Chapter 53 Community Ecology

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Chapter 53

What Is a Community?

  • Assemblage of populations of various species living close enough for potential interaction

  • Savanna community in southern Africa

  • Community’s interactions include competition, predation, herbivory, symbiosis, and disease

  • Populations are linked by interspecific interactions

Interspecific interactions


  • Species compete for a particular resource that is in short supply

Competitive Exclusion Principle

  • Two species competing for the same limiting resources cannot coexist in the same place

Ecological Niches

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

  • As a result of competition

    • A species’ fundamental niche may be different from its realized niche

Resource Partitioning

  • Differentiation of niches that enables similar species to coexist in a community

Character Displacement

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


  • One species, the predator, kills and eats the other, the prey

  • Feeding adaptations of predators include

    • Claws, teeth, fangs, stingers, and poison
  • Animals also display

    • A great variety of defensive adaptations

  • Cryptic coloration, or camouflage

    • Makes prey difficult to spot

  • Aposematic coloration

    • Warns predators to stay away from prey

  • Müllerian mimicry

    • Two or more unpalatable species resemble each other


  • Herbivore eats parts of a plant

    •  evolution of plant mechanical and chemical defenses and consequent adaptations by herbivores


  • One organism, the parasite, derives its nourishment from another organism, its host, which is harmed in the process


  • Pathogens, disease-causing agents

    • Are typically bacteria, viruses, or protists

Mutualism (mutualistic symbiosis)

  • Interspecific interaction that benefits both species


  • Dominant and keystone species exert strong controls on community structure

  • In general, a small number of species in a community

    • Exert strong control on that community’s structure

Species Diversity

  • Variety of different kinds of organisms that make up the community

    • 2 components 

  • Species richness

  • Relative abundance

    • proportion each species represents of the total individuals in the community

  • 2 different communities same species richness, but a different relative abundance

  • A community with an even species abundance

    • Is more diverse than one in which one or two species are abundant and the remainder rare

Trophic Structure

  • Feeding relationships between organisms in a community

    • key factor in community dynamics

  • Food chains

Food Webs

  • branching food chain,

  • more complex

  • Food webs can be simplified

    • By isolating a portion of a community that interacts very little with the rest of the community

Dominant Species

  • 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

Keystone Species

  • Are not necessarily abundant in a community

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

  • Sea stars, keystone species in intertidal communities

  • Observation of sea otter populations and their predation

Ecosystem “Engineers” (Foundation Species)

  • Beaver dams

    • Can transform landscapes on a very large scale

  • Some foundation species act as facilitators

  • Pollution

    • Can affect community dynamics
  • But through biomanipulation

    • Polluted communities can be restored

What Is Disturbance?

  • Changes a community

  • Removes organisms from community

  • Alters resource availability

  • The large-scale fire in Yellowstone National Park in 1988

    • Demonstrated that communities can often respond very rapidly to a massive disturbance

Human Disturbance

  • most widespread agents of disturbance

Ecological Succession

  • Sequence of community and ecosystem changes after a disturbance

  • Primary succession

    • no soil exists when succession begins
  • Secondary succession

    • where soil remains after a disturbance

  • Early-arriving species

    • May facilitate the appearance of later species by making the environment more favorable
    • May inhibit establishment of later species
    • May tolerate later species but have no impact on their establishment

  • Succession on the moraines in Glacier Bay, Alaska

    • Follows a predictable pattern of change in vegetation and soil characteristics

  • Biogeographic factors affect community diversity

  • 2 key factors correlated with a community’s species diversity

    • Are its geographic location and its size

  • Species richness generally declines along an equatorial-polar gradient

  • The greater age of tropical environments greater species richness

  • Climate

    • Is likely the primary cause of the latitudinal gradient in biodiversity

  • 2 main climatic factors correlated with biodiversity

    • solar energy input and water availability

Area Effects

  • A species-area curve of North American breeding birds

Island Equilibrium Model

  • 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

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