Competition

Competition occurs when two or more organisms depend on a resource that is in short supply, so that each one's use of it leaves less for the others. The resource can be food, water, light, soil nutrients, nesting sites, territory or mates. Because every party loses something, it harms all of them. When the competitors belong to the same species it is called intraspecific competition, and when they belong to different species it is interspecific competition.

Competition can be direct, as when animals fight over a territory or a plant releases chemicals that stunt its neighbors, called interference competition. It can also be indirect, when individuals simply use up a shared resource before others reach it, called exploitation competition. Within a species, competition grows as a population becomes crowded and helps hold it near the carrying capacity of its environment. Between species, the competitive exclusion principle says that two species cannot share exactly the same niche for long: one will eventually displace the other unless they divide the resource, for example by feeding at different times or places, a pattern called resource partitioning.

Competition is also a force of natural selection that can push related species to evolve different traits. Gause's classic experiments with two Paramecium species in one tube show exclusion on a small scale.