LB 31-299 - Does the Sarracenia purpurea inquiline community demonstrate competition and intraguild processing chain commensalism?

Background/Question/Methods:

The natural history of the inquiline community in the rainwater filled leaves of the carnivorous northern Pitcher Plant Sarracenia purpurea is well described. The species have historically been organized into a four trophic-level food web with bacteria as the basal-level supporting protozoa, a bacterivorous bdelloid rotifer Habrotrocha rosa and filter-feeding larvae of the pitcher plant mosquito Wyeomyia smithii. Rotifers and protozoa are consumed by the mosquito larvae, and rotifers, mosquitoes, protozoa and prey species are consumed by the raptorial larvae of the flesh fly Fletcherimyia fletcherii. H. rosa and protozoa consume fine particulate organic matter and bacteria. Heard established that M. knabi exerts a positive effect on W. smithii fecundity through a processing chain commensalism in which physical breakup of prey carcasses by the midge releases organic matter which is then consumed by bacteria. The increase in bacteria feeds the upper trophic-level species and causes an increase in mosquito fecundity. Because the midge and mosquito larvae occupy very similar feeding niches, we suspect that there may be competition between the two; but there have been no studies on the potential effects the mosquito larvae on midges.

Results/Conclusions:

Our first set of experiments looked at the effects of different food types and combinations on mosquito and midge larvae. The photosynthetic bacterium Rhodopseudomonas palustris produces population densities equal to control cultures grown on E.coli for both protozoa and rotifers. We are now planning an experiment, to be conducted in both artificial and natural pitchers, in which we will feed similar aged midge larvae with rotifers, protozoa (the ciliate Colpoda) and bacteria. We will then add young mosquito larvae to half of the “leaves.” Experimental units without mosquito larvae will serve as controls. Midges will be grown to pupation, and pupae will be harvested and weighed. Since pupal size in many insects is considered to be a appropriate substitute for counting actual egg numbers, we will use dry pupal weight as our metric for female fecundity. We have been unsuccessful raising midge larvae in the lab, and we find that we must provide protozoa and rotifers as live food.