By J. Madeleine Nash
Illustration ©Steve Dininno
The cartoon version of how climate change will affect mountain ecosystems goes like this. Plants and animals will try to escape rising temperatures by retreating upward until they can go no further. Then, like deserving souls transported to heaven, they will vanish into the ether. Tongue in cheek, one ecologist has labeled it “the rapture hypothesis.” And one of its main corollaries holds that the organisms to disappear first will be those that live on or near mountaintops. Chief among these is the American pika (Ochotona princeps), the fur-ball cousin of rabbits and hares that has recently morphed into a poster child for global warming.
So three summers ago, when U.S. Forest Service ecologist Connie Millar was hiking along the edges of a high meadow in the Sierra Nevada, looking for signs of pikas, she didn’t know what to expect. And initially, she says, her quarry did prove elusive. Then her husband Jeff pointed to a scattering of telltale pellets. “Is that what you’re looking for?” he asked. The next day, in the shadow of 3,981-meter-high Mount Dana, the spunky, ginger-haired scientist parked herself on a talus slope to wait out a thunderstorm. There, sheltered from lightning thatdanced on snow-streaked peaks, she heard the pika’s staccato chirps and logged nearly a dozen sightings.
Those Mount Dana sightings marked the start of a multiyear quest that sent Millar chasing pikas across 11 mountain ranges in California and adjacent Oregon and Nevada. To her surprise, American pikas proved to be not rare but common. She and colleague Robert Westfall found pikas living amid all sorts of craggy landforms: talus fields, lava flows, cliff faces, even man-made piles of rubble such as mine tailings and stone walls. “Once we started to look,” Millar says, “we found pikas nearly everywhere.”
In fact, the more ground Millar covered and the more pellets and pika calls she counted, the more resourceful and resilient these fist-sized creatures began to seem. Contrary to ingrained belief, says Millar, pikas continue to occupy a remarkably broad elevational band—broader than that of any of the pine trees she has spent most of her career studying. In the Sierras, she says, pikas can currently be found as high as 3,887 meters and as low as 1,827 meters—lower than the lowest pika population in the historical record.
So is the rapture hypothesis mere fiction? Not by a long shot. Like most generalizations, it contains more than a snippet of truth. As a rule, lower altitudes are warmer than those above. Moreover, pikas aren’t home free with regard to global warming. Data collected by wildlife biologist Erik Beever over the past 15 years strongly suggests that climate change has begun stressing pikas in the harsh interior of the Great Basin. Nine of 25 known populations have vanished within living memory. As many see it, these declines foreshadow the fate of pikas across the American West.
At present, this gloomy prognosis commands widespread acceptance. Models lend it further credence. Scott Loarie, a researcher in the Global Ecology Department of the Carnegie Institution for Science, projects that if greenhouse-gas emissions are not swiftly curtailed, American pikas could disappear from half their current range by the end of this century. No wonder it’s so hard to find a popular article about the impacts of climate change that fails to mention pikas and their plight. And yet here comes Millar, seemingly out of the blue, saying in effect: Wait a minute. It’s more complicated than that and also considerably more interesting.
Millar, as she will cheerfully tell you, is not a pika expert, though she has learned quite a bit about pika biology and behavior. Her strong suit rather is her deep knowledge of mountain ecosystems. Working out of the Forest Service’s Sierra Nevada Research Center in Albany, California, she has progressively widened the scope of her investigations, starting out as a geneticist interested in the evolution of conifers and then moving into paleoecology. Among the many hats she wears is that of cofounder of the Consortium for Integrated Climate Research in Western Mountains.
In vertiginous terrain, she often observes, the interactions between climate and biology can be almost unimaginably complex. Climate models, for example, suggest that the world’s mountains—the Alps, the Andes, the Sierras, the Himalayas—should be strongly warming. But when one looks at the actual temperature data, the picture becomes a great deal messier. Recently, for example, Jessica Lundquist, a hydrologist at the University of Washington in Seattle, and Nicholas Pepin, a climatologist at the University of Portsmouth in the U.K., examined temperature records from 1,000 weather stations in mountainous regions around the world. While a subset of the stations (including those on or near summits) recorded a globally consistent trend, others (notably those in valleys) did not. When stirred all together and plotted on a graph, the readings resembled the splatter on a painter’s drop cloth.
The explanation lies in topography. Changes in aspect, for example, are powerful modulators of climate. In spring, south-facing slopes warm before north-facing slopes, and plants start greening up weeks earlier. On the other hand, because they are sunnier, south-facing slopes tend to dry earlier. East- and west-facing slopes vary greatly in the amount of precipitation they receive and in their exposure to wind. Wind can affect both temperature and humidity. It can also move snow around, removing it from one place and depositing it in another.
