Competition

Darwin saw interspecific competition as a strong force in organic evolution.

Competition is not only between closely related species. Unrelated species can compete too: flamingoes that visit certain shallow lakes in South America compete with the fishes living there for zooplankton.

Resources need not be limiting. In interference competition, one species feeds less efficiently because the other is present, even when food and space are plentiful.

So competition is best defined by its effect: one species' fitness, measured as its r, drops significantly when the other species is around.

In laboratory experiments such as Gause's, the superior competitor eventually eliminates the other when resources are limited; evidence of this in nature is not always conclusive.

Circumstantial evidence: the Abingdon tortoise of the Galapagos Islands went extinct within a decade of goats being introduced, apparently because the goats browsed more efficiently.

Competitive release: a species confined to a small area by a superior competitor spreads widely when that competitor is removed. Connell showed that on the rocky coasts of Scotland the larger barnacle Balanus excludes the smaller Chthamalus from the intertidal zone.

Herbivores and plants seem to suffer more from competition than carnivores do.

Gause's Competitive Exclusion Principle: when two closely related species depend on the same resources, they cannot live together for ever, and the weaker competitor is eliminated. This may hold when resources are limiting, but not otherwise.

Species may instead evolve ways to co-exist, such as resource partitioning (feeding at different times or foraging differently). MacArthur showed five closely related warbler species co-existing on one tree through differences in foraging behaviour.