Wildflowers growing on land previously used for buildings and factories can accumulate lead, arsenic and other metal contaminants from the soil, which are consumed by pollinators as they feed, a new study has found.
The metals have previously been shown to damage the health of pollinators, which ingest them in nectar as they feed, leading to reduced population sizes and death. Even low nectar metal levels can have long-term effects, by affecting bees' learning and memory - which impacts their foraging ability.
Researchers have found that common plants including white clover and bindweed, which are vital forage for pollinators in cities, can accumulate arsenic, cadmium, chromium and lead from contaminated soils.
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 say soils in cities should be tested for metals before sowing wildflowers and if necessary, polluted areas should be cleaned up before new wildflower habitats are established.
The study highlights the importance of growing the right species of wildflowers to suit the soil conditions.
Reducing the risk of metal exposure is critical for the success of urban pollinator conservation schemes. The researchers say it is important to manage wildflower species that self-seed on contaminated urban land, for example by frequent mowing to limit flowering - which reduces the transfer of metals from the soil to the bees.
The results are published today in the journal Ecology and Evolution.
Dr Sarah Scott in the University of Cambridge's Department of Zoology and first author of the report, 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.
"We hope this study will raise awareness that soil health is also important for bee health. Before planting wildflowers in urban areas to attract bees and other pollinators, it's important to consider the history of the land and what might be in the soil – and if necessary find out whether there's a local soil testing and cleanup service available first."
The study was carried out in the post-industrial US city of Cleveland, Ohio, which has over 33,700 vacant lots left as people have moved away from the area. In the past, iron and steel production, oil refining and car manufacturing went on there. But any land that was previously the site of human activity may be contaminated with traces of metals.
To get their results, the researchers extracted nectar from a range of self-seeded flowering plants that commonly attract pollinating insects, found growing on disused land across the city. They tested this for the presence of arsenic, cadmium, chromium and lead. Lead was consistently found at the highest concentrations, reflecting the state of the soils in the city.
The researchers found that different species of plant accumulate different amounts, and types, of the metals. 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). These plants are all vital forage for pollinators in cities - including cities in the UK - providing a consistent supply of nectar across locations and seasons.
There is growing evidence that wild pollinator populations have dropped by over 50% in the last 50 years, caused primarily by changes in land use and management across the globe. Climate change and pesticide use also play a role; overall the primary cause of decline is the loss of flower-rich habitat.
Pollinators play a vital role in food production: many plants, including apple and tomato, require pollination in order to develop fruit. Natural 'pollination services' are estimated to add billions of dollars to global crop productivity.
Scott said: "Climate change feels so overwhelming, but simply planting flowers in certain areas can help towards conserving pollinators, which is a realistic way for people to make a positive impact on the environment."
