Mutualisms in trophic cascades: Parasitism of bumblebees and pollination service to plants

Background/Question/Methods

Researchers are increasingly recognizing the important role of mutualisms in structuring communities and are viewing positive interactions in a community context rather than as simple pairwise interactions. However, the idea of the trophic cascade, a key concept in community ecology, has never been applied to mutualisms. While many mutualisms are not trophic interactions, the idea of multitrophic effects can be extended to understand this important interaction type in a community context. Antagonists of one mutualist have the potential to negatively affect the second mutualist through negative effects on their partner, and the magnitude of such effects should vary with mutualism strength. Bumblebees are ecologically and economically important pollinators that are an ideal system to determine if trophic cascades can be mediated by mutualisms. Bumblebees are attacked by an array of parasites and predators, and interact with a range of plants that vary in their dependence on bumblebees for reproduction. We tested whether variation in parasitism rates by Nosema bombi, Crithidia bombi and conopid flies correlated with reproduction of greenhouse-raised plants placed in the field. At multiple sites over two years, we studied four plant species which varied in their reliance on bumblebees as pollinators.

Results/Conclusions

We found a remarkably consistent negative relationship between Nosema parasitism and measures of pollination for Trifolium pratense and Solanum carolinense, and plant species with high bumblebee visitation, while Rudbeckia hirta and Daucus carota, plant species with generalized pollination, experienced weaker impacts of Nosema. Conversely, both Crithidia and conopids showed inconsistent relationships with pollination service.  Although these patterns are correlational, they provide evidence that parasites of bumblebees may have negative indirect effects on plants, and that mutualism strength can moderate the magnitude of such effects.