Authors: Kendall, Liam; Nicholas, Kimberly;Brady, Mark;Olsson, Peter;Smith, Henrik;

With bumblebees facing an uncertain future under global climate change, systematic monitoring can improve their conservation by indicating vulnerable species, such as through changes in species distributions. In this study, we assessed temporal changes in bumblebee communities in semi-natural grasslands throughout Sweden over the last 15 years. The structure of bumblebee communities (i.e., density and richness) exhibited considerably temporal variability, a result driven by directional changes in the distributions of common species. In particular, the occurrence of the long-tongued species, B. hortorum, has declined considerably in the north of Sweden, whereas several short-tongued species (B. jonellus, B. lapidarius, B. pratorum and B. soroeensis), show the opposite pattern. Our results also demonstrate temporal phenological changes in several species’ activity periods. Floral resource availability was the key determinant of bumblebee densities in semi-natural grasslands. All species responded positively to increased local floral abundance, but the landscape-level availability of late-season floral resources affected bumblebee communities moreso than early season resources. Ensuring diverse floral resources throughout the year, alongside species-specific conservation plans, are needed to protect threatened bumblebee populations.