Combined effects of predation risk and food quality on freshwater detritivore insects

Abstract

Because both predation risk and resource quality have a pivotal role in ecological communities, their combined effects were assessed in stream detritivores. Bioassays were conducted using a simplified trophic chain, coupling effects of predation risk and basal resources with different nutritious qualities, namely, a predator planarian Dugesia subtentaculata, a caddisfly shredder Sericostoma vittatum, a dipteran collector Chironomus riparius, native alder Alnus glutinosa and invasive eucalypt Eucalyptus globulus. We investigated whether individual performance of C. riparius larvae is affected under predation risk and whether the response can be mediated by resource quality. We also assessed whether shredder–collector interactions are altered under these conditions. Predation risk and food quality influenced leaf decomposition and C. riparius growth independently. Chironomus riparius fed preferentially on alders, resulting in increased growth rate. Litter processing decreased under planarian presence, in both leaf types, impairing the growth of C. riparius larvae, this effect being exacerbated with eucalypts. Chironomus riparius growth was also reduced in the presence of the caddisfly, suggesting competition between these species, irrespective of planarian presence, but dependent on leaf type. The present study highlighted the importance of assessing predator influence along detritus-based processing chains, because predation risk may result in sublethal costs, with potential cascading effects.