of kingdom but below therank of DOMAIN. The prokaryotes and eu-karyotes are sometimes considered as twosuperkingdoms. See also classification; tax-onomy.

superorganism The concept of a COM-MUNITY as a kind of giant organism, withmember SPECIES closely bound together bytheir present interactions and commonevolutionary history, the function of eachpromoting the wellbeing of the whole com-munity.

supertramp species One of the earliest

colonizers of a new habitat, highly adaptedfor COLONIZATION.

surface tension A property of the sur-face of a liquid, whereby it acts as if anelastic film were stretched across the sur-face. It is caused by the attractive forces be-tween the particles within the liquid, andbetween the liquid and the gases, liquids orsolids it is in contact with. Surface tensionis responsible for the formation of waterdroplets. It slows the evaporation of water,helping plants to retain water. It is alsopartially responsible for capillarity – themovement of liquids through thin tubes –which affects the supply of water to plantsfrom the soil and its transport throughplant cell walls and the xylem.

surface-to-volume ratio The ratio ofthe surface area of an object, such as a cell,organ, or organism, to its volume. Thehigher the ratio, the easier it is for gasesand dissolved minerals to diffuse from theoutside to the interior.

surface water Water that is naturallyopen to the atmosphere – streams, rivers,springs, ponds, lakes, reservoirs, estuaries,and seas.

surplus yield model A simple model ofthe effects on a population of harvesting, inwhich the population is represented simplyby its size or BIOMASS. It ignores populationstructure (see population).

survival of the fittest See fitness; nat-ural selection.

suspended load The part of the totalsediment load of a stream or river that iscarried in suspension.

sustainability The degree to which anactivity can be sustained with causingharm to the environment or the depletionof NONRENEWABLE RESOURCES.

sustainable development Economicdevelopment that takes into account theneeds of living and future generations, butis based on the use of RENEWABLE RE-

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SOURCES rather than the depletion of NON-RENEWABLE RESOURCES, and takes accountof the environmental consequences of eco-nomic activity. It was defined by the WorldCommission on Environment and Devel-opment in 1987 as: development thatmeets the needs of the present withoutcompromising the ability of future genera-tions to meet their own needs.

swamp An area of vegetation domi-nated by trees that develops on ground thatis normally waterlogged or covered bywater all year round, such as the margin ofa lake, a river floodplain, an area of water-retentive clay in an arid region, or alongthe shores of an estuary. Swamp vegetationrepresents the early stages of a HYDROSERE

or HALOSERE. The presence of trees distin-guishes it from a marsh. The vegetationtypically consist of various reeds (e.g.Phragmites) and sedges (Cyperaceae), in-cluding papyrus (Cyperus papyrus) in partsof Africa, together with trees like gums(Nyssa), willows (Salix), alders (Alnus),and swamp cypress (Taxodium distichum).Coastal and estuarine swamps in warm cli-mates are dominated by MANGROVES. Theswamp vegetation slows the flow of waterand dead plant matter builds up, leading toa build-up of stagnant muds and bottomwater lacking in oxygen. Decay is thereforeslow, and PEAT may accumulate. Comparemarsh.

switching The tendency for a predatorto switch between prey species or ca-tegories according to their relative abun-dance

symbiogenesis See endosymbiont theory.

symbiosis (pl. symbioses) Any close as-sociation between two or more differentorganisms, as seen in parasitism, mutual-ism, and commensalism. Often one or bothorganisms is dependent on the other. Theterm is usually used more narrowly tomean mutualism, but many mutualistic as-sociations may have once been parasitic ormay become so at some stage in their lifecycle, e.g. plants and their mycorrhizalfungi.

sympatric speciation See speciation.

synecology The study of all the interac-tions between the living organisms in a nat-ural COMMUNITY and the effects of thenonliving components of the environmentupon them and on their relationships witheach other. Compare autecology.

systematic error See bias.

systematic random sampling See sam-pling.

systematics The area of biology thatdeals with the diversity of living organisms,their relationships to each other, and theirclassification. The term may be used syn-onymously with TAXONOMY.

systematic variation A variation in ameasured variable that is not due to the pa-rameter being measured, but to anotherlinked factor that causes that parameter tovary in a similar way. For example, if thedata showed an association between drink-ing coffee and coronary heart disease (i.e.that people who drink coffee are morelikely to suffer from the disease) this maynot be the true situation – in fact, peoplewho drink coffee may be more likely tosmoke, and smoking may be the true causeof the association. In this case, the linkwith smoking causes a systematic variationin the data attempting to link coffee-drinking with heart disease.

systemic Describing a chemical that isabsorbed by a plant and distributedthroughout its tissues. Systemic FUNGICIDES

and INSECTICIDES make plant tissues toxicto fungi and insects, providing built-in de-fenses. Systemic weedkillers ensure thatboth shoots and roots are killed, and areparticularly useful for plants that can re-generate from fragments of shoot or root,or which have rhizomes or runners.

systems ecology A branch of ECOLOGY

concerned with the flow of energy and cir-culation of matter in an ECOSYSTEM.

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tachytely A rate of evolution that ismuch faster than is usual for the taxonomicgroup concerned, e.g. species undergoingADAPTIVE RADIATION.

taiga See biome; boreal forest.

take back A scheme in which house-holders are required to return WASTE to re-tailers, who pass it into a private recyclingor waste disposal system rather the publicone, or where the householder receives arefund of a deposit paid at the time of pur-chase.

talus Large blocks of rock that breakaway from cliff faces and form banksagainst the base of the cliff.

Tansley, Sir Arthur George (1871–1955) British plant ecologist. Tansley’sthinking was greatly influenced by booksby Eugenius WARMING and Andreas SCHIM-PER. These – together with travels in Cey-lon, Malaya, and Egypt – stimulated hisinterest in different vegetation types. In1902 Tansley founded The New Phytolo-gist, a journal designed to promote botani-cal communication and debate in Britain.In 1913 he founded and became the firstpresident of the British Ecological Societyand four years later founded and edited theJournal of Ecology. These activities, andhis ecology courses at Cambridge Univer-sity, played a large part in establishing thescience of ecology.

tapeworms See Platyhelminthes.

taxon (pl. taxa) A group of any rank intaxonomy. Felidae (a family) and Panthera(a genus) are examples.

taxon cycle A cycle of expansion andcontraction of the geographical range andpopulation density of a species or higherTAXON.

taxonomic diversity DIVERSITY of taxo-nomic groups in a community or area. Seebiodiversity; species diversity.

taxonomy The area of SYSTEMATICS thatcovers the principles and procedures ofclassification, specifically the classificationof variation in living organisms. See classi-fication; systematics.

TBTO See tributyltin oxide.

tectonic plate A section of the Earth’scrust that is capable of movement as a re-sult of convection currents in the MANTLE.See continental drift; plate tectonics.

temperate deciduous forest A majorBIOME found in the mid-latitudes (temper-ate zone) between the Tropics of Cancerand Capricorn and extending to the Arcticand Antarctic Circles. Temperate decidu-ous forests have moderate climates withabundant and fairly evenly distributedrainfall. There is a marked seasonalchange. The trees are shorter than those ofBOREAL FOREST and are dominated bybroad-leaved trees (oak, beech, lime, birch,hazel), which shed their leaves in the fall,an adaptation to lack of available water inwinter. Many of these forests contain sev-eral different species of trees and aretermed mixed deciduous forest. Warmtemperate forests occur in south China, forexample, and cool temperate rainforestsalso exist, in New Zealand, for example. Intemperate deciduous forests there is ashrub layer beneath the CANOPY and a rich

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herbaceous ground flora. The temperate ishigh enough to decompose the leaf litter,creating rich soils. In winter, many of theanimals hibernate or become inactive andsome birds migrate to warmer climates.

