The correlation between the number of species and the size of the area is not linear. Rather, it’s a curve that slopes in a predictable way. Usually, the relationship is expressed by the formula S = cAz, where S is the number of species, A is the size of the area, and c and z are constants that vary according to the region and taxonomic group under consideration (and hence are not really constants in the usual sense of the term). The relationship counts as a rule because the ratio holds no matter what the terrain.
