BILLINGS, Mont. - A jackrabbit found throughout much of the West has disappeared from the Yellowstone area, although the reason why remains a mystery, a new study concludes.

Whatever the cause, the study suggests the white-tailed jackrabbit's disappearance has wrought major changes to Yellowstone's food chain. Coyotes and wolves, which could have depended on the rabbit as a significant food source, apparently turned their attention instead to larger prey including young elk, pronghorn antelope - even domestic livestock.

However, because the rabbit's decline went relatively unnoticed until now, quantifying that shift is virtually impossible, said the study's lead author, Joel Berger with the Wildlife Conservation Society.

The white-tailed jackrabbit - also known as the prairie hare - was once a common sight in and around Yellowstone National Park. About two feet long, the animal is distinguished by exceptionally long ears and its change in color during winter months to a stark white.

Berger's study, appearing in the latest issue of the scientific journal Oryx, tells of one inhabitant of the region encountering "jillions" of the animals near Yellowstone as late as the 1930s.

Yet by the middle of the century, sightings within the 23,000 square mile Yellowstone region grew increasingly rare. That area includes portions of Idaho, Montana and Wyoming.

Only three have been spotted by scientists since 1991 - all in Grand Teton National Park, according to Berger. He questioned area biologists and naturalists and combed through museum records and studies dating to the 1870s to reach his conclusions.

Elsewhere, the jackrabbits have continued to thrive. It is hunted in many states and listed as a species of "least concern" by the International Union for the Conservation of Nature and Natural Resources.

Where the Yellowstone rabbits went, Berger said, nobody knows. Disease, changes in weather and excessive predation were cited as possibilities.

"Since the rabbits blipped off without knowledge, there has simply been no way to get at the underlying cause," he said.

Berger said wildlife managers should consider reintroducing the jackrabbit to Yellowstone and Grand Teton. He said that would allow scientists to recreate "bottom-up" relationships between predators and their prey that were effectively lost when the animal vanished.