Interplay between dispersal determinants of larval hydropsychid caddisflies

Abstract

We examined the interactive effects of predator (perlid stoneflies) presence, epilithic algal cover, and substrate size on drift and resettlement decisions by larval hydropsychid caddisflies in a series of experiments conducted in real and artificial streams. Predator effects on drift-entry depended on algal structural complexity. Stoneflies significantly increased hydropsychid drift from biofilm-covered rocks but not from rocks covered with a thick periphyton mat or with a macroalga (Cladophora glomerata, approx. 20% surface cover). When we manipulated degree of Cladophora cover, we found that Cladophora even in small amounts (4-13% cover) nullified stonefly effects on drift and that drift was inversely related to amount of Cladophora on rocks. However, Cladophora may only be important as a dispersal determinant until retreats (i.e., immovable cases) are constructed and vulnerability to predators decreases. Hydropsychid drift rates were positively related to gravel abundance, but Cladophora presence enhanced settlement rates on the substrate. Our study shows that multiple factors jointly influence dispersal patterns of hydropsychid larvae and that the relative strength of any one factor as a dispersal determinant varies depending on conditions of other determinants.