AP Biology

Ecology Unit

Chapters 52-54

Chapter 52

• Ecology is the scientific study of the interactions between organisms and the environment

• These interactions determine distribution of organisms and their abundance

• Ecology reveals the richness of the biosphere

Ecologists work at levels ranging from individual organisms to the planet

1-Organismal ecology studies how an organism’s structure, physiology, and (for animals) behavior meet environmental challenges2-Population ecology focuses on factors affecting how many individuals of a species live in an area•A population is a group of individuals of the same species living in an area3-Community ecology deals with the whole array of interacting species in a community

•A community is a group of populations of different species in an area

4-Ecosystem ecology emphasizes energy flow and chemical cycling among the various biotic and abiotic components

•An ecosystem is the community of organisms in an area and the physical factors with which they interact

The Scope of Ecological Research

5-Landscape ecology deals with arrays of ecosystems and how they are arranged in a geographic region

•A landscape is a mosaic of connected ecosystems

6-The biosphere is the global ecosystem, the sum of all the planet’s ecosystems

•Global ecology examines the influence of energy and materials on organisms across the biosphere

Concept 52.2: Interactions between organisms and the environment limit the distribution of species

• Ecologists have long recognized global and regional patterns of distribution of organisms within the biosphere

• Biogeography is a good starting point for understanding what limits geographic distribution of species

• Ecologists recognize two kinds of factors that determine distribution: biotic, or living factors, and abiotic, or nonliving factors

Fig. 52-6

Why is species X absentfrom an area?

Does dispersallimit its

distribution? Does behaviorlimit its

distribution?

Area inaccessibleor insufficient time

Yes

No

No

No

Yes

YesHabitat selection

Do biotic factors(other species)

limit itsdistribution?

Predation, parasitism,competition, disease

Do abiotic factorslimit its

distribution?

Chemicalfactors

Physicalfactors

WaterOxygenSalinitypHSoil nutrients, etc.

TemperatureLightSoil structureFireMoisture, etc.

Flowchart of factors limiting geographic distribution

Ecologists consider multiple factors when attempting to explain the

distribution of species

Dispersal and Distribution

• Dispersal is movement of individuals away from centers of high population density or from their area of origin

• Dispersal contributes to global distribution of organisms

Species Transplants

• Species transplants include organisms that are intentionally or accidentally relocated from their original distribution

• Species transplants can disrupt the communities or ecosystems to which they have been introduced

Fig. 52-8

RESULTS

Sea urchin

100

80

60

40

20

0

Limpet

Sea

wee

d c

ove

r (%

)Both limpets and urchinsremoved

Only urchinsremoved

Only limpets removed

Control (both urchinsand limpets present)

August1982

August1983

February1983

February1984

Climate

• Four major abiotic components of climate are temperature, water, sunlight, and wind

• The long-term prevailing weather conditions in an area constitute its climate

• Macroclimate consists of patterns on the global, regional, and local level

• Microclimate consists of very fine patterns, such as those encountered by the community of organisms underneath a fallen log

Fig. 52-10c

March equinox

60ºN30ºN

0º (equator)

30ºSJune solstice

Constant tiltof 23.5º

September equinox

December solstice

Seasonal variations of light and temperature increase steadily toward the poles

Seasonality

• The angle of the sun leads to many seasonal changes in local environments

• Lakes are sensitive to seasonal temperature change and experience seasonal turnover

Concept 52.3: Aquatic biomes are diverse and dynamic systems that cover most of Earth

• Biomes are the major ecological associations that occupy broad geographic regions of land or water

• Varying combinations of biotic and abiotic factors determine the nature of biomes

• Aquatic biomes account for the largest part of the biosphere in terms of area

• They can contain fresh water or salt water (marine)

• Oceans cover about 75% of Earth’s surface and have an enormous impact on the biosphere

