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. Dirzo, E. Mendoza, in Encyclopedia of Ecology, 2008

Biodiversity – the constellation of plants, animals, fungi, and microorganisms on Earth; their genetic variation; and the communities and ecosystems of which they are a part – is a central component of Earth’s life support systems. Biodiversity encompasses several facets, including genetic diversity, endemism, diversity of functional groups, and agro-biodiversity, but these are seldom considered. The best information, on species diversity, provides some evidence about its possible magnitude and distribution. The time course of species of terrestrial organisms and the number of families of marine animals in the fossil record indicate that biodiversity has been increasing, with a pinnacle in the present era. Although some studies suggest that global species diversity may be as high as 100 million, the consensual estimate for eukaryotic organisms lies in the 5–15 million range, with a best guess of 7 million. Biodiversity exhibits its highest concentration of species in the Tropics. The distribution of individuals among species provides an additional measure of diversity. Several metrics show how localities with a more even distribution of individuals across species are ecologically more diverse. Biodiversity, manifest as the functioning of ecosystems, provides a variety of ecological services to societies, underscoring the need to understand, protect, and wisely manage biodiversity.