(N/A) The German naturalist and geographer Alexander von Humboldt observed during his pioneering and extensive explorations in the wilderness of South American jungles that within a region, species richness increases with increasing explored area, but only up to a limit.
The relation between species richness and area for a wide variety of taxa (angiosperm plants, birds, bats, freshwater fishes, etc.) 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
$A = $ Area
$Z = $ 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 (e.g., plants in Britain, birds in California, molluscs in New York).
However, if you analyze the species-area relationships among very large areas like entire continents, you will find that the slope of the regression line is much steeper ($Z$ values in the range of $0.6$ to $1.2$). For example, for frugivorous birds and mammals in the tropical forests of different continents, the slope is found to be $1.15$.