Functional robustness declines faster than structural robustness in hyperdiverse seed dispersal networks following defaunation

Abstract

Seed dispersal mediated by frugivores is a key ecological function that is being disrupted worldwide due to defaunation. Nevertheless, we know little about the impacts of frugivore loss on seed dispersal function, as most studies focus on forest ecosystems and/or use plant-disperser interactions as a proxy for seed dispersal success. The Brazilian Cerrado is a tropical savanna and a biodiversity hotspot that has lost nearly half of its vegetation cover in the last decades and faces a large defaunation rate. To understand the impacts of defaunation on the seed dispersal of Cerrado, we sampled plant-frugivore interactions in five sites and assessed seed dispersal effectiveness of each interaction. Using a network approach, we simulated frugivore extinctions and population declines to evaluate the plant community response in terms of structural (remaining plant species) and functional (remaining seed dispersal effectiveness) robustness. We found that defaunation drove sharper losses in functional, compared to structural robustness. Sharper declines in functional robustness occurred when large frugivores were extinct first, and slower declines happened when specialists were extinct first, compared to a random extinction scenario. Declines in species abundances drove more losses in robustness compared to decreases in species richness. Results indicate that function can be lost before species are extinct, and that structural robustness may underestimate losses in seed dispersal. Furthermore, it suggests an important role of large-bodied and generalist frugivore species. Finally, we reinforce the need to maintain not only frugivore species, but also effective frugivore populations to conserve seed dispersal in the Cerrado.