An analysis of bee communities in home and community gardens
Gardens have the potential to act as important habitat for urban bee communities, by provisioning forage and nesting sites. However, depending upon the plant palette and management practices used by gardeners, gardens also may act as an ecological filter (excluding specific taxa or functional groups), contributing to biotic homogenization (local reductions of rare species coupled with increased establishment of widespread species). We reviewed the data from studies of garden bee diversity in North America. Specifically, we wanted to better understand 1) the number and types of bee species that have been collected from gardens, 2) the ecological characteristics of garden bees, and 3) how garden bee communities compare to the regional pool of bee species collected from non-garden sites. We found 7 studies of bee communities in home or community gardens that both identified most bees to the species level and for which a non-garden comparison study in the same ecoregion was available. We used a cluster analysis to compare bee communities collected from garden and non-garden sites. We consulted a variety of natural history papers, journal articles and online databases to define the ecological characteristics (taxonomic family, origin (native or exotic); nest substrate; floral specificity; sociality) of garden and non-garden bees. We graphed and visually assessed the data to identify bee eco-types that were over- and under-represented in gardens, compared to non-garden sites.
Across the 7 garden studies, at least 213 bee species have been collected from home or community gardens. The most ubiquitous garden bees, collected in at least 6 of the 7 surveys, were the native bee Halictus ligatus, and the exotic bees Megachile rotundata and Apis mellifera. In the cluster analysis, garden bee communities segregated according to broad geographic region (eastern bees and western bees). When non-garden bee communities were included in the analysis, garden bee communities clustered together, within broad geographic region. Together, these results suggest that bee communities in gardens reflect the regional bee species pool, but also that gardens may contribute to biotic homogenization. Our qualitative analysis of bees’ eco-types suggests that gardens may act as ecological filters: gardens have fewer spring-flying bees and fewer soil nesters compared to non-garden sites. There are several potential explanations for this pattern including less intensive spring sampling for bees in garden studies, less availability of spring-blooming trees and other plants in gardens, and fewer undisturbed nesting sites for bees in gardens due to garden management.