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 lowerlevel organisms (such as bacteria and fungi) in west coast phytotelmata. Laboratory microcosm experiments were conducted to characterize the feeding strategies of Ae. sierrensis larvae in the presence of multiple resource types (free-swimming protozoa and substratebound particulate material). In our experiment, we quantified the effects of varying instar numbers and profile, resource type, and refuge size on predation of protozoa. Refugia were explicitly modeled in our microcosms, representing the interstitial spaces of leaf litter and the wood lining of natural tree holes. Results from these microcosm experiments suggested that: (1) Even in the absence of larvae, the majority of protozoa resided in the small-volume, resource-rich refugia. There was, however, a strong nonlinear and negative relationship between larval densities in the upper compartment and the protozoan densities in the refuge, suggesting that there was continual movement of protozoa between the two spaces. (2) Fourth instars harvested resources by filter-feeding at a higher rate than second instars. (3) As the level of substrate-bound particulate food was increased, the predation pressure 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 generalist feeding behavior of Ae. sierrensis larvae. The model system, with constant predator densities and two prey groups, exhibited full cooperativity; i.e., an increase in protozoa density resulted in a shift toward predation by filter feeding, while an increase in substrate-bound resources resulted in a shift toward predation by browsing. This indirect mutualism is mechanistically distinct from previously published systems and provides a potential mechanism for protozoan persistence in the presence of larval predation.