Whenever there are fewer resources than individuals seeking them, there can be competition. Competition is an interaction in which the use of a mutually needed resource by one individual or group of individuals lowers the availability of the resource for another individual or group. Different species of sea anemones compete for space in rocky shallows; swallows compete with one another for nesting holes in tree or cliffs; and many different species of dung beetles compete for animal droppings (some even cling to monkeys, waiting for feces to emerge). Males of many animal species commonly compete for female mates.

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In Chapter 46, we saw that in many populations the tension between intrinsic growth rate and environmental carrying capacity results in intraspecific competition, competition between individuals of a single species. The increase in competition between individuals of a species that accompanies increased population density is a main reason why population growth slows as the species’ environment approaches its carrying capacity.

Competition can also occur between individuals of different species, in which case it is interspecific competition. Whether competition occurs between individuals of one species or two, it is a lose–lose situation: Each side spends energy it would not spend in the absence of the other. Indeed, Darwin recognized that the “struggle for existence,” as he called it, is a primary driver for natural selection, leading to the evolution of traits that aid survival and reproduction by reducing competition in one way or another.