Rising levels of carbon dioxide from car and factory exhaust — which scientists say is the primary cause of global warming — could contribute to the killing off of monarch butterflies by reducing the medicinal qualities of the plants they eat, a new study has found.

The University of Michigan experiment, co-authored by a Stanford University scientist, found that higher carbon dioxide levels reduced a natural toxin in milkweed that feeding caterpillars utilize to fight off parasites.

The 77 percent reduction in parasite tolerance found in the study is the first definitive example of how monarch survival, the plants they live and feed upon and the Earth’s changing climate are intricately linked.

Leslie Decker, lead author of the study conducted for her doctoral dissertation at the University of Michigan, said climate-caused mutations like this could also affect the health of humans by degrading the many medicines that we derive from plants.

“What we found is that the monarchs got sicker when they were fed milkweed under future concentrations of CO2,” said Decker, now a chemical and disease ecologist at Stanford. “It’s important because there are many animals, including ourselves, that utilize plants for medication. This study shows that environmental changes are reducing the effect of that medicine.”

The study, published July 10 in the scientific journal Ecology Letters, means one of the largest and most colorful butterflies in the world could still be in danger of extinction even if harmful pesticides are eliminated and the lost habitat blamed for their dramatic decline is restored.

The experiments, conducted in 2014 and 2015, involved eastern monarch butterflies, famous worldwide for blanketing trees in Mexico, turning whole sections of forest into a kaleidoscope of color.

The brilliant orange and black insects spend their winters in Mexico before heading back some 2,500 miles across the United States in a remarkable multigenerational relay race toward Canada. The monarchs each fly a couple of hundred miles, lay eggs on milkweed and die. Their caterpillars feed on the plants, turn into butterflies and continue the journey, instinctively knowing which direction their parents were going.

Eastern monarchs have declined more than 90 percent since 1996, when scientists estimated there were 1 billion nesting in the trees.

Scientists believe the eastern and western migratory populations of monarchs divide at the Rocky Mountains when they head south for the winter. Western monarchs, which crowd trees along the California coast every winter, have also declined to the point that the species will likely go extinct in the next few decades if nothing is done to prevent it.

The latest count this past winter showed the western monarch population at its lowest since 2012. There are now fewer than one-sixth of the 1.2 million butterflies counted between Marin County and the Baja California peninsula 20 years ago.

In all, monarch populations in North America have plunged 95 percent since the 1980s. Researchers have blamed pesticides, herbicides and the plowing under of milkweed habitat along their migratory route.

Decker’s study confirms what scientists already suspected, that climate change probably plays a major role in the spiraling decline of this iconic species.

The research piggybacks on previous studies that showed that a compound in milkweed called cardenolide protects monarch caterpillars from parasites. In 2011, another researcher at the University of Michigan found that high levels of carbon dioxide reduce the amount of cardenolide in milkweed.

Decker’s team of scientists grew dozens of milkweed plants, half of which were grown under normal conditions. The other half were bathed from dawn to dusk in almost twice the amount of CO2 now in the atmosphere, a level climate models predict will be standard 150 years from now and could be reached this century if fossil fuel burning is not reduced.

The plants were fed to hundreds of monarch caterpillars, which were, in turn, infected with carefully controlled doses of a common monarch parasite. The protozoan, distantly related to the malaria pathogen, is known to shorten the lifespans of adult monarchs, hampering their ability to fly and reducing offspring.

The scientists waited for the caterpillars to pupate and live out their lives as butterflies, typically a month, before collecting the bodies and counting the number of parasitic spores in the carcasses.

More Information Monarchs in trouble Western monarch butterflies spend the winter in more than 300 forested groves along the California coast, including large populations in Riverside and Los Angeles counties, Pacific Grove, Monterey and at Natural Bridges State Beach in Santa Cruz. They can normally be seen from November to March. The California winter population declined an estimated 90 percent between 1997 and 2009. There are now roughly 300,000 monarchs in California compared with an estimated 10 million in 1981. The California population, first observed by a Russian expedition looking for a passage across the Arctic Ocean in 1816, is declining at an average of 7 percent a year, slightly more than the 6 percent drop seen in the eastern monarch population, according to the nonprofit Xerces Society, the University of Georgia and Tufts University in Massachusetts.

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They found that cardenolide production declined by nearly 25 percent in tropical milkweed, known scientifically as Asclepias curassavica, when it was grown under elevated CO2 levels. The caterpillars that fed on those plants were 77 percent less tolerant of parasites and died a week sooner than normal, according to the study.

Mark Hunter, Decker’s dissertation adviser and the co-author of the study, said plants are used by many animals, including birds, chimpanzees, baboons, ants and bees, to self medicate against environmental toxins and hazards. The cancer drug taxol, digitalis and aspirin are among the human medications that originally came from plants, he said.

“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,” said Hunter, who has studied monarch butterflies for more than a decade. “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.”

Peter Fimrite is a San Francisco Chronicle staff writer. Email: pfimrite@sfchronicle.com Twitter: @pfimrite