Precipitation Steepness Drives Global Patterns of Changes in Bird Community Composition Without Major Phylogenetic Signal

Aim

Understanding the processes that structure biodiversity on Earth is a major challenge in biology. Our work tests three key hypotheses driving taxonomic changes in bird communities globally, focusing on nestedness and turnover components: (1) contemporary climate, related to energy and water availability; (2) climate stability, reflecting shifts since the last glacial maximum; and (3) climatic heterogeneity, describing environmental gradients. We also examine whether these processes explain deviations in phylogenetic composition from expectations based on taxonomic changes among communities.

Location

Global.

Time Period

Present.

Major Taxa Studied

Birds.

Methods

We calculated total taxonomic dissimilarity, its nestedness and turnover components, between neighbouring cells considering all living bird species. We tested for significant phylogenetic over- and underdispersion by comparing observed phylogenetic dissimilarity to a null model. We used linear regression models to quantify the relationships between taxonomic dissimilarity and phylogenetic deviations with climatic variables representing our hypotheses.

Results

Precipitation steepness, that is, relative changes in precipitation, was strongly correlated with taxonomic changes (R² = 27%), driving both changes in local community richness (nestedness) and species replacement between different regional pools (turnover). These two processes were decoupled, with precipitation steepness driving richness differences up to 1200 mm of annual precipitation, and turnover being more relevant in hyperarid and tropical areas. Phylogenetic deviations were common (35% of global cells), resulting from both over- and underdispersion, but they lacked a climatic signal.

Main Conclusions

Our work supports the hypothesis that climatic heterogeneity, due to precipitation steepness, is the main climatic factor driving composition changes in bird communities globally, controlling local richness and transitions between regional pools. Changes in species composition often lead to phylogenetic dispersion or clustering, but the main processes responsible for taxonomic sorting are phylogenetically neutral. As such, taxonomic and phylogenetic changes between neighbouring bird communities may be driven largely by different processes.