Wild bee abundance declines with urban warming, regardless
of floral density
April L. Hamblin
Steven D. Frank
Published online: 31 January 2018
Springer Science+Business Media, LLC, part of Springer Nature 2018
As cities expand, conservation of beneficial insects is essential to maintaining robust urban ecosystem services such as pollina-
tion. 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.
Urban heat island
A ubiquitous feature of cities is that they are warmer than sur-
rounding natural areas (Oke 1973). This phenomenon, called the
urban heat island effect, is driven largely by impervious surfaces
that absorb and reradiate solar radiation (Yuan and Bauer 2007).
As small-bodied ectotherms whose metabolism depends on tem-
perature, insects are sensitive to warming (Nooten et al. 2014;
Colinetetal.2015). Indeed, urban warming can alter the physi-
ology, fitness, and abundance of insects in cities (Angilletta et al.
2007;DaleandFrank2014; Meineke et al. 2014). However, the
effects of urban warming vary across taxa, such that some species
increase in fitness and abundance whereas others decline
(Youngsteadt et al. 2016), making effects on entire communities
difficult to predict. Since urban areas are increasingly viewed as
having potential to conserve insect diversity and ecosystem ser-
vices (Hunter and Hunter 2008;New2015), understanding the
effects of urban warming on beneficial insect communities will
advance urban conservation and management.
Wild bees are among the insects most often targeted by urban
conservation actions, such as installing pollinator gardens (Hall
et al. 2016). Bees, as a group, are of conservation concern be-
cause they are important pollinators of many crops and wild
plants, yet the abundance and range sizes of some wild bees
are declining (Bartomeus et al. 2013;Kerretal.2015). Despite
these declines, relatively diverse bee communities have
persisted in cities worldwide, leading to the recognition that
cities have a role in pollinator conservation (Baldock et al.
2015;Halletal.2016). In some cities, however, bee abundance
or diversity declines with urbanization (Bates et al. 2011;Fortel
et al. 2014;Geslinetal.2016), suggesting that cities vary in
April L. Hamblin and Elsa Youngsteadt contributed equally to this work.
Electronic supplementary material The online version of this article
(https://doi.org/10.1007/s11252-018-0731-4) contains supplementary
material, which is available to authorized users.
* Elsa Youngsteadt
Department of Entomology & Plant Pathology, North Carolina State
University, Campus Box 7613, Raleigh, NC 27695, USA
Urban Ecosystems (2018) 21:419–428