Broad-leaved evergreen forest is charac-teristic of Mediterranean-type climate withhot dry summers and mild wet winters.The leaves of the trees are adapted to watershortage and high evaporation in summerand usually have thick waxy cuticles (e.g.olive trees). The forest often merges intoscrub woodland characteristic of GARRIGUE

and MAQUIS.

temperate grassland See grassland;pampas; prairie; steppe.

temperate zone The region between theTropic of Cancer and the Arctic circle orthe Tropic of Capricorn and the AntarcticCircle, i.e. latitudes 23°27′ to 66°30′ N and23°27′ to 66°30′ S.

temperature coefficient The ratio ofthe rate at which a process proceeds at onetemperature and its rate at a temperature10°C lower or higher. See Q10.

temperature inversion See inversion.

temperature profile The relationshipbetween temperature and elevation ordepth.

temporal variation Variation in envi-ronmental factors through time, e.g. on anhourly, daily, or seasonal basis.

teratogen A physical or chemical agentthat causes birth defects in a developingfetus by interfering with normal develop-ment.

termites (white ants) A group of socialinsects (order Isoptera) that contain symbi-otic microorganisms in their guts, whichhelp them digest CELLULOSE. Termites canbe highly destructive, undermining housetimbers and damaging commercial lumber.

terracing The construction of cuttingterraces (flat platforms) or broad channels

into slopes, sometimes banked up or pro-tected by embankments, into order to traprunoff for the purposes of irrigation or tochannel it to suitable outlets, thus prevent-ing soil erosion.

terra rossa A type of CALCAREOUS SOIL

that develops over limestone in parts of theMediterranean region. It is a CLAY SOIL

with low HUMUS and significant amounts ofiron oxides, which give it a bright redcolor. See soil. Compare rendzina.

terrestrial biome See biome.

territorial competition See competi-tion.

territoriality The establishment by oneor more animals of an area from whichothers of the same or different species areexcluded. Such an area is called a territory.

territory See territoriality.

Tertiary The larger and older period ofthe Cenozoic, being composed of thePaleocene, Eocene, Oligocene, Miocene,and Pliocene epochs (65–2 million yearsago). Literally the ‘third age’, it is charac-terized by the rapid evolution and expan-sion of angiosperms and the emergence ofmammals. The ferns underwent adaptiveradiation, and the angiosperms underwentan even greater explosion of adaptive radi-ation, in which flowers became more spe-cialized. This went hand in hand with theevolution of specialist insect pollinators,especially bees, butterflies, and moths. Theclimate was warm and wet at the start ofthe period, but became colder and drierlater, and this was accompanied by the ex-pansion in range of grasses. Many moderngenera of conifers arose in the Tertiary, butother gymnosperms declined. See also geo-logical time scale.

tetraethyl lead (lead tetraethyl) Seelead.

tetraploid See haploid.

thallus (pl. thalli) A simple plant body

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showing no differentiation into root, leaf,and stem and lacking a true vascular sys-tem. It may be uni- or multicellular, and isfound in the algae, lichens, bryophytes,and the gametophyte generation of ferns,clubmosses, and horsetails.

thermal A spiraling column of upward-moving warm air that forms as the landwarms faster than the air, so that warmedair next to the ground becomes less densethan the air above. Thermals are used bymany birds to gain height.

thermal pollution POLLUTION that con-tributes excess heat to the environment, forexample the cooling water discharged frompower plants. Many aquatic and marineorganisms can tolerate only a limited rangeof temperatures, and many are unable tocope with rapid changes in temperatures,because this seldom happens in naturalbodies of water. As temperature rises, theoxygen concentration in the water de-creases, yet metabolic rates also increasewith temperature, requiring greater oxygenconsumption.

thermocline See metalimnion.

thermohaline circulation Upwellingand downwelling movements of bodies orwater due to gradients of temperature andsalinity in the surface waters.

thermometer A simple instrument formeasuring temperature, based on the ex-pansion of certain liquids with tempera-ture. It consists of a graduated sealed glasstube containing mercury or dyed alcohol.See also maximum and minimum ther-mometer.

thermophile A microorganism that re-quires high temperatures (around 60°C)for growth, e.g. certain bacteria (domainARCHAEA) that grow in hot springs or com-post and manure. See mesophilic; psychro-philic. See also extremophile; halophile;methanogen.

thermophilic digestion See digestion.

thermoregulation The maintenance ofan optimum temperature range by an or-ganism. Homoiotherms (‘warm-bloodedanimals’) have internal means of generat-ing heat, and regulate temperature by bothincreasing heat generation and controllingheat loss. Poikilotherms (‘cold-blooded an-imals’) control their body heat by movingbetween warm and cool places, and byadopting a body stance and position in re-lation to the sun that helps them minimizepassive heat gain or loss.

therophyte See Raunkiaer’s life-formclassification.

third-generation insecticides Syntheticinsecticides, usually organic compounds,that aim not to harm nontargeted species.Some are synthetic versions or derivativesof naturally occurring insecticides.

thorn forest A transitional type of FOR-EST with vegetation resembling savannaand semidesert vegetation, grading intoTROPICAL FOREST. It is found in Africa, Aus-tralia, and Central and South America.

threatened species See EndangeredSpecies Act; Red Data Book.

Three Mile Island A nuclear powerstation near Harrisburg, Pennsylvania.Here, in 1979, a malfunction of the coolingsystem led to overheating and meltdown ofthe reactor core. At one point during thecrisis radioactive gas was allowed into theatmosphere. Although there was no subse-quent evidence of adverse affects on health,the incident contributed to a general mis-trust of nuclear energy.

threshold The minimum stimulus inten-sity that will initiate a physiological or en-vironmental response, or the lowest doseof a chemical or other form of pollutant atwhich a measurable effect occurs.

tidal wave See tsunami.

tide The periodic rise and fall in level ofthe oceans and other large bodies of watercaused by the relative gravitational attrac-

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tions of the Sun, Moon, and Earth. The ef-fect of the Moon is much greater than thatof the Sun, causing the cycle of rise and fall.The extent of the tidal rise and fall varieswith the time of year as a consequence ofchanges in the relative positions of the Sun,Moon, and Earth, the Sun enhancing theeffect of the Moon’s tidal pull to form thehigh spring tides and, when it opposes themoon’s pull, the neap tides (lowest tides ofthe year). Geographical variations in tidesare related to variations in the contours ofthe seabed, and to the generally uneven dis-tribution of water in some areas. Someareas have a single rise and fall in a tidalday (24 hours 50 minutes). Others havetwo periods of high and low water with aperiod of 12 hours 25minutes.

tillage 1. The process of cultivating theland.2. Land under cultivation.

tiller A shoot that develops from an ax-illary or adventitious bud at the base of astem, often in response to injury of themain stem. Tillering is characteristic ofgrasses, and is why they thrive under mod-erate grazing, which actually enhances thegrowth of the grasses by increasing thenumber of shoots produced. The tuftedhabit of many grasses is due to tillering.Tillering also occurs when a tree is cop-piced. See coppice.

timber line (Waldgrenze) The altitudi-nal limit for the growth of trees of normalheight that form a closed canopy. It lies ata lower level than the TREE LINE.

time delay (time lag) A delay in the re-sponse of a population, community, orecosystem to changes in environmental fac-tors.

