Mounting levels of atmospheric carbon dioxide pose a previously unrecognized threat to monarch butterflies, new research has revealed. CO2 reduces the medicinal properties of milkweed plants that protect the iconic insects from disease.
Monarch butterfly populations have been declining rapidly in recent years, but their plight is largely attributed to habitat loss.
Now, new research has shown that the iconic insects’ use of milkweed to help ward off predators could be playing a role in their decline.
Milkweed leaves contain bitter toxins that help monarch butterflies guard against predators and parasites. However, CO2 reduces these medicinal properties, according to the study conducted by researchers at the University of Michigan.
“We discovered a previously unrecognized, indirect mechanism by which ongoing environmental change – in this case, rising levels of atmospheric CO2 – can act on disease in monarch butterflies,” explained Leslie Decker, first author of the study, in a statement.
In a multi-year experiment at the U-M Biological Station, the researchers looked at how elevated carbon dioxide levels alter plant chemistry and how those changes affect interactions between monarchs and their parasites.
They grew four milkweed species with varying levels of their protective compounds, or cardenolides. Half the plants were grown under normal carbon dioxide levels, and half of them were bathed in nearly twice that amount. Then the plants were fed to hundreds of monarch caterpillars.
The study showed that the most protective of the four milkweed species lost its medicinal properties when grown under elevated CO2, resulting in a steep decline in the monarch’s ability to tolerate a common parasite, as well as a lifespan reduction of one week.
“If elevated carbon dioxide reduces the concentration of medicines in plants that monarchs use, it could be changing the concentration of drugs for all animals that self-medicate, including humans,” warned ecologist and study co-author Mark Hunter
Many animals, including humans, use chemicals in the environment to help them control parasites and diseases. Aspirin, digitalis, Taxol and many other drugs originally came from plants, according to the statement.
Decker added: “Our results suggest that rising CO2 will reduce the tolerance of monarch butterflies to their common parasite and will increase parasite virulence.”
Photo credit: Jerry and Pat Donaho/ CC BY-NC-ND 2.0