German naturalist and geographer Alexander von Humboldt observed that within a region species richness increased with increasing explored area, but only up to a limit. In fact, the relation between species richness and area for a wide variety of taxa (angiosperm plants, birds, bats, freshwater fishes) turns out to be a rectangular hyperbola (Figure15.2). On a logarithmic scale, the relationship is a straight line described by the equation
log S = log C + Z log A
where S= Species richness A= Area Z = slope of the line (regression coefficient) C = Y-intercept