time hypothesis The hypothesis thatBIODIVERSITY is greater in areas that haveexperienced long periods of stability thanareas of frequent disturbance.

tissue In a multicellular organism, agroup of cells that is specialized for a par-ticular function. Examples are connective

tissue, muscular tissue, and nervous tissue.Several different tissues are often incorpo-rated in the structure of each organ of thebody, e.g. the heart or, in plants, a leaf.

tolerance 1. See succession.2. The capacity of an organism to ingestcertain chemicals or be exposed to certainphysical agents without deleterious effect.The tolerance limit of a chemical or physi-cal agent is the lowest level that is harmfulto the organism.

tonnage The rate at which waste entersa LANDFILL, usually expressed as tons permonth.

top-down control See food chain.

topography The surface relief of theland, or of an object – its shape and relativeelevations.

top predator A predator that occurs atthe top of the food chain or food web –usually one that eats other predators. Forexample, the lion is one of the top preda-tors of the African savanna. See also key-stone species.

topsoil 1. The uppermost layer of SOIL,usually including the organic layer inwhich most plants root, and which isturned over during cultivation.

2. The A horizon of a soil profile. Seesoil.

tornado A relatively small (about 100 min diameter) funnel of rapidly rotating air(wind speeds may exceed 300 km h-1) thatforms around an intense low-pressure cen-ter. It is capable of sweeping up objectsfrom the ground. Tornadoes are accompa-nied by violent down-drafts.

torpor A state of inactivity, low meta-bolic rate, and lowered body temperaturedeveloped in response to adverse environ-mental conditions, especially cold or heat.For example, many insects, reptiles, andtemperate zone hummingbirds (whosesmall size and large surface-to volume ratiomakes for rapid heat loss) enter a state of

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torpor overnight to conserve energy whentemperatures fall. Torpor in swifts isthought to be linked to periods when flyinginsect prey is not available. Many poikilo-thermic vertebrates and a few homoiother-mic ones, such as polar bears, enter truehibernation or winter torpor that may lastfor weeks or months. Compare hiberna-tion.

torr A unit of pressure that equals 133.3pascals, or 1 mm Hg at 0°C.

total response of predator The percapita mortality of a prey population inresponse to changing prey density. Thisincludes the effects of the changing PREDATOR–PREY RATIO on the behavior ofboth predator and prey. For example, atlow densities a predator may selectively killweaker individuals, whereas at high densi-ties the prey may indulge in high-riskstrategies to compete for food. The shapeof this response, when plotted on a graph,gives an indication of the type of preda-tor–prey interactions occurring.

toxicity The degree of danger posed bya chemical substance to living organisms orthe environment.

toxic oil syndrome (eosinophilia myal-gia syndrome) An autoimmune diseasecaused by exposure to cooking oil contam-inated with aniline derivatives.

trace element See micronutrient.

trace fossil See fossil.

trace metal A metal element that is es-sential for growth but is required only invery small quantities.

tracheid See xylem.

tracheophyte Any plant with a differen-tiated vascular system; i.e. all plants exceptthe liverworts, mosses, and hornworts.

trade winds Persistent winds that blowtoward the Equator in the latitude belt be-tween 30°N and S (the horse latitudes) and

the DOLDRUMS. The trade winds blow fromthe northeast in the northern hemisphere,and from the southeast in the southernhemisphere.

trait See character.

transcription The process in living cellswhereby RNA is synthesized according tothe template embodied in the base se-quence of DNA, thereby converting thecell’s genetic information into a coded mes-sage (messenger RNA, mRNA) for theassembly of proteins, or into the RNAcomponents required for protein synthesis(ribosomal RNA and transfer RNA). ThemRNA transcript leaves the nucleus to di-rect protein assembly on ribosomes in thecytoplasm, in the process called TRANSLA-TION.

transect See sampling.

transgenic Describing organisms, espe-cially EUKARYOTES, containing foreign ge-netic material. Genetic engineering hascreated a wide range of transgenic animals,plants, and other organisms, for both ex-perimental and commercial purposes. Ex-amples include dairy cows that secretedrugs in their milk, herbicide-resistant cropplants, and plants that secrete pharmaceu-ticals. See recombinant DNA technology.

transhumance The moving of herds tolivestock up mountains to summer pastureand back to the shelter of the valleys inwinter.

transient polymorphism The occur-rence of two or more different forms of aspecies (i.e. groups of individuals havingdistinctive combinations of alleles) or ofparticular alleles in a population during theperiod in which one or more forms arebeing replaced by others.

transition zone See ecotone.

transit time 1. In a geochemical cycle,the average time a nutrient remains in aparticular form.2. The ratio of biomass to productivity.

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translation The process whereby the ge-netic code (sequence of nitrogenous bases)of messenger RNA (mRNA) is decipheredby the machinery of a cell to make pro-teins. This takes place in the ribosomes,where amino acids corresponding to spe-cific triplets of bases in the mRNA as as-sembled and linked together by peptidebonds. See RNA; transcription.

translocation 1. The moving of individ-uals of a species from a CAPTIVE BREEDING

program or from a habitat where thespecies is plentiful to one where it is desiredto reintroduce the species to boost its num-bers.2. The movement of mineral nutrients, re-cently synthesized food materials, and hor-mones through a plant.

transparency The degree to which asubstance allows light to pass through it.

transpiration The loss of water vaporby EVAPORATION from the surface of aplant. It has been shown that most transpi-ration occurs through the STOMATA whenthey are open for gaseous exchange, but asmall amount (typically about 5%) is lostdirectly from epidermal cells through thecuticle (cuticular transpiration) and aminute proportion through lenticels. Acontinuous flow of water, the transpirationstream, is thus maintained through theplant from the soil via root hairs, root cor-tex, xylem, and tissues such as leaf meso-phyll served by xylem. Transpiration maybe useful in maintaining a flow of solutesthrough the plant and in helping coolleaves through evaporation, but is oftendetrimental under conditions of watershortage, when WILTING may occur. It is fa-vored by low humidity, high temperatures,and moving air. A major control is the de-gree of opening of the stomata. Compareguttation.

trap crops Plants that are planted be-tween the plants of a main crop becausethey are more attractive to pests. They maysuffice to draw the pest away from themain crop, or they may simply serve to

concentrate the pests, which can then besprayed without harming the main crop.

travel time 1. The time it takes a preda-tor to move from one patch of high preyabundance to another. This is a factor indetermining when to move on – if traveltime is high, it may pay the predator tocontinue hunting in the same patch even ifprey density is relatively low. See also giv-ing-up time.2. The time it takes a pollutant to travelfrom a point source to a given point.

tree A large persistent woody plant, usu-ally with a single main stem (the trunk)that remains unbranched near the groundand doe not die back in winter or a dry sea-son. Species of trees that sometimes haveseveral trunks, such as beech (Fagus sylvat-ica) and oak (Quercus), have no ordinarybranches leaving any trunk near theground. Compare herb; shrub.

tree line (Baumgrenze) At high latitudesor high altitudes, the limit of tree growth,representing the taiga/TUNDRA and sub-alpine/alpine boundaries, respectively. Theterm also applies to lower tree limits suchas frost hollows – a point beyond whichtrees cannot grow. Trees close to the treeline are often scattered and stunted or pros-trate. See boreal forest. Compare timberline.

tree rings See annual rings.

