921 Ecology, 81(4), 2000, pp. 921–935  2000 by the Ecological Society of America GENERALIST FEEDING BEHAVIORS OF AEDES SIERRENSIS LARVAE AND THEIR EFFECTS ON PROTOZOAN POPULATIONS JOSEPH N. S. EISENBERG, 1,4 JAN O. WASHBURN, 2 and SEBASTIAN J. SCHREIBER 3 1 School of Public Health, Division of Environmental Health Sciences, University of California, Berkeley, California 94720 USA 2 Department of Plant and Microbial Biology, University of California, Berkeley, California 94720 USA 3 Department of Mathematics, Western Washington University, Bellingham, Washington 98225 USA Abstract. 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) in west coast phytotelmata. Laboratory mi- crocosm experiments were conducted to characterize the feeding strategies of Ae. sierrensis larvae in the presence of multiple resource types (free-swimming protozoa and substrate- bound 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 feed- ing 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 mech- anism for protozoan persistence in the presence of larval predation. Key words: Aedes sierrensis; biological model; filter feeding; generalist feeding; indirect mu- tualism; indirect positive effects; instar numbers and profile; microcosm experiments; mosquito larvae; Paramecium aurelia; resource type and refuge size; tree hole mosquitoes. INTRODUCTION Aquatic habitats residing on or in terrestrial plants (phytotelmata) provide a niche for a large number of microorganisms and invertebrates (Fish 1983). Many of these microcosms have been studied in order to ad- dress fundamental ecological problems, such as food web complexity (Kitching 1987, Pimm and Kitching 1987), competition (Livdahl 1982, Chambers 1985, Broadie and Bradshaw 1991), and succession (Kurihara 1983, Mogi et al. 1985). The majority of these inves- tigations have focused primarily on the invertebrate (insect) populations, especially mosquitoes, while gen- erally combining all microorganisms and detritus into a single ecological category. However, these habitats support rich protozoan populations that play a critical role in community organization (Kurihara 1983). Fur- Manuscript received 19 June 1998; revised 16 February 1999; accepted 26 February 1999; final version received 26 March 1999. 4 E-mail: eisenber@socrates.berkeley.edu thermore, protozoa occupy the midtrophic levels of phytotelmata ecosystems; they consume bacteria and are important prey for larger invertebrates, such as mosquito larvae (Fenchel 1987). Tree holes, a common microhabitat in tropical and temperate forest ecosystems, are filled with rainwater, and the communities they contain are based on an al- lochthonous energy source from leaves and other plant materials (Kitching 1971). Additional dissolved nutri- ents are supplied by material carried in stem flow (Car- penter 1982). Since leaf shed is the primary energy source for tree holes, these tend to be resource-limited habitats, resulting in a high degree of competition with- in and among resident invertebrate populations (Car- penter 1983, Livdahl 1982, Washburn et al. 1988a). In addition to resource limitations, periodic disturbances such as drying can limit the number of trophic levels; however, tree hole ecosystems generally include the larvae of one or more insect species, particularly im- mature mosquitoes (Pimm and Kitching 1987). These