There is increasing evidence that natural enemies of plants (fungal phytopathogens and insect herbivores) contribute to plant species coexistence and diversity in tropical forests. Specialist natural enemies are attracted to high concentrations of conspecific seeds and seedlings, driving negative conspecific density dependence and giving rare plant species an advantage. However, plant diversity is not uniformly distributed through the tropics and generally increases with precipitation. Precipitation also strongly influences both insect herbivores and fungal pathogens. These concurrent patterns lead to the hypothesis that increased natural enemy control of plant populations in wet climates contributes to the relationship between plant diversity and precipitation. We tested this hypothesis by suppressing insects and pathogens with biocides for 18 months at eight sites spanning the steep precipitation gradient across the Isthmus of Panama and examining the response of seedling recruitment. We quantified the effect of biocide addition on changes in diversity during the transition from seeds to seedlings and the relationship between conspecific seed density and the probability of transition from seeds to seedlings (i.e. conspecific density dependence).
At wet sites, plant species diversity increased by about 50% during the transition from seeds to seedlings when natural enemies were unmanipulated. At the drier sites, however, seeds and seedlings had similar species diversity, leading to a positive relationship between precipitation and increases in plant diversity during the seed-seedling transition. Suppressing insects or fungi diminished the increases in diversity at most sites and the slope of its relationship with precipitation. Conspecific seed density reduced seed-seedling transition probabilities at the wetter sites, indicating negative conspecific density dependence, but this effect weakened with decreasing annual precipitation. Biocide addition reduced the magnitude of this conspecific density dependence at the wet sites, and consequently, attenuated the relationship between conspecific density dependence and precipitation. These results suggest that conspecific density dependence imposed on plant populations by their natural enemies strengthens with precipitation. They imply that variation in a coexistence mechanism acting at local scales can contribute to regional patterns in plant diversity, although further work is necessary to determine if the patterns observed at this early stage persist into later life stages and are observed at other sites.