Triassic The oldest period of the Meso-zoic era, 250–215 million years ago. Dur-ing the Triassic, the climate graduallychanged from arid to more temperate.Conditions for preservation in the drier pe-riod were poor, so there are few fossils.There was a diversification of the gym-nosperms, a decrease in the number andvariety of cartilaginous fishes, and an in-crease in primitive amphibians and rep-tiles. See also geological time scale.

tributyltin oxide (TBTO) One of agroup of organic tin-containing chemicalsused as a BIOCIDES to prevent the fouling ofships’ hulls, and as stabilizers for PVC

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manufacture. It is toxic to marine life,being released into the water by bacterialaction or leached out by seawater. In shell-fish such as oysters and whelks it causes fe-males to develop secondary malecharacteristics, leading to sterility and con-sequent collapse of the population. Verylow levels have been shown to have adverseeffects on plankton. Its use is now bannedin many countries.

triploid See polyploidy.

trophic Relating to the nutrition of or-ganisms.

trophic cascade See food chain.

trophic level In complex natural com-munities, organisms whose food isobtained by the same number of energy-transfer steps are said to belong to the sametrophic or energy level. The first and low-est trophic level contains the PRODUCERS,green plants that convert solar energy tofood by photosynthesis. Herbivores oc-cupy the second trophic level and are pri-mary CONSUMERS: they eat the members ofthe first trophic level. At the third level car-nivores eat the herbivores (the secondaryconsumer level), and at the fourth level sec-ondary carnivores eat the primary carni-vores (the tertiary consumer level). Theseare general categories, and many organ-isms feed on several trophic levels, for ex-ample omnivores eat both plants andanimals. DECOMPOSERS or transformers oc-cupy a separate trophic level, which con-sists of organisms such as fungi andbacteria that break down dead organicmatter into nutrients usable by the produc-ers. See food chain.

trophic structure The structure of acommunity expressed in terms of the en-ergy flow through its various TROPHIC LEV-ELS.

trophospecies See food web.

tropical Located in or relating to theequatorial region of the Earth’s surface be-tween latitudes 23°27′ N and 23°27′ S.

tropical forest A major BIOME found be-tween the Equator and the Tropics of Can-cer and Capricorn. Tropical forests includeRAINFOREST, RIVERINE FOREST, and MON-SOON FOREST. See also thorn forest.

tropical grassland See savanna.

tropics Latitudes 23°27′ N (Tropic ofCancer) ad 23°27′ S (Tropic of Capricorn).At the summer solstice in the northernhemisphere the Sun is directly overhead atthe Tropic of Cancer, and in the southernhemisphere at the summer solstice it is di-rectly overhead at the Tropic of Capricorn.

tropopause See atmosphere.

troposphere See atmosphere.

true north The direction of the geo-graphical North Pole, which lies at thenorthernmost end of the Earth’s axis. It isnot the same direction as the magneticNorth Pole – the northernmost end of theEarth’s magnetic field, which changes posi-tion with time.

tsunami (tidal wave) A giant wavecaused by an undersea earthquake or land-slide. As travels across the sea it becomesvery large and when it reaches land mayoverrun and destroys coastal settlements.

t-test A statistical test used to comparethe means of one or more normal distribu-tions whose variance is not known butmust be estimated from the data.

tundra A major BIOME located north ofthe BOREAL FOREST in the subarctic regionsof North America, Europe, and Asia char-acterized by its lack of trees, permanentlyfrozen subsoil (permafrost), high winds,and extremely low temperatures. Tundraoccurs only in scattered localities onAntarctic islands, where it consists mainlyof mosses and lichens. The vegetation inthe northern tundra consists of low-growing grasses, lichens, and mosses. Tun-dra may also be classified as cold DESERT,the temperature rarely rising above 10°Cand the topsoil is frozen for nine months of

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the year. Thus the arctic vegetation is sub-jected to both extreme PHYSIOLOGICAL

DROUGHT (with water unavailable for mostof the year) and to extreme cold. In theshort summer, many meltwater pools con-tain insect larvae, which develop intoadults as the temperature rises, providingfood for migrating birds, e.g. waders,which return to the tundra to breed. Rein-deer and other herbivores graze on thelichens and migrate south to the boreal for-est when the winter becomes extreme, al-though they can locate lichens under thesnow cover.

Similar conditions may prevail at highaltitude in temperate and tropical regionsand similar vegetation is found (alpine tun-dra). See also gley.

turbidity current A current caused bythe slumping of sediment that has accumu-lated on the rim of the continental slope,especially at the head of a submarinecanyon. It forms a huge slurry of mixedsediment and water that spreads out atgreat speed over the ocean floor, forming alayer of unusually coarse sand in deepwater.

turnover 1. The proportion of a popula-tion that is lost or gained by deaths/emi-gration or births/immigration respectivelyover a given time.2. The changing species composition of aCOMMUNITY or habitat as species becomeextinct and their niches are occupied bynew colonizers, which may be newlyevolved local species or immigrants.3. The ratio of productivity to BIOMASS: theproportion of its biomass that is taken intoa community by primary productivity eachyear.

4. A measure of the nutrient flux in a BIO-GEOCHEMICAL CYCLE: the rate at which anutrient flows into or out of a nutrient pooldivided by the quantity of nutrient in thepool.

type specimen An individual or sampleor the whole or part of an organism that isused as an example of a particular taxo-nomic group. A type specimen is a desig-nated specimen from which the initialcriteria defining the characteristics of thegroup were taken. There are several cate-gories of type specimen. A holotype is asingle type specimen designated by the per-son who originally descried a species orsubspecies, and used to verify the status ofother specimens. It has priority over allsubsequent specimens. If the holotype isunavailable, a second specimen, called aneotype, may be selected to substitute forit. If the original describer of the group didnot designate a holotype, a specimen maybe designated by someone else later – this isa lectotype. Sometimes a collection of spec-imens was used for the original descrip-tion, but no single specimen wasdesignated as a type specimen. Such a col-lection may then be used as a whole and iscalled a syntype. Specimens that were col-lected at the same locality and at the sametime as the holotype, and used in the origi-nal designation of a new taxon, are calledparatypes. Finally, specimens collected atthe same time and from the same organismor local population as the holotype may beused as substitutes for the holotype, for ex-ample in different institutions or countries,or kept as safeguards against the destruc-tion of the holotype. Such specimens areisotypes.

typhoon See depression.

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ultra-low sulfur diesel See diesel.

ultraviolet (UV radiation) Electromag-netic radiation (see electromagnetic en-ergy) of wavelengths 4–400 nm, less thanvisible light but greater than x-rays. Lyingjust beyond the blue end of the visible spec-trum, it contains more energy than ordi-nary light, and certain forms of UVradiation have been associated with eyecataracts, genetic damage, and skin cancer.These wavelengths are to some extent fil-tered out by the OZONE LAYER.

UNCED See Earth Summit.

underdispersion See dispersion.

understory The layer of vegetation be-tween the ground cover or shrub layer andthe canopy in a forest community. It con-sists of shade-tolerant species and thesaplings of canopy and emergent species.

undulipodium (pl. undulipodia) Ciliaand flagella.

UNEP See United Nations EnvironmentProgram.

UNFCCC See United Nations Frame-work Convention on Climate Change.

ungulate A hoofed mammal.

unicellular Describing organisms thatexist as a single cell.

unitary organism See modular organ-ism.

United Nations Conference on Envi-

ronment and Development See EarthSummit.

