Suppression of fires harms rare mammal, says biologists' study

Cornell biologist Thomas Gavin indicates encroaching vegetation that isolates Northern Idaho ground squirrels to small patches of habitat. Photographs courtesy of Paul Sherman

By Roger Segelken

Cornell biologists who study dwindling populations of one of the rarest mammals in North America have found another reason to let "natural" fires burn.

Without lightning-sparked fires every 10 to 12 years, they say, pine trees are isolating Northern Idaho ground squirrels into shrinking groups where non-native plants that do not supply adequate food for the rodents have overwhelmed natural, fire-resistant, seed-laden grasses.

"Animals can run from fire, but they can't escape the disastrous effects of habitat fragmentation and starvation. Unless these squirrels can link up with neighboring populations and obtain sufficient fatty seeds to carry them through hibernation, I'm afraid they will die off altogether in our lifetime," said Paul Sherman, one of four biologists authoring a genetic study of the Northern Idaho ground squirrel (Spermophilus brunneus brunneus) in the current Journal of Mammalogy (Vol. 80, No. 1, pp 156-168).

Sherman, a Cornell professor of neurobiology and behavior, and colleagues Thomas Gavin, Eric Yensen and Bernie May offer a two-part solution for the squirrels' dilemma:

• restoration of habitat, including reopening of meadows, to reconnect squirrel populations, and

• reintroduction of fire into the ecosystem to restore native vegetational structure.

Fourteen years ago, when Yensen of Albertson College of Idaho began the squirrel study in the high-desert meadows of western Idaho, there were about 1,000 of the ground squirrels on public and private lands around Payette National Forest, the only part of North America where the rare animals live. By 1998, the total population had shrunk to about 600, and 12 of the 36 known populations were extinct, according to Gavin of Cornell's Department of Natural Resources.

The U.S. Fish and Wildlife Service has proposed listing S. b. brunneus as "threatened," deserving of protection through the Endangered Species Act.

The Northern Idaho ground squirrel depends on the fat-rich seeds of native bunch grass to sustain it through seven or eight months of hibernation. Before the federal government instituted a policy of suppressing wildfires early in the century, the meadows burned every 10 to 12 years, and the heat-resistant bunch-grass seeds then regreened Idaho's meadows.

But without natural fires to maintain the diversity of plant life, Gavin explained, a few non-native plant species took over. "The non-native plants put all their energy into rapid vegetative growth rather than making nutritious seeds," he said. "As a result, the ground squirrels cannot find enough seeds to fatten sufficiently before their long hibernation, from August to March."

Gavin added, "To make matters worse, when squirrels try to migrate in search of food or mates, their paths are blocked by dense stands of pine trees that thrive in the absence of fire. This means that groups can no longer mix when some populations expand and others drop because of disease, a hard winter or predators. Sometimes one hard winter and a little bad luck with hawks, badgers and other predators are all it takes to wipe out an isolated, local population. They cannot be rescued by dispersers from nearby populations that are doing well."

Cornell's long-term study of Idaho ground squirrels had, at first, focused on the animals' social behavior. But the focus changed as researchers returned each spring to sites where small colonies had thrived the year before -- only to find no animals emerging from their winter burrows. Analysis of blood proteins and DNA from surviving squirrels offered three clues:

• Ground squirrels in isolated populations are slightly differentiated in their genetic makeup, suggesting that they probably intermingled earlier in the century, before dense conifer forests closed in, but that intermixing no longer occurs.

• Squirrel populations as close as two miles apart are no longer mixing.

• The farther apart populations are geographically, the more different they are genetically, suggesting that they have been separated longer, and that little or no dispersal is occurring between populations today.

Gavin said that controlled "cool" fires are not a threat to ground squirrels, which are safely hibernating several feet underground when controlled fires are set.

"We know this runs counter to everything Smoky the Bear taught us," Sherman said. "But we think Smoky was worried about careless campers and cigarettes, not the natural fires that are set by lightning. Ironically, we've become so good at fire suppression that many forests are tinder boxes of underbrush and non-native vegetation. Now, when a fire gets out of control it really is a threat to wildlife.

"The Idaho ground squirrels are part of an ecosystem that functioned successfully for tens of thousands of years, and natural fires were a part of that system, too," Sherman said. "Perhaps by undoing some of mankind's tampering -- as well-meaning as it was -- and setting back the clock, we can encourage the re-establishment of native flora and fauna, especially the rodent variety, while saving the immense costs of fighting catastrophic fires every summer."

Authors of the Journal of Mammalogy report, "Population Genetic Structure of the Northern Idaho Ground Squirrel," are Gavin, Sherman, Yensen and May, formerly a research associate in Cornell's Department of Natural Resources and now a researcher in the Department of Animal Science at University of California at Davis. The genetics study was supported, in part, by the National Science Foundation, U.S. Department of Agriculture, National Geographic Society, Idaho Department of Fish and Game and the OX Ranch.