Menace to monarchs
Bad weather and shrinking habitat are being blamed for a serious decline in the number of monarch butterflies wintering at isolated Mexican mountaintops.
The cluster of monarchs in Mexico has shrunk to 1.65 acres — about the size of one and one-quarter football fields.
That is the smallest area in 20 years of record-keeping.
Because area translates into numbers, 2014 also has the smallest number of migrating monarchs, perhaps a few tens of millions, compared to as many as a billion butterflies in the winter of 1997.
The migration of these “extraordinary organisms” is noteworthy for many reasons, says Ernest Williams, a professor of biology at Hamilton College in New York. “They have a strong graceful flight, a striking orange color with black spots. They are the most recognized butterflies. This insect that weighs as much as a paper clip flies thousands of miles, to overwinter in a very small, select set of mountaintops, escaping the freezing temperature we have up here, and then migrates back. They incorporate plant toxins from milkweed as protection against predators,” he says.
Although monarchs are unlikely to go extinct as a species, the migration is tenuous, says Williams. “It’s pretty depressing how few monarchs there were. Some think we may be seeing a collapse. We are not there yet, but the large numbers we saw 20 years ago, we are not going to see again.”
Orley “Chip” Taylor, founder of Monarch Watch at the University of Kansas, agrees that the migration “will not vanish in the near term, assuming we escape some disastrous collapse in Mexico in the next few weeks due to a winter storm. If they encounter the right conditions, the number coming north should be sufficient to increase the population.”
Weather, in the north and south, accounts for much of the fluctuation in numbers, says Taylor, who has studied and advocated for monarchs for decades. “In the last three years, conditions for breeding have been less than optimal,” he says. “If we have conditions that are more normal this coming year and beyond, this population will increase, but it is not going to get back to normal in one year.”
The monarchs that winter on trees in Mexico have already traveled long distances from the East Coast, and particularly the Midwest. Scientists now know that the fir trees store solar heat in their trunks, which helps warm the butterflies.
In spring, after five months of hanging out on the branches, the weather warms and the days lengthen, and the butterflies breed, then return north to feed and deposit eggs on milkweed. The young from those eggs continue north, often producing another generation. Finally, the shortening fall days trigger a reverse migration among the grandchildren or great grandchildren of that first generation of north-bound monarchs.
Those southward-bound butterflies are several generations removed from the ancestors who departed Mexico in the spring. So how do they know the route? Taylor points to work by Steven Reppert of the University of Massachusetts, who has found that the monarchs use a “time-compensated sun compass” to orient themselves based on the sun, even though it transits the sky daily. “Reppert worked out the fact that the monarch has two internal clocks, one in the antennae, another in brain, that help keep the butterfly on course,” Taylor says.
It’s possible that the butterfly, along with many migratory birds, also sets its course by detecting earth’s magnetic field.
At any rate, after abandoning their summer breeding grounds, the monarchs head toward Texas, the gateway to Mexico. The heading must differ according to location, Taylor says. “Those that are leaving Wisconsin have a different heading than those from Minnesota or Kansas or Georgia. In each area, the monarchs use headings that are appropriate to get them to the Texas-Mexico border,” he says.
Conditions up north and down south can threaten the butterflies.
Winter blues in Mexico?
The enormous roosts of monarchs were discovered — by outsiders, at least — in the 1970s in forests in Michoacán and the Federal District of Mexico. With the monarchs from much of eastern North America crammed in a few postage-stamp locations, fire, storms or disease could have a devastating impact.
The major problems now are weather — cold winters harm the butterflies — and logging, says Williams. “The Mexican government has been fighting against commercial logging, but there is still small-scale logging. As holes are punched in the forest, the temperature changes are more extreme, and that leads to butterflies freezing and dying, or becoming too warm and using up their energy supply” before they start the northward migration, he explains.
Even the tourism that makes the reserves a source of income can harm the fragile ‘flies, which can be trampled while resting on paths in the reserves.
Great fate in the great North?
In the northern farmland where monarchs breed, two related changes help explain the population crash: use of herbicide-resistant crops, and increased planting of corn to make ethanol fuel.
Monarchs require milkweed to breed, but their breeding ground coincides with the American corn belt, Taylor says. “If you look from eastern North Dakota to Ohio, and the Canadian border to the latitude of Kansas, 37° north, that is the area where the most habitat is being lost, and that is where monarchs have the greatest breeding potential.”
Forty percent of the American corn crop — the largest single national crop in the world — is now being fermented into ethanol for blending into gasoline. A federal mandate in the 2007 Clean Energy Act to use ethanol gives farmers “an incentive to gobble up the remaining pasture, grasslands and wetland, and turn it into cropland,” says Taylor.
Some of the new land being planted with corn and soybeans was until recently protected by the Conservation Reserve Program, a federal effort to cut erosion by removing steep, hard-to-till fields from production.
Taylor calculates that since 1996, the introduction of herbicide-tolerant soy and corn removed 150 million acres of land that could have borne milkweed.
Roads and development accounted for 17 million additional acres, adding up to 167 million acres of lost monarch habitat. That’s 87 percent as big as Texas.
“When we lose native prairie, it’s not coming back,” Taylor says. “The Midwest is the biggest monarch-producing area, and this is where we have lost the most habitat. If we calculate what has happened in the most productive area, that is more than a 30 percent loss.”
Lost habitat has resulted in a “crash” in pheasant habitat in South Dakota and Nebraska,” Taylor alleges. Still, he says, “We probably have enough milkweed support three or four hectares [of monarchs] in Mexico if all other conditions are favorable.”
At an estimated density of 10 to 50 million monarchs per hectare, that wintering ground — just seven to 10 acres — could house 30 to 200 million monarchs in winter.
In Mexico, the monarchs have become minor celebrities and a tourist attraction. In the United States, they’ve been pressed into service in a variety of programs — in and out of school — devoted to the study and reproduction of the magnetic monarchs.
“Many school children are raising monarch caterpillars in class; that’s how they learn insect biology and general biology,” says Williams. “There are more people across the country who volunteer as citizen-scientists to track migration and learn more about the biology.”
Even though Monarch Watch’s “Way Station” program has registered some 7,500 gardens with milkweed for monarchs, “the scale of habitat loss exceeds anything we have been able to reverse,” Taylor says. “Just due to ethanol, we have lost 24 million acres of habitat since 2008. There is no way a few gardens are going to compensate for that. We have about 7,000 gardens, but we need 7 million.”
The garden industry concentrates on showy plants that are “beautiful, but unfortunately, provide almost nothing for pollinators,” Taylor says. “We need plants to support pollinators; we need to think more broadly about what we are putting into our gardens.”
So what are the odds that the monarch population graph will drop through the floor, and we can write off the monarch migration as we have the passenger pigeon, which went extinct 100 years ago? “If we can expand the habitat and protect it — to include nectar sources, flowers, milkweed, through the range of the monarch — that would also benefit other pollinators,” says Taylor. “If people respond to this story of decline, we can actually increase the breeding habitat, and keep working on protecting all components of the life cycle, because monarchs are being hammered at all stages.”
Finally, there is a biological reason for optimism, Williams says: “Monarchs are insects, and a female, if she survives, can lay several hundred eggs, and so the population can regrow rapidly.”
– David J. Tenenbaum