Bumblebee community composition and competition impacts reproduction of native and invasive plants

Background/Question/Methods

Our understanding of pollination network function can benefit from integrating concepts such as competition and indirect effects to better understand the how interaction-flexibility can affect ecological processes such as pollination and plant reproduction. We asked (1) whether there is evidence for competitive exclusion in short tongued bumblebees within pollination networks in an oak-savannah ecosystem, and (2) the impacts on native and invasive plant reproduction.

We examined interactions between two abundant and pollinator-attractive native plants, (Camassia quamash, C. leichtlinii) and an invasive species (Cytisus scoparius– scotch broom) and three short-tongued bumblebee at 18 sites. We examined plant-pollinator interactions by netting pollinators for a total of 40 minutes per site per sampling date and observing visit rates to our focal plants on each sample date. We then measured seed set for both Camassia species and for C. scoparius at the end of bloom. We examined how the composition of bumblebee visitors to our focal plants changed with the abundance of native and invasive plants at each site and the composition of the bumblebee community at the site. We similarly asked how these factors affected plant fitness.

Results/Conclusions

The abundance of our focal plants was the primary driver affecting the number of visits from Bombus melanopygus and Bombus bifarius. In contrast, visits by Bombus mixtus to C. quamash were negatively related to the abundance of B. bifarius at the site, suggesting that B. bifarius may exclude B. mixtus from visiting this plant. This contrasts with our preliminary data, where B. bifarius had similar impacts, but on B. melanopygus. Variation in the bumblebee community also related to reproduction in our focal plant species. For example, B. bifarius abundance was positively related to percent fruit set of C. quamash, and negatively related to percent fruit set in of C. leichtlinii. Fruit set in C. scoparius was positively related to the abundance of B. bifarius.

Our results suggest that interactions between competing bumblebee species may lead to emergent effects on pollination of plant species. High abundance of B. bifarus may reduce visit rates of B. mixtus to C. quamash, but fruit set data suggests that B. bifarius more than compensates as a pollinator. Our results also differ from our preliminary data, suggesting that interactions between Bombus species may vary from year to year.