How do macroconsumers interact with insect shredders in streams? A laboratory test using shrimp and caddisfly larvae that build cases with different organic materials

Negative interactions between macroconsumers and detritivores can influence organic matter processing in streams. We evaluated how the shrimp Macrobrachium potiuna affects the shredding activity and survival of larvae of Phylloicus major and Triplectides gracilis, two caddisfly species that build cases with leaf pieces and tree sticks, respectively. We also evaluated whether the shredding activity of the shrimp changes in the presence of potential prey. We hypothesized that (i) M. potiuna would constrain the rates of leaf consumption and FPOM production of P. major more than T. gracilis and (ii) the shredding activity of the shrimp would increase in the presence of larvae. Laboratory trials were carried out in feeding arenas containing one shrimp, three larvae of one caddisfly species, and leaf discs of Miconia chartacea as food. The visual and chemical stimuli from the shrimp did not change the rates of leaf consumption and FPOM production of either larva. However, when physical contact was allowed, the survival rate of P. major was 35.7 %, while all individuals of T. gracilis remained alive. The shredding activity of M. potiuna was low and observed only in the presence of P. major. These results suggest that although larvae of P. major are less protected against predation, M. potiuna does not constrain the activity of either shredder. Therefore, top-down effects of this macroconsumer on detritus-based food webs dominated by case-building caddisfly shredders are likely context dependent.