Recent developments in urban biodiversity research have pointed to the fact that cities serve as a refuge of sorts for bees and other insect pollinators. The bee diversity of cities can be higher than nearby agricultural areas, and at times even comparable to that of natural systems. Our work in St. Louis city has shown that bee diversity is the highest reported for an urban environment, with over 160 species of bees. Similarly to results from other locations, e.g., Berkeley, CA, Chicago, IL, New York, NY, and Northampton, UK, bee diversity at the local scale in St. Louis is highly influenced by local floral diversity. At the larger scales, biogeographic and landscape processes affect the species pool that cities can draw from. Yet, there is still significant debate about the various forces and mechanisms that affect diversity at intermediate scales in cities. In order to ascertain the contribution of spatial structuring in cities due to social processes, we sampled bees biweekly, via aerial nets at a rate of one-person hour per 0.25 ha, in four urban farms and nine community gardens in 2015 and 2016. We also surveyed managers and gardeners for information regarding practices and norms. Finally, we abstracted spatial and demographic data for each location using GIS.
We identified 122 species of bees occurring in community gardens and urban farms across the city of St. Louis. There was tremendous species turnover among locations throughout the city. The turnover was the least among gardens in neighborhoods with similar economic and ethnic composition. These neighborhoods also had similar ordinances concerning yard and lawn maintenance. Species turnover was the highest between urban farms and community gardens. This was related to the more extensive and intensive use of space in the urban farms. Still, the cultural and ethnic differences among managers of the urban farms may also contribute to the differences observed. It seems that the diversity of bees in St. Louis is been affected not just by ecological forces but also by social and cultural norms that operates at intermediate scales.