What is Biodiversity?

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Biodiversity in generic use refers to the number of species, but more appropriately, the term biodiversity refers to the concept of total biological diversity, which includes not only the number of species, but also genetic variation and diversity within species, and habitat diversity (Krohne 2001; Bush 2003). Biodiversity is a measure of the structure of a community: the number of species; ‘who’ those species are; and the relative abundance of each species. A community is a group of species that interact, directly or indirectly, in a defined area. Most natural communities – those not disturbed by human activities – have relatively high species diversity; however, the number of species varies widely from community to community. Community ecologists study patterns of biodiversity and what factors affect biodiversity.


Measures of Species Diversity

Species diversity includes measures of species abundance and evenness. Species richness is the number of species in a community. Species richness does not take into account the number of individuals of each species. Evenness is a measure of the degree to which the total number of individuals (of all species) is distributed among the species in a community. A community with 10 species in which the total number of individuals in the community is evenly distributed among the 10 species would have greater evenness than another community of 10 species in which the most of the individuals are disproportionately distributed in just a few species.

The most commonly used indices of species diversity are Simpson’s index and Shannon-Wiener index (also known as the Shannon index, and the Shannon-Weaver index in ecological literature); both incorporate species abundance and relative abundance (evenness). Simpson’s index (D) is calculated by D = 1/Σpi2, where pi is the proportion of the ith species. The Shannon-Wiener index (H') is calculated by H' = − Σ (pi × lnpi), where pi is the frequency of the ith species and ln is the natural log.


A large number of hypotheses have been proposed to explain observed patterns of biodiversity. These hypotheses include mechanisms related to:

– latitude (includes climate)

– area (size; contiguousness; isolation)

– structural heterogeneity (vegetation structure; substratum; topography)

– predation

– competition

– primary production

– disturbance (magnitude/severity; frequency)

– time (evolutionary and ecological)


Literature Cited

Bush, M.B. 2003. Ecology of a Changing Planet, Third Edition. Prentice-Hall, Inc. Upper Saddle River, New Jersey 07458.

Krohne, D.T. 2001. General Ecology, Second Edition. Brooks/Cole. 511 Forest Lodge Road, Pacific Grove, California 93950.

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