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Community Ecology

Key Concepts:

53.1 A community’s interactions include competition, predation, herbivory, symbiosis, and disease
53.2 Dominant and keystone species exert strong controls on community structure
53.3 Disturbance influences species diversity and composition
53.4 Biogeographic factors affect community biodiversity
53.5 Contrasting views of community structure are the subject of continuing debate


A community’s interactions include competition, predation, herbivory, symbiosis, and disease

  • Populations are linked by interspecific interactions that affect the survival and reproduction of the species engaged in the interaction.

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

    • Resource Partitioning: differentiation of niches so that similar species can share an environment

    • Interspecific Interactions: Interactions with other organisms

  • Competition- Interspecific competition occurs when species compete for a specific resource in short supply.  The ecological niche is the total of the organism’s use of the biotic and abiotic resources in its environment.  The competitive exclusion principle states that two species cannot coexist in the same community if their niches are identical.

  • Predation-Predation refers to an interaction in which one species, the predator, kills and eats the other, the prey.  Predation has led to diverse adaptation, including mimicry.

  • Aposematic coloration- Animals with effective chemical defenses displaying bright warning coloration.

  • Cryptic Coloration- camouflage making prey difficult to spot.

  • Batesian mimicry- a palatable or harmless species mimics an unpalatable or harmful model.

  • Mullerian mimicry- two or more unpalatable species mimic each other because the more of them that there are, the quicker predators will stop hunting them.

  • Herbivory- Herbivory, an interaction in which an herbivore eats parts of a plant or an alga, has led to the evolution of various chemical and mechanical defenses in plant species as well as consequent adaptations by herbivorous species.

  • Parasitism- In parasitism, one organism, the parasite, derives its nourishment from another organism, substantial influence on populations and the structure of communities.

    • Endoparasites - parasites living inside body

    • Ectoparasites- living outside body

    • Parasitoidism- lay eggs on/or in living hosts

  • Disease- The effects of disease on populations and communities is similar to that of parasites.  Most pathogens are microscopic.

  • Mutualism- Both species benefit

  • Commensalism- one species benefits while not affecting the other.  There are few if any cases of pure commensalism.

  • Interspecific Interactions and Adaptation- Evidence for coevolution, involving reciprocal genetic change by interacting populations, is scarce. However, general generalized adaptation of organisms to other organisms in their environment is a fundamental feature of life.


Effects on Species

Competition (-/-)

The interaction doesn't benefit either species

Predation (+/-)

Herbivory (+/-)

Parasitism (+/-)

Disease (+/-)

The interaction is beneficial to one species and detrimental to the other.

Mutualism (+/+)

The interaction is beneficial to both species.

Commensalism (+/0)

One species benefits from the interaction, and the other species is unaffected by it.

53.2 Dominant and keystone species exert strong controls on community structure

  • Species Diversity- Species diversity measures the number of species in a community- its species richness- and their 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- A key factor in community dynamics.  Food chains link the trophic levels from producers to top carnivores.  Branching food chains and complex trophic interactions form food webs.  The energetic hypothesis suggests that the length of a food chain is limited by the inefficiency of energy transfer along the chain.  The dynamic stability hypothesis proposes that long food chains are less stable than short chains.

  • Species with a Large Impact- Dominant species and keystone species exert strong controls on community structure.  Dominant species are the most abundant species in a community, or have the most biomass, and their dominance is achieved by having high competitive ability.  Keystone species are usually less abundant species that exert a disproportionate influence on community structure because of their ecological niche.  Ecosystem “engineers,” also called foundation species, exert influence on community structure through their effects on the physical environment.  They act as facilitators with positive effects on survival/reproduction for other organisms.

  • Bottom-Up and Top-Down Controls- The bottom-up model proposes a unidirectional influence from lower to higher trophic levels, in which nutrients and other abiotic factors are the main determinants of community structure, including the abundance of primary producers.  The top-down model proposes that control of each trophic level comes from the trophic level above, with the result that predators control herbivores, which in turn control primary producers.

53.3 Disturbance influences species diversity and composition

  • What is Disturbance?- Increasingly, evidence suggests that disturbance and non-equilibrium rather than stability and equilibrium are the norm for most communities.  According to the intermediate disturbance hypothesis, moderate levels of disturbance can foster higher species diversity than can low or high levels of disturbance.

    • Non Equilibrium Model- communities are constantly changing after being buffeted by disturbances

  • Human Disturbance- Humans are the most widespread agents of disturbance, and their disturbance to communities usually reduces species diversity.  Humans also prevent some naturally occurring disturbances, such as fire, which can be important to community structure.

  • 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.  Mechanisms producing community change during succession include facilitation and inhibition.

53.4 Biogeographic factors affect community biodiversity

  • Equatorial-Polar Gradients- Species richness generally declines along an equatorial-polar gradient and is especially great in the tropics.  The greater  age of tropical environments may account for the greater species richness of the tropics.  Climate also influences the biodiversity gradient through energy (heat and light) and water.

    • Evapotranspiration- evaporation of water from soil plus the transpiration of water from plants.

      • Actual- determined by amount of solar radiation, temperature, and water availability

      • Potential- measure of energy availability, not water availability

  • Area Effects- Species richness is directly related to a community’s geographic size, a principle formalized in the species-area curve.

  • Island Equilibrium Model- Species richness on islands depends on island size and distance from the mainland.  The equilibrium model of island biogeography maintains that species richness on an ecological island levels off at some dynamic equilibrium point, where new immigrations are balanced by extinctions.  The island equilibrium model has been questioned in recent years.     

53.5 Contrasting views of community structure are the subject of continuing debate

  • Integrated and Individualistic Hypotheses- The integrated hypothesis states that the species within a community are locked into particular biotic interactions.  The individualistic hypothesis proposes that communities are loosely of independently distributed species with the same abiotic requirements

  • Rivet and Redundancy Models- The rivet model suggests that all species in a  community are linked together in a tight web of interactions, so that the loss of even a single species has strong repercussions for the community.  The redundancy model proposes that if a species is lost from a community, other species will fill the gap.