Thursday, August 9, 2007: 9:50 AM
J3, San Jose McEnery Convention Center
Understanding what regulates the strength of trophic interactions in detritus-based food webs is hypothesized to rely, in part, on how consumers respond to detrital resource quality. In small, forested stream ecosystems, the primary source of energy is senesced leaf litter from streamside forests. Once in the stream, bacteria and fungi quickly colonize leaf litter and begin the decomposition process, making the leaves more palatable to detritivorous insects. It is not known, however, whether mixtures of different leaf species affect bacterial and fungal activity (e.g., respiration), or how this activity is altered by shredder activity. This is certainly a real issue, since loss of leaf litter diversity in a stream is known to alter decomposition rates. We performed a field experiment where we manipulated leaf packs comprised of monocultures or mixtures of Tulip Poplar, Green Ash, and American Beech leaf litter in a first-order piedmont stream. On three sample dates, we removed the leaf packs, separated leaf species when in mixtures, and measured microbial respiration on leaf cores both in the presence and absence of a leaf shredding amphipod. Our results show that (1) the effect of the shredder on microbial respiration was substantial, and (2) that mixing leaf species significantly altered this pattern. Such interactions were strong for Tulip Poplar, which was the most palatable species in our study. We conclude that there are appreciable interactions between detritivorous invertebrates and microbial decomposers in stream ecosystems, and that these interactions change temporally and among litter species. While theory predicts that donor-controlled food webs, such as those found in small, forested streams, should be bottom-up controlled, our research suggests that strong trophic interactions may be revealed by closely examining the role of spatial and temporal variation in basal resource quality, and detritivore-microbial interactions.