Elevation is another critical variable, but not because temperature necessarily drops with height. In Yosemite National Park, for example, the Tuolumne Meadows campground, at around 2,600 meters, is consistently colder than the slopes that rise steeply above it. “We call it cold-air pooling,” says Lundquist. “It occurs anywhere there’s a depression, a spot that’s lower than the surrounding terrain.” Cold-air pooling typically occurs at night, when surfaces re-radiate the heat they’ve absorbed during the day. As this warm air rises, it is replaced by cooler, denser air flowing downhill and often triggering condensation.
Another insight into alpine microclimates comes from botanists Christian Körner and Daniel Scherrer at the University of Basel. Starting in the summer of 2007, they deployed an infrared camera to scan soil temperatures at six sites, including Switzerland’s 2,431-meter-high Furka Pass. As an additional check, they buried temperature sensors in the ground. Both the surface and the root zone, they found, can be best described as a thermal mosaic. As expected, air temperatures did cool with height, but that difference was swamped by even larger variations that occurred at plant level within the same elevational band.
Rugged landscapes, it turns out, are rife with places that are sunnier and shadier or drier and wetter on virtually every spatial scale. The implication? Plants and animals might not need to move upslope to escape adverse conditions but instead may find the right “thermal niche” by shifting downslope or even sideways.
An example Millar likes to cite comes from 3,425-meter-high Mount Grant, the highest point in west-central Nevada’s Wassuck Range. There, the upper tree line presently consists of a sparse growth of limber pines, which are confined to north- and northeast-facing slopes. But it wasn’t always that way. By dating the dead wood found on other exposures, Millar has been able to reconstruct a long-term picture. Over the past 3,600 years, she says, these hardy, long-lived trees have hopscotched their way around the mountain.
For American pikas, the dance with climate started during the Pleistocene, some time after their Asian ancestors scampered across the Bering land bridge. And it has never stopped. During the last glacial maximum, fossil evidence suggests, American pikas inhabited lower elevations than they do today. But as the great ice sheets melted, pika habitat progressively retracted upslope.
Pikas, it is often said, are cold-adapted creatures, better at retaining heat than shedding it. In addition to thick fur, they have high basal metabolisms and body temperatures that hover just a few degrees shy of lethal. Confined to a cage on the surface, pikas quickly overheat and die, even at comparatively balmy temperatures of around 25 degrees Celsius. As the owner of six rabbits, Millar is familiar with this idiosyncrasy of the lagomorph clan. On warm days, she says, she helps her pets cope by wetting down flagstones on which they can comfortably cool.
In the wild, though, pikas are not as foolish as mad dogs and Englishmen. In the ranges Millar and Westfall surveyed, for example, pikas showed an overwhelming preference for living in or near ice-sculpted landforms, notably rock glaciers and boulder streams. In fact, 80 percent of the nearly 400 sites where researchers found pikas—or their recent signs—could be associated with these features. It’s not the presence of ice that seems to matter. It’s more that associated geophysical processes—think freeze-thaw and downslope creep—have constructed the equivalent of an air-conditioned house.
In summer, explains Western Washington University geologist Douglas Clark, chilly nighttime air sinks into the spaces between the rocks and is trapped by warmer daytime air from above. In the Sierra Nevada, Millar and Westfall found that summer temperatures in the talus were 4 to 7 degrees Celsius colder than at the surface; in Great Basin ranges, the average difference worked out to 6 degrees Celsius.
Provided they can escape from the heat, pikas can handle some rather surprising environments. At Idaho’s Craters of the Moon, for example, exposed rocks sizzle in the summer sun. Yet beneath the surface, pikas have set up house in a honeycomb of lava tubes. Since 1969, Arizona State University conservation biologist Andrew Smith has studied the pikas that hang out in the mine tailings of California’s Bodie Hills, near the Nevada border. The elevation at this former Gold Rush site, he notes, starts around 2,600 meters, which makes it “low and hot for pikas”—especially in August, when maximum daily air temperatures hit 30 degrees Celsius. “But pikas are smart,” Smith observes. “If it’s hot, they’re not active on the surface between ten in the morning and four in the afternoon, and they’re also active at night.”
Among scientists who study pikas, a consensus appears to be building that acute heat stress is probably not that important. Changes in other climatological parameters, such as snow line, are emerging as potentially more critical. Indeed, it is one of the great ironies of our warming world that these hardy herbivores, superbly tuned to Ice Age conditions, may now be in danger of freezing.
“Pikas can get into the talus to escape from extreme summer heat,” observes Chris Ray, a researcher in the University of Colorado’s Department of Ecology and Evolutionary Biology. “But without an insulating blanket of snow, they can’t escape from winter cold.” Ray, in fact, thinks that the danger to pikas may currently be greatest at mid-elevations—where summer heat is curtailing their ability to forage while a rising snow line exposes them to sub-zero temperatures in winter.
