Monarch Butterfly Populations Tumbling

Populations overwintering in Mexico may be on the decline.

THE GIST

The area of Mexican forest patches covered by overwintering butterflies has been shrinking.

Researchers say they see a long-term decline in the butterfly's numbers.

North America's beloved monarch butterfly may be sliding into a long-term decline. While monarch numbers have fluttered up and down over recent decades, one research group now says that there's enough data to spot a downward trend.

During the past 17 years, the area of Mexican forest patches covered by overwintering butterflies has been shrinking overall, says conservation biologist Ernest Williams of Hamilton College in Clinton, N.Y. He and his colleagues use the area occupied, which has averaged 7.24 hectares since the end of 1994, as a rough index of winter monarch population size.

Several menaces, including habitat loss, confront monarchs but the researchers focused on assessing trends in the populations instead of confirming the causes.

Within the overall downward trend, seven of the 10 below-average years in the study followed one another in a worrisome streak through the winter of 2010–11, the researchers say. The downward trend did not appear to be a fluke based on a couple of good or bad years; it still showed up when researchers removed the largest area (20.97 hectares in 1996–97) or the smallest (1.92 hectares in 2009–10) from the data, Williams and his colleagues report online March 21 in Insect Conservation and Diversity.

"We have enough data now to say that we are seeing a long-term decline," Williams says.

That trend in winter populations may be statistically significant, says monarch researcher Karen Oberhauser of the University of Minnesota in St. Paul, but she and other researchers are now working on a broader analysis of monarchs and the challenges the insects face throughout the year to get a better handle on whether the population is declining and, if so, why. "I am not arguing that monarch populations are not facing threats, nor am I saying I'm not concerned," she says. "I don't think the trend data clarify the situation."

That situation concerns the long-term health of what Williams calls the best-known butterfly in the world. Orange-and-black monarchs dart and waver over North American summer landscapes coast to coast and as far north as Canada, seeking milkweed plants as sites for laying eggs. The species' annual migration is perhaps its most dramatic feat, though. Butterflies that hatch during summer and in fall -- without ever having been to Mexico -- find their way to the same patches of forest their predecessors did.

In those spots, monarchs blanket the forest. "It's stunning," Williams says. "Sometimes you look at a tree trunk and you can't see the bark." When a burst of sunlight stirs the monarchs to flight, "you hear a whishing sound of their wings."

For butterflies in Mexico and in their summer range, "we see a confluence of threats," Williams says. Winter monarch retreats are officially protected, but illegal logging chews away at Mexico's forests. Plus, looming climate changes may bring more episodes of severe weather, which can hammer the butterflies. And monarchs across North America are finding less breeding habitat than they used to. Open land for milkweeds is falling to development, and researchers warn that a boom in genetically engineered crops is changing herbicide use patterns and thinning the ranks of milkweeds.

Sparser milkweeds for breeding could in theory have a major effect on monarch abundance, says insect ecologist Myron Zalucki of the University of Queensland in Brisbane, Australia, "The big problem," he says, "is disentangling the effects of all the threatening processes."