Bumblebee competition may affect pollination service to an invasive species in the endangered Garry Oak ecosystem

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 examined (1) whether there is evidence for competitive exclusion in short tongued bumblebees within pollination networks in an endangered oak-savannah ecosystem, and (2) the potential impacts on native and invasive plant reproduction.

Using interaction networks from eight sites, we examined interactions between two abundant pollinator-attractive native plants (Camassia quamash, Camassia leichtlinii) and an invasive species (Cytisus scoparius– scotch broom) and three short-tongued bumblebee species.  Landscape variation in urbanization leads to variation in the abundance of the forest-associated bumblebee Bombus bifarius. For each bumblebee species we asked whether its foraging preferences were related to the abundance of the focal plant species or the abundances of potential bumblebee competitors. We then asked whether variation in the abundances of bumblebee species might affect pollination of focal plant species by examining whether the number of potentially effective pollinators collected from flowers was related to the abundance of each bumblebee species at the site.

Results/Conclusions

Competitive interactions may impact both foraging choices of bumblebees and pollination of plants. B. bifarius and B. mixtus foraging patterns seem to be driven by the plant community – as C. quamash decreases in abundance, they forage on other plants. B. melanopygus seems to respond to competing bumblebees. As B. bifarius increases in abundance, B. melanopygus visits C. quamash less often, and visits C. scoparius instead.  For C. leichtlinii, and C. quamash, changes in the abundance of different bumblebee species was not correlated with the total abundance of potentially effective pollinators (defined as larger-bodied bees). However, increasing abundance of B. bifarius was correlated with reduced collection of potentially effective pollinators on C. scoparius.

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 indirectly improve pollination of C. scoparius, as it may cause B. melanopygus to shift its foraging preferences to this invasive species. However, C. scoparius had overall lower visitation from potentially effective pollinators at sites with high B. bifarius. This may be due to the association between B. bifarius and highly forested sites, where there are lower numbers of honeybees. Future research will explore how landscape and local interactions may influence the community in more detail.