Mapping Biodiversity

Thus far, we have focused on the so-called organizational dimension of biodiversity (Wals and Van Weelie, 1998). This refers to variation in the genetic, biochemical, anatomical, or physiological composition of organisms, and to the population and species compositions of communities and ecosystems. However, when setting priorities for conservation, we often compare the diversity of species (or ecosystems and landscapes) across areas. That gives us an idea of how biodiversity is distributed across the earth.

Species-area Curves

A comparison of species richness relative to the area sampled (the species-area relationship) is one of the most important methods for quantifying the spatial distribution of biodiversity. This can be plotted (usually logarithmically) showing the number of species against area, and it gives a species area curve. Generally speaking, as you sample a larger area, you find more species. However, this species-area relationship can vary depending on whether one is sampling a small part of a single biota or a more extensive ecosystem or landscape. Four main species-area relationships are recognized:

1. Species-area relationships among tiny pieces of a single biota; below a certain area, there might not be a close correlation with species number.

2. Species-area relationships among large pieces of a single biota; larger areas are sampled than in (1), including more habitats.

3. Species-area relationships among islands of a single archipelago; larger areas are sampled than in (2), containing more habitats, and the species-area relationship is affected by immigration and extinction of species from the islands (see Island Bio-

geography, below).

4. Species-area relationships among biogeo-graphic provinces that have had separate evolutionary histories; the species-area relationship is affected by a higher rate of spe-ciation and lower rate of extinction than the more restricted island archipelagos in (3).

Species-area relationships are important insofar as they show that any decrease in available habitat area will also result in a decrease in the number of species that can be supported by that habitat. When human activity results in the fragmentation of habitats into isolated regions of reduced area, then we expect a parallel decrease in the biodiversity of these habitats.

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