Alexander von Humboldt derived the species-area relationship and observed that within a region, species richness increased with the increasing explored area, but only up to a limit.
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.
The value of Z lies in the range of 0.1 to 0.2, regardless of the taxonomic group or the region. If we analyze the species-area relationships among very large areas like the entire continents, the slope of the line will be much steeper (Z values in the range of 0.6 to 1.2). For frugivorous (fruit-eating) birds and mammals in the tropical forests of different continents, the slope is found to be 1.15.