Ecology

For other uses, see Ecology (disambiguation).

Ecology (from Greek: , house"; -, studyof[A]) is the scientic analysis and study of interactionsamong organisms and their environment, such as the in-teractions organisms have with each other and with theirabiotic environment. Topics of interest to ecologists in-clude the diversity, distribution, amount (biomass), num-ber (population) of organisms, as well as competition be-tween them within and among ecosystems. Ecosystemsare composed of dynamically interacting parts includingorganisms, the communities they make up, and the non-living components of their environment. Ecosystem pro-cesses, such as primary production, pedogenesis, nutrientcycling, and various niche construction activities, regu-late the ux of energy and matter through an environ-ment. These processes are sustained by organisms withspecic life history traits, and the variety of organisms iscalled biodiversity. Biodiversity, which refers to the vari-eties of species, genes, and ecosystems, enhances certainecosystem services.Ecology is an interdisciplinary eld that includes biologyand Earth science. The word ecology ("kologie)was coined in 1866 by the German scientist ErnstHaeckel (18341919). Ecological thought is deriva-tive of established currents in philosophy, particularlyfrom ethics and politics.[1] Ancient Greek philosopherssuch as Hippocrates and Aristotle laid the foundationsof ecology in their studies on natural history. Mod-ern ecology transformed into a more rigorous sciencein the late 19th century. Evolutionary concepts onadaptation and natural selection became cornerstonesof modern ecological theory. Ecology is not synony-mous with environment, environmentalism, natural his-tory, or environmental science. It is closely related toevolutionary biology, genetics, and ethology. An under-standing of how biodiversity aects ecological function isan important focus area in ecological studies. Ecologistsseek to explain:

Life processes, interactions and adaptations

The movement of materials and energy through liv-ing communities

The successional development of ecosystems

The abundance and distribution of organisms andbiodiversity in the context of the environment.

Ecology is a human science as well. There are manypractical applications of ecology in conservation biol-ogy, wetland management, natural resource manage-ment (agroecology, agriculture, forestry, agroforestry,sheries), city planning (urban ecology), communityhealth, economics, basic and applied science, and hu-man social interaction (human ecology). For example,the Circles of Sustainability approach treats ecology asmore than the environment 'out there'. It is not treatedas separate from humans. Organisms (including hu-mans) and resources compose ecosystems which, in turn,maintain biophysical feedback mechanisms that mod-erate processes acting on living (biotic) and non-living(abiotic) components of the planet. Ecosystems sus-tain life-supporting functions and produce natural capitallike biomass production (food, fuel, ber and medicine),the regulation of climate, global biogeochemical cycles,water ltration, soil formation, erosion control, ood pro-tection and many other natural features of scientic, his-torical, economic, or intrinsic value.

1 Integrative levels, scope, andscale of organization

See also: Integrative levelThe scope of ecology contains a wide array of interacting

Ecosystems regenerate after a disturbance such as re, formingmosaics of dierent age groups structured across a landscape.Pictured are dierent seral stages in forested ecosystems start-ing from pioneers colonizing a disturbed site and maturing insuccessional stages leading to old-growth forests.

levels of organization spanning micro-level (e.g., cells)to planetary scale (e.g., biosphere) phenomena. Ecosys-tems, for example, contain abiotic resources and inter-acting life forms (i.e., individual organisms that aggre-gate into populations which aggregate into distinct eco-logical communities). Ecosystems are dynamic, they donot always follow a linear successional path, but they arealways changing, sometimes rapidly and sometimes soslowly that it can take thousands of years for ecologicalprocesses to bring about certain successional stages of aforest. An ecosystems area can vary greatly, from tiny to

1

2 1 INTEGRATIVE LEVELS, SCOPE, AND SCALE OF ORGANIZATION

vast. A single tree is of little consequence to the classica-tion of a forest ecosystem, but critically relevant to organ-isms living in and on it.[2] Several generations of an aphidpopulation can exist over the lifespan of a single leaf.Each of those aphids, in turn, support diverse bacterialcommunities.[3] The nature of connections in ecologicalcommunities cannot be explained by knowing the detailsof each species in isolation, because the emergent pat-tern is neither revealed nor predicted until the ecosys-tem is studied as an integrated whole.[4] Some ecologi-cal principles, however, do exhibit collective propertieswhere the sum of the components explain the propertiesof the whole, such as birth rates of a population beingequal to the sum of individual births over a designatedtime frame.[5]

1.1 Hierarchical ecology

See also: Biological organisation and Biological classi-cation

System behaviors must rst be arrayed into dierent lev-els of organization. Behaviors corresponding to higherlevels occur at slow rates. Conversely, lower organiza-tional levels exhibit rapid rates. For example, individualtree leaves respond rapidly to momentary changes in lightintensity, CO2 concentration, and the like. The growth ofthe tree responds more slowly and integrates these short-term changes.O'Neill et al. (1986)[6]:76

The scale of ecological dynamics can operate like a closedsystem, such as aphids migrating on a single tree, while atthe same time remain open with regard to broader scaleinuences, such as atmosphere or climate. Hence, ecolo-gists classify ecosystems hierarchically by analyzing datacollected from ner scale units, such as vegetation associ-ations, climate, and soil types, and integrate this informa-tion to identify emergent patterns of uniform organizationand processes that operate on local to regional, landscape,and chronological scales.To structure the study of ecology into a conceptuallymanageable framework, the biological world is organizedinto a nested hierarchy, ranging in scale from genes, tocells, to tissues, to organs, to organisms, to species, topopulations, to communities, to ecosystems, to biomes,and up to the level of the biosphere.[7] This frameworkforms a panarchy[8] and exhibits non-linear behaviors;this means that eect and cause are disproportionate, sothat small changes to critical variables, such as the num-ber of nitrogen xers, can lead to disproportionate, per-haps irreversible, changes in the system properties.[9]:14

1.2 BiodiversityMain article: Biodiversity

Biodiversity refers to the variety of life and its processes.It includes the variety of living organisms, the genetic dif-ferences among them, the communities and ecosystemsin which they occur, and the ecological and evolutionaryprocesses that keep them functioning, yet ever changingand adapting.Noss & Carpenter (1994)[10]:5

Biodiversity (an abbreviation of biological diversity)describes the diversity of life from genes to ecosystemsand spans every level of biological organization. The termhas several interpretations, and there are many ways toindex, measure, characterize, and represent its complexorganization.[11][12][13] Biodiversity includes species di-versity, ecosystem diversity, and genetic diversity and sci-entists are interested in the way that this diversity aectsthe complex ecological processes operating at and amongthese respective levels.[12][14][15] Biodiversity plays an im-portant role in ecosystem services which by denitionmaintain and improve human quality of life.[13][16][17]Preventing species extinctions is one way to preservebiodiversity and that goal rests on techniques that pre-serve genetic diversity, habitat and the ability for speciesto migrate. Conservation priorities and managementtechniques require dierent approaches and considera-tions to address the full ecological scope of biodiver-sity. Natural capital that supports populations is criti-cal for maintaining ecosystem services[18][19] and speciesmigration (e.g., riverine sh runs and avian insect con-trol) has been implicated as one mechanism by whichthose service losses are experienced.[20] An understand-ing of biodiversity has practical applications for speciesand ecosystem-level conservation planners as they makemanagement recommendations to consulting rms, gov-ernments, and industry.[21]

1.3 HabitatMain article: Habitat

The habitat of a species describes the environment overwhich a species is known to occur and the type of com-munity that is formed as a result.[22] More specically,habitats can be dened as regions in environmental spacethat are composed of multiple dimensions, each repre-senting a biotic or abiotic environmental variable; thatis, any component or characteristic of the environmentrelated directly (e.g. forage biomass and quality) or in-directly (e.g. elevation) to the use of a location by theanimal.[23]:745 For example, a habitat might be an aquaticor terrestrial environment that can be further categorizedas a montane or alpine ecosystem. Habitat shifts pro-

1.4 Niche 3

vide important evidence of competition in nature whereone population changes relative to the habitats that mostother individuals of the species occupy. For example,one population of a species of tropical lizards (Tropidurushispidus) has a attened body relative to the main pop-ulations that live in open savanna. The population thatlives in an isolated rock outcrop hides in crevasses whereits attened body oers a selective advantage. Habitatshifts also occur in the developmental life history of am-phibians and in insects that transition from aquatic toterrestrial habitats. Biotope and habitat are sometimesused interchangeably, but the former applies to a commu-nitys environment, whereas the latter applies to a speciesenvironment.[22][24][25]

Additionally, some species are ecosystem engineers, al-tering the environment within a localized region. Forinstance, beavers manage water levels by building damswhich improves their habitat in a landscape.

