Effects of Snails and Fish on Benthic Invertebrate Assemblages in a Headwater Stream

Abstract

At natural densities in enclosures in an eastern Tennessee headwater stream, snails (Elimia clavaeformis) and juvenile creek chubs (Semotilus atromaculatus) affected benthic invertebrate assemblages. Elimia reduced the total number of invertebrates without affecting total invertebrate biomass. Conversely, creek chubs reduced the total biomass, but not the total number of invertebrates. No interaction was detected between snail and fish effects on either total invertebrate biomass or abundance. Both taxa appeared to affect the benthos by direct and indirect trophic pathways. Elimia reduced the abundance of non-tanypodine chironomids, apparently by interference. Elimia also appeared to reduce the abundances of some predators of benthic insects (salamanders [Desmognathus fuscus], dragonfly nymphs [Lanthus sp.], and young-of-the-year creek chubs). Juvenile creek chubs (55-70 mm total length [TL]) excluded young-of-the-year chubs (15-25 mm TL) from the enclosures and appeared to reduce the abundance of dragonfly nymphs. These changes may have increased the abundances of herbivorous/detritivorous insects such as the stonefly Leuctra. Multiple regression analyses indicated that snail and fish mass were more important than light in determining the total biomass and numbers of invertebrates in the channels.