Rainforest consumers influences on ecosystem processes are amplified in disturbed sites

Background/Question/Methods   Disturbance is indicated as one of the main drivers of changes to forest ecosystem and successional processes. In light gaps, plants grow rapidly, and new foliage is often an ideal resource for herbivores. Since growth is rapid, litterfall is high, thus there are also increased resources for macro-detritivores. With increased resources, consumer abundance and feeding may be high, so any impacts that consumers have on ecosystem processes may be amplified. Consumers are predicted to have greater impacts to ecosystem processes under disturbances, but this has not been examined in tropical systems. To test potential impacts that invertebrate herbivores and detritivores have on ecosystem processes under disturbance in a rainforest understory, we started a 3x2 factorial design enclosure experiment manipulating canopy cover, herbivore and detritivore presence in 2005. We measured plant dimensions to estimate plant biomass over time, assessed nutrient pools over time, quantified litter microbial communities and measured decomposition rates in treatments and control plots. One year after treatments were removed, plants growing up from the seed bank were harvested to determine any legacy effects of consumer treatments on plant biomass and composition.

Results/Conclusions   Consumers had significant impacts to plant production, nutrient cycling and decomposition, but only in light gaps. Plant growth increased significantly in the presence of herbivores in light gaps. We hypothesize that herbivore consumption of higher quality plants decreases their abundance, and the nitrogen sequestered in their tissues is transferred into higher biomass in lower quality, less preferred plants. Also, leaf decomposition rates were significantly lower in the presence of herbivores, because herbivores reduced the abundance of high quality litter, and subsequently reduced litter bacterial richness and abundance. In the presence of detritivores, plant growth decreased over time, and the total amount of soil nitrogen decreased over time. Presence of detritivores did not affect decomposition rates. We hypothesize that the detritivore selectively feeds on microbial groups that are important in N-cycling (e.g. nitrifiers or myhcorrizal fungi), thus decreasing nutrients available for plant growth. Although these enclosures were simplified rainforest systems, the results of these studies show potential impacts that invertebrate consumers in light gaps may have on the ecosystems in which they reside, where their impacts to ecosystem processes and plant production may alter forest successional processes.