Biodiversity of a coral reef. Corals adapt to and modify theirenvironment by forming calcium carbonate skeletons. This pro-vides growing conditions for future generations and forms ahabitat for many other species.[26]

1.4 NicheMain article: Ecological niche

Denitions of the niche date back to 1917,[29] butG. Evelyn Hutchinson made conceptual advances in1957[30][31] by introducing a widely adopted denition:the set of biotic and abiotic conditions in which aspecies is able to persist and maintain stable populationsizes.[29]:519 The ecological niche is a central concept

Termite mounds with varied heights of chimneys regulate gas ex-change, temperature and other environmental parameters thatare needed to sustain the internal physiology of the entirecolony.[27][28]

in the ecology of organisms and is sub-divided into thefundamental and the realized niche. The fundamentalniche is the set of environmental conditions under whicha species is able to persist. The realized niche is the set ofenvironmental plus ecological conditions under which aspecies persists.[29][31][32] The Hutchinsonian niche is de-ned more technically as a "Euclidean hyperspace whosedimensions are dened as environmental variables andwhose size is a function of the number of values that theenvironmental values may assume for which an organismhas positive tness.[33]:71

Biogeographical patterns and range distributions are ex-plained or predicted through knowledge of a speciestraits and niche requirements.[34] Species have func-tional traits that are uniquely adapted to the ecologicalniche. A trait is a measurable property, phenotype, orcharacteristic of an organism that may inuence its sur-vival. Genes play an important role in the interplay ofdevelopment and environmental expression of traits.[35]Resident species evolve traits that are tted to the se-lection pressures of their local environment. This tendsto aord them a competitive advantage and discour-ages similarly adapted species from having an overlap-ping geographic range. The competitive exclusion prin-ciple states that two species cannot coexist indenitely

4 1 INTEGRATIVE LEVELS, SCOPE, AND SCALE OF ORGANIZATION

by living o the same limiting resource; one will al-ways outcompete the other. When similarly adaptedspecies overlap geographically, closer inspection revealssubtle ecological dierences in their habitat or dietaryrequirements.[36] Some models and empirical studies,however, suggest that disturbances can stabilize the co-evolution and shared niche occupancy of similar speciesinhabiting species-rich communities.[37] The habitat plusthe niche is called the ecotope, which is dened as the fullrange of environmental and biological variables aectingan entire species.[22]

1.4.1 Niche construction

Main article: Niche constructionSee also: Ecosystem engineering

Organisms are subject to environmental pressures, butthey also modify their habitats. The regulatory feedbackbetween organisms and their environment can aect con-ditions from local (e.g., a beaver pond) to global scales,over time and even after death, such as decaying logs orsilica skeleton deposits from marine organisms.[38] Theprocess and concept of ecosystem engineering is relatedto niche construction, but the former relates only to thephysical modications of the habitat whereas the latteralso considers the evolutionary implications of physicalchanges to the environment and the feedback this causeson the process of natural selection. Ecosystem engineersare dened as: organisms that directly or indirectly mod-ulate the availability of resources to other species, bycausing physical state changes in biotic or abiotic ma-terials. In so doing they modify, maintain and createhabitats.[39]:373

The ecosystem engineering concept has stimulated a newappreciation for the inuence that organisms have on theecosystem and evolutionary process. The term nicheconstruction is more often used in reference to theunder-appreciated feedback mechanisms of natural se-lection imparting forces on the abiotic niche.[27][40] Anexample of natural selection through ecosystem engi-neering occurs in the nests of social insects, includingants, bees, wasps, and termites. There is an emergenthomeostasis or homeorhesis in the structure of the nestthat regulates, maintains and defends the physiology ofthe entire colony. Termite mounds, for example, main-tain a constant internal temperature through the designof air-conditioning chimneys. The structure of the neststhemselves are subject to the forces of natural selection.Moreover, a nest can survive over successive generations,so that progeny inherit both genetic material and a legacyniche that was constructed before their time.[5][27][28]

1.5 Biome

Main article: Biome

Biomes are larger units of organization that categorizeregions of the Earths ecosystems, mainly according tothe structure and composition of vegetation.[41] There aredierent methods to dene the continental boundaries ofbiomes dominated by dierent functional types of veg-etative communities that are limited in distribution byclimate, precipitation, weather and other environmentalvariables. Biomes include tropical rainforest, temperatebroadleaf and mixed forest, temperate deciduous forest,taiga, tundra, hot desert, and polar desert.[42] Other re-searchers have recently categorized other biomes, such asthe human and oceanic microbiomes. To a microbe, thehuman body is a habitat and a landscape.[43] Microbiomeswere discovered largely through advances in moleculargenetics, which have revealed a hidden richness of mi-crobial diversity on the planet. The oceanic microbiomeplays a signicant role in the ecological biogeochemistryof the planets oceans.[44]

1.6 Biosphere

Main article: BiosphereSee also: Earths spheres

The largest scale of ecological organization is the bio-sphere: the total sum of ecosystems on the planet.Ecological relationships regulate the ux of energy, nu-trients, and climate all the way up to the planetary scale.For example, the dynamic history of the planetary atmo-spheres CO2 and O2 composition has been aected bythe biogenic ux of gases coming from respiration andphotosynthesis, with levels uctuating over time in rela-tion to the ecology and evolution of plants and animals.[45]Ecological theory has also been used to explain self-emergent regulatory phenomena at the planetary scale:for example, the Gaia hypothesis is an example of holismapplied in ecological theory.[46] The Gaia hypothesisstates that there is an emergent feedback loop generatedby the metabolism of living organisms that maintains thecore temperature of the Earth and atmospheric conditionswithin a narrow self-regulating range of tolerance.[47]

1.7 Population ecology

Main article: Population ecologySee also: Lists of organisms by population

Population ecology studies the dynamics of specie popu-lations and how these populations interact with the widerenvironment.[5] A population consists of individuals ofthe same species that live, interact and migrate through

1.8 Community ecology 5

the same niche and habitat.[48]

A primary law of population ecology is the Malthusiangrowth model[49] which states, a population will grow(or decline) exponentially as long as the environmentexperienced by all individuals in the population re-mains constant.[49]:18 Simplied population models usu-ally start with four variables: death, birth, immigration,and emigration.An example of an introductory population model de-scribes a closed population, such as on an island, whereimmigration and emigration does not take place. Hy-potheses are evaluated with reference to a null hypoth-esis which states that random processes create the ob-served data. In these island models, the rate of populationchange is described by:

dNdT = bN dN = (b d)N = rN;

where N is the total number of individuals in the popula-tion, b and d are the per capita rates of birth and deathrespectively, and r is the per capita rate of populationchange.[49][50]

Using these modelling techniques, Malthus populationprinciple of growth was later transformed into a modelknown as the logistic equation:

dN

dT= aN

1 N

K

;

where N is the number of individuals measured asbiomass density, a is the maximum per-capita rate ofchange, and K is the carrying capacity of the population.The formula states that the rate of change in populationsize (dN/dT) is equal to growth (aN) that is limited bycarrying capacity (1 N/K).Population ecology builds upon these introductory mod-els to further understand demographic processes in realstudy populations. Commonly used types of data includelife history, fecundity, and survivorship, and these areanalysed using mathematical techniques such as matrixalgebra. The information is used for managing wildlifestocks and setting harvest quotas.[50][51] In cases wherebasic models are insucient, ecologists may adopt dif-ferent kinds of statistical methods, such as the Akaikeinformation criterion,[52] or use models that can becomemathematically complex as several competing hypothe-ses are simultaneously confronted with the data.[53]

1.7.1 Metapopulations and migration

Main article: MetapopulationSee also: Animal migration

The concept of metapopulations was dened in 1969[54]as a population of populations which go extinct lo-cally and recolonize.[55]:105 Metapopulation ecologyis another statistical approach that is often used inconservation research.[56] Metapopulation models sim-plify the landscape into patches of varying levels ofquality,[57] and metapopulations are linked by the mi-gratory behaviours of organisms. Animal migration isset apart from other kinds of movement because it in-volves the seasonal departure and return of individualsfrom a habitat.[58] Migration is also a population-levelphenomenon, as with the migration routes followed byplants as they occupied northern post-glacial environ-ments. Plant ecologists use pollen records that accumu-late and stratify in wetlands to reconstruct the timing ofplant migration and dispersal relative to historic and con-temporary climates. These migration routes involved anexpansion of the range as plant populations expandedfrom one area to another. There is a larger taxonomy ofmovement, such as commuting, foraging, territorial be-haviour, stasis, and ranging. Dispersal is usually distin-guished from migration because it involves the one waypermanent movement of individuals from their birth pop-ulation into another population.[59][60]

In metapopulation terminology, migrating individuals areclassed as emigrants (when they leave a region) or immi-grants (when they enter a region), and sites are classedeither as sources or sinks. A site is a generic term thatrefers to places where ecologists sample populations, suchas ponds or dened sampling areas in a forest. Sourcepatches are productive sites that generate a seasonal sup-ply of juveniles that migrate to other patch locations.Sink patches are unproductive sites that only receive mi-grants; the population at the site will disappear unless res-cued by an adjacent source patch or environmental con-ditions become more favourable. Metapopulation modelsexamine patch dynamics over time to answer potentialquestions about spatial and demographic ecology. Theecology of metapopulations is a dynamic process of ex-tinction and colonization. Small patches of lower qual-ity (i.e., sinks) are maintained or rescued by a seasonalinux of new immigrants. A dynamic metapopulationstructure evolves from year to year, where some patchesare sinks in dry years and are sources when conditionsare more favourable. Ecologists use a mixture of com-puter models and eld studies to explain metapopulationstructure.[61][62]

1.8 Community ecology

Main article: Community ecologyCommunity ecology examines how interactions among

species and their environment aect the abundance, dis-tribution and diversity of species within communities.Johnson & Stinchcomb (2007)[63]:250

6 1 INTEGRATIVE LEVELS, SCOPE, AND SCALE OF ORGANIZATION

Interspecic interactions such as predation are a key aspect ofcommunity ecology.

