Soil Sanctuaries: Experimental Soil Surface Manipulations as Ground-Nesting Bee Habitat Across an Urban Gradient

As evidence of global pollinator declines mounts, effective conservation strategies are urgently needed. Current methods focus on planting flowers and frequently overlook nesting habitats, especially for ground nesting bees which comprise most species. Despite their prevalence, ground-nesting bee habitats remain poorly understood, with few evidence-based methods to enhance nesting habitat. This gap is problematic in areas with limited nesting sites, such as urban environments. We leveraged the limited experimental and anecdotal data to develop and test simple habitat manipulations that could improve the attractiveness or quality of nesting habitat for ground-nesting bees. We constructed experimental nesting plots at 21 sites along an urban gradient with five soil surface treatments: bare ground, leaf litter, mounded soil, pebbles and unmanipulated controls. Manipulated treatments significantly increased colonization compared to controls, differed in environmental conditions, and revealed genus-specific preferences. Site-level conditions, including slope and bare ground, strongly influenced colonization success. Urban and highly vegetated sites were more frequently colonized by ground nesting bees, indicating that our manipulations successfully attracted nest-searching females and may have alleviated limited nesting opportunities