Local extinction of large-bodied herbivores from ecosystems has a strong influence on the regulation of plant diversity. How these impacts affect the multi-scale spatiotemporal structure of diversity on forest ecosystems is currently unknown. The Janzen-Connell model of diversity suggests that natural enemies such as seed predators suffice to generate diversity amongst plant communities in forest ecosystems, without accounting for the synergic effects of seed dispersal by mutualist frugivores. We conducted a long-term multisite landscape-scale exclusion experiment across gradients of defaunation in the hyperdiverse Atlantic Forest of Brazil, and tested experimentally how two key and functionally distinct large generalist mammalian herbivore species (the lowland tapir Tapirus terrestris – a seed disperser –, and white-lipped peccary Tayassu pecari – a seed and seedling predator) affect spatiotemporal patterns of diversity of regenerating seedling communities in tropical forests. Using a database of 6947 seedlings from 162 species collected over 72 months, we evaluated defaunation impacts on seedling recruitment, incidence and abundance-based indices of beta-diversity, and alpha diversity. We also assessed how defaunation influenced the diversity structure of dominant vs. rare species so as to test for defaunation impacts on community assembly processes.
Results/Conclusions
Our results indicate that mammalian herbivores have a substantial role in regulating beta diversity in space and time through synergic effects of top-down regulation of recruitment and assisted dispersal of dominant plant species, in detriment of alpha diversity of rare species. The demography of seedlings was strongly regulated by peccary predation, but predation was a necessary yet not sufficient condition for increases in beta diversity. Abundance and incidence-based beta-diversity only decreased in response to the simultaneous exclusion of tapirs and peccaries, demonstrating a synergistic positive effect between antagonistic predation and agonistic seed dispersal on species turnover at landscape-scale. Close examination of large-herbivore impacts on dominant vs. rare species showed that defaunation impacts on beta diversity were exclusively associated to changes amongst dominant species, with rare species showing only increases in alpha diversity in response to defaunation. Hence our results indicate a strong positive association of large generalist herbivores with dominant plant species, and a strong influence of these herbivores in the interplay between stochasticity and competition at the local scale. Our study suggests that the Janzen-Connell model of diversity needs to incorporate the synergic effect of mutualists, and demonstrates that the structure and dynamics of plant communities in forest ecosystems are subordinated to the conservation of functionally complementary large-bodied vertebrates.