Plant pathogens and tropical tree diversity

At least half of all the earth's species of plants and animals live in tropical forests, and there is considerable interest in the ecological processes maintaining this extraordinary diversity. Explaining the high diversity of tropical trees is a particular challenge, because most plant species use very similar resources: just one or a few highly competitive species might be expected to dominate the forest, rather than the hundreds of species per hectare that are often observed. If we can explain the high diversity of tropical trees, then the high diversity of other taxa in tropical forests may follow. Recently, evidence has been growing that high plant diversity in tropical forests arises because of strong density-dependent seed and seedling mortality. In other words, seeds and seedlings are more likely to die when close to their parent tree and close to other seedlings of the same species. This puts locally rare plant species at an advantage, preventing any one species from reaching high abundance. It is widely suspected that fungus-like plant pathogens are responsible for seedling density-dependence. However, the evidence for their role in maintaining high plant diversity in tropical forests is largely circumstantial, and their effects on the wider structure of tropical forest communities have not been explored fully . We will test the hypothesis that plant pathogens maintain high plant diversity at our study site in Belize, Central America. We will address five key outstanding questions: (i) How widespread is pathogen mediated density-dependence? For pathogens to play an important structuring role in tropical forest communities, their effects must be widespread. Exclusion experiments using selective fungicides will reveal whether pathogens are driving density-dependent mortality. (ii) Are pathogens specialist or generalist? Density dependence is expected to have the strongest diversity-enhancing role when natural enemies show high host specificity. Molecular and experimental approaches will be used to assess the specificity of pathogen species and strains isolated from seedlings of individual tropical tree species. (iii) Do pathogens mediate indirect interactions among plant species? We will assess the extent to which pathogens are shared between pairs of plant species, and investigate the community-wide impacts of shared parasitism. (v) How do pathogens affect the relationship between host density and mortality? We will determine whether the the relationship between density and mortality is of a form likely to promote plant diversity, and we will determine the relative importance of density and distance from parent tree in influencing mortality. (iv) Does mortality from pathogens increase diversity? We will carry out the first experimental test of the hypothesis that pathogens promote plant diversity by comparing the diversity of the seed rain with the diversity of new seedlings in fungicide-treated and untreated plots.