Also known as the competitive exclusion principle, this refers to the proposition that the populations of two competing species cannot remain at stable levels over time.

When two species compete for control over a limited amount of resources, the dominant species will take advantage over its weak competitor. This will cause the weaker species to get excluded from its previous territory and its population to drop over time.

The law is named after Soviet biologist Georgii Gause although it was formulated first by American biologist Joseph Grinnell in his 1904 paper “The Origin and Distribution of the Chestnut-Backed Chickadee”.In cases where no extinction occurs, this is because adaptation to slightly different niches takes place. Ecological communities are assembled by species evolutionary differentiation and progressive adaptation to different nichesCore ideas in community ecology, such as adaptation, niche differentiation and limiting similarity, all rely on this principle.

Classical coexistence theory implicitly assumes that the strength of competition correlates positively with the degree of similarity between competing species. When one species has even the slightest advantage over another, the one with the advantage will dominate in the long term. This leads either to the extinction of the weaker competitor or to an evolutionary or behavioral shift toward a different ecological niche.

Gause’s law is valid only if the ecological factors are constant. The principle has been paraphrased in the maxim “complete competitors cannot coexist”

The competition may result in

1) Adaptive radiation of one or both species restricting them to separate niche and minimising competition.

2) Within the same or overlapping niches, equilibrium situation may be reached where one of the competitors declines in numbers to the point of extinction.

Gause law speaks of parallel evolution in terms of structural adaptations but not in terms of physiological, protective animal association, biotic and organic adaptations.