Overexploitation risks disproportionate vertebrate downsizing and declines in potential dispersal distances in frugivory networks

Poorly managed and unsustainable exploitation from either the use or trade of species can be a key driver of wildlife population declines and extirpation risk. Local extirpation of overexploited vertebrate species risks triggering cascading ecosystem effects through plant–frugivore interactions, disrupting the ecological networks they form, with potential impacts on the associated seed-dispersal services. Using a global database of the occurrence of more than 16 000 reported bird and mammal frugivory interactions across 160 binary frugivory networks containing likely overexploited species, we simulated the extirpation of overexploited species and their cascading impacts on network composition and potential ecosystem function. The loss of overexploited species triggered a sixfold greater body mass downsizing of vertebrate communities than would be expected under random species loss. By contrast, plant communities experienced comparable structural disruption in frugivory networks following overexploitation-driven versus random loss. Average seed-dispersal distances within networks declined sharply following the extirpation of overexploited species. Average distances declined 161% more after the extirpation of overexploited species relative to the equivalent randomized species loss (12.0% decline after overexploitation, 4.6% after random loss). Our results demonstrate that overexploited species have a key role in local frugivory networks, underscoring the importance of maintaining ecosystem integrity via the protection of vertebrate populations to restore ecosystem function in degraded or defaunated landscapes.