United Nations Environment Program(UNEP) A program set up in 1972 topromote international cooperation on en-vironmental issues, and provide scienificadvice to United Nations organizations,with the aim of promoting the sustainableuse of the world’s resources. It plays amajor role in organizing international con-ventions such as the MONTREAL PROTOCOL

on Substances That Deplete the OzoneLayer, The Basel Convention on the Con-trol of Transboundary Movements of Haz-ardous Wastes and Their Disposal, and theUN Convention on Biological Diversity,providing technical assistance, data, andassistance in implementing the resultingdecisions and in monitoring implementa-tion. Its Governing Council is elected bythe UN General Assembly every four years.See also sustainable development.

United Nations Framework Conven-tion on Climate Change (UNFCCC)An international treaty dating to 1992 thatcommits signatory countries to stabilizeGREENHOUSE GAS emissions due to humanactivity to ‘levels that would prevent dan-gerous anthropogenic interference with theclimate system’. It requires countries tomaintain national inventories of anthro-pogenic emissions of greenhouse gases,including those not covered by the MON-TREAL PROTOCOL. See also Earth Summit.

unstable equilibrium A state in whicha system is at present finely balanced, butsmall changes in the numbers of individu-als in populations or metapopulations or inthe concentration and availability of re-sources will lead to even larger perturba-

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tions, rather than the system returningeventually to its former state. See also equi-librium.

upwelling A upward movement ofwater, usually near coasts and driven byonshore wind, that brings deep water tothe surface, along with nutrients.

uranium (U) A toxic silvery radioactiveelement occurring naturally in variousminerals. It has three naturally occurringradioisotopes, of which the commonest(over 99%) is 239U. 235U is a major nuclearfuel and nuclear explosive, while 238U is asource of fissionable 239Pu (plutonium).See also fission; nuclear energy; isotopicdating.

urban Relating to towns and cities.

ur-gene See origin of life.

Urochordata See Chordata.

validation Determination of how well amodel fits the process or system under in-vestigation.

variable A characteristic or propertythat can posses a range of values. It may bea measured value of an environmental fac-tor, such as wind speed or temperature, orit may represent the number of individualsin a population possessing a particularcharacteristic, e.g. a particular age orheight. Variables are often used to plotgraphs showing relationships or dependen-cies. In this situation, one variable may befixed (e.g. a location along a transect) orunder the control of the experimenter (e.g.time) – this is an independent variable. Theother – the property being measured or therate of reaction – is not controlled, and is adependent variable.

variance (s2) A statistical measure of thespread (DISPERSION) of the distribution of aquantitative variable – of its average devia-tion from the mean. It is the sum of thesquares of the values of the variables di-vided by the number of samples. Comparestandard deviation.

variation The extent to which the char-acteristics of the individuals of a speciescan vary. Variation can be caused by envi-ronmental and genetic factors. Environ-mental variation (phenotypic plasticity)results in differences in the appearance ofindividuals of a species because of differ-ences in nutrition, disease, light intensity,etc. Genetic variation is caused by RECOM-BINATION and occasionally MUTATION.These differences may be favored or dis-criminated against by NATURAL SELECTION.See qualitative variation; quantitative vari-ation.

variety The taxonomic group below thesubspecies level. The term is often looselyused to describe breeds of livestock or var-ious cultivated forms of agricultural andhorticultural species. Varieties are mor-phological variants, which may differ incolor or growth habit. See also cultivar;race; species.

vascular plants Plants containing dif-ferentiated cells forming conducting tissue,which comprises the XYLEM and PHLOEM

(water- and food-conducting tissues re-spectively). All plants except mosses(Bryophyta), liverworts (Hepatophyta),and hornworts (Anthocerophyta) are vas-cular plants. They are sometimes classed asa single division, the Tracheophyta (seetracheophyte).

vector (of parasite) 1. An agent that car-ries a disease-causing organism to ahealthy plant or animal, causing the latterto become infected.2. An agent (cloning vector) used as a vehi-cle for introducing foreign DNA, for ex-ample a new gene, into host cells duringGENETIC ENGINEERING.

vegetation texture The number, den-sity, and arrangement of patches of plantsor the species composition, density, anddispersion pattern of plants in a patch ofhabitat.

vegetative propagation (vegetative re-production) Growth of any parts of aplant, other than flowers, that may get de-

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tached from the parent and become inde-pendent, although they are identical genet-ically. For example, bulbs, corms, stolons,and rhizomes.

veld Extensive open grazing land insouthern Africa. See grassland.

ventilation The process by which an or-ganism maintains a flow of air or waterover its respiratory surfaces in the lungs,gills, or other respiratory organs. This in-creases the rate at which oxygen enters andcarbon dioxide leaves the blood. Most ac-tive animals possess some form of ventila-tion mechanism, typically involvingmuscular movements (respiratory move-ments). For example, bony fishes can ac-tively pump water over their gills byalternately expanding and contracting themouth and gill cavities. In mammals, air isalternately drawn into and expelled fromthe lungs by muscular movements of therib cage and diaphragm.

ventimeter See wind.

vermiculture The culture of worms thatbreak down vegetable matter and turn itinto compost. This may be done on a com-mercial scale in a worm farm, or on a do-mestic scale in special worm bins.

vermin Animals considered to be harm-ful to game or crops, for example mice,rats, etc., and parasitic or disease-carryinginvertebrates such a fleas, lice, ticks, andparasitic worms.

vernalization The cold treatment ofungerminated or partially germinatedseeds. Certain plants will germinate andflower only if exposed to low temperatures(1–2°C) at an early period of growth, i.e.they have a chilling requirement. Thuswinter varieties of cereals will flower insummer only if sown the previous autumn.Spring-sown winter varieties remain vege-tative throughout the season unless theyhave been vernalized. Artificial vernaliza-tion is an important technique in countrieswhere severe winters can kill autumn-sowncrops.

vertical life table See life table.

vertical mixing The exchange of waterbetween surface layers and deeper layers ina body of water. This does not happenunder conditions of STRATIFICATION, but isreestablished during the SPRING OVERTURN

or fall overturn (see fall bloom). Comparestratification.

vessel See xylem.

Vienna Convention A convention heldin Vienna, in 1985, in which nationsagreed to take appropriate measure to pro-tect the environment and human healthagainst the adverse affects from human ac-tivities that modify or are likely to modifythe OZONE LAYER. It sought to encourageresearch, cooperation between nations,and the exchange of information. The ur-gency for action was stressed when evi-dence of severe depletion of the ozone layerabove Antarctica was published during1985; in 1987 the MONTREAL PROTOCOL

was drawn up.

virion See virus.

virus An extremely small infectiousagent that causes a variety of diseases inplants and animals, such as smallpox, thecommon cold, and tobacco mosaic disease.Viruses can reproduce only in living tis-sues; outside the living cell they exist as in-active particles consisting of a core of DNAor RNA surrounded by a protein coat.Most plant viruses are single-strandedRNA viruses. The inert extracellular formof the virus, termed a virion, penetrates thehost membrane and liberates the viral nu-cleic acid into the cell. Usually, the nucleicacid is translated by the host cell ribosomesto produce enzymes necessary for the re-production of the virus and the formationof daughter virions. The virions are re-leased by lysis (bursting) of the host cell.Plant viruses are transmitted by vectorssuch as aphids and nematodes. A virus thatinfects a bacterium is termed a bacterio-phage (phage). Some viruses are associatedwith the formation of tumors.

