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Wild bee abundance declines with urban warming, regardless of floral density


As cities expand, conservation of beneficial insects is essential to maintaining robust urban ecosystem services such as pollination. Urban warming alters insect physiology, fitness, and abundance, but the effect of urban warming on pollinator communities has not been investigated. We sampled bees at 18 sites encompassing an urban warming mosaic within Raleigh, NC, USA. We quantified habitat variables at all sites by measuring air temperature, percent impervious surface (on local and landscape scales), floral density, and floral diversity. We tested the hypothesis that urban bee community structure depends on temperature. We also conducted model selection to determine whether temperature was among the most important predictors of urban bee community structure. Finally, we asked whether bee responses to temperature or impervious surface depended on bee functional traits. Bee abundance declined by about 41% per °C urban warming, and temperature was among the best predictors of bee abundance and community composition. Local impervious surface and floral density were also important predictors of bee abundance, although only large bees appeared to benefit from high floral density. Bee species richness increased with floral density regardless of bee size, and bee responses to urban habitat variables were independent of other life-history traits. Although we document benefits of high floral density, simply adding flowers to otherwise hot, impervious sites is unlikely to restore the entire urban pollinator community since floral resources benefit large bees more than small bees.

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We thank Holly Menninger; Sally Thigpen; the City of Raleigh Department of Parks, Recreation, and Cultural Resources; and volunteer homeowners who helped us find study sites and let us conduct research on their property. John Ascher, Adrian Carper, Sheila Colla, Sam Droege, Joel Gardner, Jason Gibbs, and Leif Richardson shared their knowledge of bee life history and nesting. Sam Droege and Jason Gibbs identified bee specimens. Nicole Bissonnette, Bobby Chanthammavong, Catherine Croft, Laura Daly, Samantha Dietz, Karly Dugan, Morgan Duncan, Anna Holmquist, and Danielle Schmidt assisted with specimen collection, bee measurements, and database management. Laura Daly identified plants. This study was funded by an Agriculture and Food Research Initiative Competitive Grant (2013-02476) from the USDA National Institute of Food and Agriculture to SDF and EY. This work was also funded by Cooperative Agreement No. G11 AC20471, G13 AC00405, and G15AP00153 from the United States Geological Survey to SDF. Its contents are solely the responsibility of the authors and do not necessarily represent the views of the Department of the Interior Southeast Climate Science Center or the USGS. This manuscript is submitted for publication with the understanding that the United States Government is authorized to reproduce and distribute reprints for Governmental purposes. North Carolina State University Department of Entomology also contributed support for this research.

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ALH and SDF conceived the ideas and designed methodology; ALH collected the data; ALH and EY analyzed the data; ALH and EY led the writing of the manuscript. All authors contributed critically to the drafts and gave final approval for publication.

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Correspondence to Elsa Youngsteadt.

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The authors declare that they have no conflict of interest.

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Hamblin, A.L., Youngsteadt, E. & Frank, S.D. Wild bee abundance declines with urban warming, regardless of floral density. Urban Ecosyst 21, 419–428 (2018).

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  • Bee
  • Impervious surface
  • Pollinator decline
  • Pollinator
  • Urban heat island
  • Urban warming