Faltering under extreme weather and vanishing habitats, the yearly winter migration of monarch butterflies to a handful of forested Mexican mountains dwindled precipitously in December, continuing what scientists said was an increasingly alarming decline.
The migrating population has become so small — perhaps 35 million, experts guess — that the prospects of its rebounding to levels seen even five years ago are diminishing. At worst, scientists said, a migration widely called one of the world’s great natural spectacles is in danger of effectively vanishing.
The Mexican government and the World Wildlife Fund said at a news conference on Wednesday that the span of forest inhabited by the overwintering monarchs shrank last month to a bare 1.65 acres — the equivalent of about one and a quarter football fields. Not only was that a record low, but it was just 56 percent of last year’s total, which was itself a record low.
At their peak in 1996, the monarchs occupied nearly 45 acres of forest.
The acreage covered by monarchs, which has been surveyed annually since 1993, is a rough proxy for the actual number of butterflies that survive the arduous migration to and from the mountains.
Karen S. Oberhauser, a conservation biologist at the University of Minnesota who has studied monarchs for decades, called the latest estimate shocking.
“This is the third straight year of steep declines, which I think is really scary,” she said. “This phenomenon — both the phenomenon of their migration and the phenomenon of so many individuals doing it — that’s at risk.”
Mexico is the southern terminus of an age-old journey in which monarchs shuttle back and forth between far-flung summertime havens in Canada and the United States and a single winter home in Mexico’s Sierra Madre mountains.
An internal compass guides the butterflies each fall to a small cluster of mountains where ideal temperatures and humidity allow them to rest, clinging to trees by the millions like brilliant orange capes, until they begin the northward return trip each March.
By some estimates, a billion or more monarchs once made the 2,500-mile-plus trip, breeding and dying along the route north so that their descendants were actually the ones that completed the migration.
The number of surviving butterflies has varied from year to year, sometimes wildly, but the decrease in the size of the migration in the last decade has been steep and generally steady.
The latest drop is best explained by a two-year stretch of bad weather, said Chip Taylor, a biologist at the University of Kansas who has studied the butterflies for decades. But while good weather may help the monarchs rebuild their numbers, their long-term problem — the steady shrinking of habitat along their migratory route — poses a far greater danger.
The monarchs’ migratory freeway runs through the Great Plains. As they flew north from Mexico in early 2012, Dr. Taylor said, months of near-record heat sapped their endurance and skewed their migratory patterns in ways that limited their ability to reproduce.
Last spring, he said, the opposite happened: Unusual springtime cold in Texas delayed the butterflies’ northward migration, causing them to arrive late in areas where they would normally have bred weeks earlier.
“They have to arrive in the middle of a 40-day period to do really well,” Dr. Taylor said. “If they arrive too early, the population crashes, and if they arrive too late, the population crashes.”
A larger migration might have weathered the cold snap, but given their losses the previous year, “the butterflies really didn’t have the capacity to turn things around,” he said.
The loss of habitat is a far more daunting problem, Dr. Taylor and Dr. Oberhauser said.
Monarchs lay their eggs only on milkweed, and patches of the plant have rapidly disappeared from the Great Plains over the last decade. As corn prices have risen — spurred in part by a government mandate to add ethanol to gasoline — farmers have planted tens of millions of acres of idle land along the monarchs’ path that once provided both milkweed and nectar.
At the same time, growers have switched en masse to crops that are genetically engineered to tolerate herbicides. The increased use of herbicides has all but wiped out milkweed that once sprouted between rows of corn and soybean.
As a result, Dr. Taylor said, the monarchs must travel farther and use more energy to find places to lay their eggs. With their body fat depleted, the butterflies lay fewer eggs, or die before they have a chance to reproduce.
The monarchs are but the most visible victims of the habitat loss, Dr. Oberhauser said. A wide variety of pollinators and other insects, including many that are beneficial to farmers, are also disappearing, she said, along with the predators that feed on them.