Turnover of bird species along the Nullarbor Plain: Insights from taxonomic, phylogenetic, and functional beta diversity

The Nullarbor Plain constitutes one of the main biogeographic barriers of Australia, and it has been suggested to have played a key role in the disjunct distribution of numerous southern Australian species. Although previous research has shown that the origin of this barrier coincides with the timing of the speciation events in some plant lineages, it is not clear whether the uplift of this barrier promoted divergence events in vertebrates. We addressed the role of the Nullarbor barrier and its fringing semiarid habitats as drivers of beta diversity in bird assemblages. Specifically, we determined the effect of distance from the Nullarbor barrier, environmental conditions, and isolation by distance on the composition of local communities on both sides of the plain. We measured beta diversity using taxonomic, phylogenetic, and functional metrics of composition. The influence of precipitation, geographic distance, and distance to the Nullarbor barrier on these metrics was addressed using generalized dissimilarity models and a moving-window approach. We also tested for differences in local extinction, dispersal and speciation rates, and lineage diversity between two regions, southeastern (SE) and southwestern (SW) Australia. Geological and orogenetic dynamics linked to the appearance of the Nullarbor Plain may have spurred speciation events in SE. However, evidence suggests that subsequent periods in which this region was wetter and forested favored dispersal, mainly from SE to SW. Accordingly, observed dissimilarity in species composition was lower than expected at random, suggesting the existence of considerable turnover between regions. Our results suggest that precipitation deficit (and the xeric vegetation that it promotes) was the most important predictor of beta diversity, whereas the distance to the barrier explained some variation in terms of phylogenetic composition. This study shows that the uplift of the Nullarbor barrier played a minor role in shaping present-day bird diversity in southern Australia. Recent speciation events coupled with historical connectivity can explain the observed patterns.