• German naturalist and geographer, Alexander von Humboldt observed that within a region, the species richness increased utpo a limit with increased explored area. The relation between species richness and area is a rectangular hyperbola. On a logarithmic scale, it is straight line where Z is the slope of line (slope of regression).

• Z ranges from 0.1 to 0.2 for all taxa and regions except very large areas like continents.

• For larger areas, the slope of regression is steeper and Z ranges from 0.6 to 1.2.