Influence of the predatory backswimmer, Notonecta maculata, on invertebrate community structure

1. The impact of the predator, Notonecta maculata, on invertebrate community structure was investigated by a combination of a natural pool survey, an outdoor artificial pool experiment, and a laboratory prey preference experiment. 2. In a survey of forty natural pools in the Negev Desert, Israel, pelagic/neustonic species richness and overall species richness, but not benthic species richness, were negatively associated with N. maculata density. 3. In the artificial pool experiment, in which the presence or absence of Notonecta was manipulated, Notonecta caused a reduction in the richness of species colonizing the pools. Notonecta strongly reduced densities of the pelagic dipterans, Culiseta longiareolata (often 100% reduction of later-instar larvae and pupae) and ephydrids. Fewer Culiseta egg rafts were found in Notonecta pools, indicating that part of the observed reduction of Culiseta is not due to consumption of Culiseta larvae by Notonecta, but to habitat selection by ovipositing Culiseta females in response to risk of predation by Notonecta. Notonecta did not affect densities of (benthic) chironomids or the small cladoceran, Ceriodaphnia cf. quadrangula. Laboratory assessments of selectivity by Notonecta for the various prey matched patterns found in the field experiment. 4. This work provides strong evidence that N. maculata is an important organizer of community structure: it strongly reduces or even eliminates larger pelagic or neustonic species, but does not affect densities of small or benthic species.