Latitude, productivity and species richness

Abstract

Aim: Global patterns in primary productivity in natural ecosystems are important for interpreting ecological processes and patterns of biodiversity. Net primary productivity (NPP) on land has long been thought to be greatest in tropical forests and to decrease towards the poles. However, it has recently been claimed that the NPP of mid‐latitude forests is as great as, or even greater than, that of tropical forests and that ecologically relevant productivity peaks at mid‐latitudes. Here we evaluate these hypotheses by testing for relationships between latitude and productivity using a range of forest productivity datasets. Location: Global. Methods: We apply ordinary least squares regression and t‐test analyses to published latitude–productivity data for forests, specifically updated to include an expanded dataset for the previously data‐poor tropics, and we evaluate the relationship between the primary productivity of forests and modelled vascular plant species richness. Results: Contrary to the recent claims, we found strong support for a negative relationship between latitude and annual NPP of forests with all datasets, and NPP was significantly greater in tropical forests than in temperate forests. Vascular plant richness was positively correlated with NPP. Main conclusions: NPP of forests increases towards the equator. Given that species richness also increases towards the equator, and that vascular plant richness correlates with NPP, these results are consistent with recent meta‐analyses showing that the relationships between productivity and species richness of both plants and animals in natural ecosystems are predominantly positive. These results are congruent with ecological theories that predict a positive relationship between species richness and productivity, and they indicate that there is no need to explain peaked richness–productivity relationships over broad spatial extents, since they do not appear to exist.