Latitudinal biodiversity gradients at three levels: Linking species richness, population richness and genetic diversity

Motivation Theory describing biodiversity gradients has focused on species richness with less conceptual synthesis outlining expectations for intraspecific diversity gradients, that is, broad-scale population richness and genetic diversity. Consequently, there is a need for a diversity-gradient synthesis that complements species richness with population richness and genetic diversity. Review methods Species and population richness are the number of different species or populations in an area, respectively. Population richness can be totalled across species, within a species or averaged across species. Genetic diversity within populations can be summed or averaged across all species in an area or be averaged across an individual species. Using these definitions, we apply historical, ecological and evolutionary frameworks of species richness gradients to formulate predictions for intraspecific diversity gradients. Review conclusions All frameworks suggest higher average population richness at high latitudes, but similar total population richness across latitudes. Predictions for genetic diversity patterns across species are not consistent across frameworks and latitudes. New analysis methods Species range size tends to increase with latitude, so we used empirical data from c. 900 vertebrate species to test hypotheses relating species range size and richness to population richness and genetic diversity. New analysis conclusions Species range size was positively associated with its population richness but not with species-specific genetic diversity. Furthermore, a positive linear relationship was supported between species richness and total population richness, but only weakly for average population richness. Overall conclusion Through the lens of species richness theories, our synthesis identifies an uncoupling between species richness, population richness and genetic diversity in many instances due to historical and contemporary factors. Range size and taxonomic differences appear to play a large role in moderating intraspecific diversity gradients. We encourage further analyses to jointly assess diversity-gradient theory at species, population and genetic levels towards better understanding Earth's biodiversity distribution and refining biodiversity conservation.