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Competition occurs when two organisms use the same limited resource. Interspecific competition occurs between species; intraspecific competition occurs within species. Outcomes range from competitive exclusion (one species eliminates another) to coexistence through niche differentiation. The competitive exclusion principle states that two species cannot indefinitely occupy identical niches.
From your study of the niche concept, you know that every species occupies a fundamental niche — the full range of conditions and resources it could theoretically exploit — and a realized niche — the subset it actually occupies given interactions with other species. Competition is the interaction that most directly shapes the gap between the two. Whenever two organisms need the same limited resource — food, light, nesting sites, territory — using it reduces what is available for the other, and both pay a fitness cost.
Intraspecific competition (within a species) is often the most intense form because conspecifics have nearly identical resource requirements. Every deer in a forest eats the same browse, occupies the same type of habitat, and seeks the same mates. As population density rises, per capita resource availability drops, reproduction slows, and mortality increases — this is the density-dependent regulation that keeps populations near carrying capacity. Interspecific competition (between species) is often less intense because different species rarely overlap completely in their needs, but it can still powerfully shape community structure. Two species of warblers feeding on insects in the same tree may compete strongly if they forage in the same canopy zone, or weakly if one specializes on trunk bark and the other on outer branch tips.
The competitive exclusion principle, formulated by G.F. Gause from laboratory experiments with *Paramecium*, states that two species with identical niches cannot coexist indefinitely — one will always outcompete and eliminate the other. In Gause's experiments, when two *Paramecium* species were grown together on the same food source, one consistently drove the other to extinction. But nature is full of apparently similar species living side by side, which seems to contradict the principle. The resolution is niche differentiation (also called niche partitioning): coexisting species evolve or behaviorally adjust to reduce overlap in resource use. The classic example is Robert MacArthur's study of five warbler species in New England spruce forests — all ate insects, but each foraged in a different zone of the tree, from the crown to the base, dividing the resource finely enough to coexist.
Competition can take two forms mechanistically. In exploitation competition, organisms compete indirectly by depleting a shared resource — neither interacts with the other directly, but each suffers because the other reduces resource availability. In interference competition, organisms interact directly through aggression, territoriality, or chemical inhibition — think of male elk fighting for mating access or allelopathic plants releasing toxins to suppress neighbors. The outcome of competition depends on the degree of niche overlap, the relative competitive abilities of the species, and environmental variability. In some cases, competition drives character displacement: sympatric populations of competing species evolve greater morphological differences than allopatric populations, further reducing niche overlap. Understanding competition is essential for predicting community composition, interpreting species distributions, and managing ecosystems where invasive species threaten natives through competitive dominance.