Effects of density and predation risk on leaf litter processing by Phylloicus sp.

Abstract

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 m 3 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 predatorprey pairs. Population density and predation risk in Phylloicus may be important factors controlling leaf litter processing.

Leaf litter input and electrical conductivity may change density of Phylloicus sp. (Trichoptera: Calamoceratidae) in a Brazilian savannah stream

“…This is consistent with positive relationships between organic matter and shredder density in previous experiments (Grafius & Anderson, 1980;Graça, 2001) and manipulative studies (Cuffney et al, 1990;Lugthart & Wallace, 1992;Hall Junior et al, 2000). Rezende et al (2015) experimentally examined the effects of increased density of Phylloicus sp. on processing of leaf litter and hypothesized that at high densities, this species is forced to compete intraspecifically for preferential resources and may, most likely, initiate attacks to protect the food source.…”

“…In an evolutionary perspective, the main hypotheses for the low density of Phylloicus sp. in tropical streams are: i) the higher water temperatures (Dobson et al, 2002;Boyero et al, 2011;Navarro et al, 2013); ii) lower availability of high-quality leaf litter (Wantzen & Wagner, 2006;Boyero et al, 2011Boyero et al, , 2012Rawi et al, 2013; Graça et al, 2015); and iii) negative ecological interactions (e.g., competition and predation; Navarro et al, 2013;Rezende et al, 2015). Shredders belonging to orders Plecoptera and Trichoptera evolved in cold environments (Merritt & Cummins, 1996) and the low tolerance to higher temperatures imply lower abundance in tropical streams (Dobson et al, 2002;Boyero et al, 2012;Navarro et al, 2013).…”

“…on processing of leaf litter and hypothesized that at high densities, this species is forced to compete intraspecifically for preferential resources and may, most likely, initiate attacks to protect the food source. This behavior may be further stimulated by the low availability and low-quality of food resources in the environment (Moretti et al, 2009;Rezende et al, 2015).…”

“…competition and predation; Boyero & Pearson, 2006;Allan et al, 2009;Rezende et al, 2015) can affect the feeding behavior of Phylloicus. Therefore, events such as temporal changes in leaf litter input (Benson & Pearson, 1993;França et al, 2009;Gonçalves Júnior & Callisto, 2013;Gonçalves Júnior et al, 2014), as well as the physical environment (Boyero et al, 2011;Navarro et al, 2013), have been claimed to affect shredder populations.…”

Effects of litter size and quality on processing by decomposers in a tropical savannah stream

“…High concentrations of structural compounds make the case more robust and provide greater protection against predators (Rezende et al . ). In this way, litter quality can potentially influence organic matter processing by organisms (Tank et al .…”