PS 48-223 - Genera-specific analysis of ground nesting bees on an urban to rural gradient

Bees are keystone species, providing crucial pollination services to ecosystems. Reports of their decline have been a cause for concern as human land usage continues to disrupt their natural habitats. Ground-nesting bees may be particularly vulnerable to the surface-altering effects of urbanization. However, previous studies suggest the ground-nesting genus, Bombus, may prefer urban sites. This study analyzes trends in ground-nesting bee abundance over six years, to assess if other ground-nesting bees display a Bombus-like preference for urbanized areas, or if the ground-nesting community has experienced a rearrangement in spatial distribution. Furthermore, reported concentrations of Bombus and other ground-nesters will be used to determine whether a general change in local abundance has occurred over time. Bee sampling was done biweekly from early spring to late fall, using pan trap and sweep netting techniques at six sites with varying degrees of urbanization. Urbanization was estimated using three proxies: percent impervious surface, forest edge, and fragmentation index, within a 300m buffer radii. All three proxies were determined using ArcGIS. This analysis focused on the five most abundant ground-nesting wild bee genera in Southeastern Massachusetts: Lasioglossum (50.42%), Agapostemon (13.81%), Halictus (9.96%), Bombus (9.50%), and Augochlorella (4.74%).

There was a significant effect of percent impervious surface on wild ground-nesting bee abundance (F1,179=20.49, p<0.05), but not for forest edge or forest fragmentation index (p>0.05). The relationship between impervious surface and bee abundance differed among ground-nesting bees (F4,170=13.11, p<0.05), where Bombus showed a positive relationship with impervious surface (R² = 0.33) while the other genera showed negative relationships, the strongest being Augochlorella (R² = 0.6746). No statistically significant trend in yearly abundance was observed among the five genera. These data suggest that Bombus is unique among ground-nesting bees in its preference for areas with more impervious ground surface. Furthermore, the slight difference in abundance trends amongst the individual genera and the stability of the group as a whole suggests that a rearrangement within the ground-nesting community may be occurring even though bee abundance remains stable.