Community ecology is the study of the interactionsamong a collections of species that inhabit the same ge-ographic area. Research in community ecology mightmeasure primary production in a wetland in relation todecomposition and consumption rates. This requires anunderstanding of the community connections betweenplants (i.e., primary producers) and the decomposers(e.g., fungi and bacteria),[64] or the analysis of predator-prey dynamics aecting amphibian biomass.[65] Foodwebs and trophic levels are two widely employed concep-tual models used to explain the linkages among species.[5]

1.9 Ecosystem ecology

Main article: Ecosystem ecology

These ecosystems, as we may call them, are of the mostvarious kinds and sizes. They form one category of themultitudinous physical systems of the universe, whichrange from the universe as a whole down to the atom.Tansley (1935)[66]:299Ecosystems are habitats within biomes that form an

A riparian forest in theWhiteMountains, NewHampshire (USA),an example of ecosystem ecology

integrated whole and a dynamically responsive systemhaving both physical and biological complexes. The

underlying concept can be traced back to 1864 in thepublished work of George Perkins Marsh (Man andNature).[67][68] Within an ecosystem, organisms arelinked to the physical and biological components of theirenvironment to which they are adapted.[66] Ecosystemsare complex adaptive systems where the interaction oflife processes form self-organizing patterns across dier-ent scales of time and space.[69] Ecosystems are broadlycategorized as terrestrial, freshwater, atmospheric, ormarine. Dierences stem from the nature of the uniquephysical environments that shapes the biodiversity withineach. A more recent addition to ecosystem ecology aretechnoecosystems, which are aected by or primarily theresult of human activity.[5]

1.9.1 Food webs

Main article: Food webSee also: Food chain

A food web is the archetypal ecological network. Plantscapture solar energy and use it to synthesize simple sugarsduring photosynthesis. As plants grow, they accumulatenutrients and are eaten by grazing herbivores, and the en-ergy is transferred through a chain of organisms by con-sumption. The simplied linear feeding pathways thatmove from a basal trophic species to a top consumer iscalled the food chain. The larger interlocking pattern offood chains in an ecological community creates a com-plex food web. Food webs are a type of concept map ora heuristic device that is used to illustrate and study path-ways of energy and material ows.[6][70][71]

Generalized food web of waterbirds from Chesapeake Bay

Food webs are often limited relative to the real world.Complete empirical measurements are generally re-stricted to a specic habitat, such as a cave or a pond,and principles gleaned from food web microcosm studiesare extrapolated to larger systems.[72] Feeding relationsrequire extensive investigations into the gut contents oforganisms, which can be dicult to decipher, or stableisotopes can be used to trace the ow of nutrient diets

1.9 Ecosystem ecology 7

and energy through a food web.[73] Despite these limita-tions, food webs remain a valuable tool in understandingcommunity ecosystems.[74]

Food webs exhibit principles of ecological emergencethrough the nature of trophic relationships: some specieshave many weak feeding links (e.g., omnivores) whilesome are more specialized with fewer stronger feedinglinks (e.g., primary predators). Theoretical and empiri-cal studies identify non-random emergent patterns of fewstrong and many weak linkages that explain how ecolog-ical communities remain stable over time.[75] Food websare composed of subgroups where members in a commu-nity are linked by strong interactions, and the weak inter-actions occur between these subgroups. This increasesfood web stability.[76] Step by step lines or relations aredrawn until a web of life is illustrated.[71][77][78][79]

1.9.2 Trophic levels

Main article: Trophic levelA trophic level (from Greek troph, , troph, mean-

A trophic pyramid (a) and a food-web (b) illustrating ecologicalrelationships among creatures that are typical of a northernBoreal terrestrial ecosystem. The trophic pyramid roughly rep-resents the biomass (usually measured as total dry-weight) ateach level. Plants generally have the greatest biomass. Namesof trophic categories are shown to the right of the pyramid. Someecosystems, such as many wetlands, do not organize as a strictpyramid, because aquatic plants are not as productive as long-lived terrestrial plants such as trees. Ecological trophic pyramidsare typically one of three kinds: 1) pyramid of numbers, 2) pyra-mid of biomass, or 3) pyramid of energy.[5]:598

ing food or feeding) is a group of organisms acquir-ing a considerable majority of its energy from the adja-cent level nearer the abiotic source.[80]:383 Links in foodwebs primarily connect feeding relations or trophismamong species. Biodiversity within ecosystems can beorganized into trophic pyramids, in which the vertical di-mension represents feeding relations that become furtherremoved from the base of the food chain up toward toppredators, and the horizontal dimension represents theabundance or biomass at each level.[81] When the relativeabundance or biomass of each species is sorted into its re-spective trophic level, they naturally sort into a 'pyramidof numbers.[82]

Species are broadly categorized as autotrophs (orprimary producers), heterotrophs (or consumers), andDetritivores (or decomposers). Autotrophs are organisms

that produce their own food (production is greater thanrespiration) by photosynthesis or chemosynthesis. Het-erotrophs are organisms that must feed on others for nour-ishment and energy (respiration exceeds production).[5]Heterotrophs can be further sub-divided into dierentfunctional groups, including primary consumers (strictherbivores), secondary consumers (carnivorous preda-tors that feed exclusively on herbivores) and tertiary con-sumers (predators that feed on a mix of herbivores andpredators).[83] Omnivores do not t neatly into a func-tional category because they eat both plant and animal tis-sues. It has been suggested that omnivores have a greaterfunctional inuence as predators, because compared toherbivores they are relatively inecient at grazing.[84]

Trophic levels are part of the holistic or complex systemsview of ecosystems.[85][86] Each trophic level containsunrelated species that are grouped together because theyshare common ecological functions, giving a macro-scopic view of the system.[87] While the notion of trophiclevels provides insight into energy ow and top-downcontrol within food webs, it is troubled by the prevalenceof omnivory in real ecosystems. This has led someecologists to reiterate that the notion that species clearlyaggregate into discrete, homogeneous trophic levels isction.[88]:815 Nonetheless, recent studies have shownthat real trophic levels do exist, but above the herbivoretrophic level, food webs are better characterized as atangled web of omnivores.[89]:612

1.9.3 Keystone species

Main article: Keystone speciesA keystone species is a species that is connected to a

Sea otters, an example of a keystone species

disproportionately large number of other species in thefood-web. Keystone species have lower levels of biomassin the trophic pyramid relative to the importance of theirrole. The many connections that a keystone species holdsmeans that it maintains the organization and structure ofentire communities. The loss of a keystone species resultsin a range of dramatic cascading eects that alters trophicdynamics, other food web connections, and can cause theextinction of other species.[90][91]

Sea otters (Enhydra lutris) are commonly cited as an ex-ample of a keystone species because they limit the den-sity of sea urchins that feed on kelp. If sea otters are re-

8 3 RELATION TO EVOLUTION

moved from the system, the urchins graze until the kelpbeds disappear and this has a dramatic eect on com-munity structure.[92] Hunting of sea otters, for example,is thought to have indirectly led to the extinction of theStellers Sea Cow (Hydrodamalis gigas).[93] While thekeystone species concept has been used extensively as aconservation tool, it has been criticized for being poorlydened from an operational stance. It is dicult to ex-perimentally determine what species may hold a keystonerole in each ecosystem. Furthermore, food web theorysuggests that keystone species may not be common, so itis unclear how generally the keystone species model canbe applied.[92][94]

2 Ecological complexity

Main article: ComplexitySee also: Emergence

Complexity is understood as a large computational ef-fort needed to piece together numerous interacting partsexceeding the iterative memory capacity of the humanmind. Global patterns of biological diversity are com-plex. This biocomplexity stems from the interplay amongecological processes that operate and inuence patterns atdierent scales that grade into each other, such as tran-sitional areas or ecotones spanning landscapes. Com-plexity stems from the interplay among levels of bio-logical organization as energy and matter is integratedinto larger units that superimpose onto the smaller parts.What were wholes on one level become parts on a higherone.[95]:209 Small scale patterns do not necessarily ex-plain large scale phenomena, otherwise captured in theexpression (coined by Aristotle) 'the sum is greater thanthe parts.[96][97][E]

Complexity in ecology is of at least six distinct types:spatial, temporal, structural, process, behavioral, andgeometric.[98]:3 From these principles, ecologists haveidentied emergent and self-organizing phenomena thatoperate at dierent environmental scales of inuence,ranging from molecular to planetary, and these requiredierent explanations at each integrative level.[47][99]Ecological complexity relates to the dynamic resilienceof ecosystems that transition to multiple shifting steady-states directed by random uctuations of history.[8][100]Long-term ecological studies provide important trackrecords to better understand the complexity and resilienceof ecosystems over longer temporal and broader spatialscales. These studies are managed by the InternationalLong Term Ecological Network (LTER).[101] The longestexperiment in existence is the Park Grass Experiment,which was initiated in 1856.[102] Another example is theHubbard Brook study, which has been in operation since1960.[103]

2.1 Holism

Main article: Holism

Holism remains a critical part of the theoretical foun-dation in contemporary ecological studies. Holism ad-dresses the biological organization of life that self-organizes into layers of emergent whole systems thatfunction according to nonreducible properties. Thismeans that higher order patterns of a whole functionalsystem, such as an ecosystem, cannot be predicted orunderstood by a simple summation of the parts.[104]New properties emerge because the components inter-act, not because the basic nature of the components ischanged.[5]:8

