Long-term declines in early season forest plants critical to bumble bees, with a focus on Bombus affinis

Populations of bumble bees and other pollinators have declined over the past several decades due to a combination of numerous threats including habitat loss and degradation. However, we rarely can investigate the role of resource loss due to a lack of detailed long-term records of forage plants and habitats. We use 22-year repeated surveys of more than 262 sites located in grassland, forest, and wetland habitats across Illinois, USA to explore how cover and richness of bumble bee food plants have changed over the period of decline of the endangered rusty patched bumble bee (Bombus affinis). We document a loss of bumble bee plant cover within forests, which our phenology analysis suggests provides the primary nectar and pollen sources for foundress queens, a period known to be critical to bumble bee demography. By contrast, the per unit area abundance of food plants has not declined in the midsummer flowering grassland and wetland habitats, even as total grassland area has declined across the region. The results suggest a decrease in spring resources in forests may add to known stressors of bumble bees, compounding factors like agricultural intensification and novel pathogen exposure, and call for greater consideration of habitat complementarity in bumble bee conservation. We conclude that the continued loss of early-season floral resources within forests may add additional stress to critical life stages of bumble bees and limit restoration efforts if not explicitly considered.