Interspecific competition occurs when individuals of different species reduce one another's access to a limiting resource.

Competition can occur through exploitation, when species consume the same resource, or through interference, when one species directly prevents another from accessing it.

If two species depend on essentially the same limiting resource in the same way and conditions remain stable, one can eventually exclude the other. This is the competitive exclusion principle. It does not mean that similar species can never coexist; it means that complete niche overlap under persistent resource limitation is unstable.

Coexistence becomes more likely when species differ in how they use resources. Resource partitioning reduces niche overlap—for example, two bird species may feed on different parts of the same tree or at different times of day.

Example

Suppose species A and B both eat the same seeds. When alone, each population grows. Together, A is a better competitor for large seeds while B can use small seeds more efficiently. If both seed sizes are available, specialization can allow coexistence. If only large seeds remain, B may decline strongly.

The key idea is that competition is not merely 'organisms fighting.' It is a population-level consequence of shared dependence on limiting resources. Competitive outcomes depend on which resources limit growth, how strongly niches overlap and whether environmental variation creates opportunities for coexistence.