A Delicate Reprieve.(monarch butterflies)

When the storm of January 2002 struck, the small group of scientists who monitor the return of the monarch butterflies to the Trans-Volcanic Mountains in central Mexico each winter didn't think it was any big deal. A bit of snow and freezing rain is normal for the region. So why would a storm bother the hardy and migratory monarchs, who after all are tough enough to make the 2,000-mile trip from southern Canada and the United States? This time, though, cold, wet rain had soaked the butterflies hanging on the branches of the oyamel fir trees. When the weather cleared and temperatures dropped, they froze. On the ground, a thick carpet of monarchs lay dead. "We were wading in butterflies up to our knees," says Lincoln Brower, an entomologist at Sweet Briar College in Virginia. He and his colleagues estimated that 500 million monarchs had died from the storm--five times more than they thought had even existed in the colony.

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 ...