Seed dispersal increases local species richness and reduces spatial turnover of tropical tree seedlings

Abstract

<p id="d5754861e189">Dispersal is considered a key process underlying the high spatial diversity of tropical forests, with the seeds of most tropical tree species dispersed by vertebrates, particularly birds. Although it has proven very difficult to quantify the contribution of dispersal to tree species diversity, it is increasingly important to do so in the face of global declines in vertebrate disperser populations. We show that the complete loss of native seed dispersers on the island of Guam is having a major impact on tree seedling regeneration in canopy gaps, leading to species-poor and spatially aggregated seedling communities. These pronounced changes in patterns of seedling regeneration highlight the importance of dispersal in maintaining patterns of diversity in tropical forests. </p><p class="first" id="d5754861e192">Dispersal is thought to be a key process underlying the high spatial diversity of tropical forests. Just how important dispersal is in structuring plant communities is nevertheless an open question because it is very difficult to isolate dispersal from other processes, and thereby measure its effect. Using a unique situation, the loss of vertebrate seed dispersers on the island of Guam and their presence on the neighboring islands of Saipan and Rota, we quantify the contribution of vertebrate seed dispersal to spatial patterns of diversity of tree seedlings in treefall gaps. The presence of vertebrate seed dispersers approximately doubled seedling species richness within canopy gaps and halved species turnover among gaps. Our study demonstrates that dispersal plays a key role in maintaining local and regional patterns of diversity, and highlights the potential for ongoing declines in vertebrate seed dispersers to profoundly alter tropical forest composition. </p>