The Latitudinal Gradient in Geographical Range: How so Many Species Coexist in the Tropics

Abstract

The latitudinal gradient in species richness is paralleled by a latitudinal gradient in geographical-range size called Rapoport's rule. It is suggested that the greater annual range of climatic conditions to which individuals in high-latitude environments are exposed relative to what low-latitude organisms face has favored the evolution of broad climatic tolerances in high-latitude species. This broad tolerance of individuals from high latitudes has led to wider latitudinal extent in the geographical range of high-latitude species than of lower-latitude species. The existence of Rapoport's rule suggests yet another way of looking at the latitudinal gradient in species richness. If low-latitude species typically have narrower environmental tolerances than high-latitude species, then equal dispersal abilities in the two groups would place more tropical organisms out of their preferred habitat than higher-latitude species out of their preferred habitat. It is hypothesized that a larger number of "accidentals" (i.e., species that are poorly suited for the habitat) occur in tropical assemblages. The constant input of these accidentals artificially inflates species numbers and inhibits competitive exclusion.