Fig. 52-16

Littoralzone Limnetic

zone

Photiczone

PelagiczoneBenthic

zoneAphoticzone

(a) Zonation in a lake (b) Marine zonation

2,000–6,000 mAbyssal zone

Benthiczone

Aphoticzone

Pelagiczone

Continentalshelf

200 mPhotic zone

0

Oceanic zoneNeritic zone

Intertidal zone

Stratification of Aquatic Biomes

Many aquatic biomes are stratified into zones or layers defined by light penetration, temperature, and depth

• In oceans and most lakes, a temperature boundary called the thermocline separates the warm upper layer from the cold deeper water

• Many lakes undergo a semiannual mixing of their waters called turnover

• Turnover mixes oxygenated water from the surface with nutrient-rich water from the bottom

Fig. 52-17-5

Winter

4º4º

4º

4ºC

4º4º

Spring Summer Autumn

Thermocline

4º4º

4º

4ºC

4º4º

4º4º

4º

4ºC

2º0º

4ºC5º6º

8º18º

20º22º

Lakes• Oligotrophic lakes are nutrient-poor and generally

oxygen-rich• Eutrophic lakes are nutrient-rich and often depleted of

oxygen if ice covered in winter• Rooted and floating aquatic plants live in the shallow and

well-lighted littoral zone• Water is too deep in the limnetic zone to support rooted

aquatic plants; small drifting animals called zooplankton graze on the phytoplankton

Wetlands• A wetland is a habitat that is inundated by water at least

some of the time and that supports plants adapted to water-saturated soil

• Wetlands can develop in shallow basins, along flooded river banks, or on the coasts of large lakes and seas

• Wetlands are among the most productive biomes on earth and are home to diverse invertebrates and birds

Streams and Rivers

• The most prominent physical characteristic of streams and rivers is current

• A diversity of fishes and invertebrates inhabit unpolluted rivers and streams

• Damming and flood control impair natural functioning of stream and river ecosystems

Estuaries

• An estuary is a transition area between river and sea

• Salinity varies with the rise and fall of the tides

• Estuaries are nutrient rich and highly productive

• An abundant supply of food attracts marine invertebrates and fish

Video: Flapping GeeseVideo: Flapping Geese

Oceanic Pelagic Zone• The oceanic pelagic biome is a vast

realm of open blue water, constantly mixed by wind-driven oceanic currents

• This biome covers approximately 70% of Earth’s surface

• Phytoplankton and zooplankton are the dominant organisms in this biome; also found are free-swimming animals

Video: Shark Eating a SealVideo: Shark Eating a Seal

Coral Reefs

• Coral reefs are formed from the calcium carbonate skeletons of corals (phylum Cnidaria)

• Corals require a solid substrate for attachment

• Unicellular algae live within the tissues of the corals and form a mutualistic relationship that provides the corals with organic molecules

Video: Coral ReefVideo: Coral Reef Video: Clownfish and AnemoneVideo: Clownfish and Anemone

Concept 52.4: The structure and distribution of terrestrial biomes are controlled by climate and disturbance

• Climate is very important in determining why terrestrial biomes are found in certain areas

• Biome patterns can be modified by disturbance such as a storm, fire, or human activity

Fig. 52-19

Tropical forest

Savanna

Desert

Chaparral

Temperategrassland

Temperatebroadleaf forestNorthernconiferous forestTundra

High mountains

Polar ice

30ºN

Tropic ofCancerEquatorTropic ofCapricorn

30ºS

General Features of Terrestrial Biomes and the Role of Disturbance

• Terrestrial biomes are often named for major physical or climatic factors and for vegetation

• Terrestrial biomes usually grade into each other, without sharp boundaries

• The area of intergradation, called an ecotone, may be wide or narrow

• Vertical layering is an important feature of terrestrial biomes, and in a forest it might consist of an upper canopy, low-tree layer, shrub understory, ground layer of herbaceous plants, forest floor, and root layer

• Layering of vegetation in all biomes provides diverse habitats for animals

• Biomes are dynamic and usually exhibit extensive patchiness

Terrestrial Biomes• Terrestrial biomes can be characterized

by distribution, precipitation, temperature, plants, and animals

Tropical Forest• In tropical rain forests, rainfall is

relatively constant, while in tropical dry forests precipitation is highly seasonal

