Complex effects of urban landscapes on wild bee communities
Cities are now home to half of the world’s seven billion people, with the majority of the human population expected to live in cities by 2030. Considering the inevitability of urbanization, it is vital to assess its impacts on native wildlife populations and the important ecosystem services they provide. One critical ecosystem service is pollination, where bees are the primary pollinators of both wild plants and agricultural crops. Although prior work suggests that bee population persistence is primarily driven by availability of nesting sites and floral resources, very little is known about how the size, spatial configuration and availability of these resources impacts bee community composition and persistence, particularly in urban ecosystems. In this study we conducted a community-level survey of native bees along two gradients of urbanization in Austin and Dallas, TX. In each city we sampled 20 sites, collectively representing a gradient of land cover from 0 to 98 percent developed land (40 sites total). Bees were collected uniformly across sites using a combination of standardized hand-netting, pan traps and blue vane traps. Vegetation surveys were also conducted at each site, and the number of inflorescences of all flowering plants was counted. All bee and floral specimens were identified to species level, and land use composition was assessed at the local (250 m) and landscape (2 km) scale. To quantify the influence of landscape and floral resource availability on bee abundance and diversity, we used linear models to regress bee community composition on local floral resource data and local and landscape land cover data using linear models in R.
We collected a total of over 14,000 native bee specimens and surveyed more than 225 flowering plant species across our study system. Floral species richness was significantly positively related to percentage of developed land within a 2 km radius of each site. Surprisingly, neither bee species richness nor bee abundance was significantly related to either floral species richness or floral abundance across all sites. In contrast, bee species abundance was significantly negatively related to percentage urban development within a 2 km radius of each sampling point. Bee species richness was not significantly related to any floral or land use factor. Contrary to previous research, these results suggest that landscape-level nesting habitat availability is potentially a more important driver of bee community assembly and abundance than local floral abundance within rapidly urbanizing landscapes.