Competitive exclusion

Competitive exclusion is the ecological principle that two species competing for the same limiting resource cannot coexist indefinitely at constant population densities. One species will always outcompete the other, driving it to local extinction. The principle was most rigorously demonstrated by Georgii Gause in his 1934 experiments with Paramecium, and it underlies the classical theory of ecological succession: in the absence of disturbance, competitive dominants progressively eliminate subordinates until the community reaches a low-diversity equilibrium.

The competitive exclusion principle is the engine that makes the intermediate disturbance hypothesis necessary. Without competitive exclusion, disturbance would not be required to maintain diversity; species would coexist through niche partitioning and niche differentiation regardless of perturbation regime. The fact that they do not — that diversity collapses in undisturbed systems — is direct evidence that exclusion is the default and coexistence is the exception requiring explanation.