Historical and citizen-reported data show shifts in bumblebee phenology over the last century in Sweden

Summary, in English

Bumblebees are a key taxon contributing to the provision of crop pollination and ecosystem functioning. However, land use and climate change are two of the main factors causing bee decline across the world. In this study, we investigated how the flight period of bumblebee spring queens has shifted over the last century in Sweden, and to what extent such shifts depended on climate change, landscape context, latitude, and the phenology of bumblebee species. We studied ten species of bumblebees and used observations from museum specimens covering 117 years from the southernmost region in Sweden (Scania), combined with citizen-reported observations during the past 20 years across Sweden. We found that the flight period of bumblebees has advanced by 5 days on average during the last 20 years across Sweden. In the agriculture-dominated region of Scania, we found that in the late 2010s bumblebee spring queen activity in simplified landscapes had advanced by on average 14 days, compared to 100 years ago. In addition, in simplified landscapes the flight period of early species was significantly earlier compared to in complex landscapes. Our results provide knowledge on the intraspecific variation of phenological traits, indicating that early species (often common species) exhibit a higher plastic response to the environment, which may facilitate adaptation to both climate and landscape changes, compared to the late species of which many are declining.