Fluttering down for the winter, monarch butterflies face growing threats in their Mexican mountain sanctuaries
Monarchs cluster on an oyamel fir in Mexico’s Monarch Butterfly Biosphere Reserve, where their numbers have dropped by more than 80 percent.
LAST FEBRUARY, MONARCH BIOLOGISTS BREATHED A COLLECTIVE SIGH OF RELIEF when their Mexican colleagues announced that the greatest number of butterflies in five years had made it to the country, where the vast majority of North America’s migratory monarchs spend the winter. Less than two weeks later, in early March, their relief turned to dismay when a devastating storm pummeled the monarchs’ mountain sanctuaries.
The storm’s fierce winds toppled an unprecedented number of the oyamel firs monarchs depend on for survival. Many butterflies froze in place on the trees, while countless others were buried beneath a thick coat of ice. Estimates of monarch losses ranged from 3 percent of some colonies to 50 percent of others, and in spring, the number of butterflies returning to the United States—tallied by the citizen science project Journey North—approached historic lows.
The storm was just another reminder of the increasing peril monarchs face in their winter home in Mexico. At the end of October, like clockwork, the butterflies arrive in central Mexico’s Transvolcanic Belt, a chain of steep, fir-clad peaks that span some 600 miles from the Pacific Ocean to the Gulf of Mexico. The majority of monarchs overwinter on just a dozen of these peaks in an isolated area northwest of Mexico City protected as the Monarch Butterfly Biosphere Reserve. In the reserve’s cool, thin air between 9,500 and 10,800 feet, butterflies huddle together by the millions on the firs, whose dense, dark-green canopies serve as both an umbrella, shielding monarchs from rain, sleet and snow, and a blanket, providing a microclimate that is up to 12 degrees F warmer than surrounding open areas and prevents them from freezing. On warm winter days, as sunlight strikes the oyamels, rivers of orange and black butterflies swoop down the mountainsides in search of water.
Two decades ago, about a billion butterflies fluttered down to Mexico from as far as southern Canada, but since then, the number arriving in the reserve has declined by more than 80 percent. Researchers recorded the lowest population ever during the winter of 2013-2014, when only about 25 million butterflies made it. “If the population had been that low last winter, the storm would have been a disaster,” says biologist Karen Oberhauser, head of the University of Minnesota’s Monarch Lab and co-chair of the Monarch Joint Venture, a partnership of more than 50 federal and state agencies, universities and nongovernmental groups—including the National Wildlife Federation—that works to study and protect monarch butterflies.
The precipitous plunge in monarch numbers is blamed on a combination of destructive factors. During the spring and summer breeding season, a dramatic loss of milkweeds (the species’ only host plants) has limited monarchs’ ability to reproduce—especially across the U.S. Midwest, where herbicide-intensive agriculture has wiped out tens of millions of acres of milkweed. “The majority of the world’s monarchs are produced by milkweeds in the Corn Belt,” Oberhauser points out. Chip Taylor, director of Monarch Watch and a biologist at the University of Kansas, says that during the past decade, farmers in that region have plowed under an area the size of Indiana. “In much of the Corn Belt,” he says, “farming is now from road to road, with very little habitat for any wildlife remaining.” In addition, scientists suspect that large numbers of monarchs die during migration, possibly due to lack of nectar plants in fall, when single butterflies as light as paper clips can travel up to 3,000 miles and need nectar to fuel the trip.
Yet it is during winter—when tens of millions of monarchs are packed into a small geographic area—that the population as a whole is at its most vulnerable. According to Sweet Briar College biologist Lincoln Brower, who first visited the Mexican sanctuaries in 1977, this winter habitat today “is the most imperiled it has ever been.”
For several decades, large-scale, illegal deforestation wreaked havoc with the monarch’s winter home. Mexican scientists estimated the rate of forest loss in the reserve at between 2 and 5 percent a year, far exceeding the rate of reforestation and natural forest regeneration. Today, most of the majestic, old-growth oyamels are gone, and monarchs must roost in young trees that provide less protection from the elements.
