Biotic and abiotic influences on a stream invertebrate predator guild

The composition of the invertebrate predator guild (19 species in total) was found to vary greatly among eight small, steep, parallel catchments in eastern Canada. Similarity declined significantly with distance at a scale of ~50 km, but not at distances of <10 km (neighboring streams). Negative associations were found between larval abundances of two common species, Rhyacophila vibox (Trichoptera: Rhyacophilidae) and Sweltsa onkos (Plecoptera: Chloroperlidae). However, numbers of adults were not inversely related; adult R. vibox were most abundant in the absence of brook trout; and numbers of adult S. onkos were highest in low gradient streams that maintained more summer flow. Three alternative hypotheses that could explain these patterns were then tested using behavioral and growth experiments in artificial laboratory streams: 1. S. onkos is the superior competitor and R. vibox persists regionally because S. onkos does poorly in fishless streams with late summer drying; 2. R. vibox is the superior competitor and S. onkos persists in the region because it can better tolerate fish presence; 3. R. vibox and S. onkos partition food resources such that there is little interspecific competition, and each species attains higher densities where its preferred food is more abundant. The results have important implications for the spatial scale at which predatory invertebrates coexist in small streams.