Abstract: Urban ecosystems have a unique chemical signature of elevated heavy metals and other toxins, which shape ecological communities and their subsequent development and evolution. However, it is unclear why some species thrive and others decline in the face of elevated toxins. Butterflies are decreasing in abundance by 2% per year, with approximately 30% of species declining. The primary reasons for butterfly decline are habitat loss, climate change, and chemical pollution. Chemical pollutants that butterflies frequently encounter include pesticides, nitrogen, and heavy metals, but heavy metals have received little attention.We ask: 1. How does heavy metal pollution bioaccumulate in soil, plants, and butterflies? and 2. How does the heavy metal tolerance of a butterfly species correlate to its trend in long-term abundance? We collected butterflies across a gradient of heavy metal pollution throughout Minneapolis and St. Paul, MN, USA–occupied Dakota land. We measured the content of heavy metals in soil, host plants, and butterflies at these collection sites, as well as butterfly wing length as a measure of an individual’s performance. A butterfly species’ ‘tolerance to toxins’ was calculated as the slope of the reaction norm of wing length across the pollution gradient. Long-term trends in each species’ abundance were calculated from a systematic monitoring program across Ohio, USA over 20 years (extracted from Wepprich et al. 2019). We found that butterflies collected in sites that had higher heavy metal pollution in soils and plants also had higher levels of heavy metals in their bodies. We also found that there is a positive correlation between butterfly species’ tolerance to heavy metals and their abundance, where butterfly species that are experiencing widespread declines in abundance are also less tolerant to heavy metals. At the time of presentation, we will have analyzed what species level traits (coevolutionary history, life history traits, behaviors, etc.) tend to drive tolerance to heavy metal pollution. This suggests that an inability to cope with pollution may be contributing to the decline of certain butterfly species. Our research has implications for butterfly conservation, and more broadly, the remediation of urban ecological communities in ways that reduce toxin loads.
