New studies add nuance to our understanding of the effect of climate change on species migration.
When Toni Lyn Morelli went looking for Belding’s ground squirrels two summers ago, she didn’t think she would have much trouble. Anyone who has ever driven through Yosemite has seen the narrow-tailed, dun coated rodents standing sentinel above their holes and dashing about.
“At first I thought I must be going blind and deaf,” she said when she couldn’t find them in expected spots.
Morelli was looking in the places Joseph Grinnell and his researchers had noted a century earlier. These sites had once had so many Belding’s that the rodents scarcely seemed worth mentioning. The notebooks of Grinnell’s students and colleagues now line a wall 30 feet long in Berkeley’s Museum of Vertebrate Biology. Using recently developed statistical methods to quantify their observations, scientists—co-led by the Museum’s Steve Beissinger, a professor in the Department of Environmental Science, Policy, and Management—are trying to see how California is changing over the centuries. These changes are often so gradual that they’re undetectable. They become visible only when scientists create a sort of time-lapse image by comparing past and present. As more information comes in, that picture is growing sharper and more nuanced.
“With climate warming, species are supposed to move up to cooler elevations. Our goal was to test that hypothesis,” Beissinger said. “We decided to retrace the footsteps of the people who had filled those notebooks, to walk where they walked and to compare what they had seen with what we see today.”
The initial findings generally confirmed the hypothesis, and in 2008 Beissinger and his team published a paper concluding that, in Yosemite, many of the mammals were marching uphill. On the other hand, there were plenty of exceptions to be found once researchers started digging in a few years later. Take the Belding’s ground squirrel: At 42 percent of the sites where Grinnell’s team had found ground squirrels, Morelli found none. The changing climate seemed a likely suspect, and yet there wasn’t an obvious correlation. Some populations had persisted—thrived even—in the hottest locations. Overall, the outer limits of the Belding’s range hasn’t shifted.
The mystery deepened with the work of Morgan Tingley, one of Beissinger’s graduate students at the time. To his surprise, Tingley found the range of the white-headed woodpecker had risen in elevation in one region, dropped in another, and stayed the same in a third. What could be going on?
“We just scratched our heads,” Beissinger said of the bird data. “We wrestled with it for, gosh, a good year and a half.”
For Morelli, a breakthrough came while surveying around Mono Lake. At first her team found no trace of Belding’s amid the sagebrush and parched grasses. But then, they visited one more site: “Everything is dry and hot, and then we take this little side road and come to bright green Kentucky bluegrass—the Mono County Park—and the Belding’s are just everywhere. The students are running around getting drenched by the sprinklers and at that moment it was clear: There’s something about these human-modifications that are allowing the Belding’s to survive in these places where they’ve otherwise disappeared.”
If you don’t count the parks and alfalfa fields and all the human landscapes where the ground squirrels have taken refuge, the picture becomes clear: Their range has retreated up into the mountains by 250 meters.
Similarly, the bird data started to make sense when Tingley sharpened the focus, but for a different reason. Instead of only looking at the general temperature rise across the state, he looked at local changes at each site. Tracking not just temperature but also precipitation showed that 82 percent of the species had moved with the changing climate. Some have followed temperatures, others followed rainfall.
These findings suggest that the choices conservationists make about protecting habitat should be guided not just by global temperature change, but by local conditions—especially precipitation, for which future projections are much more uncertain than temperature. The studies also raise philosophical questions of what we value about wildlife and biodiversity: In 50-75 years Belding’s ground squirrels could be gone from many of California’s natural areas due to global warming, but thriving in farms and parks. Should we install sprinklers in Yosemite’s meadows to keep Belding’s—and all the predators that rely on them—around?
To help answer those questions, the scientists still have plenty of work to do to clear up the picture of what has happened since Grinnell’s time. But they’re always looking forward.
“We do this with an eye to the future as well as the past,” Beissinger said. “In 20 or 30 years when climate change has really set in, we expect people to be retracing our steps, just as we retraced Grinnell’s.”