Ecological studies are necessarily holistic as opposedto reductionistic.[35][99][105] Holism has three scienticmeanings or uses that identify with ecology: 1) the mech-anistic complexity of ecosystems, 2) the practical de-scription of patterns in quantitative reductionist termswhere correlations may be identied but nothing is un-derstood about the causal relations without reference tothe whole system, which leads to 3) a metaphysical hier-archy whereby the causal relations of larger systems areunderstood without reference to the smaller parts. Scien-tic holism diers from mysticism that has appropriatedthe same term. An example of metaphysical holism isidentied in the trend of increased exterior thickness inshells of dierent species. The reason for a thickness in-crease can be understood through reference to principlesof natural selection via predation without need to refer-ence or understand the biomolecular properties of the ex-terior shells.[106]

3 Relation to evolutionMain article: Evolutionary ecology

Ecology and evolution are considered sister disciplinesof the life sciences. Natural selection, life history,development, adaptation, populations, and inheritanceare examples of concepts that thread equally into ecolog-ical and evolutionary theory. Morphological, behaviouraland genetic traits, for example, can be mapped onto evo-lutionary trees to study the historical development of aspecies in relation to their functions and roles in dier-ent ecological circumstances. In this framework, the an-alytical tools of ecologists and evolutionists overlap asthey organize, classify and investigate life through com-mon systematic principals, such as phylogenetics or theLinnaean system of taxonomy.[107] The two disciplinesoften appear together, such as in the title of the journalTrends in Ecology and Evolution.[108] There is no sharpboundary separating ecology from evolution and they dif-fer more in their areas of applied focus. Both disci-

3.2 Cognitive ecology 9

plines discover and explain emergent and unique prop-erties and processes operating across dierent spatial ortemporal scales of organization.[35][47] While the bound-ary between ecology and evolution is not always clear,ecologists study the abiotic and biotic factors that inu-ence evolutionary processes,[109][110] and evolution can berapid, occurring on ecological timescales as short as onegeneration.[111]

3.1 Behavioural ecology

Main article: Behavioural ecologyAll organisms can exhibit behaviours. Even plants

Social display and colour variation in dierently adapted speciesof chameleons (Bradypodion spp.). Chameleons change theirskin colour to match their background as a behavioural defencemechanism and also use colour to communicate with other mem-bers of their species, such as dominant (left) versus submissive(right) patterns shown in the three species (A-C) above.[112]

express complex behaviour, including memory andcommunication.[113] Behavioural ecology is the study ofan organisms behaviour in its environment and its ecolog-ical and evolutionary implications. Ethology is the studyof observable movement or behaviour in animals. Thiscould include investigations of motile sperm of plants,mobile phytoplankton, zooplankton swimming towardthe female egg, the cultivation of fungi by weevils, themating dance of a salamander, or social gatherings ofamoeba.[114][115][116][117][118]

Adaptation is the central unifying concept in behaviouralecology.[119] Behaviours can be recorded as traits and in-herited in much the same way that eye and hair colourcan. Behaviours can evolve by means of natural selec-tion as adaptive traits conferring functional utilities thatincreases reproductive tness.[120][121]

Predator-prey interactions are an introductory conceptinto food-web studies as well as behavioural ecology.[122]Prey species can exhibit dierent kinds of behaviouraladaptations to predators, such as avoid, ee or defend.Many prey species are faced with multiple predators thatdier in the degree of danger posed. To be adapted totheir environment and face predatory threats, organismsmust balance their energy budgets as they invest in dier-ent aspects of their life history, such as growth, feeding,mating, socializing, or modifying their habitat. Hypothe-

ses posited in behavioural ecology are generally basedon adaptive principles of conservation, optimization oreciency.[32][109][123] For example, "[t]he threat-sensitivepredator avoidance hypothesis predicts that prey shouldassess the degree of threat posed by dierent predatorsand match their behaviour according to current levels ofrisk[124] or "[t]he optimal ight initiation distance oc-curs where expected postencounter tness is maximized,which depends on the preys initial tness, benets obtain-able by not eeing, energetic escape costs, and expectedtness loss due to predation risk.[125]

Symbiosis: Leafhoppers (Eurymela fenestrata) are protected byants (Iridomyrmex purpureus) in a symbiotic relationship. Theants protect the leafhoppers from predators and in return theleafhoppers feeding on plants exude honeydew from their anusthat provides energy and nutrients to tending ants.[126]

Elaborate sexual displays and posturing are encounteredin the behavioural ecology of animals. The birds of par-adise, for example, sing and display elaborate ornamentsduring courtship. These displays serve a dual purpose ofsignalling healthy or well-adapted individuals and desir-able genes. The displays are driven by sexual selection asan advertisement of quality of traits among suitors.[127]

3.2 Cognitive ecologyCognitive ecology integrates theory and observationsfrom evolutionary ecology and neurobiology, primarilycognitive science, in order to understand the eect that

10 3 RELATION TO EVOLUTION

animal interaction with their habitat has on their cognitivesystems and how those systems restrict behavior withinan ecological and evolutionary framework.[128] Until re-cently, however, cognitive scientists have not paid su-cient attention to the fundamental fact that cognitive traitsevolved under particular natural settings. With consid-eration of the selection pressure on cognition, cognitiveecology can contribute intellectual coherence to the mul-tidisciplinary study of cognition.[129][130] As a study in-volving the 'coupling' or interactions between organismand environment, cognitive ecology is closely related toenactivism,[128] a eld based upon the view that "...wemust see the organism and environment as bound togetherin reciprocal specication and selection....[131]

3.3 Social ecology

Main article: Social ecology

Social ecological behaviours are notable in the social in-sects, slime moulds, social spiders, human society, andnaked mole-rats where eusocialism has evolved. So-cial behaviours include reciprocally benecial behavioursamong kin and nest mates[116][121][132] and evolve fromkin and group selection. Kin selection explains altruismthrough genetic relationships, whereby an altruistic be-haviour leading to death is rewarded by the survival ofgenetic copies distributed among surviving relatives. Thesocial insects, including ants, bees and wasps are mostfamously studied for this type of relationship becausethe male drones are clones that share the same geneticmake-up as every other male in the colony.[121] In con-trast, group selectionists nd examples of altruism amongnon-genetic relatives and explain this through selectionacting on the group, whereby it becomes selectively ad-vantageous for groups if their members express altruisticbehaviours to one another. Groups with predominantlyaltruistic members beat groups with predominantly self-ish members.[121][133]

3.4 Coevolution

Main article: CoevolutionEcological interactions can be classied broadly into a

host and an associate relationship. A host is any entitythat harbours another that is called the associate.[134] Re-lationships within a species that are mutually or recipro-cally benecial are called mutualisms. Examples of mu-tualism include fungus-growing ants employing agricul-tural symbiosis, bacteria living in the guts of insects andother organisms, the g wasp and yucca moth pollinationcomplex, lichens with fungi and photosynthetic algae,and corals with photosynthetic algae.[135][136] If there isa physical connection between host and associate, the re-lationship is called symbiosis. Approximately 60% of allplants, for example, have a symbiotic relationship with

Bumblebees and the owers they pollinate have coevolved so thatboth have become dependent on each other for survival.

arbuscular mycorrhizal fungi living in their roots formingan exchange network of carbohydrates for mineral nutri-ents.[137]

Indirect mutualisms occur where the organisms live apart.For example, trees living in the equatorial regions ofthe planet supply oxygen into the atmosphere that sus-tains species living in distant polar regions of the planet.This relationship is called commensalism because manyothers receive the benets of clean air at no cost orharm to trees supplying the oxygen.[5][138] If the asso-ciate benets while the host suers, the relationship iscalled parasitism. Although parasites impose a cost totheir host (e.g., via damage to their reproductive organsor propagules, denying the services of a benecial part-ner), their net eect on host tness is not necessarily neg-ative and, thus, becomes dicult to forecast.[139][140] Co-evolution is also driven by competition among species oramong members of the same species under the bannerof reciprocal antagonism, such as grasses competing forgrowth space. The Red Queen Hypothesis, for exam-ple, posits that parasites track down and specialize on thelocally common genetic defence systems of its host thatdrives the evolution of sexual reproduction to diversifythe genetic constituency of populations responding to theantagonistic pressure.[141][142]

3.5 Biogeography

Main article: Biogeography

Biogeography (an amalgamation of biology and geogra-phy) is the comparative study of the geographic distribu-tion of organisms and the corresponding evolution of theirtraits in space and time.[143] The Journal of Biogeogra-phy was established in 1974.[144] Biogeography and ecol-ogy share many of their disciplinary roots. For exam-ple, the theory of island biogeography, published by themathematician Robert MacArthur and ecologist EdwardO. Wilson in 1967[145] is considered one of the funda-mentals of ecological theory.[146]

3.6 Molecular ecology 11

Parasitism: A harvestman arachnid being parasitized by mites.The harvestman is being consumed, while the mites benet fromtraveling on and feeding o of their host.

