Passage IV

NATURAL SCIENCE: This passage is adapted from the article “Silence of the Pikas” by Wendee Holtcamp (©2010 by Wendee Holtcamp).

Pikas, a diminutive alpine-dwelling rabbit relative, are unique among alpine mammals in that they gather up vegetation throughout summer—including flowers, grasses, leaves, evergreen needles, and even pine cones—and live off the hay pile throughout winter, rather than hibernating or moving downslope. But increasingly warm temperatures may drive them to the brink: the high-energy mammals can overheat and die at temperatures as mild as 25 degrees Celsius if they can’t regulate their body temperature by moving into the cooler microclimate under the talus. And since they already live near the tops of mountains, when a particular talus field’s microclimate becomes inhospitable, they simply have nowhere to go.

Sometimes called cony, mouse hare, rock rabbit, or whistling hare, the pika has a narrow niche. They live only in talus fields, and these must lie adjacent to alpine meadows or other vegetation so they have access to plants for food and hay farming. The talus rock fields must have boulders of a certain size; scree, a similar habitat with smaller rocks, won’t do. Rocks provide safe haven from pikas’ main predator, weasels. But perhaps more important, the interstices between the rocks provide both a cool, moist microclimate where pikas cool down during hot summer days and also the perfect sanctuary in which to settle during the long winter’s night. They don’t huddle together like many other mammals, as far as scientists can tell, but remain fiercely territorial and solitary throughout the winter, guarding their hay piles with their lives. As a snowpack settles over the land, it insulates the Earth and maintains a certain underground temperature at which pikas can survive, just below freezing. With warming temperatures reducing snowpack in many mountainous areas, in a strange twist of fate, global warming can cause pikas to freeze.

Biologists have dubbed mountaintop habitat patches “sky islands” because the valleys in between are as uninhabitable as the sea for nonmobile alpine species. This creates an ideal scenario to test the predictions of one of ecology’s key theories: island biogeography. Individual pikas have a relatively limited distance they can disperse, around two kilometers, so they can’t just shift from one mountain to another. At the population level, they’re stuck on a particular mountain range. In the 1990s, biologist Erik Beever and colleagues surveyed pikas throughout the hydrographic Great Basin—a heart-shaped 500,000 square kilometer intermontane plateau dotted with 314 mountain ranges, incorporating parts of California, Nevada, Utah, Oregon, Idaho and Arizona—and were unable to find pikas in 6 of 25 mountain ranges that they had occupied in the late 20th century. Was the cause of pika extirpations (disappearances) climatic, anthropogenic, or biogeographical?

Island biogeography theory says that “species are predicted to remain on large islands and islands that are not very isolated from mainland [habitat],” explains Beever, who did much of his work while a graduate student under Mary Peacock, at the University of Nevada—Reno. He and colleagues found pika populations persisted in mountain ranges with more talus habitat available—supporting one prediction of island biogeography theory—but pikas were not more likely to persist at sites closer to the Rocky Mountain or Sierra Nevada “mainland” ranges.

“Here isolation doesn’t have anything to do with whether they’re lost or not,” Beever says. Not only that, “the sheer size of a mountain range in this case isn’t very predictive of patterns of loss. [And] if we count the amount of habitat, that’s less important than these climatic influences.” Ultimately, the factors most strongly associated with pika disappearance were climatic; specifically, warmer and drier sites, which tended to be lower down the mountains. In another study published in Ecological Applications, Beever, University of Colorado researcher Chris Ray, and other colleagues revealed that acute cold stress and chronic heat stress (in other words, cold snaps and overall hotter summers) affect pika more than individual very hot days.

“The problem with global warming is that if [pikas] lose [their] snowpack, which provides insulation in winter, they freeze to death, and if the ambient air temperature heats up too much in summer, then they [overheat]. That’s the challenge,” Peacock says, who has studied pika population genetics. “They’re already at the top of the mountain. If you heat it up substantially, there’s no place for them to go.”