06/30/2010 05:35 pm ET Updated May 25, 2011

Small Creatures and Climate Change: How Warming Can "Shuffle the Deck" of Creatures That Thrive

Climate change is messing with mammals. Among other things, it is shifting their geographical ranges, changing their behavior (or, as scientists call it, their phenology), and causing disruptions up and down the food chain. Researchers often look to ancient times for information about the ways species weathered earlier periods of sudden warming. Their results provide clues to the kinds of changes we might expect with further global warming today. New research suggests that warming may shuffle the deck of biodiversity among small mammals in North America and change the types of animals that thrive in a particular place.

In northern California's Samwell Cave, in the foothills of the Cascade Mountains, researchers examined fossils and learned that populations of small mammals were severely depleted during Earth's last emergence from the ice-age world, the late Pleistocene epoch, about 12,000 years ago.

Jessica Blois, a researcher at the University of Wisconsin-Madison, and Elizabeth Hadly, a professor of biology at Stanford University, found that although small mammals didn't become extinct during Pleistocene warming, they did suffer great losses. Certain small mammal species became extremely rare while others proliferated. And the species that became king of the landscape, by virtue of its commonness, was the deer mouse.

"That is a pretty big, somewhat startling result," Hadly wrote, noting that deer mice are so common in western landscapes that most people assume they have virtually always been so. "What these data tell us is that in the Pleistocene they were not dominant at all."

Though small mammals suffered some losses, a third of their big mammal cousins (including mastodons, mammoths, and the short-faced bear) became extinct. The decline in populations led to a lasting ricochet effect. Each animal in an ecosystem plays an important role, such as spreading seeds, aerating soil, or being the hunter or the prey. The decline of gophers and squirrels, for instance, led to the increase in what Hadly calls the "weed" of small mammals: deer mice. The research indicates that when diversity shifts the effects are long-lasting. Small mammals may face huge challenges in surviving the climate changes now under way.

Creatures under strain from climate change include the American pika, a rabbitlike animal that lives on talus, or broken rock, in western alpine mountain regions. When pikas are prevented from regulating their temperature behaviorally and are exposed to even slight warming -- temperatures of 77 degrees Fahrenheit for six hours -- they die.

"The American pika may be an early-warning indicator of generally how alpine species may respond to contemporary climate change," said Erik Beever, Ph.D., a wildlife ecologist who has studied pikas for the past 16 years.

New research indicates that pika populations are developing more resilience to increases in temperature than previous models had predicted. Constance Millar, a senior scientist with the USDA Forest Service, examined sites throughout the Sierra Nevada and the southwestern Great Basin in Nevada and found that the range of the American pika was larger than prior research indicated. The drier and lower elevation sites in northwestern Nevada showed losses of pika populations.

One of the most important aspects of Millar's findings is the newly observed adaptive qualities of pika. Her research indicated that pika are able to regulate their temperature by burrowing much deeper into the talus, where cool air circulates. The conditions are much like those in an air-conditioned home in Arizona: it's scorching outside but cool indoors. Also, the pikas' foraging behavior is changing. On hot days, much like rabbits, they search for food at dawn and dusk. On cooler days, they forage all day long.

Other mammals, she said, haven't been quite as adaptive. Now concern for the survival of ground squirrels is growing. "We think these may be more at risk [than pikas]," said Millar. "They don't have this talus to escape into. They are ground dwelling, so those models about warming would far more relate to their habitat."

Climate is having an impact on all creatures, great and small. The polar bear has been the poster child of species loss risk from climate change, but the emergence of the extremely rare "grolar bear," the offspring of a grizzly and a polar bear, is new evidence of how climate affects species. Polar bears are being driven from their usual habitats on the disappearing polar ice at the same time that grizzlies are moving farther north because of global warming, resulting in cross-breeding.

Climate-science investigations will continue to aid scientists' efforts to understand the effects of global warming on animal biodiversity.