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The competitive exclusion principle, sometimes referred to as Gause's Law of competitive exclusion or just Gause's Law, is a theory which states that two species competing for the same resources cannot stably coexist. Either of the two competitors will always have an advantage over the other that leads to extinction of the second competitor or an evolutionary shift of the inferior competitor towards a different ecological niche. As a consequence, competing related species often evolve distinguishing characteristics in areas in which they coexist. This aids in mate recognition, thus maintaining each species' superiority in exploiting slightly different ecological niches.

Contents 1 Experimental Basis 2 Prediction 3 Paradoxical traits 4 Re-definition 5 See also 6 References 7 Notes

Georgii Frantsevich Gause formulated the law of competitive exclusion based on laboratory competition experiments using two species of Paramecium. Following a lag phase, one species was able consistently to drive the other to extinction.

Competitive exclusion is predicted by a number of mathematical and theoretical models, such as the Lotka-Volterra models of competition. However, for reasons that are poorly understood, competitive exclusion is rarely observed in natural ecosystems, and many biological communities appear to violate Gause's Law. The best known example is the paradox of the plankton (or short diversity 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.

Recent studies that address some of the assumptions made for the models predicting competitive exclusion have shown that these assumptions need to be reconsidered. For example, 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. ^[1]

Ecology

• Gause, G. F. (1934): The struggle for existence. Baltimore, MD: Williams & Wilkins.
• Rastetter, E. B. & Ågren, G. I. (2002): Changes in individual allometry can lead to coexistence without niche separation. Ecosystems 5: 789-801.
• Vaurie, Charles (1950): Notes from the Walter Koelz Collections, Number 6. Notes on some Asiatic nuthatches and creepers. American Museum Novitates 1472: 1-39. PDF fulltext

1. ^ Rastetter & Ågren, 2002