It doesn't take much to send a butterfly fleeing.
Just a several-degree rise in average temperatures over three decades led to a dive in the number of the colorful, fluttering insects thriving in the brisk environs of the high Sierra Nevada, according to a new study from UC Davis.
And with no higher ground to head to, prospects for alpine butterfly species such as the Small Wood Nymph and Nevada Skipper look bleak.
"There is nowhere to go except heaven," said Arthur Shapiro, a UC Davis professor of evolution and ecology.
During the past 35 years, Shapiro has built the world's largest database of regional butterfly activity from his twice-monthly observations of 159 species at 10 sites from Suisun Marsh to the peaks of the Sierra. The massive database now reveals an unmistakable pattern of butterflies traveling to higher elevations as average temperatures rise. The analysis was published last week in the Proceedings of the National Academy of Sciences.
Since the charismatic insects serve as sensitive barometers to environmental changes, the database adds to mounting evidence of how a warming world will affect wild species in the state.
The database also shows that it's a one-two punch of habitat loss and temperature increases that have sharply reduced butterfly populations in low elevations, Shapiro said. What shocked Shapiro is the local disappearance of even common species in the flatlands.
The Large Marble, with white upper wings and mottled yellow, white and black under wings, feeds on abundant weedy plants such as mustard and wild radish.
Now its population has crashed at the study's sea level sites, and the species faces regional extinction, he said. Of the 10 study sites, seven had declines in these "weedy" species of butterfly that were once common.
"I could walk two blocks from my house and find it flying along a roadside ditch," Shapiro said. "Now, to count on finding it, I have to go to the east side of the Sierra.
"It's absolutely astonishing because, as you know, there's no lack of weedy mustard or wild radish," Shapiro added.
In the areas he's studying in the Central Valley, suburban sprawl has fractured butterfly habitat, Shapiro explained. So butterflies seeking their typical meal of plant nectar might perish before finding enough food, even if the plants are still plentiful overall.
"The farther they have to go, the less likely they'll live long enough to hit a patch," Shapiro said. And warming temperatures only add to the stress, he said, even if it's just a few degrees on average.
Shapiro, who wrote "Field Guide to Butterflies of the San Francisco Bay and Sacramento Valley Regions," said that butterflies' abundance and diversity throughout the Bay Area are also likely affected in similar ways.
Terry Root, a senior fellow with the Woods Institute for the Environment at Stanford University, described Shapiro's butterfly database as "absolutely spectacular." It's rare, she explained, for a study of this duration to be conducted by one person, which reduces errors from different measurement techniques.
It also shows that the effect of warming temperatures is occurring as fast as she's been warning it would, Root said. "It makes me a little bit more worried."
While habitat loss is blamed for much of the catastrophic declines of many lowland butterfly species, Shapiro said there's little doubt temperature increases have altered the distribution of these insects in the Sierra, since there has been no habitat loss in the areas under study.
Although warming temperatures don't bode well for butterflies that have long made the chilly alpine regions of the High Sierra their home — since they have no escape — plenty of newcomers have arrived as temperatures warmed below. All of the 10 sites Shapiro and his colleagues have tracked show declines in the overall number of butterflies, except for the highest elevation site, at 9,100 feet.
So while alpine natives like Small Wood Nymph and Nevada Skipper are declining, for the first time researchers are finding butterflies there that typically live at the 7,000-foot elevation. Anna Blue and Hoffmann's Checkerspot butterflies in previous years were found only at the Donner Pass research site. But as average temperatures have increased about 7 degrees Fahrenheit in the Donner Pass area since the study began 35 years ago, they're making cooler areas at higher elevations their new home.
For Shapiro, whose fascination with butterflies dates to his childhood, that prospect means hard choices for ecologists working to preserve the diversity of butterflies in the state. Saving some species may mean planting the foods they need in their new locations — a costly proposition.
"Conservation involves a lot of choices," he said. "We're going to be called upon to play God because we can't save everything."
Suzanne Bohan covers science. Contact her at 510-262-2789. Follow her at Twitter.com/suzbohan.
Go to http://butterfly.ucdavis.edu to learn more about the work of UC Davis butterfly expert Arthur Shapiro. The site includes
photos, life histories of butterfly species, population trends, tips for creating a butterfly garden and a game for learning butterfly identification.