Tropical forests harbor the largest share of terrestrial biodiversity. Remarkably, today’s tropical forest cover is dominated by forests that have regenerated from abandoned agricultural lands. Because secondary successional forests are major features of tropical landscapes, it is critical to know what factors shape their formation. For plants, seed dispersal is the critical first step of community assembly, particularly for tropical forests undergoing secondary succession. Frugivorous animals such as birds are responsible for a large share of the seed dispersal processes in tropical forests because most tropical tree and shrub species bear fruit and seeds adapted for avian consumption and dispersal. We hypothesized that the assemblage of birds that consume fruit and disperse seeds provides a mechanism that promotes the maintenance of plant species diversity. Through their foraging preferences and movement behavior, the community of avian frugivores may disproportionately disperse fruits of plant species compared to the abundance and distribution of these species. To test this hypothesis, we conducted a seed addition experiment in an agricultural pasture containing tropical forest fragments in northwest Puerto Rico. We quantified how the relative abundance of seeds within available fruits in the community, as estimated at two spatial scales, controls local patterns of seed dispersal and plant recruitment.
Results/Conclusions
We found that the avian disperser community shaped the resultant plant community in nonrandom ways. Birds significantly increased the diversity of early successional plant communities relative to species pools within 50 m from experimental plots. Communities generated by natural avian seed dispersal were as diverse as those communities produced by experimental seed additions based on the species pool available at a scale of several square kilometers. The plant communities formed via avian dispersal experienced the smallest post-dispersal losses of diversity, thereby maintaining (and sometimes gaining) diversity relative to the other seed addition treatments. Birds may diversify regenerating tropical forests by not only delivering more species to a site than are locally available, but also bringing species that locally adapted to survive at the site. Our study suggests that the network of mutualistic interactions in tropical forests can indeed maintain biological diversity, starting from the very early stages of community formation.