Bottom-up control determines litter invertebrate community composition and abundance in Hawaii

Litter-based food chains are thought to be extreme examples of bottom-up control, because detritivores do not affect resource renewal rates.  While bottom-up control is often observed in litter food webs, this does not eliminate the possibility of top-down control, as predation pressure importance varies with litter quality.  Furthermore, if a strong linkage occurs between a predator and its prey, then manipulating the resource base could produce a response in both primary and secondary consumers.  We determined the relative importance of bottom-up and top-down control on litter invertebrate communities and leaf litter decomposition in Hawaiian lowland forests, where there is a large number of endemic and endangered litter invertebrates.  We used a 2 x 2 factorial design to determine whether leaf litter quality or an invasive invertebrate predator, Eleutherodactylus coqui (a frog endemic to Puerto Rico), has the greater influence on litter invertebrate community composition and abundance, and leaf litter decomposition rates.  We found that litter quality had a greater influence on litter invertebrate composition and abundance; and that these effects did not cascade to influence E. coqui diets, growth, or survivorship.  We also found that litter quality was more important in influencing leaf litter decomposition rates; however, E. coqui did influence leaf litter decomposition rates of a tree species of relatively low litter quality.  The results suggest that an invasive tree with a relatively high litter quality is more important in determining litter invertebrate community composition and abundance than the invasive predator, E. coqui.