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...