Herbivory and pathogens are responsible for significant amounts of leaf damage and both may impact genetic and species diversity in tropical forests. Nutrient enrichment typically increases herbivore damage and microbial diversity; however, few studies simultaneously assess these agents within a single system. We hypothesize that increasing the supply of limiting soil resources (e.g., phosphorous) will increase plant enemies (e.g. insects and bacterial pathogens) and that the identity of bacterial strains will be host specific. We also hypothesize that bacterial diversity will increase as herbivore damage increases because herbivores may provide a conduit for bacteria to enter leaf tissue. We tested these hypotheses using a well-replicated, long-term (> 10 yrs) nutrient enrichment experiment where macronutrients (N, P, K) were applied to large plots (40 x 40m) in a fully factorial design in a Panamanian tropical forest. We quantified leaf damage on seedlings of 5 tree species (Alseis blackiana, Desmopsis panamensis, Heisteria concinna, Sorocea affinis, and Tetragastris panamensis) and quantified bacterial diversity using both molecular and culture techniques.
Increasing P and K significantly increased herbivore damage and herbivore damage was even higher in plots receiving both P and K (a P x K interaction). Damage varied significantly among species. We are currently analyzing bacterial responses both among species and among the nutrient treatments; however, preliminary results suggest important host specificity. If these results hold and the bacteria are inimical to tree seedling growth and survivorship, our findings may have important implications for the maintenance of tropical forest diversity.