An ecological principle stating that two species that compete for exactly the same limiting resources in the same niche cannot stably coexist indefinitely; one species will outcompete and exclude the other under constant conditions.

Principle

Stable coexistence requires niche differentiation (differences in resource use, space, time, or other axes); perfect niche overlap for the same limiting resource leads to competitive displacement in a constant environment.

Demonstration

A laboratory experiment in which two species supplied with one limiting resource and homogeneous conditions result in one species' population declining to extinction while the other persists; field examples where resource partitioning accompanies coexistence.

Misapplication

Using the principle to predict exclusion in highly heterogeneous, fluctuating, spatially structured, or predator‑mediated systems where coexistence mechanisms (spatial refuges, temporal variation, trade-offs) allow multiple species to persist.

Consequence

Motivates study of niche differentiation, resource partitioning, and mechanisms maintaining biodiversity; informs management when invasive species share identical resource niches with natives.

Reversal

Under temporal environmental variation, spatial heterogeneity, strong frequency-dependent interactions, or when non-equilibrium dynamics operate, species with similar resource requirements can coexist without exclusion.

Boundary

Refers to competition for identical limiting resources in the same niche under constant conditions; excludes coexistence facilitated by environmental variability, habitat structure, predation, mutualisms, or non-resource limiting factors.

Semantic Tension

Tension between the idealized exclusion outcome in homogeneous, constant systems and abundant empirical cases of coexistence maintained by heterogeneity, trade-offs, and dynamic processes.

Synthesis

The competitive exclusion principle states that without niche differences or mitigating mechanisms, perfect competitors for the same limiting resource cannot persist together; in real ecosystems, coexistence typically requires spatial, temporal or functional differentiation or other stabilizing processes.