Species-Area relationships

The German naturalist and geographer Alexander von Humboldt, exploring the South American jungles, observed that within a region species richness increases as area increases, but only up to a certain limit — the relationship between species richness and area holds across very different taxa, including angiosperm plants, birds, bats and freshwater fishes, and takes the shape of a rectangular hyperbola. Plotted on a logarithmic scale instead, this curved relationship straightens into a line described by the equation log S = log C + Z log A (equivalently S = C·A^Z), where S is species richness, A is area, C is the Y-intercept, and Z is the slope of the line — also called the regression coefficient (Fig. 12.2). Across most taxonomic groups and regions, Z typically falls between 0.1 and 0.2, largely independent of which group or region is being measured. The relationship steepens sharply, however, once very large areas such as entire continents are considered — there Z rises to somewhere between 0.6 and 1.2; for frugivorous (fruit-eating) birds and mammals studied across the tropical forests of different continents, for instance, the slope works out to a notably steep 1.15. This species-area relationship gives ecologists a quantitative tool for predicting how much biodiversity a habit …