Effects of Metriocnemus knabi predation on bacterivorous ciliates in Sarracenia purpurea pitchers
Decomposition of prey in the leaves of the carnivorous pitcher plant Sarracenia purpurea provides energy that supports an inquiline community consisting of bacteria, protozoans, rotifers, mites, and dipteran larvae. The natural history of this community is well described, and it is historically organized into a four trophic-level food web with bacteria as the basal level supporting protozoa, the 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; rotifers are also consumed by the raptorial larvae of the flesh fly Fletcherimyia fletcheri. Larvae of the pitcher plant midge Metriocnemus knabi feed on drowned arthropods and, in a processing chain commensalism, promote bacterial growth and a subsequent increase in larval mosquito biomass. We recently reported that midge larvae also consume rotifers. In this study, we further tease apart the interactions among W. smithii and M. knabi larvae, H. rosa and two common bacterivorous pitcher ciliates. We reconstituted food webs in a factorial design experiment in 50 mL plastic centrifuge tubes using wood ants as “prey.” The experiment ran for two weeks; ants were added on days 1 and 7.
As expected, predation by both W. smithii and M. knabi caused significant decreases in rotifer numbers and drove them to near extinction by 14 days (p < 0.001) W. smithii also caused significant reductions in Colpoda and Cyclidium (p < 0.01). Unexpectedly, both ciliates were reduced to very low numbers by M. knabi larvae (p < .01). In each case, there were significant additive interactions between the two predators resulting in decreases in the two ciliates after one and two weeks (p < 0.03 for Colpoda; and p < 0.0001 for Cyclidium). There was a significant interaction between midges and mosquitoes for rotifers, but only after two weeks (p < 0.0001). In natural pitcher communities, it is not uncommon to find rotifers and ciliates in the presence of both dipteran consumers. We hypothesize that a spatial refugium exists within the pitcher based on the feeding behavior of the two consumers. By demonstrating that midges consume both rotifers and ciliates, we now have reason to redraw the pitcher plant inquilines food web. We suggest that both W. smithii and M. knabi together form a guild of keystone predators.