Leaves of the carnivorous pitcher plant, Sarracenia purpurea, contain a food web composed of dipteran larvae, rotifers, protozoa, mites, and bacteria. The vase-shaped, rainwater-filled leaves attract small arthropods that drown in the fluid. Larvae of the midge Metriocnus knabi and mosquito Wyeomyia smithii participate in a processing chain facilitation, in which the midges are upstream consumers chewing on sunken carcasses, while the mosquito larvae are downstream consumers filter-feeding on fine particulate organic matter (FPOM) and bacteria supported by the FPOM. The rotifer Habrotrocha rosa and various protozoa also consume FPOM and bacteria. Although it has long been assumed that the mosquito and midge larvae do not compete, we hypothesize that the lack of overt signs of competition, such as reduced larval size or numbers, may not tell the whole story. We may have an example of a more complex interaction- intraguild facilitation - in which the two predators compete for prey, but one predator maintains the prey by maintaining the resource - FPOM. In this study, we focus on the possibility of competition between predators resulting in more subtle costs in terms of fitness that can be measured by female size at pupation.
Results/Conclusions
We compared rotifer and protozoan densities in 50 mL centrifuge tubes with and without midge and/or mosquito larvae. Wood ants served as “prey” for midge larvae. Bacteria counts increased in the presence of midge larvae alone, and decreased with mosquitoes. Bacteria numbers were the same in the control and in the presence of both midge and mosquito larvae. Rotifers were reduced to less than 10 per mL and 20 per mL in midge and mosquito treatments respectively. In the controls and in communities with both consumers, bacterial counts were > 600 per mL thus demonstrating the facilitation effect. Protozoa were eliminated from the community in the presence of midges and/or mosquitoes. The time period for these effects was between 3 and 5 days. Our current study looks at these same experimental communities in natural pitchers. Since rotifers are found in over 50% of the pitchers at our field site, we expect that there is a spatial refuge near the base of the leaf in which the protozoa and rotifers can hide. Ciliates on the other hand, are much rarer in the field, and we know from previous field studies that ciliates are rapidly driven to extinction by W. smithii larvae.