• Tropical forests are vertically layered and competition for light is intense

• Tropical forests are home to millions of animal species, including an estimated 5–30 million still undescribed species of insects, spiders, and other arthropods

Desert

• Precipitation is low and highly variable, generally less than 30 cm per year; deserts may be hot or cold

• Desert plants are adapted for heat and desiccation tolerance, water storage, and reduced leaf surface area

• Common desert animals include many kinds of snakes and lizards, scorpions, ants, beetles, migratory and resident birds, and seed-eating rodents; many are nocturnal

Savanna• Savanna precipitation and temperature

are seasonal• Grasses and forbs make up most of the

ground cover• Common inhabitants include insects and

mammals such as wildebeests, zebras, lions, and hyenas

Chaparral• Chaparral climate is highly seasonal,

with cool and rainy winters and hot dry summers

• The chaparral is dominated by shrubs, small trees, grasses, and herbs; many plants are adapted to fire and drought

• Animals include amphibians, birds and other reptiles, insects, small mammals and browsing mammals

Fig. 52-21d

An area of chaparralin California

Northern Coniferous Forest• The northern coniferous forest, or taiga, extends

across northern North America and Eurasia and is the largest terrestrial biome on Earth

• Winters are cold and long while summers may be hot• The conical shape of conifers prevents too much snow

from accumulating and breaking their branches• Animals include migratory and resident birds, and large

mammals

Temperate Broadleaf Forest• Winters are cool, while summers are hot and

humid; significant precipitation falls year round as rain and snow

• A mature temperate broadleaf forest has vertical layers dominated by deciduous trees in the Northern Hemisphere and evergreen eucalyptus in Australia

• Mammals, birds, and insects make use of all vertical layers in the forest

• In the Northern Hemisphere, many mammals hibernate in the winter

Tundra

• Tundra covers expansive areas of the Arctic; alpine tundra exists on high mountaintops at all latitudes

• Winters are long and cold while summers are relatively cool; precipitation varies

• Permafrost, a permanently frozen layer of soil, prevents water infiltration

• Vegetation is herbaceous (mosses, grasses, forbs, dwarf shrubs and trees, and lichen) and

supports birds, grazers, and their predators

Fig. 52-21h

Denali National Park, Alaska,in autumn

Fig. 52-UN1Why is species X absent from an area?

Does dispersal limit its distribution?Area inaccessible orinsufficient time

Yes

Yes

Yes

No

No

No

Does behavior limit its distribution?

Do biotic factors (other species)limit its distribution?

Do abiotic factors limit its distribution?

Habitat selection

Predation, parasitism,competition, disease

Water, oxygen, salinity, pH,soil nutrients, etc.

Chemicalfactors

Physicalfactors

Temperature, light, soilstructure, fire, moisture, etc.

Fig. 52-T1

Fig. 52-UN3

Fig. 52-UN2

Sierra NevadaGreat Basin

Plateau

Mea

n h

eig

ht

(cm

)A

ltit

ud

e (m

)

Seed collection sites

100

50

0

3,000

2,000

1,000

0

Chapter 53

Population ecology is the study of populations in relation to environment, including environmental influences on density and distribution, age structure, and population size

Concept 53.1: Dynamic biological processes influence population density, dispersion, and

demographics

• A population is a group of individuals of a single species living in the same general area

• Density is the number of individuals per unit area or volume

• Dispersion is the pattern of spacing among individuals within the boundaries of the population

• Density is the result of an interplay between processes that add individuals to a population and those that remove individuals

• Immigration is the influx of new individuals from other areas

• Emigration is the movement of individuals out of a population

Fig. 53-3

Births

Births and immigrationadd individuals toa population.

Immigration

Deaths and emigrationremove individualsfrom a population.

