Effects of the General Predator, Notonecta (Hemiptera) Upon a Freshwater Community

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Abstract
In laboratory experiments attack rates and preferences of several instars of the predatory backswimmer, N. hoffmani were estimated. The prey included mosquito larvae, surface prey and 3 spp. of zooplankton: Ceriodaphnia reticulata, Daphnia pulex and Moina sp. Notonecta''s predatory behavior was stereotyped: neither level of hunger nor previous diet significantly influenced preference. Attack rate increased exponentially and handling time decreased exponentially with temperature. These laboratory results and others reported elsewhere were used to explain the results of field experiments in which treatment prey populations in stock tanks on ranches in southern California [USA] were exposed to predation by Notonecta populations at natural densities while controls were not. In these field experiments, Notonecta strongly influenced the structure and dynamics of the community. It drove mosquito larvae and D. pulex extinct and reduced the density of Moina sp., C. reticulata and several other species. Notonecta either destabilized its prey populations by driving them extinct or did not affect the degree of temporal variability. Relationship of these results to prevailing views on the importance of predation by invertebrates in aquatic communities, extinction in natural communities and the question of whether predators in general act to stabilize or destabilize prey population are discussed. The qualitative results obtained in the field were well-predicted by the information obtained in the laboratory but caution is needed in extrapolating estimates of absolute attack rates from the laboratory to the field.