Biogeography has a long history in the natural sciencesconcerning the spatial distribution of plants and animals.Ecology and evolution provide the explanatory contextfor biogeographical studies.[143] Biogeographical patternsresult from ecological processes that inuence range dis-tributions, such as migration and dispersal.[146] and fromhistorical processes that split populations or species intodierent areas. The biogeographic processes that resultin the natural splitting of species explains much of themodern distribution of the Earths biota. The splittingof lineages in a species is called vicariance biogeogra-phy and it is a sub-discipline of biogeography.[147] Thereare also practical applications in the eld of biogeographyconcerning ecological systems and processes. For exam-ple, the range and distribution of biodiversity and invasivespecies responding to climate change is a serious concernand active area of research in the context of global warm-ing.[148][149]

3.5.1 r/K-Selection theory

Main article: r/K selection

A population ecology concept is r/K selection theory,[D]one of the rst predictive models in ecology used toexplain life-history evolution. The premise behind ther/K selection model is that natural selection pressureschange according to population density. For example,when an island is rst colonized, density of individu-als is low. The initial increase in population size isnot limited by competition, leaving an abundance ofavailable resources for rapid population growth. These

early phases of population growth experience density-independent forces of natural selection, which is calledr-selection. As the population becomes more crowded,it approaches the islands carrying capacity, thus forcingindividuals to compete more heavily for fewer availableresources. Under crowded conditions, the population ex-periences density-dependent forces of natural selection,called K-selection.[150]

In the r/K-selection model, the rst variable r is the intrin-sic rate of natural increase in population size and the sec-ond variableK is the carrying capacity of a population.[32]Dierent species evolve dierent life-history strategiesspanning a continuum between these two selective forces.An r-selected species is one that has high birth rates, lowlevels of parental investment, and high rates of mortal-ity before individuals reach maturity. Evolution favourshigh rates of fecundity in r-selected species. Manykinds of insects and invasive species exhibit r-selectedcharacteristics. In contrast, a K-selected species has lowrates of fecundity, high levels of parental investment inthe young, and low rates of mortality as individuals ma-ture. Humans and elephants are examples of speciesexhibiting K-selected characteristics, including longevityand eciency in the conversion of more resources intofewer ospring.[145][151]

3.6 Molecular ecologyMain article: Molecular ecology

The important relationship between ecology and geneticinheritance predates modern techniques for molecularanalysis. Molecular ecological research became morefeasible with the development of rapid and accessible ge-netic technologies, such as the polymerase chain reac-tion (PCR). The rise of molecular technologies and in-ux of research questions into this new ecological eld re-sulted in the publication Molecular Ecology in 1992.[152]Molecular ecology uses various analytical techniques tostudy genes in an evolutionary and ecological context. In1994, John Avise also played a leading role in this areaof science with the publication of his book, MolecularMarkers, Natural History and Evolution.[153] Newer tech-nologies opened a wave of genetic analysis into organ-isms once dicult to study from an ecological or evolu-tionary standpoint, such as bacteria, fungi and nematodes.Molecular ecology engendered a new research paradigmfor investigating ecological questions considered other-wise intractable. Molecular investigations revealed pre-viously obscured details in the tiny intricacies of natureand improved resolution into probing questions aboutbehavioural and biogeographical ecology.[153] For ex-ample, molecular ecology revealed promiscuous sexualbehaviour and multiple male partners in tree swallowspreviously thought to be socially monogamous.[154] In abiogeographical context, the marriage between genetics,ecology and evolution resulted in a new sub-discipline

12 5 RELATION TO THE ENVIRONMENT

called phylogeography.[155]

4 Human ecologyMain article: Human ecology

The history of life on Earth has been a history of inter-action between living things and their surroundings. To alarge extent, the physical form and the habits of the earthsvegetation and its animal life have been molded by the en-vironment. Considering the whole span of earthly time,the opposite eect, in which life actually modies its sur-roundings, has been relatively slight. Only within the mo-ment of time represented by the present century has onespecies man acquired signicant power to alter the natureof his world.Rachel Carson, Silent Spring[156]

Ecology is as much a biological science as it is a humanscience.[5] Human ecology is an interdisciplinary inves-tigation into the ecology of our species. Human ecol-ogy may be dened: (1) from a bio-ecological standpointas the study of man as the ecological dominant in plantand animal communities and systems; (2) from a bio-ecological standpoint as simply another animal aectingand being aected by his physical environment; and (3)as a human being, somehow dierent from animal life ingeneral, interacting with physical and modied environ-ments in a distinctive and creative way. A truly interdis-ciplinary human ecology will most likely address itselfto all three.[157]:3 The term was formally introduced in1921, but many sociologists, geographers, psychologists,and other disciplines were interested in human relationsto natural systems centuries prior, especially in the late19th century.[157][158]

The ecological complexities human beings are facingthrough the technological transformation of the planetarybiome has brought on the Anthropocene. The unique setof circumstances has generated the need for a new uni-fying science called coupled human and natural systemsthat builds upon, but moves beyond the eld of humanecology.[104] Ecosystems tie into human societies throughthe critical and all encompassing life-supporting func-tions they sustain. In recognition of these functions andthe incapability of traditional economic valuation meth-ods to see the value in ecosystems, there has been a surgeof interest in social-natural capital, which provides themeans to put a value on the stock and use of informationand materials stemming from ecosystem goods and ser-vices. Ecosystems produce, regulate, maintain, and sup-ply services of critical necessity and benecial to humanhealth (cognitive and physiological), economies, and theyeven provide an information or reference function as aliving library giving opportunities for science and cogni-tive development in children engaged in the complexity

of the natural world. Ecosystems relate importantly tohuman ecology as they are the ultimate base foundationof global economics as every commodity and the capac-ity for exchange ultimately stems from the ecosystems onEarth.[104][159][160][161]

4.1 Restoration and managementMain article: Restoration ecologySee also: Natural resource management

Ecosystem management is not just about science nor is itsimply an extension of traditional resource management;it oers a fundamental reframing of how humans maywork with nature.Grumbine (1994)[162]:27

Ecology is an employed science of restoration, repair-ing disturbed sites through human intervention, in nat-ural resource management, and in environmental im-pact assessments. Edward O. Wilson predicted in 1992that the 21st century will be the era of restoration inecology.[163] Ecological science has boomed in the in-dustrial investment of restoring ecosystems and theirprocesses in abandoned sites after disturbance. Natu-ral resource managers, in forestry, for example, employecologists to develop, adapt, and implement ecosystembased methods into the planning, operation, and restora-tion phases of land-use. Ecological science is usedin the methods of sustainable harvesting, disease andre outbreak management, in sheries stock manage-ment, for integrating land-use with protected areas andcommunities, and conservation in complex geo-politicallandscapes.[21][162][162][164][165]

5 Relation to the environmentMain article: Natural environment

The environment of ecosystems includes both physicalparameters and biotic attributes. It is dynamically in-terlinked, and contains resources for organisms at anytime throughout their life cycle.[5][166] Like ecology, theterm environment has dierent conceptual meaningsand overlaps with the concept of nature. Environment"... includes the physical world, the social world of humanrelations and the built world of human creation.[167]:62The physical environment is external to the level of bio-logical organization under investigation, including abioticfactors such as temperature, radiation, light, chemistry,climate and geology. The biotic environment includesgenes, cells, organisms, members of the same species(conspecics) and other species that share a habitat.[168]

The distinction between external and internal environ-

5.3 Radiation: heat, temperature and light 13

ments, however, is an abstraction parsing life and envi-ronment into units or facts that are inseparable in reality.There is an interpenetration of cause and eect betweenthe environment and life. The laws of thermodynamics,for example, apply to ecology by means of its physicalstate. With an understanding of metabolic and thermo-dynamic principles, a complete accounting of energy andmaterial ow can be traced through an ecosystem. In thisway, the environmental and ecological relations are stud-ied through reference to conceptually manageable andisolated material parts. After the eective environmen-tal components are understood through reference to theircauses, however, they conceptually link back together asan integrated whole, or holocoenotic system as it was oncecalled. This is known as the dialectical approach to ecol-ogy. The dialectical approach examines the parts, but in-tegrates the organism and the environment into a dynamicwhole (or umwelt). Change in one ecological or environ-mental factor can concurrently aect the dynamic stateof an entire ecosystem.[35][169]

5.1 Disturbance and resilience

Main article: Resilience (ecology)

Ecosystems are regularly confronted with natural envi-ronmental variations and disturbances over time and ge-ographic space. A disturbance is any process that re-moves biomass from a community, such as a re, ood,drought, or predation.[170] Disturbances occur over vastlydierent ranges in terms of magnitudes as well as dis-tances and time periods,[171] and are both the cause andproduct of natural uctuations in death rates, species as-semblages, and biomass densities within an ecologicalcommunity. These disturbances create places of renewalwhere new directions emerge from the patchwork of natu-ral experimentation and opportunity.[170][172][173] Ecolog-ical resilience is a cornerstone theory in ecosystem man-agement. Biodiversity fuels the resilience of ecosystemsacting as a kind of regenerative insurance.[173]

5.2 Metabolism and the early atmosphere

Metabolism the rate at which energy and material re-sources are taken up from the environment, transformedwithin an organism, and allocated to maintenance, growthand reproduction is a fundamental physiological trait.Ernest et al.[174]:991

The Earth was formed approximately 4.5 billion yearsago.[175] As it cooled and a crust and oceans formed,its atmosphere transformed from being dominated byhydrogen to one composed mostly of methane andammonia. Over the next billion years, the metabolic ac-tivity of life transformed the atmosphere into a mixture of

carbon dioxide, nitrogen, and water vapor. These gaseschanged the way that light from the sun hit the Earthssurface and greenhouse eects trapped heat. There wereuntapped sources of free energy within the mixture ofreducing and oxidizing gasses that set the stage for prim-itive ecosystems to evolve and, in turn, the atmospherealso evolved.[176]

The leaf is the primary site of photosynthesis in most plants.

