Other Environmental Themes

A New Tag of Monarchs

Hundreds of millions of monarch butterflies spend the winter months in the oyamel fir tree forests of central Mexico, painting the verdant woods with bright patches of orange. The mountain sanctuaries are largely protected by federal decree, yet the forest is still being degraded by illegal logging, agriculture, and even tourism. A program launched in 2002 aims at saving the monarch's winter haven by providing financial incentives for sparing trees, rather than chopping them down.

The threats monarchs face when they fly north from Mexico are less well documented, mainly because the butterflies disperse themselves widely throughout the United States and Canada. Environmentalists would like to know where the insects breed to establish the most sensible conservation priorities. For the past 30 years, monarch watchers have tracked migrations by placing adhesive tags on butterfly wings. This strategy, unfortunately, suffers from a low return, as only about one in a thousand tagged butterflies is ever recovered.

Canadian researchers Leonard Wassenaar and Keith Hobson think there's a better way: Every butterfly, they say, carries its own chemical tag—with distinct concentrations of hydrogen and carbon isotopes—that can identify its birthplace. The idea behind the tags is that water of a particular locale has a characteristic isotopic pattern that gets incorporated in milkweed plants, monarch catterpillar's main food, and is eventually incorporated in the tissue of adult butterflies. Wassenaar and Hobson showed that half the butterflies examined in Mexico were born in a limited part of the breeding range—a band of midwestern states ranging from Kansas to Ohio. That's the region where conservation efforts would yield the biggest payoff, they note.

The technique is not restricted to monarchs. A 1998 Science report coauthored by Hobson showed that the breeding success of a migratory songbird, the American redstart, depended on the quality of their winter habitat in Jamaica, which was reflected in the isotopic signature of carbon. Birds that spent their winters in poor, dry habitats—as opposed to moist, fertile accommodations—fared poorly the next summer when they attempted to breed in New Hampshire.