Global patterns in the divergence between phylogenetic diversity and species richness in terrestrial birds

Aim The conservation value of sites is often based on species richness (SR). However, metrics of phylogenetic diversity (PD) reflect a community's evolutionary potential and reveal the potential for additional conservation value above that based purely on SR. Although PD is typically correlated with SR, localized differences in this relationship have been found in different taxa. Here, we explore geographical variation in global avian PD. We identify where PD is higher or lower than expected (from SR) and explore correlates of those differences, to find communities with high irreplaceability, in terms of the uniqueness of evolutionary histories. Location Global terrestrial. Methods Using comprehensive avian phylogenies and global distributional data for all extant birds, we calculated SR and Faith's PD, a widely applied measure of community PD, across the terrestrial world. We modelled the relationship between avian PD for terrestrial birds and its potential environmental correlates. Analyses were conducted at a global scale and also for individual biogeographical realms. Potential explanatory variables of PD included SR, long-term climate stability, climatic diversity (using altitudinal range as a proxy), habitat diversity and proximity to neighbouring realms. Results We identified areas of high and low relative PD (rPD; PD relative to that expected given SR). Areas of high rPD were associated with deserts and islands, while areas of low rPD were associated with historical glaciation. Our results suggest that rPD is correlated with different environmental variables in different parts of the world. Main conclusions There is geographical variation in avian rPD, much of which can be explained by putative drivers. However, the importance of these drivers shows pronounced regional variation. Moreover, the variation in avian rPD differs substantially from patterns found for mammals and amphibians. We suggest that PD adds additional insights about the irreplaceability of communities to conventional metrics of biodiversity based on SR, and could be usefully included in assessments of site valuation and prioritization.