The impact of black-tailed deer on plant and bumblebee communities in Garry oak ecosystems

Background/Question/Methods

Only 5% of Garry Oak and Associated Ecosystems (GOAE) remain in Canada. Home to over 100 Species at Risk, these culturally and ecologically significant landscapes require urgent attention to guide future management. Hyperabundant black-tailed deer pose a threat to the persistence of these ecosystems. While the impact of hyperabundant deer on plant communities is well-documented, their impact on pollinators (essential for the restoration and conservation of GOAEs) is still unknown. In this study we test the hypothesis that deer browsing has an indirect effect on bumblebee populations through their direct negative impact of browsing on floral communities. We surveyed deer density, floral communities, and bumblebee abundances across ten islands with differing levels of deer presence in the Salish Sea, British Columbia. Simple Linear Regressions (SLR) and General Linear Mixed Models (GLMM) were used to assess the relationships between deer, floral resources, and bumblebees. A Piece-wise Structural Equation Model (PSEM) was then used to determine direct and indirect relationships within this system.

Results/Conclusions

Our results indicate that deer density has a negative relationship with all flowering stems and an even greater impact on flowers that bumblebees forage from (P=0.016, F1,48=6.3). Mean female bumblebee abundances were 225% higher on islands with no deer present, and our PSEM confirmed that the presence of deer was indirectly impacting female bees through the depletion of floral resources. These findings highlight the importance of immediate deer management to prevent the loss of native pollinators and further, potentially irreversible ecological degradation of GOAEs.