Influence of nymphal Anax imperator (Odonata: Aeshnidae) on oviposition by the mosquito Culiseta longiareolata (Diptera: Culicidae) and community structure in temporary pools.

We assessed the overall (consumptive plus non-consumptive) and non-consumptive effects of nymphal Anax imperator (Aeshnidae: Odonata) on experimental pool communities by comparing three treatments: (1) control (no Anax); (2) free Anax (Anax was not constrained); and (3) caged Anax (Anax, enclosed within a cage, could not consume prey outside the cages, but could possibly influence them via perceived risk of predation). Fewer egg rafts of the mosquito Culiseta longiareolata were found in the free Anax treatment compared to the other two treatments. There was no statistically significant difference in the number of egg rafts between control and caged Anax pools. Thus, while Culiseta females apparently oviposit fewer egg rafts in the presence of unconstrained Anax, they did not respond to predation risk from the caged Anax. Larval Culiseta densities were drastically reduced by free Anax; there was nearly a 100% reduction in the number reaching metamorphosis (pupae) and a 100% reduction in emergence (pupal exuviae). There were no significant treatment effects on densities of crustaceans, Daphnia magna and Heterocypris sp., or on chironomid pupal exuviae. Ceratopogonid pupal abundance was higher in free Anax pools than in the other two treatments toward the end of the experiment. Free Anax caused a trophic cascade, presumably by strongly reducing the dominant periphyton grazer, Culiseta larvae; periphyton mass was greater in the free Anax treatment than in the control. However, there was no behavioral trophic cascade, i.e., no difference in periphyton abundance between the control and caged Anax treatment.