Deaths

Emigration

Patterns of Dispersion

• Environmental and social factors influence spacing of individuals in a population

• Three types of Dispersion:• Clumping is most common, individuals aggregate in

patches, may be influenced by resource availability and behavior

• Uniform is one in which individuals are evenly distributed, may be influenced by social interactions such as territoriality

• Random is the position of each individual is independent of other individuals, it occurs in the absence of strong attractions or repulsions

Fig. 53-4

(a) Clumped

(b) Uniform

(c) Random

Demographics

• Demography is the study of the vital statistics of a population and how they change over time

• Death rates and birth rates are of particular interest to demographers

• A life table is an age-specific summary of the survival pattern of a population

• It is best made by following the fate of a cohort, a group of individuals of the same age

• The life table of Belding’s ground squirrels reveals many things about this population

Table 53-1

Survivorship Curves

• A survivorship curve is a graphic way of representing the data in a life table

• The survivorship curve for Belding’s ground squirrels shows a relatively constant death rate

Fig. 53-5

Age (years)20 4 86

10

101

1,000

100

Nu

mb

er o

f su

rviv

ors

(lo

g s

cale

)

Males

Females

• Survivorship curves can be classified into three general types:– Type I: low death rates during early and middle

life, then an increase among older age groups

– Type II: the death rate is constant over the organism’s life span

– Type III: high death rates for the young, then a slower death rate for survivors

Fig. 53-6

1,000

100

10

10 50 100

II

III

Percentage of maximum life span

Nu

mb

er

of

su

rviv

ors

(lo

g s

ca

le)

I

Reproductive Rates

• For species with sexual reproduction, demographers often concentrate on females in a population

• A reproductive table, or fertility schedule, is an age-specific summary of the reproductive rates in a population

• It describes reproductive patterns of a population

Table 53-2

Concept 53.2: Life history traits are products of natural selection

• An organism’s life history comprises the traits that affect its schedule of reproduction and survival:

– The age at which reproduction begins

– How often the organism reproduces

– How many offspring are produced during each reproductive cycle

• Life history traits are evolutionary outcomes reflected in the development, physiology, and behavior of an organism

Evolution and Life History Diversity

• Life histories are very diverse

• Species that exhibit semelparity, or big-bang reproduction, reproduce once and die

• Species that exhibit iteroparity, or repeated reproduction, produce offspring repeatedly

• Highly variable or unpredictable environments likely favor big-bang reproduction, while dependable environments may favor repeated reproduction

Fig. 53-7

“Trade-offs” and Life Histories

• Organisms have finite resources, which may lead to trade-offs between survival and reproduction

• In animals, parental care of smaller broods may facilitate survival of offspring

Fig. 53-8

MaleFemale

100

RESULTS

80

60

40

20

0Reduced

brood sizeNormal

brood sizeEnlarged

brood sizePar

ents

su

rviv

ing

th

e fo

llo

win

g w

inte

r (%

)

• Some plants produce a large number of small seeds, ensuring that at least some of them will grow and eventually reproduce

• Other types of plants produce a moderate number of large seeds that provide a large store of energy that will help seedlings become established

Fig. 53-9

(a) Dandelion

(b) Coconut palm

Concept 53.3: The exponential model describes population growth in an idealized,

unlimited environment

• It is useful to study population growth in an idealized situation

• Idealized situations help us understand the capacity of species to increase and the conditions that may facilitate this growth

Per Capita Rate of Increase

• If immigration and emigration are ignored, a population’s growth rate (per capita increase) equals birth rate minus death rate

• Zero population growth occurs when the birth rate equals the death rate

• Most ecologists use differential calculus to express population growth as growth rate at a particular instant in time:

Nt

rN

where N = population size, t = time, and r = per capita rate of increase = birth – death

Exponential Growth

• Exponential population growth is population increase under idealized conditions

• Under these conditions, the rate of reproduction is at its maximum, called the intrinsic rate of increase

• Equation of exponential population growth:

dNdt

rmaxN

Exponential population growth results in a J-shaped curve

• The J-shaped curve of exponential growth characterizes some rebounding populations

Fig. 53-10

Number of generations

0 5 10 150

500

1,000

1,500

2,000

1.0N =dNdt

0.5N =dN

dt

Po

pu

lati

on

siz

e (N

)

