| Popis: |
SUMMARY (1) In laboratory experiments we estimated the attack rates and preferences of several instars of the predatory backswimmer, Notonecta hoffmani. The prey included mosquito larvae, surface prey, and three species of zooplankton: Ceriodaphnia reticulata, Daphnia pulex and Moina sp. Notonecta's predatory behaviour was shown to be stereotyped: neither level of hunger nor previous diet significantly influenced preference. Attack rate increased exponentially, and handling time decreased exponentially, with temperature. (2) 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 were exposed to predation by Notonecta populations at natural densities while controls were not. (3) 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. (4) We discuss the 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 populations. 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. |
