Seed dispersal determines the abiotic and biotic environments that plants experience throughout their lifetimes. Roughly half of plant species are dispersed by animals and up to 90% of tree species are dispersed by animals in the world's most diverse forests. Therefore animal seed dispersers have critical impacts on spatial patterns of plant communities and may in turn influence plant coexistence by determining the local abiotic and biotic environments that plants experience. We consider the role of seed dispersing mutualists in mediating plants interactions with space, and the resulting influence on plant coexistence, using an unplanned ecosystem-scale manipulation of vertebrate seed dispersal on Guam. The brown treesnake was introduced to Guam in the 1940s and through the next half century caused the functional extirpation of seed dispersers on the island. Nearby islands, with forests and frugivores similar to those historically on Guam, offer an accidental experiment to assess the role of seed dispersers on plant coexistence. To assess the predictions that dispersers influence local diversity patterns and limit coexistence, we recorded seedling recruitment within canopy gaps, used observations and experiments on the dispersal process and its impacts on plant demography, and simulated forest community dynamics in the presence and absence of seed dispersers
Results/Conclusions
Seed disperser loss on Guam reduces local richness within canopy gaps and creates greater turnover among gaps relative to islands where seed dispersers are present, indicating strong influences on diversity patterns by decreasing alpha but increasing beta diversity. Forest simulation models and recent forest censuses indicate that disperser loss will lead to reductions in plant diversity at the scale of Guam’s forests through strong negative impacts on a subset of animal-dispersed plant species. Plant species' life history strategy appears to predict the vulnerability of individual species to mutualist loss. The most vulnerable plant species depend heavily on seed dispersers to reach favorable microhabitats (typically fast-growing, light-dependent, and small-seeded species), and these species are currently highly underrepresented in Guam's forests relative to forests on other islands. These findings are consistent with greater fitness differences among plant species in biotic environments without seed dispersers. Animal seed dispersers therefore play a role in plant species coexistence by impacting spatial processes within heterogeneous environments.