Soil sanctuaries: Experimental manipulations enhance ground-nesting bee habitat across an urban gradient

O_LIAs evidence of 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. The lack of evidence-based methods to enhance nesting habitat presents a significant challenge, particularly in environments with limited natural nesting sites, such as urban landscapes. C_LIO_LITo determine whether soil manipulations enhance bee colonization rates, we constructed experimental nesting plots at 21 sites along an urban gradient and implemented five soil surface treatments: bare ground, leaf litter, mounded soil, pebbles, and unmanipulated controls. We collected nesting individuals from experimental plots through observation and emergence traps to investigate preferences for soil surface substrates, and the impact of plot-level and site-level conditions on colonization. C_LIO_LIManipulated treatments significantly increased colonization compared to controls and revealed genus-specific preferences. Treatments also resulted in differences in environmental conditions including temperature, moisture, and soil compaction. C_LIO_LISite-level conditions including slope and urbanization increased colonization while hard soil compaction and bare ground decreased 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. C_LIO_LIPolicy Implications: Simple soil surface manipulations, or soil sanctuaries, represent a practical tool to enhance an essential component of bee habitat and provide a safe nesting location that can improve conservation outcomes for ground-nesting bees across diverse environments. Incorporating ground-nesting bee habitat enhancements into urban planning and land management policies could address habitat limitations for many species. Conservation strategies should prioritize the inclusion of nesting features alongside floral resources to create comprehensive pollinator-supportive landscapes, benefiting both biodiversity and critical ecosystem services. C_LI