That spring only about 20 percent of the usual number of monarchs made their way north. Last summer, monarch sightings in the United States and Canada were below the norm. When the scientists went back in December to see how many of the butterflies had returned, they were afraid of what they would find. They climbed to the mountaintops, ran their tape measures around the half-dozen or so monarch colonies and calculated how many insects were nesting in the trees. The results, released last week, were shocking–delightfully so. In one year, the devastated monarch population had returned to normal.
The news is good for the butterflies, but scientists are ambivalent. They’ve warned for years that the loss of forest habitat in Mexico makes the butterflies increasingly vulnerable. A year ago they lamented tragedy. Now they worry that the quick recovery will damage their credibility in the eyes of the public, and perhaps give people a false sense that the monarchs are not as vulnerable as they’ve been made out to be.
The news certainly is a testament to the robustness of these amazing insects. Monarchs migrating north from Mexico don’t live long enough–only about four weeks–to make the trip in a single generation. Along the way, females lay their eggs on milkweed, a common northern plant and their only source of sustenance–as many as 500 eggs in four weeks, which may have something to do with the quick jump in numbers. In a few months, the monarchs appear to have multiplied fourfold, migrating all the while.
To a great extent, though, the butterflies may just have gotten lucky. Last spring the weather along the migratory path up the Gulf Coast, through Texas and up into the northern latitudes, was unusually mild. It was also a bumper year for milkweed. Had the weather been poor, or had the milkweed been thin, the monarchs might not have recovered quite so well.
And why did so many monarchs die during the storm? Brower thinks illegal logging had much to do with it. Monarchs have a kind of natural antifreeze that protects them from below-freezing temperatures. But if they get wet, the antifreeze doesn’t work. During the storm, says Brower, the forest, thinned from logging, provided too little protection against dampness and wind.
A few months ago the Mexican government increased the area under protection from 16,000 to 56,000 hectares, but researchers say the decree hasn’t made much of a difference. “There’s illegal logging all over the place,” says Brower. “The Mexican government doesn’t have the resources to stop it.”
The insects are especially dependent on their Mexican habitat. Although the monarchs spread out during the summer throughout the eastern United States and Canada, they return each winter to the same 12 mountaintops within a 50-by-100-kilometer section of Mexican forest. What’s more, the butter-flies congregate only on the southwestern slope of the mountains, in a narrow band between 3,100 and 3,300 meters in elevation. A bit of logging in the wrong place can ruin a great deal of monarch habitat. Illegal logging in this area has been a problem ever since scientists first discovered the butterflies’ winter hideaways in 1975. Conservationists have called on President Vicente Fox to send the Army to police the area. “The monarch butterfly is an environmental symbol of Mexico,” says the poet Homero Aridjis. And thus far, a lucky one at that.
