or biodiversity, the number of species in a given habitat. Scientists have variously estimated that there are from 3 to 30 million extant species, of which 2.5 million have been classified, including 900,000 insects, 41,000 vertebrates, and 250,000 plants; the remainder are invertebrates, fungi, algae, and microorganisms. Although other species remain to be discovered, many are becoming extinct through deforestation, pollution, and human settlement. Much of this diversity is found in the world's tropical areas, particularly in the forest regions. A habitat in equilibrium has a balance between the number of species present and its resources. Diversity is affected by resources, productivity, and climate. The more pristine a diverse habitat, the better chance it has to survive a change or threat—either natural or human—because that change can be balanced by an adjustment elsewhere in the community; damaged habitats may be destroyed by breaking the food chain with removal of a single species. Thus, biological diversity helps prevent extinction of species and helps preserve the balance of nature. At the 1992 United Nations Conference on Environment and Development, more than 150 nations signed a treaty intended to protect the planet's biological diversity. See also ecology.

See E. O. Wilson, ed., Biological Diversity (1988); N. Eldredge, Life in the Balance (1998).

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