Pitcher‐Plant Midges and Mosquitoes: A Processing Chain Commensalism

Abstract

Larvae of the midge Metriocnemus knabi and of the mosquito Wyeomyia smithii are found only inside the water-filled leaves of the carnivorous pitcher plant, Sarracenia purpurea, where they feed on decaying invertebrate carcasses. I examined the interaction between the two species in a natural population in western Newfoundland, Canada.Experimental manipulations of rates of prey capture by pitcher-plant leaves indicated that both insect species are limited by carcass supply. However, the interaction between them is commensal rather than competitive. Midge growth was unaffected by experimental quadrupling of mosquito density in otherwise unmanipulated leaves. Mosquito growth, on the other hand, increased with midge density in both natural leaves and artificial leaf microcosms.This interaction is an example of a processing chain commensalism. Although both species feed on carcass material, they use it in different stages of decay: midges feed by chewing on solid material, while mosquitoes filter-feed on particles derived from the decaying matter. Consumption of particles by mosquitoes does not affect resource availability for midges, but feeding by midges does influence particle availability. In artificial leaf microcosms, high bacterial densities occurred sooner in artificial leaves with midges present than in identical leaves without them. Bacteria are a direct food source for mosquitoes, and high bacterial densities also indicate that other organic material is being comminuted, providing particles for mosquitoes and surface area for bacterial growth. Although midges remove some of the carcass resource that also limits mosquitoes, their net effect on mosquitoes is positive because they accelerate conversion ofthe remaining resource to particles.

Organic-matter loading determines regime shifts and alternative states in an aquatic ecosystem

“…This resource base of captured prey supports a five-trophic level food web that includes bacteria, protozoa, the bdelloid rotifer Habrotrocha rosa Donner, and larvae of several obligate Diptera (25,33,34). The Sarracenia food web has been characterized as a processing chain commensalism (33), but the top predators in the system-larvae of the pitcher plant mosquito, Wyeomyia smithii (Coq. ), and the pitcher plant flesh-fly, Fletcherimyia fletcheri (Aldrich)-are not critical for breakdown of prey and translocation of nutrients to the plant; the microbes by themselves efficiently decompose and mineralize nearly all of the captured prey biomass (29).…”

“…The plant has pitcher-shaped leaves that open during the growing season, fill with rainwater, and capture invertebrate prey, primarily ants (32). This resource base of captured prey supports a five-trophic level food web that includes bacteria, protozoa, the bdelloid rotifer Habrotrocha rosa Donner, and larvae of several obligate Diptera (25,33,34). The Sarracenia food web has been characterized as a processing chain commensalism (33), but the top predators in the system-larvae of the pitcher plant mosquito, Wyeomyia smithii (Coq.…”

Are natural microcosms useful model systems for ecology?

“…Mosquito occurrence at the pitcher scale is affected by bottom-up effects, such as carcass provisioning [43], carcass processing by midges [47] and pitcher size [43,48]. Importantly, these local effects on mosquito occurrence are modified by landscape-scale features, such as pitcher density and location [43,48,49], probably because of the behaviour of adults during oviposition [43,49].…”

Top predators affect the composition of naive protist communities, but only in their early-successional stage

“…The rainwater-filled leaves of Sarracenia purpurea is one such model system for aquatic communities (e.g., Miller and Kneitel 2005). This system shows the typical dynamics of larger aquatic food webs, but on small spatial and short time scales (e.g., Addicott 1974;Heard 1994;Kneitel and Miller 2002;Gotelli and Ellison 2006;Gray et al 2006;Hoekman 2007). Whole communities can be easily sampled and used in experiments both in the field and in the laboratory.…”