Recent declines, extinctions, and range contractions of vertebrates has not only led to increasingly empty forests, but also affected important ecological functions such as seed dispersal. We study one of the world's most extreme examples of an empty forest, the Pacific island of Guam, where the invasive brown treesnake is responsible for the extirpation of all native vertebrate seed dispersers. Using comparisons of Guam to nearby islands with birds and bats, experiments, and a forest model, we identified the importance of seed dispersal to forest composition and structure. We also developed a strategy for restoring ecological function to the island's forests. To do this, we used the seed dispersal effectiveness framework to identify preferred avian candidates for restoration on Guam, and explored the potential for utilizing existing non-native species on Guam - the feral pig (Sus scrofa) and black rat (Rattus rattus) - as seed dispersers in this novel ecosystem.
Results/Conclusions
We demonstrate that Guam’s forests will become less diverse and less able to recover from disturbance without frugivores present to disperse seeds. The frugivorous bird species of the Marianas vary in their effectiveness as dispersers, with two species being preferred candidates for reintroduction due to their broad diet, positive impact on germination, and movement across habitat types. We propose to expand the range of one bird species that is still present on Guam in an area under snake control, and to reintroduce a species found on nearby islands. Birds could be protected from snake predation using a combination of nest boxes, fencing, and toxicants. The non-native pig is also an effective disperser, and may be useful as a stop-gap measure, whereas the non-native rat does not appear to be beneficial as a disperser. Restoration of ecological function in the presence of the brown treesnake may be an achievable goal and could serve as a guide for rewilding efforts in other places with empty forests and persistent threats.