A new study found that white-tailed jack rabbits have vanished from the Greater Yellowstone Ecosystem, where the bunnies were once abundant.

No one knows what caused the rabbits to disappear, according to the study conducted by the Bronx Zoo-based Wildlife Conservation Society. It could be disease, extreme weather, predation or other factors, said Wildlife Conservation Society conservationist Joel Berger, a professor at the University of Montana.

Historical records from more than 130 years ago indicate that white-tailed jack rabbits were once common in Greater Yellowstone, a 60,000 square kilometer (23,166 square mile) ecosystem that contains both Grand Teton and Yellowstone National Parks. The study found no jack rabbit sightings could be confirmed in Yellowstone since 1991 and only three in Grand Teton since 1978.

The jack rabbits' departure may be having significant impacts on the whole region, including both predators of the rabbits and other prey species, Berger said.

The absence of jack rabbits may be causing elevated predation by coyotes on juvenile elk, pronghorn and other ungulates, according to the study. Elsewhere, when rabbit densities drop predators often turn to preying more on livestock. But without baseline data on rabbit numbers in Greater Yellowstone, assessing the impacts on predators such as grey wolves, which were reintroduced to the parks in 1995, becomes more difficult.

Berger said wildlife managers should consider reintroducing white-tailed jack rabbits into Grand Teton and Yellowstone National Parks. Reintroduction may restore the dynamic ecological processes that were intact before the rabbits vanished from the ecosystem, he said.

The study will appear in latest issue of the journal Oryx.