A Sideways Look at Patterns in Species Richness, or Why There Are So Few Species Outside the Tropics

Abstract

What causes the widely observed latitudinal gradients in species richness remains one of the key unanswered questions in ecology. Despite numerous hypotheses generating a wealth of tests, no consensus has yet been reached. We do not add to the list either of tests or of hypotheses, but instead examine how some of the implicit (and occasionally explicit) assumptions of most research into the problem may be hindering its resolution. We discuss four ideas that could aid progress in this regard. First, we observe that view depends on viewpoint, and that altering the latter often leads to changes in the former. The most common view of richness gradients is that the tropics are unusually species rich. Viewing the richness of the tropics as the norm causes more than a simple rephrasing of the question: rather, it alters the perspective on other assumptions about richness gradients. Second, we suggest that the latitudinal gradient in species richness may actually arise from more than one process. The amount of energy flowing into a region must set an upper bound on the amount of the raw material of life the region can support. However, this raw material can be viewed as first being converted into individuals, and these individuals as subsequently distributed into species. Both processes will contribute to the determination of how many species inhabit a region. Third, we argue that the species richness of a region will not be independent of the mean body size and mean abundance (density) of the species, because all these variables depend on division of the same basic raw material. The energy entering a region may be viewed as being allocated to the number, abundance and size of species, but the allocation to each will not be independent. Spatial variation in mean abundance and mean body size will interact with spatial patterns in species richness. Fourth, we argue that, given the second and third points, it is unlikely that a single cause for species richness gradients will be identified. Effectively, this amounts to trying to give one answer to two quite different questions. © 1996 Blackwell Science Ltd.