Interactive effects of urbanization and local habitat characteristics influence bee communities and flower visitation rates.

Current declines in the abundance and diversity of bees and other pollinators has created uncertainty in their ability to reliably deliver pollination services. Recent studies examining urban bee diversity have provided conflicting results, with some studies identifying parts of cities with high bee diversity and others documenting reduced diversity with high levels of urbanization, with potential effects on surrounding agricultural areas. However, these studies have not specifically investigated pollination services, or examined the influence of local habitat conditions on these services. We surveyed urban gardens and city parks across the metropolitan region of Toledo, Ohio (USA) to understand how urbanization (impervious surface) and local habitat characteristics (herbaceous cover, floral abundance and color, tree abundance, canopy cover, soil moisture, garden size) impact bee communities (abundance, diversity, composition) and pollination services (visitation frequency). We collected 729 bees representing 19 genera and 57 species. We found that bee community composition was strongly associated with percent impervious surface. Bee abundance declined with increased canopy cover and impervious surface, while declines in bee diversity with increasing impervious surface were greatly reduced by increases in floral resources. Visitation rates were positively correlated with bee abundance and diversity, declining with increased impervious surface, but increasing with floral resource availability. These results suggest that increasing floral resources at high impervious sites may counteract the negative effects of impervious surface on bee diversity and pollination services in cities similar to Toledo, OH.