� Chapter 53

� Community~ an assemblage of populations living close enough together for potential interaction

� Richness (number of species) & abundance…….

� Species diversity

Figure 53.1

� Hypotheses:

•Individualistic~ chance assemblage with similar abiotic

requirements

•Interactive~ assemblage locked into association by mandatory biotic interactions

� Interspecific

(interactions between populations of different species within a community):

•Predation including parasitism; may involve a keystone species/predator

•Competition•Competition

•Commensalism

•Mutualism

� Cryptic (camouflage)

coloration

� Aposematic (warning)� Aposematic (warning)

coloration

� Mimicry~ superficial resemblance to another species

√ Batesian~ palatable/ harmless species mimics an

unpalatable/ harmful model

√ Mullerian~ 2 or more unpalatable, aposematically colored species resemble each other

� Interference~ actual fighting over resources

�

� Exploitative~ consumption or use of similar resources

� Competitive Exclusion Principle (Lotka / Volterra)~ 2 species with similar needs for the same limiting resources cannot coexist

in the same place

� √Gause experiment

� Resource partitioning~ sympatric species consume slightly different foods or use other resources in slightly different ways

Ex: Anolis lizard sp. perching sites in the

Dominican RepublicDominican Republic

� Character displacement~ sympatric species tend to diverge in those characteristics that overlap

Ex: Darwin’s finch beak size on the Galapagos Islands

� Ecological niche~ the sum total of an organism’s use of biotic and abiotic resources in its environment; its

“ecological role”

√ fundamental~ the set of √ fundamental~ the set of resources a population is theoretically capable of using under ideal conditions

√ realized~ the resources a population actually uses

� Thus, 2 species cannot coexist in a community if their niches are identical Ex: Barnacle sp. on the coast of Scotland

� Ecological succession~ transition in species composition over ecological time

� Primary~ begun in lifeless area; no � Primary~ begun in lifeless area; no soil, perhaps volcanic activity or retreating glacier

� Secondary~ an existing community has been cleared by some disturbance that leaves the soil intact

� Certain species have an especially large impact on the structure of entire communities� Either because they are highly abundant or because they

play a pivotal role in community dynamicsplay a pivotal role in community dynamics

� Are those species in a community that are most abundant or have the highest biomass

� Exert powerful control over the occurrence and distribution of other species

� One hypothesis suggests that dominant species� Are most competitive in exploiting limited resources

� Another hypothesis for dominant species success� Is that they are most successful at avoiding predators

� Are not necessarily abundant in a community

� Exert strong control on a community by their ecological roles, or niches

� Field studies of sea stars� Exhibit their role as a keystone species in intertidal

communities

Figure 53.16a,b

(a) The sea star Pisaster ochraceous feeds preferentially on mussels but will consume other invertebrates.

With Pisaster (control)

Without Pisaster (experimental)N

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(b) When Pisaster was removed from an intertidal zone, mussels eventually took over the rock face and eliminated most other invertebrates and algae. In a control area from which Pisasterwas not removed, there was little change in species diversity.

� Observation of sea otter populations and their predation

� Some organisms exert their influence

� By causing physical changes in the environment that affect community structurecommunity structure

Beaver dams

� Can transform landscapes on a very large scale

Figure 53.18

� Some foundation species act as facilitators� That have positive effects on the survival and

reproduction of some of the other species in the community

8

Figure 53.19

Salt marsh with Juncus(foreground)

With Juncus

Without Juncus

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Conditions

� The bottom-up model of community organization� Proposes a unidirectional influence from lower to higher

trophic levels

� In this case, the presence or absence of abiotic nutrientsIn this case, the presence or absence of abiotic nutrients� Determines community structure, including the

abundance of primary producers

� The top-down model of community organization� Proposes that control comes from the trophic level above

� In this case, predators control herbivores� Which in turn control primary producers

� Long-term experiment studies have shown� That communities can shift periodically from bottom-up

to top-down

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Figure 53.20

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� Studies of species richness

FIELD STUDY

RESULTS

Ecologists Robert MacArthur and E. O. Wilson studied the number of plant species on the Galápagos Islands, which vary greatly in size, in relation to the area of each island.

200

400

� Species richness on islands� Depends on island size, distance from the

mainland, immigration, and extinction

� Studies of species richness on the Galápagos Islands� Support the prediction

that species richness increases with island size

The results of the study showed that plant species richness increased with island size, supporting the species-area theory.

CONCLUSION

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Area of island (mi2) (log scale)

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0.1 1 10 100 1,000

5

Figure 53.28

� The equilibrium model of island biogeography maintains that

� Species richness on an ecological island levels off at some dynamic equilibrium point

Figure 53.27a–c

Number of species on island

(a) Immigration and extinction rates. The equilibrium number of species on anisland represents a balance between the immigration of new species and theextinction of species already there.

(b) Effect of island size. Large islands may ultimately have a larger equilibrium num-ber of species than small islands because immigration rates tend to be higher and extinction rates lower on large islands.

Number of species on island Number of species on island

(c) Effect of distance from mainland. Near islands tend to have largerequilibrium numbers of species thanfar islands because immigration ratesto near islands are higher and extinctionrates lower.

Equilibrium number Small island Large island Far island Near island

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