Adult dragonflies are indicators of environmental metallic elements

Adult dragonflies (Insecta; Odonata) are aerial predatory arthropods that occur globally except in the polar regions. However, we know of no research on adult dragonflies as potential indicators of metallic elements in the environment or metallic element concentrations and relative contribution patterns between sites, species, size classes, habitat types, and relation to possible pollution sources. There is also no information available about adult dragonflies and their responses to toxic metals. However, metallic elements are toxic in elevated concentrations to all organisms. We predict that adult dragonflies would be suitable indicators of elemental concentrations. We analysed 105 adult male dragonflies from 21 sites in South Africa for 33 metallic elements including Hg, As, Pb, Cr, Cu, Cd, Ni, Se, Al, and Au. The results indicated that all species of dragonflies, regardless of body size, are suitable indicators. Furthermore, different aquatic habitat types did not affect the metallic element concentrations at the scale of this study. Dragonflies collected near wastewater treatment plants showed concentrations of certain elements such as Au higher than from elsewhere. Elements such as As and Pb were found at elevated concentrations (relative to the other sites) in dragonflies collected near mines. Dragonflies from sampling sites near potential pollution sources, but had seemingly isolated water sources, showed lower metallic element concentrations when compared with other sites. We conclude that adult dragonflies would be good indicators of environmental metallic elements.