Summer migration pathways for the Monarch butterfly |
By Michael Wines, The New York Times, March 13, 2013
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.
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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