Reed Noss, "Indicators for Monitoring Biodiversity: A Hierarchial Approach", Conservation Biology 4(4):355-364. 1990.

Biodiversity is not simply the number of genes, species, ecosystems, or any other group of things in a defined area...A definition of biodiversity that is altogether simple, comprehensive, and fully operational (i.e. responsive to real- life management and regulatory questions) is unlikely to be found. More useful than a definition, perhaps, would be a characterization of biodiversity that identifies the major components at several levels of organization.

...(C)omposition, structure, and function...determine, and in fact constitute, the biodiversity of an area. Composition has to do with the identity and variety of elements in a collection, and includes species lists and measures of species diversity and genetic diversity. Structure is the physical organization or pattern of a system, from habitat complexity as measured within communities to the pattern of patches and other elements at a landscape scale. Function involves ecological and evolutionary processes, including gene flow, disturbances, and nutrient cycling.

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