Competitive exclusion principle

In community ecology, 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, if the ecological factors are constant. Either of the two competitors will always take over the other which leads to either the extinction of one of the competitors or its evolutionary or behavioural shift towards a different ecological niche.

Experimental basis
Russian ecologist Georgii Frantsevich Gause, formulated the law of competitive exclusion based on laboratory competition experiments using two species of Paramecium, the Paramecium aurelia and Paramecium caudatum. Following a lag phase, the Paramecium aurelia was consistently able to drive the other to extinction. The conditions were to add fresh water everyday and input a constant flow of food. On the other hand, Gause was able to let the Paramecium caudatum survive by driving differently the environmental parameters (food, water): that explains why the Gause law is valid only if the ecological factors are constant.

Prediction
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.

Paradoxical traits
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.

Re-definition
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.