Roadside pollinator plantings: Valuable resource or deadly ecological trap?

Roadsides are popular sites for the installation of pollinator plantings nationwide. The extent of road networks within the US highlights the potential of these habitat strips to reduce patchiness and provide valuable resources in fragmented landscapes. Many agencies encourage or require planting pollinator seed mixes when revegetating roadsides. However, roadsides are associated with elevated heavy metals, from vehicle emissions and residual leaded gasoline, and roadside soils and plants often have elevated levels of lead, zinc, copper, and other metals. If roadside pollinator plantings are contaminated with heavy metals they may be acting as an unintentional ecological trap, harming the pollinators utilizing them. We measured the heavy metal content of pollen nectar, and bees, from roadsides across Minnesota that varied in traffic volume from 100-40,000+ vehicles per day. We sampled nectar from five species commonly used by bees, and pollen loads from four bumblebee species. Samples were taken at varying distances from the road to examine how far heavy metal pollutants are transported. Preliminary results indicate that nectar and pollen from high traffic roads had elevated concentrations of Zinc and Copper. However, concentrations of metals in nectar were generally much lower than in leaf tissue, and several metals of concern along roadsides were at non-detectable levels. This suggests that roadside pollinator plantings have the potential to act as ecological traps, but likely only along high traffic roads. Future work needs to clarify the level of metal exposure where roadside pollinator plantings can turn from boon to detriment for pollinator health.