This multifaceted study uses the aquatic community inhabiting the water-filled leaves of the carnivorous pitcher plant Sarracenia purpurea as a natural model system for studying and testing metapopulation and metacommunity dynamics, and for evaluating the close interplay between processes at both the local and regional scales. Interactions among the pitcher plant mosquito Wyeomyia smithii, the three most abundant and frequently observed protozoa, and pitcher bacteria have been described previously. The data presented here complete the survey of local interactions within pitchers by examining the role of H. rosa within the pitcher community.
Results/Conclusions
Field and laboratory studies show that rotifer populations are regulated by larvae of the mosquito Wyeomyia smithii via competition and intraguild predation, and possibly via egg predation by larvae of the midge Metriocnemus knabi. The relationship between H. rosa and pitcher protozoa has not been clearly established. Here I report laboratory and field study data indicating that H. rosa does not significantly impact the protozoan community. Rotifers collected from pitchers often have small mixotrophic chrysophytes throughout the gut, but it is not clear that these are digested. Since many chrysophytes bear siliceous scales on the cell surface, the animals may be unable to break open the cells with trophi adapted for a diet of bacteria and detritus. In a laboratory study, H. rosa had a slight but significant negative effect on Colpoda a common pitcher ciliate (one-tailed t-test, p < 0.01); however, this effect was probably the result of competition rather than predation since rotifers were never directly observed capturing and consuming Colpoda. Two other common pitcher protists, Cyclidium and Bodo, were unaffected by H. rosa. Thus, in the context of the full pitcher food web, it seems reasonable to treat H. rosa as strictly a bacterivore that has significant direct and indirect effects on bacterial community structure. H. rosa population dynamics vary significantly according to bacterial species availability. In a laboratory study, grazing by H. rosa resulted in a marginally significant decrease in total bacterial cell density (one-tailed t-test, p = 0.057) while the relative abundances of nine bacterial taxa were significantly altered (profile analysis of variance, p < 0.0001). These grazer-induced changes in bacterial species abundance patterns may explain rotifer-mediated competitive interactions among ciliates in the pitcher community. Bacterial community structure was significantly affected directly by rotifer grazing and indirectly through higher-order interactions among all food web components, and this may have a strong impact on nutrient flow through the community.
