Effects of Red-Spotted Newts (Notophthalmus Virldescens) on the Densities of Invertebrates in a Permanent, Fish-Free Pond: A One-Month Enclosure Experiment

The effects of predation by adult red-spotted newts (Notophthalmus viridescens) on littoral-zone benthic invertebrates were examined by experimental manipulations of newt density. To minimize enclosure effects and facilitate comparisons with similar studies of fish, the experiment was limited to one month. The study was carried out in September and October to maximize the potential response by benthic insects, many of which feed and grow rapidly at this time of year. The abundance of chironomid larvae was significantly reduced by newt predation, while the damselflies Lestes eurinus and Enallagma aspersum showed similar but nonsignificant responses to newts. On the other hand, density of the ostracod Cypria was positively related to newt density. This indirect effect was probably mediated through a three-level trophic cascade. Newts had no apparent effect on size distributions of prey, although they tended to deplete mobile prey that inhabit macrophytes. In this study, newts appear to have a minor short-term impact as predators relative to fish. This pond system is apparently dominated by strong invertebrate interactions and structural complexity