|Abstract or Summary
- The role of insect herbivores in the nutrient cycling dynamics of forest
ecosystems remains poorly understood. Although past research in herbivory has
focused primarily on the deleterious effects that insects can have on tree growth and mortality, the overall effects of herbivory are more complex. Herbivores have the potential to alter key ecosystem processes in a number of ways. One is by altering the flow of nutrients from the canopy to the forest floor. The studies presented here examine the effects that frass (insect excreta), greenfall (green leaf fragments) and herbivore modified throughfall, have on decomposition processes and soil nutrient dynamics. Both studies were carried out in a lower montane tropical rain forest near the El Verde Field Station, Luquillo Long-Term Ecological Research (LTER) site, Puerto Rico.
The first study presented here tested the hypotheses that green leaves are of higher quality and decompose more rapidly than senesced leaves. Green and senesced leaves of four common native tree species (Dacryodes excelsa, Manilkara bidentata, Guarea guidonia and Cecropia schreberiana), were collected and analyzed for C, N, and complex C compounds. Litterbags containing green and senescent leaves of each species were placed in the field for up to 16 weeks in order to compare rates of litter decomposition. Green leaves contained significantly higher (p < 0.05) concentrations of N and lower lignin:N ratios than senescent leaves for all four species. Decomposition rates were significantly higher (p < 0.05) for green leaves compared to senescent leaves for all four species. These results demonstrate that insect herbivores may influence key ecosystem processes via the production of greenfall. The relevance of this study extends to green leaf deposition resulting from multiple sources, including high wind and rain events, as well as disturbance by larger canopy organisms. The second study tested a direct link between herbivory and soil processes. By altering levels of herbivory on a common understory plant (Piper glabrescens), using a prevalent folivore (Lamponius portoricensis), this study tested for effects of herbivory on nutrient inputs to the soil, as well as rates of litter decomposition. Enclosures were constructed around P. glabrescens individuals in the field and assigned to one of four treatments: herbivore exclusion, control, low herbivory and high herbivory. Total leaf area loss and greenfall production were measured for each plant using a sub-sample of randomly chosen leaves. Litterbags and ion exchange resin bags, placed under each plant, recorded rates of litter decomposition and the flow of NO3, NH4 and PO4 to the forest floor during this 76-day experiment. The treatments were effective in establishing a wide range of herbivory. Both the total leaf area removed and greenfall deposition demonstrated significant positive correlations (p <0.05) with NO3 transfer to the forest floor, but not with NH4 or PO4. Although decomposition rates showed no significant correlation with total leaf area losses or the greenfall component, a significant correlation was found between decay rates and frass related inputs (defined as the total leaf area removed minus the portion removed as greenfall). This study clearly demonstrates the ability of insect herbivores to alter nutrient cycling in tropical forest ecosystems and is the first study to demonstrate a direct link between herbivory and decomposition processes. These experiments provide clear evidence that insect herbivores can alter
nutrient cycling and soil processes in forest systems. Such findings elucidate the
need to more fully consider the effects of herbivores in both ecosystem models and
management issues of tropical forest ecosystems.