INTERACTIONS IN A LAKE OUTLET STREAM COMMUNITY - DIRECT AND INDIRECT EFFECTS OF NET-SPINNING CADDIS LARVAE

A field experiment was performed in a small lake outlet stream in order to examine the effects of net-spinning caddisfly larvae (Hydropsyche siltalai) on insect community structure. The experimental design allowed the effects of the larvae per se to be distinguished from the effects of their nets. H. siltalai larvae and nets affected all common insect taxa. The density of Ephemerella ignita (Ephemeroptera) nymphs was reduced by H. siltalai larvae, while the density of Simulium truncatum (Simuliidae) larvae was reduced by the presence of nets. Rhyacophila nubila (Trichoptera) and chironomid larvae increased their densities in response to nets of H. siltalai larvae. Laboratory experiments showed that predation was the most important mechanism whereby H. siltalai larvae affected E. ignita nymphs and S. truncatum larvae. Immigration and emigration rates of S. truncatum larvae were increased in the presence of H. siltalai nets. Data from the laboratory experiments and a simple colonization model were used to generate quantitative predictions of the effects of H. siltalai larvae and their nets on E. ignita nymphs and S. truncatum larvae. The predictions agreed fairly well with field data, indicating that the mechanisms included in the model, immigration. emigration and mortality caused by H. siltalai larvae, are sufficient to explain the effects found in the field experiment.