Linking Shredders and Leaf Litter Processing: Insights from an Acidic Stream Study

We performed a seven-month study on beech leaf breakdown and associated fauna along a forested stream showing a gradient in acidification level (4 stations with mean pH ranging from 5.3 to 7.2). We completed this approach by comparing in situ the shredding efficiency of the main invertebrate species present at each station, the acid-tolerant Protonemura sp. (Plecoptera: Nemouridae) and the acid-sensitive Gammarus fossarum (Amphipoda: Gammaridae). Results showed that beech leaf breakdown rate was severely depressed under acidic conditions. Not only did the acidic station show lower biomass of shredders, but Protonemura sp., the main shredder presented significantly lower feeding activity than G. fossarum in the non-acidic stations. At the other three stations, feeding activities of G. fossarum were similar and differences in litter breakdown rates appeared to be related to G. fossarum densities. By combining both fine-resolution studies of individual species and ecosystem level studies, our results suggest that the functional role played by G. fossarum in the studied system seems to be unique and not fulfilled by other species in the acidified upper part of the stream.