Throughout history, the Earths atmosphere andbiogeochemical cycles have been in a dynamic equi-librium with planetary ecosystems. The history ischaracterized by periods of signicant transformationfollowed by millions of years of stability.[177] Theevolution of the earliest organisms, likely anaerobicmethanogen microbes, started the process by convertingatmospheric hydrogen into methane (4H2 + CO2 CH4+ 2H2O). Anoxygenic photosynthesis reduced hydrogenconcentrations and increased atmospheric methane, byconverting hydrogen sulde into water or other sulfurcompounds (for example, 2H2S + CO2 + hv CH2O +H2O + 2S). Early forms of fermentation also increasedlevels of atmospheric methane. The transition to anoxygen-dominant atmosphere (the Great Oxidation) didnot begin until approximately 2.42.3 billion years ago,but photosynthetic processes started 0.3 to 1 billion yearsprior.[177][178]

5.3 Radiation: heat, temperature and lightThe biology of life operates within a certain range of tem-peratures. Heat is a form of energy that regulates temper-ature. Heat aects growth rates, activity, behaviour andprimary production. Temperature is largely dependent onthe incidence of solar radiation. The latitudinal and lon-gitudinal spatial variation of temperature greatly aectsclimates and consequently the distribution of biodiversityand levels of primary production in dierent ecosystemsor biomes across the planet. Heat and temperature re-late importantly to metabolic activity. Poikilotherms, forexample, have a body temperature that is largely regu-lated and dependent on the temperature of the externalenvironment. In contrast, homeotherms regulate their

14 5 RELATION TO THE ENVIRONMENT

internal body temperature by expending metabolic en-ergy.[109][110][169]

There is a relationship between light, primary produc-tion, and ecological energy budgets. Sunlight is theprimary input of energy into the planets ecosystems.Light is composed of electromagnetic energy of dier-ent wavelengths. Radiant energy from the sun generatesheat, provides photons of light measured as active en-ergy in the chemical reactions of life, and also acts asa catalyst for genetic mutation.[109][110][169] Plants, algae,and some bacteria absorb light and assimilate the energythrough photosynthesis. Organisms capable of assimilat-ing energy by photosynthesis or through inorganic xa-tion of H2S are autotrophs. Autotrophs responsiblefor primary production assimilate light energy whichbecomes metabolically stored as potential energy in theform of biochemical enthalpic bonds.[109][110][169]

5.4 Physical environments5.4.1 Water

Main article: Aquatic ecosystem

Wetland conditions such as shallow water, high plant pro-ductivity, and anaerobic substrates provide a suitable en-vironment for important physical, biological, and chemi-cal processes. Because of these processes, wetlands playa vital role in global nutrient and element cycles.Cronk & Fennessy (2001)[179]:29

Diusion of carbon dioxide and oxygen is approximately10,000 times slower in water than in air. When soilsare ooded, they quickly lose oxygen, becoming hypoxic(an environment with O2 concentration below 2 mg/liter)and eventually completely anoxic where anaerobic bacte-ria thrive among the roots. Water also inuences the in-tensity and spectral composition of light as it reects othe water surface and submerged particles.[179] Aquaticplants exhibit a wide variety of morphological and phys-iological adaptations that allow them to survive, competeand diversify in these environments. For example, theirroots and stems contain large air spaces (aerenchyma) thatregulate the ecient transportation of gases (for exam-ple, CO2 and O2) used in respiration and photosynthe-sis. Salt water plants (halophytes) have additional spe-cialized adaptations, such as the development of specialorgans for shedding salt and osmoregulating their internalsalt (NaCl) concentrations, to live in estuarine, brackish,or oceanic environments. Anaerobic soil microorgan-isms in aquatic environments use nitrate, manganese ions,ferric ions, sulfate, carbon dioxide and some organic com-pounds; other microorganisms are facultative anaerobesand use oxygen during respiration when the soil becomesdrier. The activity of soil microorganisms and the chem-istry of the water reduces the oxidation-reduction poten-

tials of the water. Carbon dioxide, for example, is re-duced to methane (CH4) by methanogenic bacteria.[179]The physiology of sh is also specially adapted to com-pensate for environmental salt levels through osmoreg-ulation. Their gills form electrochemical gradients thatmediate salt excretion in salt water and uptake in freshwater.[180]

5.4.2 Gravity

The shape and energy of the land is signicantly af-fected by gravitational forces. On a large scale, thedistribution of gravitational forces on the earth is un-even and inuences the shape and movement of tectonicplates as well as inuencing geomorphic processes suchas orogeny and erosion. These forces govern many ofthe geophysical properties and distributions of ecologi-cal biomes across the Earth. On the organismal scale,gravitational forces provide directional cues for plant andfungal growth (gravitropism), orientation cues for ani-mal migrations, and inuence the biomechanics and sizeof animals.[109] Ecological traits, such as allocation ofbiomass in trees during growth are subject to mechanicalfailure as gravitational forces inuence the position andstructure of branches and leaves.[181] The cardiovascularsystems of animals are functionally adapted to overcomepressure and gravitational forces that change according tothe features of organisms (e.g., height, size, shape), theirbehaviour (e.g., diving, running, ying), and the habitatoccupied (e.g., water, hot deserts, cold tundra).[182]

5.4.3 Pressure

Climatic and osmotic pressure places physiological con-straints on organisms, especially those that y and respireat high altitudes, or dive to deep ocean depths. Theseconstraints inuence vertical limits of ecosystems inthe biosphere, as organisms are physiologically sensi-tive and adapted to atmospheric and osmotic water pres-sure dierences.[109] For example, oxygen levels decreasewith decreasing pressure and are a limiting factor for lifeat higher altitudes.[183] Water transportation by plants isanother important ecophysiological parameter aectedby osmotic pressure gradients.[184][185][186] Water pres-sure in the depths of oceans requires that organismsadapt to these conditions. For example, diving animalssuch as whales, dolphins and seals are specially adaptedto deal with changes in sound due to water pressuredierences.[187] Dierences between hagsh species pro-vide another example of adaptation to deep-sea pressurethrough specialized protein adaptations.[188]

5.4.4 Wind and turbulence

Turbulent forces in air and water aect the environmentand ecosystem distribution, form and dynamics. On a

5.4 Physical environments 15

The architecture of the inorescence in grasses is subject to thephysical pressures of wind and shaped by the forces of naturalselection facilitating wind-pollination (anemophily).[189][190]

planetary scale, ecosystems are aected by circulationpatterns in the global trade winds. Wind power and theturbulent forces it creates can inuence heat, nutrient,and biochemical proles of ecosystems.[109] For exam-ple, wind running over the surface of a lake creates tur-bulence, mixing the water column and inuencing theenvironmental prole to create thermally layered zones,aecting how sh, algae, and other parts of the aquaticecosystem are structured.[191][192] Wind speed and turbu-lence also inuence evapotranspiration rates and energybudgets in plants and animals.[179][193] Wind speed, tem-perature and moisture content can vary as winds travelacross dierent land features and elevations. For exam-ple, the westerlies come into contact with the coastal andinterior mountains of western North America to producea rain shadow on the leeward side of the mountain. Theair expands and moisture condenses as the winds increasein elevation; this is called orographic lift and can causeprecipitation. This environmental process produces spa-tial divisions in biodiversity, as species adapted to wet-ter conditions are range-restricted to the coastal mountainvalleys and unable to migrate across the xeric ecosystems(e.g., of the Columbia Basin in western North America)to intermix with sister lineages that are segregated to theinterior mountain systems.[194][195]

5.4.5 Fire

Main article: Fire ecology

Forest res modify the land by leaving behind an en-vironmental mosaic that diversies the landscape intodierent seral stages and habitats of varied quality (left).Some species are adapted to forest res, such as pinetrees that open their cones only after re exposure (right).

