Changes in climate and land-use are modifying biodiversity worldwide. Yet it remains unclear how both drivers interact to structure communities and determine patterns in taxonomic, functional, and phylogenic diversity at local scales. We focused on bird diversity and asked: how do precipitation and forest cover gradients interactively structure these elements of avian diversity? We quantified changes in the abundance and composition of bird communities along independent gradients of regional precipitation, local forest cover, and landscape forest cover that serve as proxies for climate drying and habitat conversion. We conducted point counts at 150 sites and statistically accounted for imperfect detection to test how environmental variation shaped community-wide diversity metrics. This work was conducted in Guanacaste, Costa Rica.
Results/Conclusions
We found that the three dimensions of diversity diverged in their responses to environmental gradients. Specifically, species richness increased linearly with precipitation, reached maximum values with intermediate tree cover at local scales, and peaked at high levels of landscape tree cover. While phylogenetic diversity did not vary strongly across any gradient, functional diversity increased monotonically with both local and landscape-level tree cover. Maximum values of functional diversity only occurred in large patches of forest where the tree cover was >75% at both local and landscape scales. Contrary to our expectations, we did not detect significant interactions between precipitation and tree cover gradients on any metric of local bird diversity.
Our findings suggest that taxonomic diversity was more sensitive to environmental gradients than functional and phylogenetic diversity. The lack of synergies between precipitation and forest cover gradients, at least in this system, simplifies the prospects of predicting future biodiversity change at local scales as our results suggest that climate change and land-use act independently of one another.