monarch decline
Isotopes to the rescue
glider theory

Saving the monarch migration
Monarch butterflies are a common and durable critter. In most years hundreds of millions of the black-and-orange arthropods live in North America, so they aren't likely to go extinct anytime soon.

But the outlook for the marvelous migration between the United States, Canada and Mexico is another matter. Habitat declines on both ends of the migration are pinching the living resources monarchs need to survive: food and shelter.

The isotope research we examined earlier showed that half of all migrating monarchs in eastern North America spend their summers and reproduce in some of the most heavily farmed land in the world -- the corn and soybean belt of the U.S. Midwest. "About 85 percent of that land is agricultural land, and fairly intensive agriculture," says Orley Taylor, director of Monarch Watch and a professor of entomology at the University of Kansas.

The stages of monarchy: From egg to caterpillar and, finally, to adult, monarchs depend almost entirely on milkweed, a plant that may one day be at risk because of increasing use of herbicides in some parts of the world.
Photos by Karen Oberhauser and Mary Kodet.

© 1987, Monarchs in the Classroom, Monarch Butterfly Sanctuary Foundation.

caterpillarHe says a major new threat is the widespread use of corn and soybeans that are genetically resistant to herbicides like Roundup. Roundup-resistant crops allow farmers to spray Roundup, which kills just about anything green.

If the herbicide becomes virtually ubiquitous, the side effects could be enormous. "We are cleaning up U.S. agriculture, the weedy fields, in a way that they've never been cleaned up before," Taylor says. "Monarchs, birds, and other animals depend on weediness" in crop fields. Yet the potential of herbicide-resistant crops is to eradicate weeds.

Nonetheless, Taylor does not pin this winter's 80 percent monarch decline in Mexico on herbicide usage in the United States, but rather on fires set during the winter of 1997-1998, when El Niño caused a massive drought in Mexico. (However, it's also important to remember that the decline could reflect a natural fluctuation in population instead.)

female monarchSouth of the border
Monarchs fly from North America (east of the Rockies) to a few isolated mountaintops west of Mexico City. Although Mexico has set aside more than 40,000 acres on five mountains for the migration, Lincoln Brower, a veteran monarch scientist and champion, says a surging human population is harming the reserves. "People are setting fires [to clear agricultural land]. The fires get out of hand and get up into the preserves. [During the winter of 1998-1998] there were dozens of fires in and near the butterfly areas, and not one was from natural causes."

Two hundred acres of protected land in one of the pristine reserves burned in spring, 1998. That may not sound like much, but the cores of the monarch reserves cover only 10 to 20 acres on each mountain. When occupied during the winter, an acre of fir forest shelters an estimated 4 million butterflies.

milkweedLogging and firewood cutting are also causing problems. Brower, a research professor of biology at Sweet Briar College in Virginia, says monarchs depend on the oyamel firs to prevent the loss of radiant energy from their bodies on cold, clear nights, and get shelter from rain or snow. "It's clear that the intact forest acts like an umbrella and a blanket, keeping them dry and preventing them from freezing."

Environmentalists worry that increased U.S. interest in importing logs of fir species, including the oyamel that protects monarchs (see "Pest Risk Assessment..." in the bibliography), will lead to more logging.

Brower, who has labored for years to study and protect the butterflies, is not optimistic about the future. "I project that within 20 years, at the present rate of destruction of forest in Mexico, the migration may be gone," he says. While habitat destruction in the United States is problematic, only five or six "really healthy forests are left" in Mexico. Threats to those patches of firs, he says, are the "real Achilles heel."

How do monarchs actually find their refuges?


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