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The variety and variability among living organisms from all sources, including inter alia, terrestrial, marine and other aquatic ecosystems and the ecological complexes of which they are part; this includes diversity within species, between species and of ecosystems. Diversity indices are measures of richness (the number of species in a system) and, to some extent, evenness (variances of species’ local abundance). They are therefore indifferent to species substitutions that may, however, reflect ecosystem stresses (such as those due to high fishing intensity).

The variety of species in a community, sometimes expressed by various quantitative measures that reflect not only the total number of species present but also the degree of domination of the system by a small number of species.