Concept 53.4: The logistic model describes how a population grows more slowly as it

nears its carrying capacity• Exponential growth cannot be sustained for long in any

population• A more realistic population model limits growth by

incorporating carrying capacity• Carrying capacity (K) is the maximum population size the

environment can support• In the logistic population growth model, the per capita

rate of increase declines as carrying capacity is reached

• The logistic model of population growth produces a sigmoid (S-shaped) curve

Fig. 53-12

2,000

1,500

1,000

500

00 5 10 15

Number of generations

Po

pu

lati

on

siz

e (

N)

Exponentialgrowth

1.0N=dN

dt

1.0N=dN

dt

K = 1,500

Logistic growth1,500 – N

1,500

The Logistic Model and Real Populations

• The growth of laboratory populations of paramecia fits an S-shaped curve

• These organisms are grown in a constant environment lacking predators and competitors

Fig. 53-13a

1,000

800

600

400

200

00 5 10 15

Time (days)

Nu

mb

er

of

Pa

ram

ec

ium

/mL

(a) A Paramecium population in the lab

The Logistic Model and Life Histories

• Life history traits favored by natural selection may vary with population density and environmental conditions

• K-selection, or density-dependent selection, selects for life history traits that are sensitive to population density

• r-selection, or density-independent selection, selects for life history traits that maximize reproduction

Population Change and Population Density

• In density-independent populations, birth rate and death rate do not change with population density

• In density-dependent populations, birth rates fall and death rates rise with population density

Density-Dependent Population Regulation

• Density-dependent birth and death rates are an example of negative feedback that regulates population growth

• They are affected by many factors, such as competition for resources, territoriality, disease, predation, toxic wastes, and intrinsic factors

Competition for Resources

• In crowded populations, increasing population density intensifies competition for resources and results in a lower birth rate

Territoriality

• In many vertebrates and some invertebrates, competition for territory may limit density

• Cheetahs are highly territorial, using chemical communication to warn other cheetahs of their boundaries

• Oceanic birds exhibit territoriality in nesting behavior

Fig. 53-17

(a) Cheetah marking its territory

(b) Gannets

Disease

• Population density can influence the health and survival of organisms

• In dense populations, pathogens can spread more rapidly

Predation

• As a prey population builds up, predators may feed preferentially on that species

Toxic Wastes

• Accumulation of toxic wastes can contribute to density-dependent regulation of population size

Intrinsic Factors

• For some populations, intrinsic (physiological) factors appear to regulate population size

Population Dynamics

• The study of population dynamics focuses on the complex interactions between biotic and abiotic factors that cause variation in population size

• Stability and Fluctuation• Long-term population studies have challenged

the hypothesis that populations of large mammals are relatively stable over time

• Weather can affect population size over time• Changes in predation pressure can drive

population fluctuations-Next slide

Fig. 53-19

Wolves Moose

2,500

2,000

1,500

1,000

500

Nu

mb

er o

f m

oo

se

0

Nu

mb

er o

f w

olv

es

50

40

30

20

10

01955 1965 1975 1985 1995 2005

Year

Concept 53.6: The human population is no longer growing exponentially but is still

increasing rapidly

• No population can grow indefinitely, and humans are no exception

• Though the global population is still growing, the rate of growth began to slow during the 1960s

Regional Patterns of Population Change

• To maintain population stability, a regional human population can exist in one of two configurations:– Zero population growth =

High birth rate – High death rate– Zero population growth =

Low birth rate – Low death rate

• The demographic transition is the move from the first state toward the second state

Fig. 53-24

1750 1800 1900 1950 2000 2050

Year

1850

Sweden MexicoBirth rate Birth rate

Death rateDeath rate0

10

20

30

40

50B

irth

or

dea

th r

ate

per

1,0

00 p

eop

le

• The demographic transition is associated with an increase in the quality of health care and improved access to education, especially for women

• Most of the current global population growth is concentrated in developing countries

Age Structure

• One important demographic factor in present and future growth trends is a country’s age structure

