Enhancing ground-nesting bee habitat through substrate manipulations across an urban gradient

Enhancing ground-nesting bee habitat is crucial for sustaining bee diversity. However, our knowledge of the nesting requirements of ground-nesting bees remains incomplete, posing challenges for conservation efforts. Planting pollinator gardens in urban parks is a common strategy to provide bee habitat, but flowers only support bee populations if adequate nesting sites and materials are available within their foraging range. Urban landscapes may lack adequate nesting resources for ground-nesting bees, resulting in lower diversity of these species. Here, we examine the effectiveness of manipulating soil surface substrates to attract ground-nesting bees across an urban gradient. We established plots at 21 sites in the Columbus, Ohio metropolitan area with three replicated treatments per site of soil surface manipulations: bare ground, leaf litter, pebbles, mounded soil, and an unmanipulated control. We collected 196 bees representing seven ground-nesting and two cleptoparasitic genera across 19 sites. We found that bees preferentially colonized, and were significantly more abundant, within bare ground and leaf litter manipulations. Our landscape-level analysis revealed that urbanization negatively affected plot colonization and that, at the plot level, soil temperature positively affected bee abundance. These results fill gaps in our knowledge of bee nesting preferences and provide valuable insight and tools for enhancing bee habitats.