The rainforest of Yasuni National Park, Ecuador, is among the most diverse on earth. Various mechanisms have been hypothesized to explain the local coexistence of such a large number of canopy tree species. Many of these proposed mechanisms invoke particular patterns of seedling recruitment, growth, and mortality that reduce the likelihood that dying canopy trees will be replaced by only one or a few dominant species. This dissertation research evaluates these hypotheses using a two-pronged approach. First, natural rates of recruitment, growth, and mortality of seedlings are being quantified in permanent census plots distributed throughout the study forest. These plots, established in 2002, are monitored annually. Second, based on these censuses, focal sets of common and rare species will be selected for use in field experiments. The spatial distributions and densities of seeds and seedlings of these species will be manipulated to directly test the hypotheses that diversity is maintained by 1) the restriction of different species' seedlings to particular soil types or microhabitats, 2) spatially limited dispersal of seeds; particularly by competitively dominant species, or 3) frequency-dependent or density-dependent mortality that prevents a dominant competitor from excluding relatively rare species. These experiments will determine whether species of differing abundances (i.e., relatively rare vs. relatively common) exhibit responses to experimental manipulation consistent each proposed mechanism of diversification. The study forest is more species-rich than sites of previous studies of rainforest diversity. Thus, the research will test the generality of conclusions drawn from other tropical forests. An added benefit of the project is that it involves active collaboration between Ecuadorian and North American scientists.
