Speciesarea relationships

The number of species generally increases with the area sampled, as has been well documented using collections from such discrete units as islands (MacArthur and Wilson 1967) and lakes (Barbour and Brown 1974). The relationship typically is log-linear, according to the equation:

where S is the species richness, A is the area of habitat, and c and z are parameters determined from the data. The slope parameter z quantifies the rate of increase in number of species with area surveyed, and frequently falls between 0.2 and 0.4. Several studies document that Equation 10.1 applies to rivers, although estimates of z vary widely. Freshwater mussels fall well within the expected range, with z = 0.32 (Figure 10.1).

The principal mechanism underlying the species-area relationship probably is that larger rivers harbor greater habitat diversity due to their

Regional Patterns in Species Diversity

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