The loss of at-risk morphologically unique avian frugivores diminishes seed dispersal function and natural restoration potential in the tropics

Seed-dispersing, frugivorous bird species play critical roles in maintaining the diversity of ecosystems but many are threatened by habitat loss and overexploitation, with unknown consequences for seed-dispersal functioning at the global scale. By combining data on the ecology, morphology, geographical distribution, and threats of frugivorous birds, we show that extinctions of at-risk species will drive declines in functional richness, and shifts in the functional composition, of avian frugivore assemblages across a large part of the world's tropics. These impacts will be strongest in the Neotropics and the tropical Indo-Pacific where species threatened by rapid habitat loss and degradation are concentrated, with the contribution of hunting to these patterns especially high in many parts of mainland and insular Southeast Asia. Furthermore, a substantial proportion of tropical areas with medium or high natural forest restoration potential coincide with areas where the functional diversity of avian frugivore assemblages are at greatest risk. Our results suggest that the defaunation of frugivorous birds will substantially diminish the ability of many tropical ecosystems to recover fully after disturbance, highlighting the need to enhance efforts to halt or reverse losses in this important functional group.