• Age structure is the relative number of individuals at each age

• Age structure diagrams can predict a population’s growth trends

• They can illuminate social conditions and help us plan for the future

Fig. 53-25

Rapid growthAfghanistan

Male Female Age AgeMale Female

Slow growthUnited States

Male Female

No growthItaly

85+80–8475–7970–74

60–6465–69

55–5950–5445–4940–4435–3930–3425–2920–2415–19

0–45–9

10–14

85+80–8475–7970–74

60–6465–69

55–5950–5445–4940–4435–3930–3425–2920–2415–19

0–45–9

10–14

10  10 8 866 4 422 0Percent of population Percent of population Percent of population

66 4 422 08 8 66 4 422 08 8

Limits on Human Population Size

• The ecological footprint concept summarizes the aggregate land and water area needed to sustain the people of a nation

• It is one measure of how close we are to the carrying capacity of Earth

• Countries vary greatly in footprint size and available ecological capacity

• The carrying capacity of Earth for humans is uncertain

• The average estimate is 10–15 billion

• Our carrying capacity could potentially be limited by food, space, nonrenewable resources, or buildup of wastes

Chapter 54

• A biological community is an assemblage of populations of various species living close enough for potential interaction

Concept 54.1: Community interactions are classified by whether they help, harm, or have no

effect on the species involved

• Ecologists call relationships between species in a community interspecific interactions

• Examples are competition, predation, herbivory, and symbiosis (parasitism, mutualism, and commensalism)

• Interspecific interactions can affect the survival and reproduction of each species, and the effects can be summarized as positive (+), negative (–), or no effect (0)

Competition

• Interspecific competition (–/– interaction) occurs when species compete for a resource in short supply

• Strong competition can lead to competitive exclusion, local elimination of a competing

species• The competitive exclusion principle states that

two species competing for the same limiting resources cannot coexist in the same place

Ecological Niches

• The total of a species’ use of biotic and abiotic resources is called the species’ ecological niche

• An ecological niche can also be thought of as an organism’s ecological role

• Ecologically similar species can coexist in a community if there are one or more significant differences in their niches

• Resource partitioning is differentiation of ecological niches, enabling similar species to coexist in a community

Fig. 54-2

A. ricordii

A. insolitus usually percheson shady branches.

A. distichus perches on fenceposts and other sunny surfaces.

A. aliniger

A. distichus

A. insolitus

A. christophei

A. cybotes

A. etheridgei

• As a result of competition, a species’ fundamental niche may differ from its realized niche

Fig. 54-3

Ocean

Chthamalus

Balanus

EXPERIMENT

RESULTS

High tide

Low tide

Chthamalusrealized niche

Balanusrealized niche

High tide

Chthamalusfundamental niche

Low tideOcean

Character Displacement

• Character displacement is a tendency for characteristics to be more divergent in sympatric populations of two species than in allopatric populations of the same two species

• An example is variation in beak size between populations of two species of Galápagos finches

Fig. 54-4

Los Hermanos

G. fuliginosa G. fortis

Beakdepth

Daphne

G. fuliginosa,allopatric

G. fortis,allopatric

Sympatricpopulations

Santa María, San Cristóbal

Beak depth (mm)

Per

cen

tag

es o

f in

div

idu

als

in e

ach

siz

e cl

ass

60

40

20

0

60

40

20

0

60

40

20

08 10 12 14 16

Predation

• Predation (+/– interaction) refers to interaction where one species, the predator, kills and eats the other, the prey

• Some feeding adaptations of predators are claws, teeth, fangs, stingers, and poison

• Prey display various defensive adaptations

• Behavioral defenses include hiding, fleeing, forming herds or schools, self-defense, and alarm calls

• Animals also have morphological and physiological defense adaptations

• Cryptic coloration, or camouflage, makes prey difficult to spot

Video: Seahorse CamouflageVideo: Seahorse Camouflage

Fig. 54-5

Canyon tree frog

(a) Crypticcoloration

(b) Aposematiccoloration

Poison dart frog

(c) Batesian mimicry: A harmless species mimics a harmful one.

Hawkmothlarva

Green parrot snakeYellow jacketCuckoo bee

Müllerian mimicry: Two unpalatable speciesmimic each other.

