When two populations compete for the same limited, vital resource, one will always outcompete the other and thus bring about the local extinction of the latter species.
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. Either of the two competitors will always have an advantage over the other that leads to extinction of the second competitor or an evolutionary shift of the inferior competitor towards a different ecological niche. As a consequence, competing related species often evolve distinguishing characteristics in areas in which they coexist.