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visibility The distance that can be seen –the most distant object toward the horizonthat can be seen with the naked eye. It is ameasure of the transparency (or opacity) ofthe medium (air or water). See also turbid-ity current.

vitamins Organic chemical compoundsthat are essential in small quantities for me-tabolism but are not synthesized by ani-mals, which therefore need to obtain themfrom food or microorganisms. The vita-mins have no energy value; lack of vitaminsresults in the breakdown of normal bodilyactivities and produces disease symptoms.Such deficiency diseases can usually beremedied by including the necessary vita-mins in the diet. Plants can synthesize vita-mins from simple substances, but animalsgenerally require them in their diet, thoughthere are exceptions to this.

vitrification The encasing of somethingin a glassy material by heat and fusion. It isused to create fuel pellets for nuclear reac-

tors, and also to immobilize and store nu-clear wastes to minimize leaching intogroundwater.

volatile organic compounds (VOCs)Organic compounds that readily evaporateat room temperature, especially those thattake part in atmospheric photochemical re-actions. See also photochemical smog.

volcanism (vulcanism; volcanic activity)The processes by which magma and associ-ated gasses, or hot water and steam, riseinto the Earth’s crust, travel along lines ofweakness, and are extruded onto the sur-face or ejected into the atmosphere. It en-compasses the formation of volcanoes,fumaroles, and geysers on the Earth andother planets. See also plate tectonics;earthquake.

vortex See polar vortex.

vulnerable species See EndangeredSpecies Act.

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Waldgrenze See timber line.

Wallacea A zone of mixing between an-imal species from the Oriental (SoutheastAsian) faunal region and the Australasianregion, bounded to the west by Huxley’sline (a modification of Wallace’s line thatruns east of Bali and Java and north be-tween Borneo and Sulawesi, then eastagain, passing to the west of the Philip-pines, with the exception of Palawan,marking the easternmost spread of marsu-pials), and to the east by Lydekker’s line,which follows the edge of Australia’s con-tinental shelf, marking the westernmostspread of Oriental fauna.

Wallace’s line See Wallacea.

warm event See El Niño.

Warming, (Johannes) Eugenius Bülow(1841–1924) Danish botanist. In 1895 hepublished his book Plantesamfund. An en-larged and revised English translation waspublished in 1925 as Oecology of Plants.Warming is regarded as one of thefounders of the subject of plant ecology.

warm temperate forest See temperatedeciduous forest.

warning coloration See aposematism.

waste Domestic or commercial refuse,including garbage, unwanted materials leftover from industrial processes and con-struction work, spent nuclear fuel, and soon. Waste may be gaseous, liquid, or solid.See also commercial waste; hazardouswaste; radioactive waste; wastewater.

waste disposal Methods of dealing

with WASTE. See activated sludge process;incineration; landfill; recycling.

wastewater Water that as been used fora process and released because it is nolonger required (e.g. industrial effluent,SEWAGE).

water (H2O) A colorless liquid thatfreezes at 0°C and, at atmospheric pres-sure, boils at 100°C. The maximum den-sity of water occurs at 3.98°C, which iswhy ice floats on water. Without floatingice to insulate against further temperaturefall and severe weather, aquatic communi-ties in temperate and polar regions wouldperish. Water is a polar liquid, and themost powerful solvent known, partly be-cause of its high dielectric constant andpartly its ability to hydrate ions.

water balance index See Langelierindex.

water culture See hydroponics.

water cycle (hydrological cycle; hydro-logic cycle) The cycling of water throughthe atmosphere, biosphere, lithosphere,and hydrosphere: water in the sea andlakes and rivers evaporates, forming cloudsthat deposit rain on the land, and the waterruns off the land or percolates through soiland rocks to the water table, eventually re-turning to the sea, lakes, and rivers.

water-flow measurements The sim-plest methods for measuring water flowmeasure the time taken for a float to travelbetween two fixed points. The Lagrangianmeasurements measure only the flow of thesurface waters. For measurement at depth,current meters are used – these are called

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Eulerian measurements. A current meterhas a propeller that is driven by the watermovement, its revolutions per unit time in-creasing with the rate of water flow.

Water flow is measured in units of dis-charge (Q), usually m3s–1. These are calcu-lated from the flow velocity (m s–1) and thecross-sectional area of the riverbed, pipe,or channel (m2). The rate at which sedi-ment is transported in a river (the instanta-neous suspended sediment discharge) iscalculated by: water discharge (m3s–1)times suspended sediment concentration(mg l–1). Regular measurements of depthand flow may be automated at gauging sta-tions anchored in the water at intervalsalong a river.

water hyacinth A floating aquatic plant(Eichhornia crassipes) with showy bluish-purple flowers, native to tropical America.Water hyacinth is a serious pest in thesouthern United States and in many coun-tries to which it has been introduced, form-ing dense masses and choking rivers,ponds, and waterways, suffocating fishand blocking formerly navigable channels.

water potential See osmosis.

water sampler A manual or automaticdevice for collecting samples of water, con-sisting of an open tube of fixed volume thatis lowered to the desired depth, then a re-leasing mechanism triggers two tightlyclosing rubber stoppers to seal its ends.

watershed See divide.

water table The upper surface of theGROUNDWATER, below which the soil androcks are saturated with water. The topog-raphy of the water table usually mirrorsthat of the ground surface, but with less ex-aggerated peaks and troughs. In places thewater table may be above the surface of theground, forming lakes, rivers, or springs.

water vapor The gaseous form ofWATER, especially when below its boilingpoint and diffused in the atmosphere. It isthe most abundant GREENHOUSE GAS in theatmosphere, and helps regulate global tem-perature, weather, and climate throughcloud formation.

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water vapor

evaporation

ice and snow

sublimation

water vapor intothe atmosphere condensation

advection

precipitation ontoland 119,000 km3

precipitation ontoocean 458,000 km3

evaporation fromocean 502,800 km3

transpiration

evaporation fromland 74,000 km3

lake

rock

river runoff

groundwater flow

infiltration

Water cycle

wave power The generation of electric-ity using the kinetic energy of waves todrive turbines or rocking floats.

WCMC See World Conservation Moni-toring Center.

WCU See World Conservation Union.

weather The state of the ATMOSPHERE

over a short period of time – air movement(wind), air pressure, humidity, cloud cover,the nature of precipitation, air tempera-ture, and periodic disturbances such asstorms or hurricanes. Weather occursmainly in the troposphere, but is to a smallextent influenced by events in the upper at-mosphere. It is also affected by topogra-phy, especially by large bodies of water andmountains.

weathering The physical and chemicalbreakdown of rock and its componentminerals. It occurs both on bare rock sur-faces and at the underground boundary be-tween rock and soil.

Chemical weathering breaks down theminerals and cement in rocks, so looseningtheir structure and making them more li-able to crumble. Chemicals present in theatmosphere (e.g. oxygen) and in rain (e.g.carbonic acid and ACID RAIN) act on ex-posed rocks, and solutions of chemicalspercolating through rocks and sedimentsmay selectively dissolve out minerals andredeposit them in other sediments.

Physical weathering includes the effectsof differential expansion and contractionof the surface layers of rocks when exposedto the sun or to frost. Water in the inter-stices of rocks, and in cracks and faults, ex-pands when it freezes, forcing blocks ofrock apart, leading to the formation ofscrees and landslides. Runoff is an impor-tant lubricating factor in landslides, help-ing loosened blocks of rocks fall away,exposing new surfaces to weathering.

