Biodiversity

Easures and Maintenance of Biodiversity

There are various means of measuring and conceptualizing biodiversity, and each possesses its own merit depending on the objective of the research. Alpha diversity refers to the number of species within a particular area or plot. In a 10-meter × 10-meter plot in a chaparral plant community, the alpha diversity of shrubs is simply a compilation of the number of shrub species encountered in the area. Beta diversity, on the other hand, refers to the change in species from one plot or area to another in an ecosystem. Also known as species turnover, beta diversity reveals much more about underlying diversity than simply a numbers count. For example, in a 1-ha (hectare) coniferous forest plot, a total of seven species of trees are identified. In a second 1-ha plot a few kilometers away, seven coniferous tree species are also recorded. However, six of the seven species encountered in the first plot are different from those encountered in the second plot. Thus, whereas the alpha diversity of the two plots is equal, the high beta diversity (difference in species composition between the two plots) suggests that the overall diversity of the ecosystem is quite high. A third type of diversity is referred to as gamma or landscape-level diversity. This is similar to beta diversity but refers to the total biodiversity of the various communities within a landscape. In this case, for example, we could be measuring the biotic diversity of the Columbia River watershed. Finally, epsilon diversity describes species richness at the broadest possible scale, that is, a region that encompasses many landscape types.

An important dimension of biodiversity at the local scale is community evenness. In this case, the issue is not only the number of species but also the number of individuals represented by each of the respective species. Two of the traditional means of measuring this feature are the Simpson and the Shannon-Weiner Indices. For example, two tropical forest plots are found to have equal alpha diversities, 25 tree species each, and an equal total number of trees, 100. However, in one plot, the 100 trees are equally divided among the 25 species, with 4 trees in each species. In the second plot, one species is represented by 76 individual trees, whereas the other 24 species are represented by only 1 tree each. The biodiversity of the first plot, as measured by community evenness, is thus much higher than the second plot.

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