Introduction

Community ecology is the branch of ecology that studies how interactions among species and between species and the abiotic environment affect community structure, including species richness, species diversity and patterns of species abundance.

Important interspecific interactions

Important interactions among species include competition, predation, and mutualism.

Competition

When species compete with each other for limited resources, all participating species are affected negatively with respect to population growth rate, population size, or population biomass. Thus, competition is defined as a - - (double negative) interaction. There are two mechanisms of interspecific competition. Interference competition occurs when one species directly affects the ability of a second species to live or consume resources. Examples of interference competition would include a lion chasing a hyena away from a kill or a plant that releases allelopathic chemicals that reduced the growth rate of a competing species. Exploitative competition occurs via the consumption of resources. When an individual of one species consumes a resource (e.g., food, hiding place, female, a photon of light, etc), it is no longer available to be consumed by a member of a second species. Exploitative is the most common mechanism of competition in nature. Competition may limit average population size and the number of species that coexist in a community

Predation

Predation occurs when one individual uses another for food. Because the preditor species benefits while the prey species is harmed, predation is considered to be a + - (positive-negative) interaction. Many animal predators kill their prey before they eat them (e.g., a hawk eating a mouse). Parasitism is an type of predation in which the parasites do not kill their prey when they feed on them (e.g., vampire bat feeding on the blood of a cow). Herbivory occurs when an animal feeds on a plant (e.g., a deer browsing on a shrub). Predation may influence the population size of both predators and prey and influence the number of species coexisting in a community.

Mutualism

A mutualism is a symbiotic interactions between two species in which both species benefit. Mutualisms are considered to be + + (double positive) interactions. Examples of mutualisms include Rhizobium bacteria growing in nodules on the roots legume plants, insects pollinating the flowers of angiosperms, or cleaner fish/client fish.

Direct and indirect effects

Species may be affected by both direct or indirect effects. An individual of one species may affect an individual of second species directly by interacting with that species, such as in predation or mutualism. Indirect effects occur when one species influences a second species through its effect on a third species. For example, foxes eat rabbits and rabbits eat grass. Thus, foxes indirectly affect grass by eating rabbits; increasing the population size of foxes would result in a smaller population of rabbits which in turn would allow the grass population to increase. Exploitative competition is an example of an indirect effect.

Community structure

Species richness and species diversity

Species diversity is a measure of the diversity within an ecological community that incorporates both species richness (the number of species in a community) and the evenness of species' abundances. For a given number of species, greater evenness corresponds to greater diversity. For instance, a two-species community with in which the population of one species is 10 times larger than the population of the other species is less diverse than a two-species community in which the two populations have equal sizes. A current major focus of community ecologists is to understand the factors that affect species diversity within a community.

Species abundance

Within a community, some species may be common (i.e. large populations) whereas others may be rare (i.e. small populations). Community ecologists are interested in understanding the factors that affect patterns of species abundance.

Further reading

• Campbell, N.A., J.B. Reece, and L.G. Mitchhell. 2006. Biology. Addison Wesley Longman, Inc. Menlo Park, CA. ISBN: 080537146X
• Raven, P.H., G.B. Johnson, J.B. Losos, K.A. Mason, and S.R. Singer. 2008. Biology, 8th edition. McGraw Hill, New York, NY. ISBN: 0073227390