Yellowstone vexed by disappearing antelope

A mystery is building in Yellowstone National Park, and the central figure in the puzzle is one with dark horns, a goat-like shape, and Maserati speed - the pronghorn antelope.

Inexplicably, the herd at America's first national park is dwindling, even as biologists deploy sleuthing methods to figure out what's causing the decline. In fact, the pronghorn is in more peril of extinction than any other species in Yellowstone.

"To anybody that is a serious Yellowstone-watcher, you've got to be concerned about the long-term survival of these pronghorn," says John Varley, director of the National Park Service's Yellowstone Center for Resources.

Last year's count tallied only 209 pronghorn in the herd, down from nearly 600 about 10 years ago.

Antelope generally favor open areas where they can take advantage of their speed and keen eyesight, but the pronghorn in the park traverse rugged terrain where they are vulnerable to lurking predators.

"They have a long migratory path. They swim the Yellowstone River and go through forests, things that pronghorn are not expected to do," Mr. Varley says.

Pronghorn antelope can be spotted in the northern part of the 2.2 million-acre park from around Mammoth Hot Springs to the Lamar Valley, popular as a wolf watching area.

Citing the dwindling numbers, complex migration routes through forests and rivers, and development that has largely cut the herd off from its cousins to the north, wildlife biologists fear the worst is possible for this distinct group. Even if other pronghorn were introduced into the park, they say, the newcomers might not be able to learn quickly enough the current herd's unique habits to be able to survive.

Still, Varley says it's hard to drum up concern about the herd's decline in Yellowstone because the pronghorn is seen often in the rest of Wyoming and surrounding states.

This ungulate is anything but ordinary. Both sexes have the distinct prong-shaped horns, and they are unique among antelopes in shedding their horns annually, like antlered deer and elk.

Weighing 90 to 125 pounds, the pronghorn is built for speed. It can sprint at 55 miles per hour, bested only by the cheetah, but its speed cannot be matched by any other creature in the distance run. With keen vision on the open plains, adult antelope are well prepared to elude predators. Hardiness in the face of desert heat and brutal cold allow the antelope to make its home from the plains to the rugged country of northern Yellowstone.

The park's herd dropped from 600 in 1991 to 235 in 1995. But what baffles wildlife biologists is that the population has not rebounded.

Researchers are currently tracking does with radio collars in two studies regarding fawn survival and habitat use, especially on the pronghorn's winter range. The antelope spend the summer in the Lamar Valley, where reintroduced wolf packs reign, and winter in the Gardiner Basin, where the Yellowstone River flows north from the park.

Although wolves kill an occasional antelope, the park's top carnivore generally sticks with meatier elk to feed an entire pack, says Bill Edwards, an instructor for the Yellowstone Association Institute.

The coyote is the usual suspect, as the main predator of the antelope, but researchers note that the coyote population has dropped by roughly 50 percent in the northern part of the park since wolves were reintroduced in 1995.

For the past 10 years, conservationists and wildlife agencies have worked to protect winter range north of the park. But even with sufficient winter range, the herd remains largely cut off from antelope grazing on the plains farther north, scientists say.

With a large, healthy population, that might not be a problem. But lack of genetic diversity and occasional ecological hardships can seriously threaten small herds. "With this low of a population, if we lose [the herd's migration] patterns, would we ever be able to reestablish the knowledge of the pronghorn for the landscape?" asks Glenn Plumb, a park biologist. "Unlikely."

(c) Copyright 2001. The Christian Science Monitor