The effects of canopy herbivory on soil microarthropods in a tropical rainforest

H. Bruce Rinker (2004)

This dissertation examined the effects of canopy herbivory on soil decomposition in a tropical rainforest at Luquillo Experimental Forest in eastern Puerto Rico (an LTER site). Specifically, the project established quantifiable links between arthropod activity above- and below-ground via additions of frassfall, greenfall, and throughfall. Litterbag samples were measured for mass loss due to decomposition at six sample dates through a 36-week treatment period. They were also analyzed for their abundance and diversity of springtails, three suborders of mites (oribatids, prostigmatids, and mesostigmatids), pseudoscorpions, nematodes, and "other" soil mesofauna. Additions of frassfall, greenfall, and throughfall, as herbivore-derived inputs, promoted the abundance and diversity of some soil microarthropods and other mesofauna. No significant treatment effects, however, were observed on litter decomposition for this experiment though each sample date showed significant mass loss. During the sample period, numbers of most organisms increased except at the transition between dry and wet seasons. On the other hand, numbers of mesostigmatids and "other" mesofauna continued to rise. A positive response was achieved among total mesofauna to frass additions. Frassfall also had a dramatic effect on the densities of microarthropods relative to those of the control groups. Pseudoscorpions showed a positive effect from throughfall additions. Numbers of nematodes were negligible so it was difficult to ascertain treatment effect on these organisms. In terms of treatment effect on estimated numbers of mesofauna per m 2 of forest litter, frassfall had the greatest relative impact, followed by greenfall and then throughfall. When contrasted again the controls, oscillations in the densities of some mesofauna in the treatment litterbags indicated a predator-prey feedback system, especially for frassfall and greenfall additions. Further, densities of "other" mesofauna increased over time, especially for frassfall additions; importantly, these "other" fauna included the larvae of numerous kinds of macro-invertebrates, suggesting detrital succession in the litterbag microcosms. Hence, herbivore-derived inputs play a significant role in the spatial and temporal dynamics of soil mesofauna in Puerto Rico that then have important consequences for decomposition and, ultimately, for the health of the forest ecosystem.