Native butterflies are vanishing from parts of the U.S. Northeast, a new Harvard University study finds, while warmer-natured southerners are increasingly invading their turf. This coincides with a half-century warming trend in the region, raising concerns that these rapid ecological shifts are a symptom of climate change.

Published Aug. 19 in the journal Nature Climate Change, the study is based on two decades of research by amateur experts — aka "citizen scientists" — who logged species counts during 19,000 expeditions with the Massachusetts Butterfly Club. Not only did these data fill a critical gap in the scientific record, Harvard researchers say, but they also revealed a troubling trend among New England's native butterflies.

"Over the past 19 years, a warming climate has been reshaping Massachusetts butterfly communities," lead author Greg Breed says in a statement released Sunday. More than three-quarters of the state's northernmost butterfly species are now in decline, he notes, despite a population boom among subtropical and warm-climate species that were virtually nonexistent there as recently as the late 1980s.

The giant swallowtail and zabulon skipper, for example, were rare or absent in Massachusetts 25 years ago, but they're now undergoing a "highly significant" pattern of increase, according to the study. Meanwhile, 17 of 21 cold-climate butterfly species across the state are in steep decline, especially those that spend winter as eggs or small larvae — suggesting these developmental stages could be much more vulnerable to drought or reduced snow cover, the researchers report.

The results seem to fit with previous research into how climate change affects butterfly distribution. The giant swallowtail was also found in Montreal earlier this year, for instance, even farther from its traditional range of Central and South America. In Colorado's Rocky Mountains, researchers have found that warmer weather in late winter can cause flowers to bloom too early, throwing off the life cycles of native butterflies. And since yearly average temperatures have risen by 2 degrees in the Northeastern U.S. since 1970 — with winter temperatures rising at double that rate — the Harvard study hints at similar phenomena in New England.

The study was limited to Massachusetts, so it doesn't address whether the state's dwindling cold-climate butterflies might also be moving north into New Hampshire or upstate New York. But as Breed points out, it does raise the possibility that simply protecting a species' habitat might not be enough to save it as the planet heats up.

"For most butterfly species, climate change seems to be a stronger change agent than habitat loss," Breed says. "Protecting habitat remains a key management strategy, and that may help some butterfly species. However, for many others, habitat protection will not mitigate the impacts of warming."

Aside from possibly uncovering an ecological effect of climate change, the Massachusetts Butterfly Club's research also shows the value and growing role of citizen science, says study co-author and Harvard ecologist Elizabeth Crone.

"Careful data sets from amateur naturalists play a valuable role in our understanding of species dynamics," Crone says in a statement. "Scientists constantly ask questions, but sometimes the data just isn't there to provide the answers, and we can't go back in time to collect it. This study would not have been possible without the dedication and knowledge of the data collectors on those 19,000 club trips."

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Russell McLendon ( @russmclendon ) writes about humans and other wildlife.

Butterflies move north as climate warms
Climate change may be at the heart of why warm-climate butterflies are quickly replacing some native species in Massachusetts, a new study suggests.