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.

The nature of the relationship between productivity and species richness has remained controversial for at least two decades. Recently authors have favored the suggestion that the form of this relationship is highly variable and scale dependent. However, this conclusion is not universally accepted. Here we present the results of a meta-analysis of animal productivity-species-richness relationships (PSRR) in terrestrial and freshwater ecosystems. Initially, 374 separate cases from 273 published studies were identified as potential tests of the animal PSRR. After critically assessing each study, 115 cases were accepted as robust tests of the relationship, and of these 95 had data available for formal meta-analysis. Contrary to expectation, we found no support for the form of the relationship being scale dependent; positive relationships predominated at all scales (geographical extents and grains). Furthermore, positive relationships were the most common form of the animal PSRR in both terrestrial and freshwater ecosystems and among vertebrates, invertebrates, homeotherms and poikilotherms. Therefore, our results also contrast with previous reviews that suggest no particular form of the PSRR is predominant. We demonstrate that the method used for classifying the form of PSRRs is critical to the result and that previous reviews may have been too liberal toward classifying the form of relationships as unimodal. The tendency for positive relationships between productivity and species richness across diverse animal taxa has important implications for understanding the mechanisms behind the latitudinal gradient in species richness.

Slower genetic evolution in microbial thermophiles has been attributed to internal mutation control mechanisms in very high temperatures, whereas the tempo of plant microevolution has been positively correlated to ambient thermal conditions. Here, using a global dataset of 136 teleost fish species, contrasting warm and cool water species, and controlling for any differences between species in mutation control mechanisms, we found mitochondrial genetic evolution was 1.61 times faster in warm water species. These results suggest that temperature-mediated reduction in mutation rate is only important in extreme thermal regimes.

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