Earth & Conservation

Monarch Butterflies Are Threatened by Rising Carbon Dioxide Levels

Elevated CO2 levels in the atmosphere could be reducing the medicinal properties of the milkweed plants that monarchs eat.

Researcher Leslie Decker with a monarch butterfly in a University of Michigan laboratory | Austin Thomason/Michigan Photography
Researcher Leslie Decker with a monarch butterfly in a University of Michigan laboratory | Austin Thomason/Michigan Photography

A troubling ecology study published this week contains some bad news for monarch butterflies … and maybe humans, too.

According to new research conducted at the University of Michigan, elevated levels of atmospheric carbon dioxide are reducing the medicinal properties of milkweed plants. The compromised plants, in turn, are threatening the health of monarch butterflies, who rely on specific chemicals in milkweed to fend off disease and parasites.

The research details some very specific threats for monarch butterflies, who are already experiencing population decline due to habitat loss. But researchers warn that the study also suggests a much bigger potential problem for other species — including humans.

"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 co-author Mark Hunter in press materials released with the new research.

Published Tuesday in the journal Ecology Letters, the research examines how elevated carbon dioxide levels alter the chemistry in milkweed plants, and how those changes in turn affect monarch butterflies. Milkweed leaves contain bitter toxins that help monarchs ward off predators and parasites.

Two monarch butterflies on a milkweed plant in a University of Michigan laboratory | Austin Thomason/Michigan Photography

In a multi-year experiment, researchers grew four different species of milkweed and subjected the plants to varying levels of atmospheric carbon dioxide. Using 40 different growth chambers, the research exposed milkweed plants to two different carbon dioxide levels. Twenty chambers were maintained at current global CO2 concentrations of around 400 parts per million. The other 20 chambers were ramped up to 760 ppm — a level that researchers say could be reached before the end of the century.

The study showed that some milkweed species essentially lost their medicinal properties when grown under the elevated CO2 levels. This triggered a steep decline in the monarch's ability to fight off a common parasite and reduced average lifespan by one week. That might not seem like a big deal, except that monarch butterflies only live two to six weeks.

RELATED: The Number of Monarch Butterflies in Western North America Is Plummeting

“We've been able to show that a medicinal milkweed species loses its protective abilities under elevated carbon dioxide,” said Leslie Decker, first author of the study. “Our results suggest that rising CO2 will reduce the tolerance of monarch butterflies to their common parasite and will increase parasite virulence.”

As to the broader implications of the study, Hunter notes that any animals, including humans, use chemicals in the environment to help them control parasites and diseases. What's more, nearly half of all human pharmaceuticals now in use were originally derived from natural sources.

“When we play Russian roulette with the concentration of atmospheric gases, we are playing Russian roulette with our ability to find new medicines in nature,” Hunter said.