(d)

• Animals with effective chemical defense often exhibit bright warning coloration, called aposematic coloration

• Predators are particularly cautious in dealing with prey that display such coloration

• In some cases, a prey species may gain significant protection by mimicking the appearance of another species

• In Batesian mimicry, a palatable or harmless species mimics an unpalatable or harmful model

• In Müllerian mimicry, two or more unpalatable species resemble each other

Herbivory

• Herbivory (+/– interaction) refers to an interaction in which an herbivore eats parts of a plant or alga

• It has led to evolution of plant mechanical and chemical defenses and adaptations by herbivores

Symbiosis

• Symbiosis is a relationship where two or more species live in direct and intimate contact with one another

Parasitism

• In parasitism (+/– interaction), one organism, the parasite, derives nourishment from another organism, its host, which is harmed in the process

• Parasites that live within the body of their host are called endoparasites; parasites that live on the external surface of a host are ectoparasites

Mutualism

• Mutualistic symbiosis, or mutualism (+/+ interaction), is an interspecific interaction that benefits both species

• A mutualism can be– Obligate, where one species cannot survive

without the other– Facultative, where both species can survive

alone

Video: Clownfish and AnemoneVideo: Clownfish and Anemone

Commensalism

• In commensalism (+/0 interaction), one species benefits and the other is apparently unaffected

• Commensal interactions are hard to document in nature because any close association likely affects both species

Concept 54.2: Dominant and keystone species exert strong controls on community

structure

• In general, a few species in a community exert strong control on that community’s structure

• Two fundamental features of community structure are species diversity and feeding relationships

Trophic Structure

• Trophic structure is the feeding relationships between organisms in a community

• It is a key factor in community dynamics• Food chains link trophic levels from

producers to top carnivores

Video: Shark Eating a SealVideo: Shark Eating a Seal

Fig. 54-11

Carnivore

Carnivore

Carnivore

Herbivore

Plant

A terrestrial food chain

Quaternaryconsumers

Tertiaryconsumers

Secondaryconsumers

Primaryconsumers

Primaryproducers

A marine food chain

Phytoplankton

Zooplankton

Carnivore

Carnivore

Carnivore

Food Webs

• A food web is a branching food chain with complex trophic interactions

• Species may play a role at more than one trophic level

• Food webs can be simplified by isolating a portion of a community that interacts very

little with the rest of the community

Fig. 54-12

Humans

Smallertoothedwhales

Baleenwhales

Spermwhales

Elephantseals

Leopardseals

Crab-eaterseals

Birds Fishes Squids

Carnivorousplankton

CopepodsEuphausids(krill)

Phyto-plankton

Limits on Food Chain Length

• Each food chain in a food web is usually only a few links long

• Two hypotheses attempt to explain food chain length: the energetic hypothesis and the dynamic stability hypothesis

• The energetic hypothesis suggests that length is limited by inefficient energy transfer

• The dynamic stability hypothesis proposes that long food chains are less stable than short ones

• Most data support the energetic hypothesis

Species with a Large Impact

• Certain species have a very large impact on community structure

• Such species are highly abundant or play a pivotal role in community dynamics

Dominant Species

• Dominant species are those that are most abundant or have the highest biomass

• Biomass is the total mass of all individuals in a population

• Dominant species exert powerful control over the occurrence and distribution of other species

• One hypothesis suggests that dominant species are most competitive in exploiting resources

• Another hypothesis is that they are most successful at avoiding predators

• Invasive species, typically introduced to a new environment by humans, often lack predators or disease

Keystone Species

• Keystone species exert strong control on a community by their ecological roles, or niches

• In contrast to dominant species, they are not necessarily abundant in a community

Foundation Species (Ecosystem “Engineers”)

• Foundation species (ecosystem “engineers”) cause physical changes in the environment that affect community structure

• For example, beaver dams can transform landscapes on a very large scale

• Some foundation species act as facilitators that have positive effects on survival and reproduction of some other species in the community

Concept 54.3: Disturbance influences species diversity and composition

• Decades ago, most ecologists favored the view that communities are in a state of equilibrium

