Protected area coverage of vulnerable regions to conserve functional diversity of birds.

Global change drivers are increasing the rates of species extinction worldwide, posing a serious threat to ecosystem functioning. Preserving the functional diversity of species is currently a priority to mitigate abrupt biodiversity loss in the coming decades. Therefore, understanding what factors better predict functional diversity loss in bird assemblages at a global scale and how existing protected areas cover the most vulnerable regions is of key importance for conservation. Here, we examine what environmental factors are associated with the risk of functional diversity loss under three different scenarios of bird species extinction based on species: (i) distribution range size, (ii) generation length, and (iii) IUCN conservation status, and identify those regions that deserve a special conservation focus. We also assess how efficiently extant terrestrial protected areas preserve these particularly vulnerable bird assemblages. We found that the vulnerability of bird functional diversity increases as net primary productivity, land-use diversity, mean annual temperature, and elevation decrease. Low values of these environmental factors are connected to a higher risk of functional diversity loss worldwide through two mechanisms: a direct one irrespective of species richness (i.e., reduced niche packing and increased functional dissimilarity among species) and an indirect one (i.e., via reduced species richness and increased probability of extinction). We also show that existing protected areas are ineffective at protecting regions with a higher risk of losing functional diversity in the coming decades. Our results unveil global predictors and the underlying mechanisms that influence functional vulnerability in bird assemblages, allowing us to design better strategies aimed at preserving bird-driven functions worldwide. This article is protected by copyright. All rights reserved.