Urban heavy metal contamination limits bumblebee colony growth

Abstract

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 analysed 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.

Exposure to Urban Heavy Metal Contamination Diminishes Bumble Bee Colony Growth

“…Colonies fed cadmium for 30 days were four times more likely to have dead brood compared to control colonies; chromium fed colonies had 15 times higher likelihood of having dead brood, and colonies fed all four test metals had 4.5 times higher likelihood of containing dead brood (Cd: p< 0.01, OR= 3.91 (95% C.I: 2.71, 5.68); Cr: p< 0.01, OR= 15.2 (95% C.I: 9.61, 24.81); All heavy metals: p< 0.01, OR= 4.56 (95% C.I: 3.14, 6.69).DiscussionBumble bee colonies exposed to environmentally relevant concentrations of heavy metals experienced increased brood mortality. This causal link supports the correlations between heavy metal contamination and brood mortality demonstrated in previous studies(Moroń et al 2014;Sivakoff et al 2020). The goal of our study was to isolate the impacts of oral heavy metal exposure on bumble bee colony tness from other factors that may in uence colony tness and survival.…”

“…2 We predicted that consuming environmentally relevant concentrations of heavy metal contaminated provisions represents a tness cost for bumblebee colonies through increased brood mortality and decreased colony weight. To examine if urban heavy metal contamination was a true threat to city dwelling bee health, we measured the growth of colonies fed heavy metal concentrations detected in the provisions of bumblebees foraging within the city of Cleveland, Ohio (Sivakoff et al 2020). Surprisingly, oral exposure to each individual heavy metal resulted in similar brood mortality for all four metals tested, three to four times higher compared to control colonies.…”

“…We tested four heavy metals commonly found in urban environments and correlated with lethal and sublethal effects on bees: As, Cd, Cr, and Pb (Jennings et al 2002;Sharma et al 2015;Sivakoff et al 2020). As contaminated environments oftentimes have multiple heavy metals, we also evaluated a treatment of all heavy metals combined.…”

“…While these reclaimed, converted greenspaces support arthropod biodiversity (Flor et al 2017 (Jennings et al 2002;Perry et al 2020). For example, bumble bee workers and brood from hives located within Cleveland were more likely to contain detectable levels of heavy metals than workers from suburban or rural hives (Sivakoff et al 2020). Alarmingly, negative correlations between the presence of some heavy metals in bees and their provisions, and worker and brood abundance were also found, raising the question if legacy cities with widespread heavy metal contamination are an appropriate context for urban bee conservation investment (Sivakoff et al 2020).…”

Impacts of multiple pollutants on pollinator activity in road verges

“…Although visitation was reduced, pollinators did frequently consume even the greatest experimental concentrations of metals. Further research is needed to understand if and how this might affect them, though concentrations of Pb have been negatively related to bumblebee colony growth in urban areas (Sivakoff et al., 2020), concentrations of Cd, Pb and Zn have been linked to wild bee abundance, diversity and forewing structure (Moroń et al., 2012; Szentgyörgyi et al., 2017) and concentrations of Zn between 55 and 1,158 ppm (a similar range to that measured in road verge flowers) have been found to affect caterpillar survival and development (Shephard et al., 2020). A recent study estimated that the LC50 after 7 days for bumblebees feeding on contaminated sugar water was 0.83 ppm for Cd and 66.55 ppm for Cu (Rothman et al., 2020).…”