Competitive Exclusion Principle
Two species competing for the exact same limited resource cannot stably coexist.
When one species has even the slightest advantage over another, the one with the advantage will dominate in the long term.
Two species requiring approximately the same resources are not likely to remain long evenly balanced in numbers in the same habitat.
Grinnell, 1915
This leads either to the extinction of the weaker competitor or to an evolutionary or behavioral shift toward a different ecological niche. If two competing species coexist in a stable environment, they do so as a result of differentiation of their niche (i.e. they may consume different foods, or use different areas of the environment).
A solution…
Niche partitioning: This is where the niche is divided, so all competing species use the environment differently in such a way that helps them to coexist and therefore survive.