Effects of Metriocnemus knabi predation on Habrotrocha rosa populations in Sarracenia purpurea pitchers
Decomposition of prey in the leaves of the carnivorous pitcher plant Sarracenia purpurea provides the energy that supports an inquiline community consisting of bacteria, protozoans, rotifers, mites, and dipteran larvae. This community has been studied for decades, and its natural history and food web structure have been well described. It is organized into a four trophic-level food web in which the bacterivorous bdelloid rotifer Habrotrocha rosa is consumed by filter-feeding larvae of the pitcher plant mosquito Wyeomyia smithii and raptorial larvae of the flesh fly Fletcherimyia fletcheri. No other interactions have been documented. Heard (1994) demonstrated a processing chain commensalism between larvae of the pitcher plant midge Metriocnemus knabi and W. smithii in which midges produced an increase in bacterial density by feeding on prey within the leaf. In response to this food increase, mosquito biomass increased significantly. We hypothesized a similar interaction occurs between midges and rotifers; we predicted that prey consumption by M. knabi larvae would cause an increase in H. rosa density due to the proliferation of bacteria. We compared rotifer densities in 50 mL centrifuge tubes with and without midge larvae. Wood ants served as “prey” for midge larvae.
Bacterial densities in tubes containing midge larvae were significantly higher than densities in tubes without midges (p < 0.01); however, instead of causing an increase in rotifer density, midge larvae caused a decrease in H. rosa populations from an initial 25 rotifers/mL to less than 1 rotifer/mL after five days. In the absence of midges, rotifers increased significantly to 407.8 + 48.24 rotifers/mL (p < 0.0001). We hypothesized that M. knabi was directly consuming rotifers, and our suspicions were confirmed through direct microscope observations. When W. smithii and M. knabi larvae were both introduced as predators, they produced an additive effect. As expected, rotifers disappeared even more rapidly in the presence of two predators in centrifuge tubes. How, then, does H. rosa coexist with both mosquito and midge larvae in a natural pitcher? Initial studies suggest that the rotifers occupy a spatial refugium either where the leaf cavity tapers at the bottom, or at intermediate water levels below mosquito larvae feeding near the surface, but well above the reach of bottom dwelling midge larvae. We plan to repeat this study in pitcher plant leaves, and we will also look into the possibility of interactions between H. rosa and protozoa.