“Two closely related species competing for the same resources cannot co-exist indefinitely when the resources are limiting.” This principle is known as

Gause’s Competitive Exclusion Principle states that two closely related species competing for the same resources cannot coexist indefinitely when the resources are limiting.
Competitive release is a phenomenon in which a species experiences reduced competition and an increase in the use of resources due to the removal or reduction of a competing species.
Character displacement refers to the evolution of phenotypic differences between species to lessen competition. Resource partitioning is the process by which different competing species use different portions or aspects of the same resource to reduce competition and coexist in the same habitat. This can be achieved in several ways, such as by using different parts of the same plant, feeding at different times of the day, or occupying different areas of the habitat.