A top predator affects community dynamics of predator-naive Sarracenia pitcher plant communities

Background/Question/Methods

Sarracenia purpurea is a carnivorous pitcher plant and a model system for aquatic communities. The leaves of this plant species form a pitcher shape and, once they open, they fill with rainwater. An aquatic community of bacteria, protozoans and rotifers form a food web in these detritus-fueled ecosystems. In the plant’s native range of North America, this food web also includes an insect larvae top predator, which is endemic to the Sarracenia pitchers. In the non-native range of Switzerland, Sarracenia was introduced by seed, so the native food web and its top predator were not imported. The Swiss Sarracenia communities have thus developed without a top predator and are therefore top predator naive. We tested the effect of top predator presence on early and late succession protozoan and bacterial communities that were either naive or non-naive to the top predator. We collected early and late succession communities from four climatically similar sites. Two of these sites were from North America (non-naive) and two sites were from Switzerland (naive). We then conducted a common garden experiment in which we followed changes in protozoan community composition and bacterial density through time, with and without the presence of the top predator.

Results/Conclusions

We found that the intermediate trophic level (protozoan) diversity was never significantly altered by top predator presence. However, in naive, early succession communities, there was a change in protozoan community composition when the top predator was present. All late succession and non-naive protozoan communities were not susceptible to the top predator. This result suggests that top predators could have an effect on the community composition of naive prey, but only during specific stages of community development. This effect filtered down to the bottom trophic level. When a top predator was present, bacterial density was higher, possibly due to a strong effect of top-down control between the top predator and the protozoan communities. Our results highlight the effect that non-native top predators can have on naive food webs and that understanding the magnitude of these effects is dependent on the successional stage of the naive community.