Revisiting Rapoport’s rule and its relationship to the latitudinal diversity gradient in tree species

Background/Question/Methods The increase of range size in species at higher latitudes - “Rapoport’s rule” - is among the most discussed of macroecological patterns, partly because it may be involved in latitudinal gradients in species diversity. The large majority of studies investigating Rapoport’s rule, however, have used only a weak unidimensional proxy for range size, the latitudinal extent of occurrence. Considering actual geographic range area, we both revisit Rapoport’s rule and investigate the association between range size and species richness with a focus on North American trees. To do so we used both cross-species analysis and phylogenetic independent contrasts. Regarding species range sizes and the diversity gradient, we also tested whether null models may generate a pattern similar to the observed one.

Results/Conclusions We show that Rapoport’s rule applies to North American tree species (i.e. those species ranging anywhere from Canada to Central America, n=599), and we checked that this pattern is not affected by phylogenetic relatedness among species. Focusing on the entire flora of tree species in the North American temperate zone (33°N to 74°N, n=461), we further show that tree species richness and mean species range area are negatively covariant across latitudes. Our results suggest the possibility that the causes of the increase in range area at higher latitudes may in turn underpin the decreasing latitudinal trend in species diversity as well. More generally, this study opens the possibility of discovering robust laws about variation in species range area and species diversity that are founded on known causes rooted in species-environment interactions.