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competitive-exclusion principle (exclusion principle, Gause principle) The principle that no two species will occupy the same ecological niche; i.e. two or more resource-limited species, having identical patterns of resource use, cannot coexist in a stable environment because one species will be better adapted and will out-compete or otherwise eliminate the others. The concept was derived mathematically from the logistic equation by Lotka and Volterra, working independently, and was first demonstrated experimentally by G. F. Gause in 1934 using two closely related species of Paramecium. When grown separately, both species populations showed normal S-shaped growth curves; when grown together, one species was eliminated.