Plants generally suffer less enemy damage and are more defended in a copper mine than in a closely adjacent site

Abstract

Heavy metal pollution in soil can impact the relationships between plants and their natural enemies. Enemy attack from herbivores and pathogens is predicted to be lower in metal-contaminated areas such as mine sites. However, whether this is the case is remains to be tested. It is also unknown how defense traits differ in polluted sites compared to adjacent sites. To address this gap in knowledge, we compared the standing leaf damage in populations of two invasive and five native plant species at the abandoned Jiuhua copper mine and an adjacent site. We also compared physical and chemical defense traits of the populations. Herbivory on four plant species was significantly lower in a copper mine than in an adjacent site. Overall, plants growing in the mine were more physically and chemically defended than plants growing adjacent to the mine. Copper hyperaccumulator species (Cynodon dactylon and Kummerowia stipulacea) increased defense levels in the mine for only one of the traits. In contrast, defenses were higher in the mine site for most traits relative to their respective adjacent populations in the non-hyperaccumulator species. Our results suggest that the damage herbivores inflict on plants may be strongly influenced by how plants respond to stressors in mine site environments, such as metal pollution. Metal hyperaccumulation and increased non-elemental defenses may be alternate responses used by plants to simultaneously deal with metal pollution and natural enemies in contaminated sites.