Herbivore effects on nutrient fluxes in a tropical rain forest in Puerto Rico

Forest canopy herbivores have been shown to increase rates of nutrient fluxes to the forest floor in a number of temperate and boreal forests, but few previous studies have demonstrated herbivore effects on nutrient fluxes in tropical forests. We simulated herbivore-induced fluxes in a tropical rainforest in Puerto Rico by augmenting greenfall (fresh foliage fragments resulting from chewing), frassfall (insect feces deposited on the forest floor), and throughfall (precipitation enriched with leachates from chewed leaves) in experimental plots on the forest floor. Background rates of greenfall and frassfall were measured monthly using litterfall collectors and augmented by adding 10X greenfall or 10X frassfall to designated plots the following month. Throughfall rate was doubled using published rates of throughfall NO3, NH4 and PO4. Control plots received only background rates. We measured treatment effects as NO3, NH4 and PO4 adsorption in ion-exchange resin bags and as decomposition rate of leaf litter in litterbags. Frass addition significantly increased NO3 and NH4 fluxes, and frass and throughfall additions significantly reduced decay rate, compared to controls. Reduced decay rate suggests that nitrogen flux was sufficient to inhibit microbial decomposition activity. Our treatments represented fluxes expected from low-moderate herbivore outbreaks and demonstrate herbivore ability to significantly affect ecosystem-level N and P fluxes in this tropical rainforest.