Individual and interactive effects of a predator and controphic species on mosquito populations

Species sharing the same trophic level as mosquito larvae (hereafter, controphic species) may have complex effects on mosquitoes by sharing both predators and food resources. We conducted an outdoor artificial pool experiment to assess the individual and interactive effects of a predator (Anax imperator) and controphic species (primarily Daphnia magna) on larval populations of two common mosquitoes, Culex pipiens and Culiseta longiareolata. Controphic species did not significantly affect survival to pupation of C. pipiens but did increase time to metamorphosis and reduce size at metamorphosis. Culex pipiens and D. magna, both primarily filter feeders, probably compete for food resources. Controphic species caused a small reduction (21.9%) in C. longiareolata survival, an unexpected result given that C. longiareolata larvae are thought to be primarily periphyton grazers while D. magna is a filter feeder. Controphic species did not affect C. longareolata time to, or size at, pupation. Anax imperator reduced C. longiareolata survival to pupation (78%) and size at pupation (11.5%) while increasing development time in males only (11.3%). Anax imperator caused a smaller (32.4%), but statistically significant, reduction in the number of C. pipiens surviving to the pupal stage but did not significantly affect size at or time to pupation. The predator did not reduce controphic species densities, and controphic species did not result in increased predator growth. We predicted that controphic species, by serving as competitors, would result in fewer C. pipiens surviving to pupation in the absence of predators. In the presence of the predator, the negative competitive effect of controphic species would be attenuated by the positive effect of serving as alternative prey. Our results followed this pattern although the predator X controphic species interaction was not statistically significant (P = 0.157). Because previous studies showed that D. magna did not affect predation rates by A. imperator on C. longiareolata larvae, and because controphic species did not have a strong competitive effect, we did not predict and did not find a predator X controphic species interactive effect.