Complex effects of urban land use on native bee communities in a rapidly growing metropolis

By 2030, nearly 60% of the human population is expected to live in cities. Rapid urbanization has been implicated as causes of population declines in numerous native plant and animal species, which can often lead to decreasing ecosystem functionality and services. One critical ecosystem service is pollination, with bees being the primary pollinators of both agricultural crops and native plants. Native bees are experiencing worldwide declines and are thought to be heavily impacted by increasing urbanization.  Despite growing public concern over these pollinator declines, very little is known about the contribution of urbanization to native bee population declines and changes in bee community composition.  In this study, we investigated how urban land use impacts native bee abundance, species richness, and community composition. We collected native bees living in and near the heavily urbanized area of Austin, Texas, USA, in two habitat types:  urban farms and gardens, and urban nature preserves, with an equal number of agricultural and natural sites outside of the urban area, 20 sites in total.  We measured land-cover type, percent canopy cover, and floral diversity and abundance using on-the-ground measurements and GIS land cover data in and around each sampling site.

We found that bee species richness, diversity, and abundance of native bees were significantly negatively correlated with increasing urbanization; measured by increasing percent impervious surface cover, decreasing tree canopy cover, and decreasing floral diversity and density.  We also found that native bee community composition shifted dramatically across the landscape, with larger-bodied cavity nesting species occurring more often within urban areas and smaller-bodied ground nesting species occurring more often outside the urban area. Overall, these results suggest that urbanization is negatively impacting native bee populations. Future work will focus on quantifying genetic diversity and population structure of native bee species in urban areas to determine if increasing urbanization impacts populations connectivity and gene flow.