Invertebrate herbivore and detritivore effects on microbial community composition and leaf decomposition rates under light gaps and closed canopies in a tropical forest

Background/Question/Methods Herbivores and macro-detritivores are often considered unimportant to leaf decomposition in tropical forests, despite their known importance to leaf decomposition in non-tropical ecosystems.  By feeding on vegetation and litter, herbivores and macro-detritivores may impact forest litter quality and quantity.  Litter quality and quantity may in turn play an important role in determining the biomass and composition of the litter microbial community. This pathway has been previously overlooked, especially in terrestrial tropical systems.  In disturbance-driven forests, disturbances may amplify the effects of these groups.  This study examined how an invertebrate folivore (walking stick, Lamponius portoricensis) and a detritivore (snail, Megalomastoma croceum) affect litter microbial communities and leaf decomposition rates under two different levels of canopy cover in a disturbance-driven forest.  A fully crossed, factorial experiment (herbivore presence/absence x detritivore presence/absence x light gaps/forested sites) began in 2005.  Herbivores and detritivores were stocked at natural abundances inside enclosures in a Puerto Rican rainforest.  In these experimental enclosures, we measured decomposition rates (k) of four different litter types (two senesced single species of leaves, and fresh and senesced mixed leaves) with litterbags in 2006-2007.   Subsequently, a second litterbag experiment in 2007-2008 was conducted to determine the composition of fungal and bacterial communities in the litter.  Relative number and abundance of fungal and bacterial groups were determined using TRFLP and ARISA.  Both experiments were analyzed with repeated measures ANOVA.

Results/Conclusions Herbivore presence significantly reduced decomposition rates, but the presence of detritivores did not significantly affect k.  This reduction was amplified in light gaps and in litterbags containing the fastest decomposing litter type.  Preliminary results from the second litterbag experiment show that herbivore presence reduces the number and relative abundances of fungal groups.  It is hypothesized that by preferentially feeding on the faster decomposing plants, herbivores reduce the quantity of nutritious, high quality litter on the forest floor, which may then reduce microbial activity and biomass.  In disturbance-driven forests, increased light availability to the understory in light gaps promotes plant growth.  Herbivore feeding increases in response to increased resources, thus amplifying their impact on decomposition.  This research emphasizes the need for further studies investigating the role of different trophic levels, especially consumers, on the process of decomposition in tropical forests.