Where rocks lie under vegetation, as onlichen-covered rocks or at the soil–bedrockinterface, biological weathering is impor-tant, plants secreting organic acids that eatinto the rocks. Plant roots can exert extra-ordinary pressures as they grow, especially

where a tree takes root in a rock crevice.They can force apart walls and push upsidewalks.

weather station A collection of instru-ments for recording and reporting meteo-rological observations at a single site. Thesimplest measure atmospheric pressure,temperature (or maximum and minimumtemperatures), precipitation, and windspeed and direction.

weed Any plant growing where it is notwanted. Many weed species are adapted tocolonize disturbed ground, such as a newlyplowed field, and have rapid growth, goodseed dispersal, and are frequently self-pollinated. Others have persistent rhi-zomes or roots that can regenerate fromfragments, e.g. bindweed (Convolvulus).

weedkiller See herbicide.

weighted Describing data that havebeen adjusted for the fact that some itemsare less reliable or to reduce the effect of aparticular bias.

Westerlies The prevailing winds in themiddle latitudes that blow from a westerlydirection as a result of the Earth’s rotationSee Coriolis force. Compare trade winds.

wet-and-dry-bulb thermometer Seehumidity.

wetfall Nutrients that wash into theecosystem in rain, snow, and fog.

wetland An area that is waterlogged formost of the year with surface or ground-water and supports vegetation adapted forsuch conditions. See bog; fen; estuary;marsh; salt marsh; swamp.

whirling psychrometer See humidity.

white ant See termites.

white goods Large (usually white) elec-trical appliances such as refrigerators,stoves, washing machines, washing-up ma-chines, and air conditioners, for which spe-

wave power

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cial waste disposal and recycling facilitiesare often made available.

wildfire A destructive and rapidlyspreading natural conflagration, especiallyin a wilderness area. Such fires are oftenstarted by lightning and are seasonal oc-currences in such habitats as grasslandsand dry scrub, and in certain forests whosetrees give off flammable aromatic oils.

wildlife management The manage-ment of land for the benefit of wildlife, in-cluding, where necessary, the culling ofpopulations of animals to ensure the con-tinued good health of the population or toprevent overcrowding leading to disease,starvation, or destruction of the habitat.

wildlife refuge An area designated forthe protection of wild animals, withinwhich hunting and fishing are strictly con-trolled or prohibited.

Wilson, Edward Osborne (1929– )American entomologist, ecologist, and so-ciobiologist. Wilson collaborated withRobert MACARTHUR in developing a theoryon the equilibrium of island populationsfrom which emerged their Theory of IslandBiogeography (1967). To test such ideasWilson conducted a number of remarkableexperiments with Daniel Simberloff in theFlorida Keys. They selected six small man-grove clumps and made a survey of thenumber of insect species present. They thenfumigated the islands to eliminate all the75 insect species found. Careful monitor-ing over the succeeding months revealedthat the islands had been recolonized bythe same number of species, thus confirm-ing the prediction that a dynamic equilib-rium number of species exists for anyisland. Later Wilson caused some contro-versy with his book Sociobiology (1975);he has also written a work on ecology, TheDiversity of Life (1992), and a revealingautobiography, Naturalist (1994).

wilting The loss of turgor in a plant dueto lack of water.

wind A directional movement of air.

Wind speed can be estimated by observingthe effects of the wind on objects. TheBEAUFORT SCALE of wind speed, used inweather and shipping forecasts, is based onthis. Wind speed may be measured by ahand-held instrument called a ventimeter,which consists of a calibrated tube overwhich wind passes, causing a reduction inthe pressure inside the tube that induces apointer to rise in proportion to wind speed.Anemometers are fixed or hand-held windspeed measurers. They are used inWEATHER STATIONS, mounted in locationswhere wind is not obstructed by buildingsor other objects. An anemometer consistsof three revolving cups connected to ameter that counts the number of rotationsper unit time and converts this to windspeed. The strength and direction of thewind varies at different heights above theEarth’s surface. The variation in windspeed with elevation is called the wind pro-file.

wind chill The cooling effect of thewind in disrupting the boundary layer be-tween the land and the atmosphere andcarrying away heat that has moved into theair by convection. It makes the air feelmuch colder than it really is, an effect (thechill factor) that increases with wind speed.

wind power The generation of electric-ity using propeller-shaped wind vanesmounted on tall supports (windmills) todrive turbines that generate electricity.Groups of windmills are usually sited to-gether and linked to a common generatingcenter – such clusters are called windfarms.

wind profile See wind.

Winkler titration A laboratory methodthat uses an iodine titration to determinethe dissolved oxygen content of a sample ofwater.

wood The hard fibrous structure foundin woody PERENNIALS such as trees andshrubs. It is normally formed from the sec-ondary XYLEM and thus found in plantsthat show secondary thickening (the addi-

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tion of extra layers of tissues, includingvascular tissues, to allow increase in girthas the plant grows taller), mainly conifersand dicotyledons. Water and nutrients aretransported only in the outermost youngestwood, termed the sapwood. The nonfunc-tional compacted wood of previous sea-sons’ growth is called the heartwood and itis this that is important commercially.Wood is classified as hardwood or soft-wood depending on whether it is derivedfrom dicotyledons, e.g. oak (Quercus), orconifers, e.g. pine (Pinus). Hardwood isgenerally harder than softwood but the dis-tinction is actually based on whether or notthe wood contains fibers and vessels in ad-dition to tracheids and parenchyma. Seealso annual rings.

woodland A plant COMMUNITY similarto a FOREST but whose large trees are morewidely spaced, and their crowns do notform a closed CANOPY. The ground floramay consist of grass, herbs, heath, orscrub. While there may be scattered under-story species, the understory does not formsuch a dense and well-defined layer as in atrue forest.

woody perennial See perennial.

World Conservation Monitoring Cen-ter (WCMC) An information service setup by the World Conservation Union, theWorldwide Fund for Nature, and theUnited Nations Environment Program tocompile information on conservation andthe sustainable use of the world’s naturalresources. The WCMC collects data onthreatened and endangered species andhabitats, National Parks and nature re-serves, international agreements, and con-servation and environment programs. Italso assists in the development of other in-

formation sources and the training of staffto support them. See Red Data Book.

World Conservation Union (WCU)Formerly the International Union for theConservation of Nature (IUCN), foundedin 1948, an organization that brings to-gether states, government agencies, and arange of nongovernmental organizationsto promote conservation and the sustain-able use of natural resources. Expert vol-unteers collect information on particularspecies and biodiversity conservation pro-jects, providing a base of expertise that en-ables the WCU to advise countries onnational conservation strategies, assistingthem in devising strategy and managementplans, and providing technical support.The WCU has an expanding network of re-gional and country offices, especially in de-veloping countries.

World Health Organization (WHO)An agency of the United Nations that pro-motes international cooperation to im-prove global health, providing guidance onand finance for the control of epidemic andendemic diseases, quarantine measures,and public health, sending teams into thefield to train medical and health workers,and disseminating information on researchrelating to health and disease.

Worldwide Fund for Nature Thelargest independent conservation organiza-tion in the world, comprising a globalnetwork of National Organizations, Asso-ciates, and Program Offices. It providesconservation services based on global pol-icy, fieldwork, and scientific information.Its goal is to protect genetic, species andecosystem diversity, ensure the sustainableuse of resources, and promote actions toreduce pollution.

