Heavy metal pollution is a widespread threat to urban ecosystems. Although many types of heavy metals are required by organisms at trace levels, the environmental availability of heavy metals is increasing to potentially toxic levels due to human activities such as mining, industrial combustion, and automobile use. A key goal for ecotoxicologists, evolutionary biologists, and conservation biologists is to understand the potential for organisms to adapt to heavy metal pollution. Although extreme heavy metal tolerance has been noted in some species of plants, aquatic organisms, and terrestrial invertebrates, we know little about the potential for tolerance in species of conservation concern.
Heavy metal pollution poses a potential threat to insect pollinators, such as the declining monarch butterfly (Danaus plexippus). Zinc in particular is a widespread heavy metal pollutant in roadside areas, which are currently targeted for monarch butterfly habitat restoration in the Midwestern USA. Along roadsides, zinc from vehicle residue accumulates in soil where it is incorporated into leaf tissue of milkweed, the food plant of monarch caterpillars. Habitat restoration for monarchs along roadsides therefore creates a potential ecological trap scenario: monarch butterflies may be drawn to roadside milkweed to lay their eggs, and the offspring may be harmed by zinc pollution in their diet.
A recent study by Shephard et al. at the University of Minnesota experimentally tested the effects of exposure to dietary zinc pollution in monarch caterpillars. The authors used an artificial diet approach to precisely manipulate a range of zinc doses to feed monarch caterpillars derived from a wild-caught population in St. Paul Minnesota (Figure 1). For comparison, the authors also tested the effects of zinc exposure in the cabbage white butterfly (Pieris rapae), a common pest species in North America. The authors found that dietary zinc exposure reduced monarch caterpillar survival at concentrations as low as 344 mg/kg (Figure 2). Yet, zinc positively affected cabbage white caterpillar survival at 227 and 738 mg/kg. Despite these positive survival impacts, however, zinc exposure slowed cabbage white growth rate, prolonged development time, and reduced adult body size. This suggests that zinc tolerance in cabbage whites came at a cost of reduced investment in other components of fitness.
The results of the study by Shephard et al. show that zinc tolerance can vary across pollinator species. While cabbage white butterflies are highly tolerant of zinc, monarchs are less tolerant. Although the reasons for this difference in tolerance are not known, the authors point to several possibilities. For instance, cabbage white caterpillars might be adapted to tolerate higher metal concentrations, given that their dietary host plants tend to naturally accumulate higher metal concentrations than milkweed. Additionally, the cabbage white is a disturbance-adapted agricultural pest species that is commonly targeted with pesticides. It is possible that recent selection for pesticide detoxification could pre-adapt the cabbage white to better tolerate heavy metal toxicity.
Although monarchs were more susceptible to zinc pollution than cabbage whites, the Shephard et al. study suggests that zinc pollution alone is unlikely a risk for monarch caterpillars in roadside habitats. Field data from roadsides in Minnesota show that zinc pollution is higher in milkweed plants closer to the road and in plants along busier roadways. Yet, these field levels rarely reach zinc concentrations shown to induce significant mortality in the laboratory. However, the authors point out that in nature, monarchs likely experience zinc pollution in combination with a range of other potential stressors, such as sodium from road de-icing salt or pesticide residue from agricultural fields. Future work considering the interactive effects of metals and these other potential stressors is required to achieve a deeper understanding of how monarchs are being affected by polluted urban ecosystems.
Want to know more? Read the study by Shephard et al. here:
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Image from Ceeec is available on Wikimedia Commons (https://commons.wikimedia.org/wiki/File:Monarch_butterfly_caterpillar_on_milkweed.jpg)
I am broadly interested in ecological and developmental factors influencing how organisms adapt to novel, rapidly changing environments.
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