• This view was supported by F. E. Clements who suggested that species in a climax community function as a superorganism

• Other ecologists, including A. G. Tansley and H. A. Gleason, challenged whether communities were at equilibrium

• Recent evidence of change has led to a nonequilibrium model, which describes communities as constantly changing after being buffeted by disturbances

Characterizing Disturbance

• A disturbance is an event that changes a community, removes organisms from it, and alters resource availability

• Fire is a significant disturbance in most terrestrial ecosystems

• It is often a necessity in some communities

• The large-scale fire in Yellowstone National Park in 1988 demonstrated that communities can often respond very rapidly to a massive disturbance

Fig. 54-21

(a) Soon after fire (b) One year after fire

Ecological Succession

• Ecological succession is the sequence of community and ecosystem changes after a disturbance

• Primary succession occurs where no soil exists when succession begins

• Secondary succession begins in an area where soil remains after a disturbance

• Early-arriving species and later-arriving species may be linked in one of three processes:– Early arrivals may facilitate appearance of later

species by making the environment favorable– They may inhibit establishment of later species– They may tolerate later species but have no

impact on their establishment

• Retreating glaciers provide a valuable field-research opportunity for observing succession

• Succession on the moraines in Glacier Bay, Alaska, follows a predictable pattern of change in vegetation and soil characteristics

Fig. 54-22-1

Pioneer stage, withfireweed dominant

1

1941

1907

1860

1760

Alaska

GlacierBay

Kilometers5 10 150

Fig. 54-22-2

Pioneer stage, withfireweed dominant

1

1941

1907

1860

1760

Alaska

GlacierBay

Kilometers5 10 150

Dryas stage2

Fig. 54-22-3

Pioneer stage, withfireweed dominant

1

1941

1907

1860

1760

Alaska

Kilometers5 10 150

Dryas stage2

Alder stage3

GlacierBay

Fig. 54-22-4

Pioneer stage, withfireweed dominant

1

1941

1907

1860

1760

Alaska

GlacierBay

Kilometers5 10 150

Dryas stage2

Alder stage3Spruce stage4

Human Disturbance

• Humans have the greatest impact on biological communities worldwide

• Human disturbance to communities usually reduces species diversity

• Humans also prevent some naturally occurring disturbances, which can be important to community structure

Concept 54.4: Biogeographic factors affect community biodiversity

• Latitude and area are two key factors that affect a community’s species diversity

Latitudinal Gradients

• Species richness generally declines along an equatorial-polar gradient and is especially great in the tropics

• Two key factors in equatorial-polar gradients of species richness are probably evolutionary history and climate

• The greater age of tropical environments may account for the greater species richness

• Climate is likely the primary cause of the latitudinal gradient in biodiversity

• Two main climatic factors correlated with biodiversity are solar energy and water availability

• They can be considered together by measuring a community’s rate of evapotranspiration

• Evapotranspiration is evaporation of water from soil plus transpiration of water from plants

Area Effects

• The species-area curve quantifies the idea that, all other factors being equal, a larger geographic area has more species

• A species-area curve of North American breeding birds supports this idea

Island Equilibrium Model

• Species richness on islands depends on island size, distance from the mainland, immigration, and extinction

• The equilibrium model of island biogeography maintains that species richness on an ecological island levels off at a dynamic equilibrium point

• Studies of species richness on the Galápagos Islands support the prediction that species richness increases with island size

Concept 54.5: Community ecology is useful for understanding pathogen life cycles and

controlling human disease

• Ecological communities are universally affected by pathogens, which include disease-causing microorganisms, viruses, viroids, and prions

• Pathogens can alter community structure quickly and extensively

Pathogens and Community Structure

• Pathogens can have dramatic effects on communities• Human activities are transporting pathogens around

the world at unprecedented rates• Community ecology is needed to help study and

combat them

Community Ecology and Zoonotic Diseases

• Zoonotic pathogens have been transferred from other animals to humans

• The transfer of pathogens can be direct or through an intermediate species called a vector

• Many of today’s emerging human diseases are zoonotic