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
When species compete with other taxa for limited resources, all participating species are affected negatively with respect to population growth rate, population size, or population biomass. Thus, competition can be viewed 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 thrive or consume resources. Examples of interference competition would include a lion chasing a hyena away from a kill or a plant releasing allelopathic chemicals that reduce 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 competition is more common in nature. Competition may limit average population size and the number of species that coexist in a community
Predation occurs when one individual uses another for food. Because the predator species benefits while the prey species is harmed, predation may be thought of as 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.
A mutualism is a symbiotic interaction between two species in which both species benefit, and is therefore a double positive interaction. Examples of mutualism include Rhizobium bacteria growing in nodules on the roots of legume plants, insects pollinating the flowers of angiosperms, or cleaner fish and 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 effects on other species. For example, foxes eat rabbits and rabbits eat clover. 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 clover population to increase. Exploitative competition is an example of an indirect effect.
Species richness and species diversity
Species diversity is a measure of diversity within an ecological community that incorporates both the number of species in the community (species richness) and the their relative abundance, or evenness. Even with the same number of species, a community with greater evenness is considered to be more diverse. For instance, a two-species community within 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.
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.