Competitive Exclusion

The competitive exclusion principle states that two species that occupy the same biological niche cannot coexist. Another way of expressing this idea is that "complete competitors" cannot coexist. That is because when two species occupy precisely the same niche, and compete for precisely the same resources, one species will inevitably be better at exploiting those resources than the other. The more effective species will outcompete the other and eliminate it from the habitat. The competitive exclusion principle was first stated in this form in 1934 by G. F. Gause, although other biologists, starting as early as Charles Darwin, appear to have had similar thoughts.

The competitive exclusion principle is actually a μtheoretical result derived from mathematical equations for competition called the Lotka-Volterra equations. However, there appears to be empirical, or factual, support for the idea as well. Studies of coexisting species always show that they differ in at least one important aspect of their niche. In one famous study, Robert MacArthur examined the habitat use of five species of coexisting forest warblers and found that each species foraged for food on a different part of the tree.

Similar studies of lizards of the genus Anolis suggest that in this group, species that are found in the same place tend to be of different sizes, prefer to forage at different branch heights, or use branches of different thicknesses. In certain large lakes of the African Rift Valley, several hundred species of cichlid fish may coexist. Studies of these species suggest that each is specialized to exploit a different food resource.

Competition between species is an interspecific interaction (that is, one that occurs between individuals of different species) that harms both players involved. Consequently, species tend to evolve in such a way as to avoid competition. When two competing species coexist in the same habitat, they tend to shift their niches in such a way as to overlap less. The niche that a species is able to exploit in the absence of any competitors is called its fundamental niche. The resources that are actually exploited by a species in a specific habitat represent its realized niche. The realized niche is always smaller than the fundamental niche and a subset of it.

If competition occurs over long periods of time, character displacement may occur. Character displacement describes a situation where two species are more morphologically different in habitats where they coexist than in habitats where they do not coexist. Character displacement is often interpreted as evidence for past (and perhaps continuing) competition. Character displacement has been observed among species of Anolis lizards as well as among the Galapagos finches.

see also Competition.

Jennifer Yeh

Bibliography

Futuyma, Douglas J. Evolutionary Biology. Sunderland, MA: Sinauer Associates, 1998.

Krebs, Charles J. Ecology: The Experimental Analysis of Distribution and Abundance. New York: Harper Collins College Publishers, 1994.

Pianka, Eric R. Evolutionary Ecology. New York: Addison Wesley Longman, 2000.

COMPETITIVE EXCLUSION PRINCIPLE

This principle defines the concept that when populations of two different species compete for the same limited resources, one species will use the resources more efficiently than its competitor. This provides the more efficient species with a reproductive edge, so that the second species will eventually be eliminated.