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 orde r to examine the effects of net-spinning caddisfly larvae (Hydropsyche siltalai) on insect community structure. The experimental design allo wed the effects of the larvae per se to be distinguished from the effe cts of their nets. H. siltalai larvae and nets affected all common ins ect taxa. The density of Ephemerella ignita (Ephemeroptera) nymphs was reduced by H. siltalai larvae, while the density of Simulium truncatu m (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 show ed 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 pre sence of H. siltalai nets. Data from the laboratory experiments and a simple colonization model were used to generate quantitative predictio ns of the effects of H. siltalai larvae and their nets on E. ignita ny mphs and S. truncatum larvae. The predictions agreed fairly well with field data, indicating that the mechanisms included in the model, immi gration. emigration and mortality caused by H. siltalai larvae, are su fficient to explain the effects found in the field experiment.