A test of Metabolic Theory as the mechanism underlying broad-scale species-richness gradients

Aim To test whether the temperature dependence of individuals' metabolic rates is the mechanism shaping broad-scale species-richness gradients as proposed in the Metabolic Theory of Ecology recently proposed by Allen, Gillooly and Brown. Location North America, north of Mexico. Methods Metabolic Theory predicts that the natural logarithm of species richness will be a linear function of environmental temperature ((kT)(-1), where k is Boltzmann's constant and T is temperature in K) with a slope of -0.78. We tested these predictions using the broad-scale variation in richness of amphibians, reptiles, trees, tiger beetles, butterflies and blister beetles. We tested whether the temperature-richness relationship was linear or curvilinear, and determined the range of temperature values (and geographical area) where the instantaneous slope of the curvilinear temperature-richness relationship was statistically indistinguishable from -0.78, after correcting for spatial autocorrelation. Results We found that for all taxa, temperature-richness relationships were curvilinear. Moreover, for five of six taxa, the slope of this relationship was close to the predicted value for only a narrow range of temperatures. Blister beetles displayed the widest temperature range that is consistent with the Metabolic Theory, covering 45% of the study's geographical area. For the remaining taxa, the geographical range in which the slope is consistent with the predicted value amounts to only 10-20% of North America. Main conclusions For a wide array of taxa in North America, temperature-richness relationships deviate from the pattern predicted by Metabolic Theory. These results demonstrate that the temperature dependence of individuals' metabolic rates is not the sole cause of broad-scale diversity gradients. Even in areas where factors other than temperature do not influence productivity, the data do not suggest that richness patterns are determined by the temperature dependence of metabolic rate.