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 turns out to be a rectangular hyperbola. 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 $\mathrm{A}=\mathrm{ }\mathrm{A}\mathrm{r}\mathrm{e}\mathrm{a}\mathrm{ }\mathrm{Z}\mathrm{ }=$ slope of the line (regression coefficient) $C = Y-$ intercept Ecologists have discovered that the value of Z lies in the range of $0.1$ to $0.2$ , regardless of the taxonomic group or the region (whether it is the plants in Britain, birds in California or molluscs in New York state). But, if you analyse the species-area relationships among very large areas like the entire continents, you will find that the slope of the line to be much steeper. For example, for frugivorous (fruit-eating) birds and mammals in the tropical forests of different continents, the slope is found to be $1.15$ . In this graph line 2 is steeper than line $1$ . Thus Z value is higher.