The competitive exclusion principle, sometimes referred to as Gause's
Law of competitive exclusion or just Gause's Law, states that two
species that compete for the exact same resources cannot stably coexist.
One of the two competitors will always have an ever so slight
advantage over the other that leads to extinction of the second
competitor in the long run (in a hypothetical non-evolving system) or
(in the real world) to an evolutionary shift of the inferior competitor
towards a different ecological niche.