Butterfly displaced by climate change?

Edith's checkerspot butterflies inhabit patches of fields, rocky hills, and alpine terraces from Baja California to British Columbia. But throughout their lives, they don't roam far. An entire population can confine its existence for decades to a piece of land 100 by 100 meters. Yet as a species, new data show, this butterfly is moving northward-big time.

Camille Parmesan of the National Center for Ecological Analysis and Synthesis, at the University of California, Santa Barbara, has just completed a census of 151 previously reported populations scattered along the west coast of North America. Because they are such stay-at-homes, the butterflies' apparent northward trek actually reflects large numbers of populations dying off at the southern end of their range and presumably new populations in the north.

Mexican populations, for instance, were four times more likely to be extinct than ones in Canada. Low-altitude populations are also losing ground.

She says that the slow, difficult movement that she's documented in this species "tracks changes over the last 100 years."

Biologists have argued that if Earth's climate warms, whole ecosystems would begin moving into what had been cooler zones. While there has been evidence for several plant species that such a shifting may have begun (SN: 6/18/94, p.

399), Parmesan says those data came from only a fraction of each species' global population. Establishing a true shift requires canvasing the entire range of a plant or animal-which she has now done for Edith's checkerspot.

The result: "The clearest indication to date that global climate warming is already influencing species' distributions," she reports in the Aug. 29 Nature.

Her conclusions make sense, says Paul R. Ehrlich of Stanford University. A renowned observer of this butterfly, he notes that even a small change in the microclimate-affecting moisture, temperature, or sunlight-can disrupt "the very complex relationship between [this checkerspot] and its food plants," causing extinctions locally.