The principle of competitive exclusion (also known as the principle of competitive exclusion) in ecology means that two species cannot survive in the same ecological niche at the same time, or for a long time. This is because there is competition between the two, resulting in one of them winning and being able to remain in its original ecological niche. In order to continue to survive, the other side will change its habitat, or change its diet, or change its habits (e.g., change its diurnal behavior to nocturnal, to achieve temporal separation from the winning side), and evolve adaptations in order to continue to live. In other words, two species with similar ecological habits cannot occupy the same ecological niche at the same time. Complete extinction of the loser side is relatively rare. These changes are also contributing to adaptive radiation: the process of evolving from primitive generalist species to diverse, specialized species, each adapted to a unique way of life.

Gause (Gause) used the protozoans Dinoflagellate and Macroflagellate as competitors to observe the results of competition between these two species, which are close in taxonomy and ecological habits. When cultured separately in yeast medium, dinucleate tussock worms grew faster than macro tussock worms. When both species were added to the same culture vessel, the dinucleate grasshopper dominated, and eventually the large grasshopper died and disappeared.