Generalist feeding behaviors of Aedes sierrensis larvae and their effects on protozoan populations

The generalist feeding strategy of larvae of the western tree hole mosquito , Aedes sierrensis, is central to understanding the community-level effects of the tritrophic interactions among mosquito larvae, midsized organisms ( such as protozoa), and lower-level organisms (such as bacteria and fungi) i n west coast phytotelmata. Laboratory microcosm experiments were conducted to characterize the feeding strategies of Ae. sierrensis larvae in the pres ence of multiple resource types (free-swimming protozoa and substrate-bound particulate material). In our experiment, we quantified the effects of var ying instar numbers and profile, resource type, and refuge size on predatio n of protozoa. Refugia were explicitly modeled in our microcosms, represent ing the interstitial spaces of leaf litter and the wood lining of natural t ree holes. Results from these microcosm experiments suggested that: (1) Eve n in the absence of larvae, the majority of protozoa resided in the small-v olume, resource-rich refugia. There was, however, a strong nonlinear and ne gative relationship between larval densities in the upper compartment and t he protozoan densities in the refuge, suggesting that there was continual m ovement of protozoa between the two spaces. (2) Fourth instars harvested re sources by filter-feeding at a higher rate than second instars. (3) As the level of substrate-bound particulate food was increased, the predation pres sure by filter-feeding on the protozoa decreased. (4) As the refuge volume increased, the predation pressure on the protozoa decreased. We constructed a three-state-variable mathematical model describing the gen eralist feeding behavior of Ae. sierrensis larvae. The model system, with c onstant predator densities and two prey groups, exhibited full cooperativit y; i.e., an increase in protozoa density resulted in a shift toward predati on by filter feeding, while an increase in substrate-bound resources result ed in a shift toward predation by browsing. This indirect mutualism is mech anistically distinct from previously published systems and provides a poten tial mechanism for protozoan persistence in the presence of larval predatio n.