Universality in biodiversity patterns: causes of the variation in species-temperature and species-productivity relationships

Temperature and productivity appear as universal positive correlates of species richness. However, the strength and the shape of species-temperature (STR) and species-productivity (SPR) relationships vary widely, and the causes of this variation are poorly known. We analysed (1) published species richness data for multiple taxa sampled in various regions and (2) different clades within vertebrate classes globally, to test for the effects of spatial scale and characteristics of examined taxa and regions on the strength and direction of STRs and SPRs. Up to 55% of the variation in the strength and direction of STRs and SPRs can be explained by the variables tested, although there are striking differences in the variation of the relationships among types of data, between ectotherms and endotherms and also between STRs and SPRs. When focusing on multiple taxa sampled in different regions, only the variables concerning examined region affect the strength of the relationships: STRs are stronger in colder regions, while SPRs are stronger in warmer regions, and both relationships are strong within regions in which temperature and productivity are positively correlated. Additionally, STRs are stronger in regions with wider range of temperature. When focusing on vertebrate clades, there is additional effect of clade size: the relationships are stronger for larger clades. Moreover, clade size is the only variable consistently affecting the strength of SPRs for endotherms. The finding that both the relationships are weak in regions where temperature and productivity are not correlated indicates that both temperature and productivity simultaneously affect spatial variation in species richness. However, species richness appears to be more universally driven by productivity than temperature, as SPRs are stronger, do not vary as much as STRs, and this variation is not tightly linked to the examined factors.