Competitive exclusion principle

The competitive exclusion principle, sometimes referred to as Gause's Law of competitive exclusion or just Gause's Law, states that two species that compete for the exact same resources cannot stably coexist. One of the two competitors will always have an ever so slight advantage over the other that leads to extinction of the second competitor in the long run (in a hypothetical non-evolving system) or (in the real world) to an evolutionary shift of the inferior competitor towards a different ecological niche. As a consequence, competing related species often evolve distinguishing characteristics in areas where they both coexist. This aids in mate recognition, thus maintaining each species' superiority in exploiting slightly different ecological niches. The Western and Eastern Rock Nuthatches are a famous example; in areas where they do not coexist, they are virtually indistinguishable, but in areas where both species occur, they have formed local subspecies in which the otherwise slight differences are prominent to the extent that the species to which an individual belongs can be readily determined even at a glimpse (Vaurie, 1950); for a thorough discussion, see niche differentiation.

The competitive exclusion principle is a theoretical concept that follows from abstract mathematical modeling. The conditions under which competitive exclusion must hold are not very well understood; several natural ecosystems are known in which competitive exclusion seems to be violated. The best known example is the paradox of the plankton (or short plankton paradox): All plankton species live on a very limited number of resources, primarily solar energy and minerals that are dissolved in the water. According to the competitive exclusion principle, only a small number of plankton species should be able to coexist on these resources. Nevertheless, large numbers of plankton species coexist within small regions of open sea.

A partial solution to the paradox lies in raising the dimensionality of the system. Spatial heterogeneity, multiple resource competition, competition-colonization trade-offs, and lag prevent exclusion (ignoring stochastic extinction over longer time-frames). However, such systems tend to be analytically intractable. In addition, many can theoretically support an unlimited number of species. A new paradox is created: Most well-known models that allow for stable coexistence allow for unlimited number of species to coexist, yet in nature, any community contains just a handful of species.

Perhaps most popular of Gauss's experiments involving the Law of competitive exclusion was that concerning 2 species of paramicium. Following a lag phase, one species was able to consistently drive the other to extinction.

Recent studies that address some of the assumptions made for the models predicting competitive exclusion have shown that these assumptions need to be reconsidered. E.g. a slight modification of the assumption of how growth and body size are related leads to a different conclusion, namely that for a given ecosystem a certain range of species may coexist while others become outcompeted (Rastetter & Ågren, 2002).

See also: Ecology