The Monarch butterfly is disappearing as a result of a triple punch of disappearing habitat in Mexico, disappearing milkweed in the U.S., and the Polar vortex. Scientists say they are seriously alarmed by the butterfly's current situation.
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 in January 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, that is also the year the monarch survey began.
Karen S. Oberhauser, a conservation biologist at the University of Minnesota who has studied monarchs for decades, called the latest estimate "shocking," according to a New York Times story.
Monarchs shuttle back and forth between far-flung summertime havens in Canada and the United States to their 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.
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, 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.