Hyperabundant black-tailed deer impact endangered Garry oak ecosystem floral and bumblebee communities

Garry oak (Quercus garryana) and associated ecosystems (GOAE) are culturally and ecologically significant landscapes home to over 100 Species at Risk. With less than 5% of their original extent remaining in Canada, GOAE require urgent attention to guide management of remnant habitats and protect them from ongoing cumulative threats. The loss of top predators has played a large role in a trophic cascade where hyperabundant black-tailed deer (Odocoileus hemionus columbianus) are reshaping plant and animal communities, even within protected areas. The impacts of deer browsing on plant communities is well-documented; however, the impact on pollinators that are essential for the restoration and conservation of GOAEs is still unknown. We tested the hypothesis that deer browsing has an indirect impact on bumblebee populations through their direct negative impact 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. We found that deer had a negative effect on the abundance of flowering plants, with an even greater effect on native species that bumblebees rely on for foraging. Deer presence had an indirect negative impact on female bumblebee abundances through the depletion of floral resources. Deer also had a negative impact on male bumblebee abundances, potentially caused by lowered colony success over the season on islands with less 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.