A trophic cascade effect of the predatory mosquitoToxorhynchites rutilus on aquatic bacteria
Feeding by container-dwelling mosquito larvae may negatively affect microorganism richness or abundance. However, indirect effects of mosquito predators on microorganisms are poorly studied. Larvae of the predator Toxorhynchites rutilus prey on larval Aedes triseriatus, which feed on bacteria. We postulated that a trophic cascade arises from T. rutilus predation on A. triseriatus larvae, impacting bacterial abundance within water-filled containers. We conducted a laboratory study in which we stocked experimental microcosms with oak leaf infusion as a nutrient source for bacteria and 100 first-instar A. triseriatus larvae. Treatments were (1) A. triseriatus alone (2) A. triseriatus with one T. rutilus larva, (3) A. triseriatus with water that had held T. rutilus larva feeding on A. triseriatus larvae (predatory cues). Controls were (1) infusion alone (2) Infusion plus predatory cues.
Bacterial abundance, measured at 7 and 14 days via 3H leucine incorporation, was greatest in the predator treatment (2) but did not significantly differ between predator (2) and predatory cues (3) treatments. We hypothesized that predator and predatory cues treatments contain partially eaten prey, increasing bacteria abundance. We tested this in an experiment in which A. triseriatus were crushed, crushed and removed, or subject to real predation. Controls received 100 first instar A. triseriatus larvae. Surprisingly, bacterial abundance was greater with real predation than in all other treatments. We suggest that presence of feces from the predator (and possibly the prey) or reduced foraging induced by predator cues also contributed to bacterial growth. Thus, effects producing this trophic cascade may go beyond the typical density-mediated impact of predators on prey populations, and include trait-mediated effects of predators on prey.