The number of monarch butterflies that completed an annual migration to their winter home in a Mexican forest sank this year to its lowest level in at least two decades, due mostly to extreme weather and changed farming practices in North America, the Mexican government and a conservation alliance reported on Wednesday.
Travis Morisse/ The Hutchinson News via Associated Press
The monarch population has declined with extreme weather and changes in farming that have diminished its source of food.
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The area of forest occupied by the butterflies, once as high at 50 acres, dwindled to 2.94 acres in the annual census conducted in December, Mexico's National Commission of Natural Protected Areas disclosed at a news conference in Zitácuaro, Mexico.
That was a 59 percent decline from the 7.14 acres of butterflies measured in December 2011.
Because the insects cannot be counted, the combined size of the butterfly colonies is used as a proxy in the census, which is conducted by the commission and a partnership between the World Wildlife Fund and the Mexican cellphone company Telcel.
"We are seeing now a trend which more or less started in the last seven to eight years," Omar Vidal, the head of the wildlife group's Mexico operations, said in an interview. Although insect populations can fluctuate greatly even in normal conditions, the steady downward drift in the butterfly's numbers is worrisome, he said.
The latest decline was hastened by drought and record-breaking heat in North America when the monarchs arrived last spring to reproduce. Warmer than usual conditions led the insects to arrive early and to nest farther north than is typical, Chip Taylor, director of the conservation group Monarch Watch at the University of Kansas, said in an interview. The early arrival disrupted the monarchs' breeding cycle, he said, and the hot weather dried insect eggs and lowered the nectar content of the milkweed on which they feed.
That in turn weakened the butterflies and lowered the number of eggs laid.
But an equally alarming source of the decline, both Mr. Taylor and Mr. Vidal said, is the explosive increase in American farmland planted in soybean and corn genetically modified to tolerate herbicides.
The American Midwest's corn belt is a critical feeding ground for monarchs, which once found a ready source of milkweed growing between the rows of millions of acres of soybean and corn. But the ubiquitous use of herbicide-tolerant crops has enabled farmers to wipe out the milkweed, and with it much of the butterflies' food supply.
"That habitat is virtually gone. We've lost well over 120 million acres, and probably closer to 150 million acres," Mr. Taylor said.
A rapid expansion of farmland — more than 25 million new acres in the United States since 2007 — has eaten away grasslands and conservation reserves that supplied the monarchs with milkweed, he said.
The monarchs' migration is seen as a natural marvel and, for Mexico, a huge tourist attraction. But naturalists regard the butterflies as a forward indicator of the health of the food chain. Fewer butterflies probably means there are fewer other insects that are food for birds, and fewer birds for larger predators.
Mr. Vidal and Mr. Taylor said December's record-low census does not necessarily constitute a knockout blow against the butterfly. The Mexican government has halted what was once extensive logging in the monarchs' winter home, and there remains the prospect that conservationists and state and local governments will replenish some of the milkweed lost to development and changed farming habits.
Mr. Vidal said that American and Canadian officials should move quickly. "Mexico is doing its part," he said. "Mexico has invested resources, and it's eliminated this massive illegal logging in the reserve. But on the other hand, I think the United States has to do much more."
Mr. Taylor said a further decline could cross a tipping point at which the insects will be unusually vulnerable to outside events like a Mexican cold snap or more extreme heat that could put them in peril.
"Normally, there's a surplus of butterflies and even if they take a big hit, they recover," he said. But if their current 2.94-acre wintering ground drops below 2.5 acres, bouncing back could be difficult.
"This is one of the world's great migrations," he said. "It would be a shame to lose it."
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