Differentiating the roles of shrimp and aquatic insects in leaf processing in a Neotropical stream

There is interest in the relationships between macroconsumers (e.g. shrimp) and aquatic insects, as well as their role in ecological processes, including leaf breakdown, in tropical freshwater environments. Many studies have shown that shrimp have the capacity to reduce the abundance of aquatic insects (by predation and behavioural inhibition) and promote leaf breakdown as shredders. To discriminate between these effects, we used fresh leaves of Erythroxylum pulchrum and manipulated the presence of shrimp and insects by electric exclusion at high and low intensities of electric field. In the control treatment (no electric exclusion) shrimp (the brushing collector Potimirim brasiliana and the shredding omnivore Macrobrachium olfersii) and aquatic insects (including shredders) were present. The low-intensity electric field excluded only shrimp, whereas the high-intensity electric field excluded both shrimp and medium- and large-sized aquatic insects (>2mm). Leaf processing was approximately twice as fast in the absence of shrimp and in the presence of insects than when both or neither were present. This implied a trophic cascade of shrimp acting as potential predators of insect shredders, but not acting as shredders themselves. We postulate that increased leaf processing was caused by a behavioural response of the putative shredders to the absence of shrimp; abundant leptophlebiid ephemeropterans were the most likely shredders.