Aim Spatial patterns of phylogenetic diversity (PD) aid our ability to discern diversification rate mechanisms underlying hypotheses for the large-scale distribution of biodiversity. We develop a predictive framework for the way in which spatial patterns of PD vary with those of species richness, depending on the balance between speciation and extinction rates. Within this framework, diversification processes thought to underlie the productive energy, ambient energy, topographic variability and habitat variety hypotheses predict that gradients of increase in species richness will be associated with: (1) decreasing extinction rates where driven by productive energy, hence increasing relative PD (i.e. PD controlling for species richness, or PDrel); (2) a similar positive relationship between ambient energy and PDrel; (3) increasing speciation rates where driven by topographic variability, hence decreasing PDrel; and (4) no consistent relationship between PDrel and habitat variety when driven by the latter. We test these predictions using distributional data on parrots.
Location Neotropical, Afrotropical, Indo-Malayan and Australasian realms.
Methods Spatial models were used to test the predictions.
Results Globally, a positive association between productive energy and PDrel confirms prediction (1). However, within realms, hump-shaped relationships suggest the importance of decreasing extinction rates up to a threshold level of productive energy, and the increasing importance of speciation rates thereafter. Ambient energy is positively associated with PDrel in Australasia, Indo-Malaya, and globally, supporting prediction (2). However, this is driven by the coincidence of highest PDrel in areas of high ambient energy and intermediate productive energy (i.e. in seasonal tropical environments), which may be characterized by relatively low speciation and extinction rates. In the Neotropics, increasing topographic variability is associated with decreasing PDrel and increasing species richness, suggesting an increasing gradient of speciation, supporting prediction (3). Elsewhere, the signal of this mechanism may be obscured by collinearities with energy gradients. The lack of an overall relationship between habitat diversity and PDrel confirms prediction (4).
Main conclusions Spatial patterns of PDrel in relation to environmental gradients may be sensitive to collinearities among those gradients. Nevertheless, patterns emerge which have implications for the relative importance of speciation and extinction processes in generating latitudinal diversity gradients.