, the German naturalist and geographer Alexander von Humboldt observed during his explorations in South American jungles that species richness increases with increasing explored area, but noted that this relationship holds only up to a limit . This relationship between species richness and area for various taxa, such as birds and freshwater fishes, typically results in a rectangular hyperbola . When plotted on a logarithmic scale, this relationship becomes a straight line expressed by the equation log S = log C + Z log A . In this equation, S represents species richness while Z represents the slope of the line or regression coefficient .