Long-term surveys support declines in early season forest plants used by bumblebees

Populations of bumblebees and other pollinators have declined over the past several decades due to numerous threats, including habitat loss and degradation. However, we can rarely investigate the role of resource loss due to a lack of detailed long-term records of forage plants and habitats. We used 22-year repeated surveys of more than 262 sites located in grassland, forest, and wetland habitats across Illinois, USA to explore how the abundance and richness of bumblebee food plants have changed over the period of decline of the endangered rusty patched bumblebee Bombus affinis. We documented a decline in abundance of bumblebee forage plants in forest understories, which our phenology analysis suggests provide the primary nectar and pollen sources for foundress queens in spring, a critical life stage in bumblebee demography. By contrast, the per-unit area abundance of food plants in primarily midsummer-flowering grassland and wetland habitats had not declined. However, the total area of grasslands had declined across the region resulting in a net loss of grassland resources. Synthesis and applications. Our results suggest a decline in spring-flowering forest understorey plants is a previously unappreciated bumblebee stressor, compounding factors like agricultural intensification, novel pathogen exposure and grassland habitat loss. These findings emphasize the need for greater consideration of habitat complementarity in bumblebee conservation. We conclude that the continued loss of early season floral resources may add additional stress to critical life stages of bumblebees and limit restoration efforts if not explicitly considered in pollinator conservation.

Automated summary

Populations of bumblebees and other pollinators have declined due to threats like habitat loss, with a notable decline in abundance of bumblebee forage plants in spring-flowering forest understories. However, per-unit area abundance of food plants in grasslands and wetlands has not declined significantly, but overall loss of grassland area has resulted in a net loss of resources. These findings emphasize the importance of considering habitat complementarity in bumblebee conservation efforts.