Frugivorous birds are thought to perform critical services for fleshy-fruited trees. By consuming fruit, they can move seeds away from the parent, where mortality is often high, and increase germination through pulp removal or scarification. However, there are few large-scale examples of disrupted fruit-frugivore mutualisms that demonstrate the importance of generalist birds for fleshy-fruited tree species. Our research takes advantage of the mass extirpation of forest birds from the island of Guam caused by the introduction of the Brown Treesnake (Boiga irregularis) to investigate how the loss of frugivorous birds affects plant recruitment. We measured the seed shadows for two native tree species on Guam and three nearby islands with healthy bird populations using seed rain traps placed under the canopy and at 1, 5, 10 and 20 meters from the canopy. Traps were collected monthly for one year. For the same two species, we also planted seeds handled by birds alongside seeds unhandled by birds to determine whether bird handling influences germination rates. Finally, to determine if differences in dispersal distance could be observed at the seedling stage, we measured the distance between a randomly selected seedling and its nearest conspecific adult (i.e. most likely parent) on the same four islands.
Results from the seed rain traps indicate a significant shift in the dispersal kernels between Guam and nearby islands with birds; seeds fall closer to the parent tree on Guam than on islands with birds. The strength of the response varies by species, however. In our germination experiments, bird handling increased germination rates for one focal species. The effect of disperser loss is also apparent at the seedling stage, as seedlings are much closer to a conspecific adult on Guam than on islands with birds. Collectively, these results suggest that birds are important for plant recruitment of fleshy-fruited species, but also that the severity of the impact of bird loss varies by species. Since approximately 70% of tree species in the native forests of Guam have fruits dispersed by birds, the forests are likely to be significantly affected by the loss of birds. And with the possibility of large-scale avian extinction looming on many islands, and many bird populations in decline globally, understanding the consequences of frugivorous bird loss may be critical for preventing widespread changes in other forest systems.