The milkweed plants growing in 40 cube-shaped chambers on a hilltop at the University of Michigan Biological Station in Pellston, Michigan, provide a glimpse into the future that allows researchers to ask a question: How will monarch butterflies fare?
Carbon dioxide gas is pumped into half of the transparent, plastic-covered boxes to simulate the atmosphere that’s likely to exist more than a century from now if levels of the heat-trapping greenhouse gas continue to rise due to the burning of fossil fuels.
Milkweed plants from the chambers were fed to hundreds of monarch caterpillars this summer. Milkweed is a monarch caterpillar’s only food, satisfying its nutritional needs while providing an invaluable medicinal boost. The plant’s leaves contain a bitter toxin that helps the insects ward off predators and parasites.
But previous work at U-M’s northern Michigan biological outpost, in the laboratory of ecologist Mark Hunter, showed that some species of milkweed produce lower levels of the protective toxins, called cardenolides, when grown under elevated carbon dioxide conditions.
That finding caught the attention of U-M doctoral student Leslie Decker, who with Hunter designed a multi-year follow-up study, underway at the Biological Station, that is the focus of her dissertation in the Department of Ecology and Evolutionary Biology.
“When I heard that, it really set off an alarm,” Decker said. “If toxins that are very active against parasites are decreasing under elevated CO2, what does that mean for the susceptibility of monarchs in the future?”
Most discussions of the monarch’s plight focus on habitat loss: the logging of trees in the Mexican forest where monarchs spend the winter, as well as the loss of wild milkweed plants that sustain them during their annual migration across North America.
The U-M study examines a different type of potential threat. It is one of the first experiments to assess the likely impacts of elevated carbon dioxide levels — the same mounting gases blamed for human-caused climate change — on the health of future monarch populations.
But getting to the answers is not a simple proposition. It required the creation of an experimental system that allows Decker and Hunter to manipulate and measure all the key links in the chain: carbon dioxide levels, toxin concentrations in milkweed leaves, infection by parasites and monarch susceptibility to those parasites.
“It’s a nice little trick that allows us to explore how well the medicine works,” said Hunter, who’s been studying monarchs at the Biological Station for nine years. During that time, he’s seen monarch numbers at the station drop, as they have elsewhere in the Midwest.
By “medicine,” Hunter is referring to the cardenolide toxins in milkweed leaves. In the wild, parasite-infected female monarch butterflies sometimes seek out milkweed plants with high toxin levels and lay their eggs there, a process known as trans-generational self-medication.
“It turns out that many kinds of animals self-medicate,” said Hunter, a professor in the Department of Ecology and Evolutionary Biology. “In other words, they use chemicals in the environment to help them with their own parasites and diseases. And of course, humans have been doing that for a very long time — we still obtain about half of our new pharmaceutical drugs from plants.