To climb a mountain is to ascend through distinct worlds. The dry grasslands of the plains change to thick ponderosa forests in the foothills. Further up, Douglas firs briefly dominate the landscape before flora better suited for the subalpine environs, such as Engelmann spruce, takes over. Finally, that scraggly foliage gives way to the alpine tundra. Throughout the journey, temperatures grow cooler.

Each of those layers, even the barren tundra, supports a complex system of organisms—all well-adapted to the plants, predators, and fluctuations in temperature and precipitation within each habitat layer. But according to a study published earlier this year in the peer-reviewed journal Ecology, climate change is rapidly altering these once-predictable patterns, forcing small Colorado mammals higher and higher into the Rockies—and possibly out of the state entirely.

The study comes out of the McCain Mountain Lab, a University of Colorado Boulder research facility investigating the impact human activity has on biodiversity, especially in the Rocky Mountains. Christy McCain, the professor who runs the facility, realized that mountain landscapes could act as a living lab while serving in the Peace Corps in Honduras. “I would go backpacking in the mountains, and I saw how drastically the climate, as well as organisms’ patterns, changed as you go up in elevation,” she says. “All these different organism communities are just a short hike from one another, so it’s easier to study.”

That epiphany accompanied another realization that had been sweeping through the scientific community since the 1980s—that the earth’s climate is changing at an unprecedented rate due to human activity. By the time McCain moved to Colorado in the mid-aughts to study mountain habitats, research detailing climate change’s impact on animal behavior were already rolling in, including a 2008 study showing that many small mammals in Yosemite were appearing at higher and higher elevations.

McCain thought she knew what to expect of the Rocky Mountain-specific data when she began collecting in 2007. She and her team predicted that small mammal species would be living at slightly higher elevations than their ancestors because the cool temperatures they craved could only be found higher and higher up on the mountain as Earth warmed.

The results, though, were more drastic. McCain expected populations to have shifted upwards 100 feet or so, maybe 200. Instead, the 47 mammal species studied had, on average, moved 430 feet higher in elevation. For species living at higher elevations already, such as the golden-mantled ground squirrel and the Uinita chipmunk, the shift was even more extreme: On average, those animals saw a 1,135-foot upward shift in elevation. “This is happening so much faster than I was expecting,” McCain says. “It’s truly alarming.”

Risk of Local Extinction

To unveil those dramatic changes, McCain and her team compared the current elevational ranges at which different species of small mammals live to those same species’ historical stomping grounds (i.e., the elevation ranges they inhabited in the Rockies between 1886 and 1979). Luckily for McCain and her team, humans have been preserving specimens for hundreds of years, so they were able to examine 4,700 specimens of 47 different species at Northern American museums, including the CU Museum of Natural History, where McCain works as a curator of vertebrates.

Gathering new data for those critters—among them the yellow belly marmot, the adorably long-eared Abert squirrel, and the pygmy shrew, the smallest mammal in North America—required quite a bit of legwork. Over the course of 13 years, teams traveled to different elevations in Colorado’s Front Range and San Juan mountains, where they trapped, tagged, and released small mammals to determine each species’ range.

For 26 species (including the golden-mantel ground squirrel, the yellow bellied marmot, the pygmy shrew, and several species of chipmunk) on the list of 47, the population had shifted further up the mountain. “What’s happening,” says McCain, “is that the populations that live at higher altitudes survive, while populations at lower altitudes die off.”

The results were slightly more complex than a skyward mass exodus. McCain’s team also found that six mammal species didn’t change their elevational range, while 11 actually moved lower in elevation. Four were locally extinct, which means they no longer live in the Rocky Mountains. More studies are needed to determine what, exactly, happened to the unlucky quartet: the olive-backed pocket mouse, the silky pocket mouse, the canyon mouse, and the Ord’s kangaroo rat.

Still, the trend points up the mountain, not down, especially for high-altitude animals like the yellow-bellied marmot and the Western jumping mouse. “Most of their evolutionary history has shaped them for the cold,” McCain says. “They tend to be extra sensitive to warming temperatures.”

And while certain peaks seem to stretch on forever overhead, even the state’s most looming of 14ers do, eventually, top out. McCain fears that once a species reaches those levels, the heat will chase them out of Colorado entirely. “At a certain point, they won’t be able to survive in the habitat this state can offer any longer,” she says.

McCain doesn’t know exactly when that point will come. It’s possible that, for certain species, it already has.

When those small mammals disappear, they take with them all the benefits they contribute to their environs: They’ll no longer be a food source for birds of prey; they’ll no longer drag pollen and seeds from place to place on their fur, helping to pollinate the mountains.

But McCain emphasizes that it isn’t too late. “I like to communicate that it’s not a hopeless cause,” she says. “We can be more efficient in heating and cooling our houses. We can drive cars that are more efficient. We can decide to take the bus once a week or take one fewer flight each year. And we can push for bigger change by voting for candidates that support policies for cleaner air and cleaner emission. What’s going on is incredibly depressing, but there are still things we can do to make a difference.”

(Read More: ​​How To Live More Sustainably In Colorado)

Angela Ufheil
Angela Ufheil
Angela Ufheil is a Denver-based journalist and 5280's former digital senior associate editor.