Leaf decomposition and invertebrate colonization responses to manipulated litter quantity in streams

Resource availability is an important ecosystem attribute that can influence species distributions and ecosystem processes. We manipulated the quantity of leaf litter, a critical resource in streams, in a replicated field experiment to test whether: 1) greater litter quantity promotes microbial leaf decomposition (through greater microbial inoculum potential), and 2) reduced litter quantity enhances decomposition by leaf-shredding invertebrates (because shredders aggregate on rare resource patches). In each of 3 streams, we identified reaches in which litter quantity was either: 1) augmented, 2) depleted, or 3) left unchanged. We determined decomposition rates and macroinvertebrate colonization of alder leaves placed in coarse- and fine-mesh litter bags, an approach intended to allow or prevent access to leaves by leaf-shredding macroinvertebrates. Responses to litter manipulations were complex. In 2 streams, litter quantities differed among treatments, but high quantities of litter in the control reach of the 3(rd) stream produced an overall variable pattern. Microbial decomposition was similar across litter treatments. In contrast, in the 2 streams where litter manipulation was successful, decomposition in coarse-mesh bags tended to be faster where litter was scarce than where it was abundant. Abundances of total and leaf-shredding macroinvertebrates in litter bags did not differ among litter manipulations in these 2 streams. However, a litter-consuming amphipod (Gammarus fossarum) tended to be most abundant in bags placed in litter-depleted reaches in the 2 streams, indicating that this large and highly mobile shredder might have been instrumental in causing differences in decomposition in response to litter manipulations. Overall, the effects caused by alteration of litter quantities on leaf decomposition and macroinvertebrate colonization were relatively weak. Nevertheless, results from 2 of the 3 streams where litter manipulation was successful were consistent with the hypothesis that short-term changes in resource availability might influence ecosystem processes by determining the spatial distribution of key consumers.