Even pikas living in the far north may not be safe, says David Hik, an ecologist at the University of Alberta in Edmonton, Canada. Hik studies collared pikas, close relatives of American pikas, in the Yukon’s Ruby Range. There, in 1999 and 2000, two exceptionally warm winters turned snow to rain, which then turned to ice. In response, the pika population at one of his study sites crashed by 80 percent. The population has since recovered, but many—including Hik—view the sudden collapse as a cautionary tale. In the Sierra, warmer winters that produce rain instead of snow are projected to become more common.
At present, the pika’s cup really does seem to be simultaneously half empty and half full, and the same could be said for myriad other species. For as the greenhouse juggernaut bears down, the individual components of present-day ecosystems will likely unravel like threads in a tapestry, then come together to create new designs. Some organisms will explode in number. Others will dwindle to a precious few. But in the mountains, even these stand a chance of finding some sanctuary where they can hunker down until climate—from their perspective—improves.
At least for a while. As time passes, the capacity of mountains to serve as biodiversity arks will be severely strained, says Daniel Fagre, an ecologist at the U.S. Geological Survey’s Northern Rocky Mountain Science Center. “Because of their geomorphological heterogeneity, mountains do offer many more options for survival than more homogeneous landscapes. But we also can’t ignore the fact that the mountains, too, are changing.”
Millar would not disagree. For despite her seemingly contrarian position on pikas, she is no Pollyanna when it comes to global warming. Over the long sweep of time, she says, “climate change has sent species migrating up and down ranges, expanding across basins or contracting into fragmented populations.” And today, she says, the impacts are likely to be all the greater—due both to the accelerating speed of change and to a witch’s brew of other factors introduced by humans, ranging from the alteration of fire and grazing regimes to the spread of exotic pests.
These days, when she talks to those in charge of public lands, Millar whips out a palette of suggestions to ensure that as many species as possible make it into the next century. In some cases, such as an invasive plant creeping upslope, the best course of action might be to fight the incursion. In others, Millar envisions building resilience into important biomes—perhaps thinning overly dense forests to decrease the threat posed by fire and disease. The goal, she says, is to develop a policy of “no regrets,” meaning that responses should not be driven by any particular climate scenario but rather seem reasonable regardless of what happens.
Millar has even thought about what might be done to help pikas navigate the treacherous times ahead. For example, she says, to ensure that core populations of pikas remain viable, managers of national parks and forests could consider erecting rocky corridors between talus slopes that lie perhaps a few kilometers apart on opposite sides of a mountain meadow. It would be a form of assisted migration, to be sure, but one with the advantage of being
locally focused and allowing the pikas to decide whether to move.
But before trying to combat or mitigate the impacts of climate change, scientists caution, it is first essential to understand at a much deeper level the changes underway. It’s not enough to check for the presence and absence of pikas and other creatures, says Robert Klinger, a U.S. Geological Survey ecologist based in Bishop, California. “We need to know the reasons why certain populations are blinking out, and why others are persisting.”
Looking for answers, Millar buried over a hundred temperature-data loggers at four pika-occupied talus slopes last year and then left them to overwinter. As always, she is anticipating surprises. In February, she drove to California’s Mono Lake area, clipped on a pair of skis, and glided six miles over crusty snow past majestic stands of lodgepole pines to check on her sites. When she arrived at the first talus slope, she was startled—and a little alarmed—to find a big expanse of bare rocks. But suddenly, she broke into a grin. For just then she noticed snow draped across the bottom of the slope—right where the resident pikas like to stash their food. ❧
J. Madeleine Nash is a science writer based in San Francisco, California. Her articles have appeared in TIME, Smithsonian, The New York Times, and High Country News.
Further Reading:
Beever, E.A. et al. 2010. Testing alternative models of climate-mediated extirpations. Ecological Applications 20(1):164-178.
Millar, C.I. and R.D. Westfall. 2010. Distribution and climatic relationships of the American pika (Ochotona princeps) in the Sierra Nevada and Western Great Basin, U.S.A.; Periglacial landforms as refugia in warming climates. Arctic, Antarctic, and Alpine Research, 42(1):76-88.
Pepin, N.C., and J.D. Lundquist. 2008. Temperature trends at high elevations: Patterns across the globe. Geophysical Research Letters, 35, DOI: 10.1029/2008GL034026.
Scherrer, D. and C. Körner. 2009. Infra-red thermometry of alpine landscapes challenges climatic warming projections. Global Change Biology, DOI: 10.1111/j.1365-2486.2009.02122.x
