Urban heavy metal contamination limits bumblebee colony growth.
Post-industrial shrinking cities contain abundant vacant land and are increasingly recognized for their pollinator conservation potential. At the same time, the industrial legacies of these urban ecosystems have resulted in elevated levels of heavy metals in surface soils, which could negatively affect bee populations. We investigated whether foraging within heavy metal contaminated landscapes represents a fitness cost for the common Eastern bumblebee Bombus impatiens, by placing colonies in residential backyards along an urban to rural gradient extending south and east from the city of Cleveland, Ohio, USA. Bees foraged in the landscape for 3 weeks, after which time we counted the total number of workers and larvae present in the colony and analyzed castes for the presence of heavy metals. We then assessed the relationship between landscape composition, heavy metal loads and caste abundance. Colonies located in urban landscapes were more likely to be exposed to lead (Pb). Elevated concentrations of Pb within workers were negatively correlated with both the number of workers and the number of larvae present. Synthesis and applications: Our findings raise concern that lead (Pb) contamination could represent a significant challenge to bee conservation in cities. To elucidate risks posed by this pollutant, we highlight a need to quantify lethal and sub-lethal effects of Pb exposure using laboratory and field-based studies. Further, identifying routes of exposure and landscape factors that influence exposure risk is necessary to implement mitigation strategies as part of urban conservation initiatives.