monarch decline
Isotopes to the rescue
glider theory

bunch o' butterflies

male monarch

Active overwintering monarchs [above right] fly to open areas to get nectar from flowers.
Photo by Tom Trower. Male monarch [above] by Karen Oberhauser.

All images on this page © 1987, Monarchs in the Classroom, Monarch Butterfly Sanctuary Foundation.

Monarch's menace
24 MARCH 1999. Monarch butterflies are the long-ranging kings of insect migration. In spring, they fly from winter refuges on a few mountaintops in Mexico. In fall, their descendants somehow find the same isolated mountains, where they huddle by the millions in a massive mingling of insects.

Even though individuals weigh a half-gram each, boughs bend beneath the masses of butterflies. On a warm day, butterflies cloud the sky. On a cool morning, you must tread carefully to spare half-frozen butterflies littering the ground.

Sadly, the monarch migration may be in trouble. This winter, there are an estimated 60 million butterflies in Mexico. That's an 80 percent drop from recent averages, according to monarch authority Lincoln Brower, a biologist from Sweet Briar College in Virginia.

For several decades, monarchs have been tagged to track the path and destination of their migration.
Photo by Karen Oberhauser.
tag, you're itLike much else in Mexico, monarchs are being pressured by a human population explosion. Poor farmers are cutting logs and gathering firewood ever closer to the butterfly reserves. Commercial logging is picking up speed. As trees are thinned out, the butterflies get less shelter.

But monarchs could be facing problems north of the border, too. Questions about the effects of changes in their summer range in Canada and the United States are easy to ask, but difficult to answer.

Until now, there's never been a good way to find out exactly where the Mexican monarchs spend their summers and reproduce. Butterfly tagging programs are popular. But only a few tagged butterflies are found among the millions of monarchs in the wintering grounds. Butterflies are too small to track with radios or satellites.

Now, from the science of geology, comes an appealing solution called stable isotope analysis.

So what's an isotope, and how do they tell us about monarchs' summer habits?


The Why Files
There are 1 2 3 4 5 pages in this feature.
Bibliography | Credits | Feedback | Search

©1999, University of Wisconsin, Board of Regents.

The Why Files Staff includes: Terry Devitt, editor; Darrell Schulte, webmaster; David Tenenbaum, feature writer