German naturalist and geographer Alexander von Humboldt proposed graph given below .Identify  A in the graph ?

Species richness.

German naturalist and geographer 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 (angiosperm plants, birds, bats, freshwater fishes) 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.