Plants convert carbon dioxide into biomass and emit oxy-gen into the atmosphere. By approximately 350 millionyears ago (the end of the Devonian period), photosynthe-sis had brought the concentration of atmospheric oxygenabove 17%, which allowed combustion to occur.[196] Firereleases CO2 and converts fuel into ash and tar. Fire isa signicant ecological parameter that raises many issuespertaining to its control and suppression.[197] While theissue of re in relation to ecology and plants has beenrecognized for a long time,[198] Charles Cooper broughtattention to the issue of forest res in relation to the ecol-ogy of forest re suppression and management in the1960s.[199][200]

Native North Americans were among the rst to inu-ence re regimes by controlling their spread near theirhomes or by lighting res to stimulate the production ofherbaceous foods and basketry materials.[201] Fire cre-ates a heterogeneous ecosystem age and canopy struc-ture, and the altered soil nutrient supply and clearedcanopy structure opens new ecological niches for seedlingestablishment.[202][203] Most ecosystems are adapted tonatural re cycles. Plants, for example, are equipped witha variety of adaptations to deal with forest res. Somespecies (e.g., Pinus halepensis) cannot germinate until af-ter their seeds have lived through a re or been exposedto certain compounds from smoke. Environmentally trig-gered germination of seeds is called serotiny.[204][205] Fireplays a major role in the persistence and resilience ofecosystems.[172]

5.4.6 Soils

Main article: Soil ecology

Soil is the living top layer of mineral and organic dirtthat covers the surface of the planet. It is the chief or-ganizing centre of most ecosystem functions, and it is ofcritical importance in agricultural science and ecology.The decomposition of dead organic matter (for exam-ple, leaves on the forest oor), results in soils contain-ing minerals and nutrients that feed into plant produc-tion. The whole of the planets soil ecosystems is calledthe pedosphere where a large biomass of the Earths bio-diversity organizes into trophic levels. Invertebrates that

16 6 HISTORY

feed and shred larger leaves, for example, create smallerbits for smaller organisms in the feeding chain. Col-lectively, these organisms are the detritivores that reg-ulate soil formation.[206][207] Tree roots, fungi, bacte-ria, worms, ants, beetles, centipedes, spiders, mammals,birds, reptiles, amphibians and other less familiar crea-tures all work to create the trophic web of life in soilecosystems. Soils form composite phenotypes where in-organic matter is enveloped into the physiology of a wholecommunity. As organisms feed and migrate through soilsthey physically displace materials, an ecological processcalled bioturbation. This aerates soils and stimulates het-erotrophic growth and production. Soil microorganismsare inuenced by and feed back into the trophic dynam-ics of the ecosystem. No single axis of causality can bediscerned to segregate the biological from geomorpho-logical systems in soils.[208][209] Paleoecological studiesof soils places the origin for bioturbation to a time beforethe Cambrian period. Other events, such as the evolutionof trees and the colonization of land in the Devonian pe-riod played a signicant role in the early development ofecological trophism in soils.[65][207][210]

5.4.7 Biogeochemistry and climate

Main article: BiogeochemistrySee also: Nutrient cycle and Climate

Ecologists study and measure nutrient budgets to un-derstand how these materials are regulated, ow, andrecycled through the environment.[109][110][169] This re-search has led to an understanding that there is globalfeedback between ecosystems and the physical pa-rameters of this planet, including minerals, soil, pH,ions, water and atmospheric gases. Six major ele-ments (hydrogen, carbon, nitrogen, oxygen, sulfur, andphosphorus; H, C, N, O, S, and P) form the constitu-tion of all biological macromolecules and feed into theEarths geochemical processes. From the smallest scaleof biology, the combined eect of billions upon billionsof ecological processes amplify and ultimately regulatethe biogeochemical cycles of the Earth. Understandingthe relations and cycles mediated between these elementsand their ecological pathways has signicant bearing to-ward understanding global biogeochemistry.[211]

The ecology of global carbon budgets gives one exampleof the linkage between biodiversity and biogeochemistry.It is estimated that the Earths oceans hold 40,000 giga-tonnes (Gt) of carbon, that vegetation and soil hold 2070Gt, and that fossil fuel emissions are 6.3 Gt carbon peryear.[212] There have been major restructurings in theseglobal carbon budgets during the Earths history, regu-lated to a large extent by the ecology of the land. For ex-ample, through the early-mid Eocene volcanic outgassing,the oxidation of methane stored in wetlands, and seaoorgases increased atmospheric CO2 (carbon dioxide) con-centrations to levels as high as 3500 ppm.[213]

In the Oligocene, from 25 to 32 million years ago, therewas another signicant restructuring of the global carboncycle as grasses evolved a new mechanism of photosyn-thesis, C4 photosynthesis, and expanded their ranges.This new pathway evolved in response to the drop in at-mospheric CO2 concentrations below 550 ppm.[214] Therelative abundance and distribution of biodiversity altersthe dynamics between organisms and their environmentsuch that ecosystems can be both cause and eect in re-lation to climate change. Human-driven modicationsto the planets ecosystems (e.g., disturbance, biodiversityloss, agriculture) contributes to rising atmospheric green-house gas levels. Transformation of the global carbon cy-cle in the next century is projected to raise planetary tem-peratures, lead to more extreme uctuations in weather,alter species distributions, and increase extinction rates.The eect of global warming is already being registeredin melting glaciers, melting mountain ice caps, and risingsea levels. Consequently, species distributions are chang-ing along waterfronts and in continental areas where mi-gration patterns and breeding grounds are tracking theprevailing shifts in climate. Large sections of permafrostare also melting to create a new mosaic of ooded ar-eas having increased rates of soil decomposition activitythat raises methane (CH4) emissions. There is concernover increases in atmospheric methane in the context ofthe global carbon cycle, because methane is a greenhousegas that is 23 times more eective at absorbing long-waveradiation than CO2 on a 100-year time scale.[215] Hence,there is a relationship between global warming, decom-position and respiration in soils and wetlands producingsignicant climate feedbacks and globally altered biogeo-chemical cycles.[104][216][217][218][219][220]

6 HistoryMain article: History of ecology

6.1 Early beginnings

Ecology has a complex origin, due in large part to itsinterdisciplinary nature.[221] Ancient Greek philosopherssuch as Hippocrates and Aristotle were among the rstto record observations on natural history. However, theyviewed life in terms of essentialism, where species wereconceptualized as static unchanging things while varietieswere seen as aberrations of an idealized type. This con-trasts against the modern understanding of ecological the-ory where varieties are viewed as the real phenomena ofinterest and having a role in the origins of adaptations bymeans of natural selection.[5][222][223] Early conceptionsof ecology, such as a balance and regulation in nature canbe traced to Herodotus (died c. 425 BC), who describedone of the earliest accounts of mutualism in his observa-tion of natural dentistry. Basking Nile crocodiles, he

6.1 Early beginnings 17

noted, would open their mouths to give sandpipers safeaccess to pluck leeches out, giving nutrition to the sand-piper and oral hygiene for the crocodile.[221] Aristotle wasan early inuence on the philosophical development ofecology. He and his student Theophrastus made exten-sive observations on plant and animal migrations, bio-geography, physiology, and on their behaviour, giving anearly analogue to the modern concept of an ecologicalniche.[224][225]

Ecological concepts such as food chains, population regu-lation, and productivity were rst developed in the 1700s,through the published works of microscopist Antonivan Leeuwenhoek (16321723) and botanist RichardBradley (1688?1732).[5] Biogeographer Alexander vonHumboldt (17691859) was an early pioneer in eco-logical thinking and was among the rst to recognizeecological gradients, where species are replaced or al-tered in form along environmental gradients, such asa cline forming along a rise in elevation. Humboldtdrew inspiration from Isaac Newton as he developed aform of terrestrial physics. In Newtonian fashion, hebrought a scientic exactitude for measurement into nat-ural history and even alluded to concepts that are thefoundation of a modern ecological law on species-to-area relationships.[226][227][228] Natural historians, suchas Humboldt, James Hutton and Jean-Baptiste Lamarck(among others) laid the foundations of the modern eco-logical sciences.[229] The term ecology (German: Oe-kologie, kologie) is of a more recent origin and was rstcoined by the German biologist Ernst Haeckel in his bookGenerelle Morphologie der Organismen (1866). Haeckelwas a zoologist, artist, writer, and later in life a professorof comparative anatomy.[230][231]

By ecology, we mean the whole science of the relations ofthe organism to the environment including, in the broadsense, all the conditions of existence.... Thus the the-ory of evolution explains the housekeeping relations oforganisms mechanistically as the necessary consequencesof eectual causes and so forms the monistic groundworkof ecology.Ernst Haeckel (1866)[230]:140 [B]

Ernst Haeckel (left) and Eugenius Warming (right), twofounders of ecology

Opinions dier on who was the founder of modern eco-logical theory. Some mark Haeckels denition as thebeginning;[232] others say it was Eugenius Warming with

the writing of Oecology of Plants: An Introduction tothe Study of Plant Communities (1895),[233] or Carl Lin-naeus' principles on the economy of nature that maturedin the early 18th century.[234][235] Linnaeus founded anearly branch of ecology that he called the economy ofnature.[234] His works inuenced Charles Darwin, whoadopted Linnaeus phrase on the economy or polity of na-ture in The Origin of Species.[230] Linnaeus was the rstto frame the balance of nature as a testable hypothesis.Haeckel, who admired Darwins work, dened ecology inreference to the economy of nature, which has led someto question whether ecology and the economy of natureare synonymous.[235]

The layout of the rst ecological experiment, carried out in agrass garden at Woburn Abbey in 1816, was noted by CharlesDarwin in The Origin of Species. The experiment studied theperformance of dierent mixtures of species planted in dierentkinds of soils.[236][237]

From Aristotle until Darwin, the natural world was pre-dominantly considered static and unchanging. Priorto The Origin of Species, there was little appreciationor understanding of the dynamic and reciprocal re-lations between organisms, their adaptations, and theenvironment.[222] An exception is the 1789 publicationNatural History of Selborne by Gilbert White (17201793), considered by some to be one of the earliest textson ecology.[238] While Charles Darwin is mainly noted forhis treatise on evolution,[239] he was one of the foundersof soil ecology,[240] and he made note of the rst ecologi-cal experiment in The Origin of Species.[236] Evolutionarytheory changed the way that researchers approached theecological sciences.[241]

