Invertebrate consumer influences on ecosystem processes in a rainforest understory.

摘要

Consumer organisms are frequently considered passive components of tropical forests with negligible effects on ecosystem processes. However, non-tropical consumers play known important roles by changing resources available to detrital food webs. In rainforests, plants sequester most nutrients, and microbes facilitate important nutrient transformations. Consequently, only plants and microbes are assumed to affect how rainforests function. If tropical consumers alter detrital resources like their non-tropical counterparts, they may also have important effects on rainforest functioning, particularly in light gaps, where increased resources due to rapid plant growth and litterfall lead to higher consumption rates.;This dissertation examines how common herbivores (walking sticks, Lamponius portoricensis) and detritivores (snails, Megalomastoma croceum) affect a Puerto Rican rainforest under two different canopy covers. I hypothesized that herbivores prefer faster decomposing plants would shift plants to a slower decomposing plant community. This shift would reduce resource quality for decomposers, consequently reducing decomposition, nutrient availability and plant production. I predicted that detritivores would accelerate decomposition by fragmenting litter, consequently increasing nutrient availability and primary production. Lastly, I expected consumer effects to be greatest in light gaps because of higher feeding rates. To test these hypotheses, I conducted enclosure experiments manipulating canopy, herbivore and detritivore presence. I measured consumer effects on plant growth, nutrient cycling, and decomposition. These data were used to parameterize a trophic interaction model, which examined the role of trophic interactions on nutrient cycling in this rainforest.;The experiments and models show that consumers significantly affect this rainforest, but almost exclusively in light gaps. Herbivores increased plant growth and nutrient availability, but decreased litter decomposition rates by driving a shift to a slower decomposing plant community. This shift supplied poorer quality resources to microbial decomposers, leading to less abundant and rich litter bacteria. Detritivores reduced plant growth and nutrient availability, but did not affect decomposition rates. These detritivores may selectively feed on microbial groups crucial to N-cycling, thus reducing nutrient availability. These results challenge the common assumption that consumers are unimportant to rainforest functioning by providing evidence that consumers alter rainforest processes, particularly after disturbances.