Can altitudinal diversity gradients be explained by a reduction in area with altitude?

Abstract

Species richness at higher altitudes of a region typically decreases with altitude, the usual explanation being that environmental conditions become harsher as altitude increases. On conical or ridge shaped mountains the surface area available within equally spaced altitude bands declines as altitude increases. It has been suggested (Ogden 1995) that this may be responsible for the decrease in species richness. The phenomenon of decreasing species richness with altitude has been further interpreted by Ogden as lending support to the equilibrium theory of island biogeography (MacArthur and Wilson 1967) which predicts that larger areas will contain more species. The hypothesis that a decrease in area is responsible for the decline in diversity with altitude is here considered critically, and the evidence presented in support of it is found to be lacking.