While destructive small-scale logging continued in recent years, illegal clear-cutting seemed to be under control. But in 2015, a local environmentalist discovered an extensive new clear-cut in the Sierra Chincua, one of the most important monarch sanctuaries. According to Brower and other biologists who documented the damage in American Entomologist, 25 acres were razed in one of the few areas of the reserve where mature forest remained. The logging is particularly worrisome, they say, because it occurred at a location where monarchs gather before heading north as winter wanes.
Brower worries even more, however, about a decision by Mexican authorities to permit “salvage” logging of downed trees inside the reserve following last spring’s storm. As this issue went to press in October 2016, trucks had hauled out “thousands and thousands of logs,” he said, and were scheduled to continue until the end of the month when the reserve would open for tourism. Brower calls this salvage logging operation “more damaging to the monarchs’ overwintering area than anything that has happened in the past.”
In another disturbing development, the country’s largest mining conglomerate, Grupo México, has decided to move forward with plans to reopen a sprawling mine that snakes beneath the monarchs’ rugged mountain sanctuaries near the town of Angangueo. Among other things, scientists worry that the huge volumes of water needed to extract copper and other metals from the ore could dry up springs crucial to the survival of oyamels and that resulting toxic wastewater could pollute the water sources monarchs rely on.
At the same time monarch habitat is degrading, the threat from severe weather is growing. The storm last March followed even more extreme weather events in 2002, 2004 and 2010. In 2002, an estimated 500 million monarchs were wiped out—more than this year’s entire winter population of about 150 million. As climate change intensifies, the situation is expected to get worse. An analysis by Oberhauser and Town Peterson of the University of Kansas, published in the Proceedings of the National Academy of Sciences, projects that winter precipitation will increase, and with it, the risk that butterflies will be exposed to the deadly one-two punch of wet weather followed by freezing temperatures.
While winter storms have always lashed these mountains, they are particularly worrisome today, scientists say, as the number of butterflies reaching Mexico has dwindled. A study published in Scientific Reports last spring concludes that if the U.S. government fails in an ambitious plan to restore habitat across the monarch’s breeding range, there is a “substantial probability” that during the next two decades the number of butterflies will fall so low that a single storm could bring an end to the migration.
Already, the oyamel firs monarchs rely on are suffering from hotter, drier conditions in spring as a result of climate change, and scientists say the situation will worsen. According to research published in Forest Ecology and Management, global warming is likely to shrink suitable habitat for the tree species by 69 percent within 15 years. If greenhouse gas emissions continue to rise rapidly, 87.6 percent of the habitat will no longer meet the oyamel’s needs by 2060.
In response to the crisis, Mexican scientists are pinning their hopes on a bold plan to move oyamels progressively higher up the mountainsides in a race to save the tree—and the butterfly migration that depends on it. “We have to act now,” says the plan’s architect, forest geneticist Cuauhtémoc Sáenz-Romero of the Universidad Michoacana de San Nicolás de Hidalgo. “Later will be too late because the trees will be dead or too weak to produce seeds in enough quantity for large reforestation programs.”
As a first step, Sáenz-Romero and his colleagues last summer planted a few hundred oyamel seedlings at an elevation of 11,286 feet, where conditions suitable for the tree are expected by 2030. With the likelihood of catastrophic habitat decline just a few decades away, the small trial planting will need to be followed soon by a massive effort to move the fir to even higher altitudes. Sáenz-Romero expects surrounding communities that survive on monarch-based ecotourism to do the actual planting, with assistance from foreign governments and private organizations. The end of the monarch migration, he notes, would be particularly tragic for local people “who would lose their only chance to make a decent living” that also safeguards the reserve’s forest.
