Mounting levels of atmospheric carbon dioxide pose a previously unrecognised threat to monarch butterflies, by reducing the medicinal properties of milkweed plants that protect the iconic insects from disease, a study has found.
Milkweed leaves contain bitter toxins that help monarchs ward off predators and parasites, and the plant is the sole food of monarch caterpillars.
Half the plants were grown under normal carbon dioxide (CO2)levels, and half of them were bathed, from dawn to dusk, 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.
It did not examine the climate-altering effects of the heat-trapping gas emitted when fossil fuels are burned.
"We discovered a previously unrecognised, indirect mechanism by which ongoing environmental change - in this case, rising levels of atmospheric CO2 - can act on disease in monarch butterflies," said Leslie Decker, first author of the study published in the journal Ecology Letters.
"Our results emphasise that global environmental change may influence parasite-host interactions through changes in the medicinal properties of plants," said Decker, who is now a postdoctoral researcher at Stanford University in the US.
"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," said Hunter.
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