The Natural History Reader in Evolution

By Niles Eldredge | Go to book overview

9
Competition or Peaceful Coexistence?

JOHN A. WIENS

Examples of competition between species seem to abound in nature. A calliope hummingbird feeding at a nectar-rich flower in a mountain meadow is apt to be supplanted by a larger broad-tailed hummingbird, which, in turn, may be chased from the flower by a rufous hummingbird. Deer mice on islands that have no Meadow mice seem to occupy a broader range of habitats and exploit more types of foods than do deer mice in adjacent mainland areas where meadow mice are present. As one travels up a mountainside in western North America, the scrub jays common at lower elevations may suddenly disappear, replaced by ecologically similar Steller's jays.

These kinds of observations have led many ecologists to conclude that competition between species is commonplace and that it determines, to a great extent, how natural communities are put together. According to this view, species are likely to compete if they require similar resources, such as food, habitat, or breeding sites, and if those resources are in limited supply. This competition will lead to the exclusion of one species by another or, over evolutionary time, to divergence in the species' use of resources until competition is minimized. Competition thus limits the number and kinds of species that may coexist in an environment.

These views are by no means new. To Charles Darwin, competition between and within species was a fundamental component of the "struggle for existence." In On the Origin of Species, he noted: "We have reason to believe that species in a state of nature are limited in their ranles by the competition of other organic beings quite as much as, or more than, by adaptation to particular climates." Elsewhere in this work, Darwin provided examples of such competitive exclusion:

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