Metal pollution and floral abundance influence the diversity of flower visiting insects

Soil pollution with metals is a worldwide problem with negative effects on ecosystem health. Metals are persistent and can accumulate in the food web, with potential toxic effects for most organisms. Some metal-tolerant plants can accumulate metals in their aboveground parts, including the pollen and nectar of their flowers. Therefore, insects visiting these flowers are exposed to these toxic elements. They can also be influenced by the plant species turnover observed along pollution gradients. Surprisingly, the impact of soil metal contamination on pollinating insect diversity has rarely been studied. To fill in this gap, we carried out a study in a former mining valley in the French Pyrenees with metal contamination by Zn, Pb and Cd. We estimated the richness and community composition of insects visiting the flowers of metal-tolerant and metal-intolerant plant species in 96 plots along metal pollution gradients. Insect richness did not change along the pollution gradient and was similar between metal-tolerant and metal-intolerant plant species. On the contrary, insect community composition changed along the gradient and differed between plant types. We also found that insect species richness increased with the flower abundance of the focal plant. This study showed that flowers of metal-tolerant plants, including hyper-accumulator species, are visited by several insect species and harbour a distinct pollinator community. The ingestion of metal-rich nectar or pollen may have lethal or sub-lethal effects for these flower-visiting insects. Further experimental studies are needed to determine if metal-rich floral resources act as an ecological trap for some of these species.