In general, mutualistic associations expand the range of conditions beyond those in which either species could live alone, whereas parasitism tends to restrict the range of conditions under which the host can live. Consequently, associations have strongly influenced the distribution and abundance of organisms, and indeed the basic structure of ecosystems. Mutualistic associations have allowed associated species to dominate their environments, and to colonize new environments that would be unsuitable for either species alone.
The evolutionary diversification and dominance of terrestrial ecosystems by flowering plants has been aided by several such associations. Insects and other animals pollinate flowers much more efficiently than does the wind, and the specificity of their behavior has fostered reproductive isolation and diversification of plant species. The roots of most terrestrial plants are intimately associated with mutualistic fungi (mycorrhizae) that enhance their uptake of nutrients from the soil. One of the most ecologically important mutualisms between plants and microbes involves the fixation of organic nitrogen by bacteria (rhi-zobia) in the roots of legumes. This association introduces new usable nitrogen into soils, is important in plant succession and community organization, and provides a source of limiting nitrogen to grazing livestock and other animals. Reef-building corals similarly thrive in extremely nutrient-poor ocean waters, and form the foundation of the most diverse marine ecosystems on Earth, as a result of the tight recycling of nutrients by their endosymbiotic algae.
Mutualisms have also fostered dominance of certain mobile consumers. Leafcutter ants are the major herbivores of the New World tropics because their elaborate farming of fungi on harvested foliage allows them to use virtually any plant species as nourishment, in stark contrast to most herbivorous insects, which use one or a few closely related plant species as food. In lower termites, symbiotic bacteria and protists within the gut produce cellulases that allow them to feed on one of the poorest food sources in the world, wood. Similarly, the mutualis-tic gut flora of ruminant grazers such as wildebeest and bison contribute to their high densities despite relatively poor nutritional quality of their forage. These densities in turn give large grazing vertebrates a fundamental role in determining the structure and functioning of their ecosystems.
Human welfare depends critically on association with certain beneficial organisms, and is threatened by its associations with others. Over the last few thousand years, various crop plants, livestock, and household animals have been domesticated as humans selectively bred them for characteristics we consider desirable. The food security and physical power provided by our association with these domestic organisms has been central to making humans the most abundant and powerful species in Earth's history. Remarkably, similar agricultural associations have also evolved in several groups of insects, helping them dominate their environments.
See also: Coevolution; Community.
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