Natural enemies have been implicated as agents of negative density dependence (NDD) in tropical forests, but their relative contributions to NDD and thus the maintenance of diversity are largely unknown. We assessed the influence of natural enemies on density-dependent interactions among tropical seedlings, monitoring survival and relative growth rates in plots that excluded fungal pathogens, insects, small mammals, or large mammals. Only fungal pathogens caused density dependent mortality, and their exclusion reduced species diversity. Insects reduced relative growth rates when stem density was high, but because this was not driven by conspecific density, insects had little effect on species diversity. Mammals did not cause NDD interactions. We conclude that both fungal pathogens and insects drive NDD interactions among seedlings. Even so, only fungal pathogens increased species diversity, and consequently contribute critically to the structure of tropical tree communities.