Nowhere can one see more clearly illustrated what maybe called the sensibility of such an organic complex,--expressed by the fact that whatever aects any speciesbelonging to it, must speedily have its inuence of somesort upon the whole assemblage. He will thus be madeto see the impossibility of studying any form completely,out of relation to the other forms,--the necessity for tak-ing a comprehensive survey of the whole as a conditionto a satisfactory understanding of any part.Stephen Forbes (1887)[242]

18 7 SEE ALSO

6.2 Since 1900

Modern ecology is a young science that rst attractedsubstantial scientic attention toward the end of the 19thcentury (around the same time that evolutionary studieswere gaining scientic interest). Notable scientist EllenSwallow Richards may have rst introduced the term"oekology" (which eventually morphed into home eco-nomics) in the U.S. as early 1892.[243]

In the early 20th century, ecology transitioned froma more descriptive form of natural history to a moreanalytical form of scientic natural history.[226][229]Frederic Clements published the rst American ecologybook in 1905,[244] presenting the idea of plant commu-nities as a superorganism. This publication launched adebate between ecological holism and individualism thatlasted until the 1970s. Clements superorganism conceptproposed that ecosystems progress through regular anddetermined stages of seral development that are analo-gous to the developmental stages of an organism. TheClementsian paradigm was challenged by Henry Glea-son,[245] who stated that ecological communities developfrom the unique and coincidental association of indi-vidual organisms. This perceptual shift placed the fo-cus back onto the life histories of individual organismsand how this relates to the development of communityassociations.[246]

The Clementsian superorganism theory was an overex-tended application of an idealistic form of holism.[35][106]The term holism was coined in 1926 by Jan Christi-aan Smuts, a South African general and polarizing histor-ical gure who was inspired by Clements superorganismconcept.[247][C] Around the same time, Charles Elton pi-oneered the concept of food chains in his classical bookAnimal Ecology.[82] Elton[82] dened ecological relationsusing concepts of food chains, food cycles, and food size,and described numerical relations among dierent func-tional groups and their relative abundance. Eltons 'foodcycle' was replaced by 'food web' in a subsequent ecolog-ical text.[248] Alfred J. Lotka brought in many theoreticalconcepts applying thermodynamic principles to ecology.In 1942, Raymond Lindeman wrote a landmark paper onthe trophic dynamics of ecology, which was publishedposthumously after initially being rejected for its theo-retical emphasis. Trophic dynamics became the founda-tion for much of the work to follow on energy and ma-terial ow through ecosystems. Robert E. MacArthuradvanced mathematical theory, predictions and tests inecology in the 1950s, which inspired a resurgent schoolof theoretical mathematical ecologists.[229][249][250] Ecol-ogy also has developed through contributions from othernations, including Russias Vladimir Vernadsky and hisfounding of the biosphere concept in the 1920s[251] andJapans Kinji Imanishi and his concepts of harmony innature and habitat segregation in the 1950s.[252] Scien-tic recognition of contributions to ecology from non-English-speaking cultures is hampered by language and

translation barriers.[251]

This whole chain of poisoning, then, seems to rest on abase of minute plants which must have been the originalconcentrators. But what of the opposite end of the foodchainthe human being who, in probable ignorance ofall this sequence of events, has rigged his shing tackle,caught a string of sh from the waters of Clear Lake, andtaken them home to fry for his supper?Rachel Carson (1962)[253]:48

Ecology surged in popular and scientic interest duringthe 19601970s environmental movement. There arestrong historical and scientic ties between ecology, en-vironmental management, and protection.[229] The his-torical emphasis and poetic naturalistic writings advo-cating the protection of wild places by notable ecolo-gists in the history of conservation biology, such as AldoLeopold and Arthur Tansley, have been seen as far re-moved from urban centres where, it is claimed, the con-centration of pollution and environmental degradation islocated.[229][254] Palamar (2008)[254] notes an overshad-owing by mainstream environmentalism of pioneeringwomen in the early 1900s who fought for urban healthecology (then called euthenics)[243] and brought aboutchanges in environmental legislation. Women such asEllen Swallow Richards and Julia Lathrop, among others,were precursors to the more popularized environmentalmovements after the 1950s.In 1962, marine biologist and ecologist Rachel Carson'sbook Silent Spring helped to mobilize the environmen-tal movement by alerting the public to toxic pesticides,such as DDT, bioaccumulating in the environment. Car-son used ecological science to link the release of envi-ronmental toxins to human and ecosystem health. Sincethen, ecologists have worked to bridge their understand-ing of the degradation of the planets ecosystems withenvironmental politics, law, restoration, and natural re-sources management.[21][229][254][255]

7 See alsoMain article: Outline of ecology

Agroecology Chemical ecology Circles of Sustainability Cultural ecology Dialectical naturalism Earth science Ecological death

19

Ecological psychology Ecology movement Ecosophy Euthenics Industrial ecology Information ecology Landscape ecology Natural resource Political ecology Restoration ecology Sensory ecology Spiritual ecology Sustainable development

Lists

Glossary of ecology Index of biology articles List of ecologists Outline of biology Terminology of ecology

8 Notes1. ^ In Ernst Haeckels (1866) footnote where the term

ecology originates, he also gives attribute to AncientGreek: khr "", meaning dwellingplace, distributional area - quoted from Stauer(1957)

2. ^ This is a copy of Haeckels original denition(Original: Haeckel, E. (1866) Generelle Morpholo-gie der Organismen. Allgemeine Grundzige der or-ganischen Formen- Wissenschaft, mechanisch be-griindet durch die von Charles Darwin reformirteDescendenz-Theorie. 2 vols. Reimer, Berlin.)translated and quoted from Stauer (1957).

3. ^ Foster & Clark (2008) note how Smuts holismcontrasts starkly against his racial political views asthe father of apartheid.

4. ^ First introduced in MacArthur & Wilsons (1967)book of notable mention in the history and theoret-ical science of ecology, The Theory of Island Bio-geography

5. ^ Aristotle wrote about this concept in Metaphysics(Quoted from The Internet Classics Archive transla-tion by W. D. Ross. Book VIII, Part 6): To returnto the diculty which has been stated with respectboth to denitions and to numbers, what is the causeof their unity? In the case of all things which haveseveral parts and in which the totality is not, as itwere, a mere heap, but the whole is something be-side the parts, there is a cause; for even in bodiescontact is the cause of unity in some cases, and inothers viscosity or some other such quality.

9 References[1] Eric Laferrire; Peter J. Stoett (2 September 2003).

International Relations Theory and Ecological Thought:Towards a Synthesis. Routledge. pp. 25. ISBN 978-1-134-71068-3.

[2] Stadler, B.; Michalzik, B.; Mller, T. (1998). Linkingaphid ecology with nutrient uxes in a coniferous for-est. Ecology 79 (5): 15141525. doi:10.1890/0012-9658(1998)079[1514:LAEWNF]2.0.CO;2. ISSN 0012-9658.

[3] Humphreys, N. J.; Douglas, A. E. (1997). Partitioningof symbiotic bacteria between generations of an insect:a quantitative study of a Buchnera sp. in the pea aphid(Acyrthosiphon pisum) reared at dierent temperatures.Applied and Environmental Microbiology 63 (8): 32943296. PMC 1389233. PMID 16535678.

[4] Liere, Heidi; Jackson, Doug; Vandermeer, John; Wilby,Andrew (20 September 2012). Ecological Complex-ity in a Coee Agroecosystem: Spatial Heterogeneity,Population Persistence and Biological Control. PLoSONE 7 (9): e45508. Bibcode:2012PLoSO...745508L.doi:10.1371/journal.pone.0045508. PMC 3447771.PMID 23029061.

[5] Odum, E. P.; Barrett, G. W. (2005). Fundamentals ofEcology. Brooks Cole. p. 598. ISBN 978-0-534-42066-6.

[6] O'Neill, D. L.; Deangelis, D. L.; Waide, J. B.; Allen, T. F.H. (1986). A Hierarchical Concept of Ecosystems. Prince-ton University Press. p. 253. ISBN 0-691-08436-X.

[7] Nachtomy, Ohad; Shavit, Ayelet; Smith, Justin (2002).Leibnizian organisms, nested individuals, and unitsof selection. Theory in Biosciences 121 (2): 205.doi:10.1007/s12064-002-0020-9.

[8] Holling, C. S. (2004). Understanding the complexity ofeconomic, ecological, and social systems. Ecosystems 4(5): 390405. doi:10.1007/s10021-001-0101-5.

[9] Levin, S. A. (1999). Fragile Dominion: Complexity andthe Commons. Reading, MA: Perseus Books. ISBN 978-0-7382-0319-5.

[10] Noss, R. F.; Carpenter, A. Y. (1994). Saving NaturesLegacy: Protecting and Restoring Biodiversity. IslandPress. p. 443. ISBN 978-1-55963-248-5.

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[12] Scholes, R. J.; Mace, G. M.; Turner, W.; Geller, G. N.;Jrgens, N.; Larigauderie, A.; Muchoney, D.; Walther, B.A.; Mooney, H. A. (2008). Toward a global biodiver-sity observing system (PDF). Science 321 (5892): 10441045. doi:10.1126/science.1162055. PMID 18719268.

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