Lead (Pb) concentrations across 22 species of butterflies correlate with soil and air lead and decreased wing size in an urban field study

Pollution is a global issue contributing to biodiversity loss, climate change, and human health concerns. Lead (Pb) has long been recognized as a toxic heavy metal pollutant but few studies have investigated the impact and routes of exposure to lead in field conditions and across multiple species. We collected 22 common species of butterflies across a gradient of lead pollution in the Twin Cities metropolitan area (Minneapolis and St. Paul, MN, USA). We measured their thorax lead concentrations and their body condition including wing area, number of eggs, and brain mass. We quantified lead in the soil, host plant leaves, and air (through lichen bio-monitors) at sites where the butterflies were collected to investigate potential routes of exposure. We found a negative correlation between sublethal lead concentrations and butterfly wing size across all species. Contrary to expectations from previous literature, we did not find correlations between butterfly lead concentration and number of eggs or brain mass. Our data indicate that routes of lead exposure for butterflies are particularly pronounced through soil and air, relative to exposure through their host plants, as there were positive correlations between butterfly lead and lead in nearby soil and air, but not that of host plants. Such sublethal effects of lead, even at low levels of pollution, underline the importance of continuing to reduce emissions and impacts of pollutants to protect biodiversity.