Fundamental Causes of the Global Patterns of Species Range and Richness1

Abstract

Global patterns of species range and richness are a consequence of many interacting factors, including environmental conditions, competition, geographical area, and historical/evolutionary development. Two widely studied global patterns of distribution are the latitudinal and elevation gradients of species range and richness. The fundamental mechanisms by which environment and physiology of the plants themselves interact to generate global-scale correlations between increased species range or decreased species richness and latitude/elevation have not previously been established. This paper develops the hypothesis that the primary climatic variables determining global-scale gradients in ectotherm species range and richness are temperature (T) and temperature variability (ΔT), and that the primary physiological variable defining adaptation of ectotherms to temperature is respiratory energy metabolism. This hypothesis is based on a postulate that adaptation of ectotherms to latitudinal/altitudinal gradients of T and ΔT leads to corresponding gradients in properties of energy metabolism. The gradients of metabolic properties give rise to gradients of species range and richness that are observed on a global scale. We demonstrate that natural selection results in ectotherms with metabolic properties matched to their environment and that energy use efficiency and the temperature range allowing growth are inversely related. Thus, opposing selective pressures to increase metabolic energy-use-efficiency or to increase the probability of surviving climate extremes control adaptation of ectotherms to climate. The principles developed in this paper yield fundamental laws of ecology that allow calculation of the contributions of global temperature patterns to the formation of gradients of species range and diversity. Relative values of richness and range are calculated solely from data on abiotic variables. Predictions agree with known patterns of ectotherm distribution.