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In community ecology, the competitive exclusion principle, sometimes referred to as Gause's Law of competitive exclusion or just Gause's Law, is a theory which states that two species competing for the same resources cannot stably coexist, if the ecological factors are constant. Either of the two competitors will always take over the other which leads to either the extinction of one of the competitors or its evolutionary or behavioural shift towards a different ecological niche.

Niche - one species will outcompete the other excluding it from the niche. Like the guy above says if the excluded one can't adapt to another niche in the environment it won't survive.