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.
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.
See also: Ecology