Wildflowers growing in contaminated urban soils absorb toxic metals and pass them on to pollinators through their nectar, a new study has suggested.
Researchers at the University of Cambridge discovered that plants such as white clover and bindweed absorbed trace metals — including arsenic, cadmium, chromium and lead — from contaminated soils. When bees foraged on these wildflowers, they inadvertently ingested the metals.
Pollinators such as bees play a vital role in global food production, including crops like apples and tomatoes that rely on insect pollination to bear fruit.
“Metal contamination is an issue in the soils of cities worldwide, with the level of contamination usually increasing with the age of a city. The metals come from a huge range of sources, including cement dust and mining,” the researchers said in a statement.
The study was conducted on vacant, post-industrial plots in Cleveland, Ohio, in the United States. These lands were previously used for iron and steel production, oil refining and automobile manufacturing — activities believed to have caused the soil to be contaminated with heavy metals.
Researchers extracted nectar from a range of plant species growing on these sites. Analysis revealed that different plants absorbed varying types and amounts of metals. The variation was attributed to species-specific strategies such as metal avoidance, detoxification and biochemical tolerance.
“Overall, the bright blue-flowered chicory plant (Cichorium intybus) accumulated the largest total metal concentration, followed by white clover (Trifolium repens), wild carrot (Daucus carota) and bindweed (Convolvulus arvensis),” the study said.
Lead was found in the highest concentration across all nectar samples tested, followed by chromium, cadmium and arsenic.
Among the tested species, only the common dandelion (Taraxacum officinale) did not show a significant variation in metal uptake, suggesting it may have an inherent ability to limit the transfer of metals into its nectar compared with other floral species in the same location.
Individually, Asclepias syriaca, also known as common milkweed, absorbed the highest levels of arsenic. Wild carrot recorded the highest cadmium concentrations, chicory accumulated the most chromium and white clover had the highest levels of lead in its nectar.
Previous studies have shown these metals can impair pollinators’ health when they consume nectar, which lowers population sizes and raises mortality rates. “Even low nectar metal levels can have long-term effects by affecting bees’ learning and memory – which impacts their foraging ability,” the latest paper stated.
Sarah Scott, lead author and a researcher in the University of Cambridge’s Department of Zoology, said: “It’s really important to have wildflowers as a food source for the bees and our results should not discourage people from planting wildflowers in towns and cities.”
The authors hoped the study would raise awareness about urban soil quality and pollinator health. They recommended assessing the historical use of land before planting wildflowers in urban areas.
The study also cited growing evidence of sharp declines in wild pollinator populations, by more than 50 per cent in the past 50 years, due to changes in land use. Climate change and pesticide use were also identified as contributing factors, particularly by accelerating the loss of flower-rich habitats essential for pollinators.