A bigger challenge than tree planting, he says, will be convincing skeptics that this “assisted migration” project is essential to oyamel survival. At a time when invasive plants and animals pose one of the gravest threats to natural areas, some ecologists remain wary of moving a species to a habitat where it is not native. Yet attitudes are changing quickly as the implications of climate change become clear. “As an ecologist, I’m nervous about assisted migration of the oyamel,” Oberhauser says. But she believes “it is an important part of our toolkit when a species is faced with the total loss of its habitat.” Most other scientists agree—with an important caveat: “This longer-term approach is needed,” says Scott Black, executive director of the Xerces Society for Invertebrate Conservation and Monarch Joint Venture co-chair. “But if the Mexican government does not stem logging and deal with mining, there may be no monarchs to move uphill by 2030.”
According to Sáenz-Romero and his colleagues, the oyamel could completely run out of room in the reserve by 2090. In that case, scientists may consider planting the trees at higher altitudes on loftier volcanoes southeast of Mexico City, called “smoking mountains” by native peoples. Small groups of monarchs have been spotted near these peaks, including around the impressive, 17,800-foot Popocatépetl, which regularly belches plumes of ash and lava. Yet biologists fear that smaller expanses of forest in these areas, along with a lack of protected status, would make it difficult for large butterfly colonies to develop.
Meanwhile, some eastern monarchs already have stopped migrating to Mexico, instead spending the winter in warm coastal locations in the southern United States. “If a few monarchs have figured out that they don’t need to go all the way to Mexico, how long will it be before the rest of them do?” asks University of Georgia biologist Andy Davis. Davis, who edits the journal Animal Migration, suspects that mounting climate change will cause more butterflies to winter along the Southeast coast in the coming decades, just as monarchs that breed west of the Rocky Mountains now winter along California’s Pacific coast.
Monarchs “are simply adapting to human influence right now,” Davis says. The species itself “will live on.” But tragically, one of the world’s most wondrous natural history phenomena—the monarch migration—could be lost forever.
An adult monarch (right) feasts on nectar from native butterfly weed. Gardeners throughout the species’ breeding and migratory ranges can help these struggling butterflies. Here’s how:
Plant milkweeds native to your area: Because plants coevolved with your region’s wildlife, native species are best. Studies show that one nonnative—tropical milkweed—harms monarchs. Unlike indigenous species, tropical milkweed does not die back where it stays warm in winter, encouraging monarchs to breed year-round and exposing them to a debilitating parasite, called OE, that caterpillars ingest when they feed on leaves.
Cultivate native nectar plants: Nectar sources are particularly critical during spring and fall when monarchs need to fuel their long migratory flights, which can reach up to 3,000 miles in fall.
Eliminate pesticides: In particular, avoid systemic insecticides such as neonicotinoids, which are taken up by plants’ vascular systems, leaving caterpillars and butterflies that eat leaves, nectar and pollen exposed to the poison long after it has been applied.
Participate in citizen science: Help biologists learn more about monarchs by tagging butterflies for Monarch Watch, reporting migration sightings for Journey North, monitoring for parasites for Monarch Health or checking milkweed for caterpillars for the Monarch Larva Monitoring Project.
Restoring Monarch Habitat
Since signing a 2015 Memorandum of Understanding with the U.S. Fish and Wildlife Service to collaborate on monarch conservation, the National Wildlife Federation has rallied thousands of people nationwide to restore monarch habitat through multiple campaigns and programs, including Butterfly Heroes™, the Mayors’ Monarch Pledge and NWF’s Garden for Wildlife™ program. Efforts by the Federation and its state affiliates have targeted the 14 states that make up the monarch’s central flyway between Canada and Mexico. NWF is also working with state transportation departments to encourage habitat restoration along highways within the central flyway and, at the national and international levels, meeting with U.S., Canadian and Mexican officials to coordinate monarch conservation efforts continent wide. To learn more, visit www.nwf.org/pollinators.
Janet Marinelli wrote about cultivars of native plants, or “nativars,” in the June–July 2016 issue.
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