Reconstructing the glacial–interglacial biogeographic history of closely related North American Andrena Fabricius, 1775 bees

Over the past 2.6 million years there have been ~52 glacial-interglacial cycles, which have had profound implications for species’ distributional changes through time. Despite the ecological importance of bees, the long-term historical distributional shifts of most species have not been studied using ecological niche models (ENM) or analyzed in depth biogeographically. To date, ENM studies have been limited for bees in the genus Andrena . However, Andrena is an extraordinarily diverse group represented by > 1,700 species, offering a unique opportunity to address complex biogeographical questions through ENM. The species restricted to North America in the subgenus Plastandrena represent an ideal system for such study due to their common ancestry, occupancy of diverse environments, and polylectic nature. We utilized Chelsa-bioclim V2.1 1981–2010 and Chelsa-TraCE21k climate data in combination with the maximum entropy method (Maxent) to model the potential suitable area for each species during the modern day and Last Glacial Maximum (LGM). Our findings suggest that each of the five species potential distributions may have experienced significant expansions from the LGM to modern day, even though each species occupies distinct environments. Furthermore, we document the species-level biogeography of each taxon, identify new areas of potential discovery, and address regions where colonization may be limited by barriers such as the Sonoran Desert and species-specific phenologies. Future research should test these interpretations through population genetic studies. First detailed study of Andrena bees using ecological niche models with hindcasting to the Last Glacial Maximum (LGM). Potential significant post-glacial expansions were identified for all five species from the LGM to the present even though each species occupies distinct environments. New potential localities for species discovery are documented across North America. Barriers to colonization, including the Sonoran Desert and specific phenologies, likely restrict Andrena argemonis and Andrena mellea from coastal regions of the Western United States. Andrena crataegi may have undergone splitting into three or more populations during the LGM, followed by a limited reconnection during the modern interglacial period. Our results demonstrate the unique biogeography of these understudied bee species and illustrate how dramatic glacial-interglacial cycles have driven distributional changes, underscoring the need for future population genetic research.