Responses of aquatic invertebrate assemblages and leaf breakdown to macroconsumer exclusion in Amazonian "terra firme" streams

Abstract

Many authors have reported a lack of insect shredders in tropical streams and some have suggested that macroconsumers, such as fish and shrimp, are potential substitutes for insect shredders. We experimentally excluded macroconsumers (fish and shrimp) from leaf packs to examine their role in determining the rate of leaf breakdown and their effects on the associated invertebrate community. The experiment was designed in blocks and replicated in two reaches of four streams. Temperature of all stream reaches studied was 24 °C (without variation), and water conductivity was low, varying from 8.8 to 10.8 μs/cm. Fish were never observed near the leaf packs and therefore the effects of the macroconsumer treatment were attributed to shrimps. We found a significant effect on leaf breakdown, with greater leaf breakdown (i.e., less mass remaining after 17 days) in the control (65 % leaf mass remaining) compared to the macroconsumer exclusion (70 % leaf mass remaining). However, the mechanism for this effect was not clear. Considerable variation in leaf decomposition occurred among blocked stream sites, suggesting that some factors differing among these sites, perhaps macroinvertebrate shredder abundance, was contributing to decomposition. Leaves were visually inspected at the conclusion of the experiment and there was no sign of shrimp directly feeding on the leaves. There was no difference in insect shredder abundance between treatments. There was, however, a significantly greater amount of non-mining chironomids in the absence of macroconsumers. This is probably due to the release from predation by shrimp.