What is the relationship between productivity and animal species richness? A critical review and meta-analysis
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Understanding spatial patterns of species diversity is a central goal in ecology. Species richness has often been shown to correlate strongly with ambient energy, available energy or primary productivity. Theories that invoke energy as an underlying factor driving species richness have received much attention. However, the relationship between species richness and energy is not always linear and can vary with scale. Here I present the results of a meta-analysis of published animal–productivity species richness relationships (A–PSRRs). Initially, 387 separate cases from 267 published studies were identified as potential tests of the A–PSRR. After critically assessing each study, 141 separate cases were accepted as robust tests of the A–PSRR, of which 112 had data available for re-analysis. Positive A–PSRRs were found to predominate at all scales (geographical extents and grains), in terrestrial and freshwater ecosystems and homeo- and poikilotherms. Marine ecosystems contrasted with the general patterns with unimodal relationships being the most common form of the A–PSRR. The results reported in the present study contrast with predictions that the true form of PSRR is unimodal, and with a previous review that found that no particular form of the A–PSRR was dominant. Importantly, the previous review has been criticised for its treatment of scale, surrogates for measuring productivity, relaxed criteria for including studies in the analyses and statistical methods. The present study addresses these issues and finds the contrast with the previous review of the A–PSRR is related largely to the statistical methods used for classifying relationships and, to a lesser degree, the use of strict study selection criteria in the present study. The predominance of positive A–PSRRs found in the present study compares with a recent review of the plant–PSRR which also reported that positive relationships predominate after addressing the issues of scale, surrogates, selection criteria and statistical methods. The results of the plant–PSRR and A–PSRR are consistent with evidence that a number of taxa (poikilotherms and homeotherms) have faster rates of molecular evolution in warmer and more productive environments.