The allochthonous detritus that accumulates in the substrate of streams is used by aquatic invertebrate shredders for shelter and food. Shredders are considered rare in tropical systems, and little information is available about the role of density effects and predation risk (associated with the perception of predators by prey) in relationship to the resources used by these organisms. The aim of this study was to examine experimentally the effects of increased predation risk and of the density of Phylloicus sp. (i.e. of two types of biological relationships) on the processing of the leaf litter of Nectandra megapotamica (Spreng.) Mez. Phylloicus sp. can use leaf litter for case building and as a food resource. The density effect was measured using four treatments that differed only in the number of individuals (one, two, three or four). A second experiment with five treatments was performed to test the risk of non‐lethal predation on detritus consumption (shelter and food) by Phylloicus sp. (T1: Caddisfly; T2: Mayfly; T3: Astyanax sp./fish; T4: Damselflies; T5: Stonefly). A single Phylloicus and one other organism (a potential predator blocked with 0.5 mm fine mesh) were placed in each tank (0.002 m³ volume). We observed a negative effect of density on per capita litter consumption (experiment 1). The low density of Phylloicus may be a natural factor that decreases intraspecific competition. In the presence of fish, Phylloicus showed the lowest amount of litter processing observed in the experiment, indicating top‐down control (experiment 2). In treatments that involved the presence of invertebrates (non‐predatory and predatory), Phylloicus showed the highest amount and an intermediate amount of leaf litter processing, respectively (experiment 2). This observation also suggests that the predation effect is more probable for specific predator–prey pairs. Population density and predation risk in Phylloicus may be important factors controlling leaf litter processing.