Temperature- and species-specific infection could modify stream insect communities.

To maintain diverse communities under changing environmental conditions, species must be able to concentrate density feedback within species compared to between species. Density feedback can occur via multiple mechanisms, including predator partitioning, and is often mediated by the physical environment. If species respond differently to the environment (e.g. susceptibility to predators changes under different conditions) then environmental fluctuations can provide the necessary separation of density feedback to promote coexistence. Additionally, directional change in environmental conditions (as with climate change) can yield overall shifts in community composition. We assess possible pathways that a fungal-like parasite (Saprolegnia spp.) could modify these effects in a community of Hydrobiosidae caddisflies. We exposed caddisfly egg masses from four species to four controlled temperature regimes in the laboratory and determined mortality from infection. Saprolegnia spp. infection in caddisfly egg masses is species specific, a signature for predator partitioning which can influence coexistence. The caddisfly species also differ in how temperature alters their infection rates, with dramatic shifts in relative infection rates depending on the duration and timing of periods of high temperature. Our findings suggest that Saprolegnia spp. infection could limit dominant species, shift community composition, and help stabilise coexistence under fluctuating temperatures. While we show key conditions of coexistence are met, quantifying the effect of these temperature-mediated infection rates would require measuring the extent to which they modify density-dependent feedback.