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xenobiota Any organisms displacedfrom their normal habitat.

xenon (Xe) A colorless odorless inertgas that occurs in trace amounts in air (seeatmosphere).

xeric Relating to or describing an envi-ronment that is dry. Compare hydric;mesic.

xeromorphic Structurally adapted towithstand dry conditions. See xerophyte.

xerophyte Any plant adapted to grow-ing in dry conditions or in a physiologicallydry habitat, such as an acid bog, sandydesert (see dune), or a salt marsh, or an ex-posed very windy situation. Xerophytesstore available water, reducing water loss,or possess a deep root system. Succulents,such as cacti and agaves (family Aga-vaceae), have thick fleshy stems or leavesthat store water. Features associated withreducing water loss include: shedding ordieback of leaves, e.g. ocotillo (Fouquieriasplendens); waxy leaf coatings coupledwith closure or plugging of stomata, e.g.Kalanchoe; sunken or protected stomata,e.g. marram grass (Ammophila arenaria);folding, rolling, or repositioning of leavesto reduce sunlight absorption, e.g. marramgrass; a dense covering of white reflectivespines, e.g. cacti; and the development of adense hairy leaf covering, e.g. Espeletia.Compare hydrophyte; mesophyte. See alsohalophyte; physiological drought.

xerosere Any plant community in a suc-cession that starts in dry conditions. Theeffect of increasing vegetation on the envi-ronment can lead toward more mesophyticconditions. The seeds of XEROPHYTES estab-

lish a pioneer community called a PSAM-MOSERE, dominated by plants that can tol-erate abrasion by blown sand, high winds,temperature extremes by day and night,and in coastal areas, salt spray. Various xe-rophytic grasses such as marram grass(Ammophila arenaria) grow and trap sandparticles, raising the level of the DUNE.Their rotting remains contribute to soilformation. See sere; succession.

xylem The water-conducting tissue invascular plants. Xylem consists of deadhollow cells (the tracheids and vessels),which are the conducting elements. It alsocontains additional supporting tissue in theform of fibers and sclereids and some livingpacking tissue (parenchyma). The sec-ondary cell walls of xylem vessels and tra-cheids eventually become thickened withlignin to give greater support. Comparephloem.

yield The total amount of BIOMASS pro-duced by an ecosystem, community, orpopulation in a given time (usually a year).It may be calculated by multiplying thedensity of a population by the mean weightper individual.

zero net growth isocline (ZNGI) AnISOCLINE along which the rate of popula-tion growth is zero.

zinc A micronutrient that is toxic inlarge quantities and can pollute the soil, al-lowing only certain tolerant plants togrow.

ZNGI See zero net growth isocline.

zonation The distribution of species andhabitats along environmental gradients.

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XYZ

zone of depletion An area around anindividual plant from which its roots drawnutrients and water. Where the zones ofdepletion of adjacent plants overlap, theremay be competition for resources.

zoogeography The study of the geo-graphical distribution of animal species.Such study shows that the earth can be di-vided into distinct geographical regions,each having its own unique collection of animal species (see AUSTRALASIA,ETHIOPIAN, NEARCTIC, NEOTROPICAL, ORIEN-TAL, PALAEARCTIC). For example, the conti-nents of the southern hemisphere –Australia, Africa (south of the Sahara), andSouth America – each have a characteristicfauna not found elsewhere. Anteaters,sloths, and armadillos are native to SouthAmerica; marsupial and monotreme mam-mals are characteristic of Australia; while

Africa shows a greater diversity of faunathan any other region.

zooid An individual module of a modu-lar organism such as a colonial coral, hy-droid, or bryozoan.

Zoological Code of NomenclatureSee International Codes of Nomenclature.

zoology The study of animals, includingtheir classification, structure, morphology,physiology, interrelationships, behavior,population dynamics, and evolution.

zooplankton See plankton.

zooxanthellae See corals; symbiosis.

zygote The diploid cell resulting fromthe fusion of two haploid gametes. See sex-ual reproduction.

zone ofdepletion

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APPENDIXES

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SI Units

Appendix I

BASE AND DIMENSIONLESS SI UNITS

Physical quantity Name of SI unit Symbol for SI unitlength meter mmass kilogram(me) kgtime second selectric current ampere Athermodynamic temperature kelvin Kluminous intensity candela cdamount of substance mole mol*plane angle radian rad*solid angle steradian sr*supplementary units

DERIVED SI UNITS WITH SPECIAL NAMES

Physical quantity Name of SI unit Symbol for SI unitfrequency hertz Hzenergy joule Jforce newton Npower watt Wpressure pascal Paelectric charge coulomb Celectric potential difference volt Velectric resistance ohm Ωelectric conductance siemens Selectric capacitance farad Fmagnetic flux weber Wbinductance henry Hmagnetic flux density tesla Tluminous flux lumen lmilluminance (illumination) lux lxabsorbed dose gray Gyactivity becquerel Bqdose equivalent sievert Sv

DECIMAL MULTIPLES AND SUBMULTIPLES USED WITH SI UNITS

Submultiple Prefix Symbol Multiple Prefix Symbol10–1 deci- d 101 deca- da10–2 centi- c 102 hecto- h10–3 milli- m 103 kilo- k10–6 micro- µ 106 mega- M10–9 nano- n 109 giga- G10–12 pico- p 1012 tera- T10–15 femto- f 1015 peta- P10–18 atto- a 1018 exa- E10–21 zepto- z 1021 zetta- Z10–24 yocto- y 1024 yotta- Y

The following are university department websites:

University of California, Irvine, Department of Ecology and Evolutionary Biologyecoevo.bio.uci.edu

University of Connecticut, Department of Ecology and Evolutionary Biologywww.eeb.uconn.edu

Harvard University, Department of Organismic and Evolutionary Biologywww.oeb.harvard.edu

Princeton University, Department of Ecology and Evolutionary Biologywww.eeb.princeton.edu

University of Tennessee, Department of Ecology and Evolutionary Biologyeeb.bio.utk.edu

Yale University, Department of Ecology and Evolutionary Biologywww.eeb.yale.edu

There are many museum sites on the Web:

American Museum of Natural History www.amnh.orgHarvard Museum of Natural History www.hmnh.harvard.eduSmithsonian Institution National Museum of Natural History

www.mnh.si.eduThe Natural History Museum, London www.nhm.ac.ukYale Peabody Museum of Natural History www.peabody.yale.edu

Useful ecology sites are:

The British Ecological Society www.britishecologicalsociety.orgThe Ecological Society of America www.esa.orgThe National Center for Ecological Analysis and Synthesis

biology.usgs.gov

Websites dealing with the environment include:

The US Environmental Protection Agency www.epa.gov/enviroedEnvironmental Sciences Division, Oak Ridge National Laboratory

www.esd.ornl.govSmithsonian Environmental Research Center www.serc.si.edu

A general site for biology resources is:

The US Geological Survey biology.usgs.gov

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Webpages

Appendix II

Begon, M.; Harper, J. L.; & Townsend, C. R. Ecology: Individuals, Populations andCommunities. Oxford, U.K.: Blackwell Science, 1996

Evans, G. M. & Furlong, J. C. Environmental Biotechnology: Theory and Application.New York: Wiley, 2002

Glasson, J.; Therivel, R.; & Chadwick, A. Introduction to Environmental Impact Assess-ment. 2nd ed. London: UCL Press, 2001

Gotelli, N. J. A Primer of Ecology. 3rd ed. Sunderland, Mass.: Sinauer Associates, 2000

Harrison, R. M. Pollution: Causes, Effects and Control London: Royal Society of Chem-istry, 2001

Pepper, D. Modern Environmentalism: A Critical Assessment. 2nd ed. London: Taylor &Francis , 1999

Smith, A. The Weather London: Arrow Books, 2000

Southwood, T. R. E. & Henderson, P. A. Ecological Methods. 3rd ed. Oxford, U.K.:Blackwell Science, 2000

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