BILLINGS, Mont. — A federal appeals court ruled Tuesday that grizzly bears in the Yellowstone region need continued protection under the Endangered Species Act due to the decline of a tree species that serves as a key food source for some of the animals.

The ruling from the 9th U.S. Circuit Court of Appeals blocks the federal government's effort to lift protections on about 600 threatened grizzlies across 19,000 square miles of Montana, Wyoming and Idaho. Such a move would have turned over management of the animals to state wildlife agencies and allowed hunting for grizzlies for the first time in decades.

But the judges said a significant threat to the bears' recovery has emerged in recent years as whitebark pine trees have declined.

Whitebark pine nuts are a key food source for some bears. Beetle infestations brought on by a warmer climate have devastated huge stands of the trees at high elevations across the Yellowstone region.

After a judge in Montana pointed to the trees' decline as an unacknowledged threat to grizzlies, biologists from the U.S. Fish and Wildlife Service argued in the appeal to the 9th Circuit that grizzlies could adapt and find other food sources.

Members of the three-judge 9th Circuit panel offered harsh criticism of that approach in Tuesday's ruling. And they said they were unconvinced by the government's pledge that protections could be restored at a later date if the bears' recovery falters.

"Now that this threat has emerged, the Service cannot take a full-speed ahead, damn-the-torpedoes approach to delisting," Judge Richard Tallman wrote. "We reject out of hand any suggestion that the future possibility of relisting a species can operate as a reasonable justification for delisting."

Tallman was joined in his opinion by Judge Susan Graber.

The 9th Circuit panel's third member, Judge Sidney Thomas of Billings, issued a dissenting opinion. Thomas agreed with the other judges on the whitebark issue, and also said the government had failed to set up adequate regulatory measures to protect grizzly bears once they come off the threatened species list.

Graber and Tallman said the regulatory framework was sufficient. That part of the case was sent back to a lower court for reconsideration.

Fish and Wildlife spokesman Chris Tollefson said the agency was reviewing the ruling and officials have not decided how to proceed. But the agency's grizzly bear coordinator, Chris Servheen, said an appeal was unlikely.

Servheen said the agency would start working on a new proposal to lift protections based on research not available to the 9th Circuit.

"We can provide more information on the whitebark issue as they requested," he said.

Since the case began in 2007, the grizzly population "hasn't declined at all," Servheen said. "We're seeing the bears pushing out to where they haven't been in decades, and the density of bears in the center of the ecosystem is very high."

Grizzly bears were once found across much of the West. Their numbers dwindled rapidly in the late 1800s and early 1900s as intensive hunting and trapping exterminated grizzlies across more than 98 percent of their historic range.

By the time grizzlies were given protections under the Endangered Species Act across the lower 48 states in 1975, as few as 136 bears survived in the Yellowstone region.

Protections were lifted in 2007 following a protracted and costly recovery effort that also has resulted in a sharp increase in the frequency of bear-human conflicts. Four people have been killed by grizzly bears in Montana and Wyoming over the past two years.

A Montana conservation group, the Greater Yellowstone Coalition, challenged the government's decision in a 2007 lawsuit.

After U.S. District Judge Donald Molloy sided with the group in a 2009 ruling that restored protection, the government appealed, setting the stage for Tuesday's opinion.

An attorney for the Greater Yellowstone Coalition said the ruling revealed that the government had been "hell-bent" on lifting protections despite the scientific evidence on whitebark.

"Whitebark is the key food source that has a dramatic effect on whether grizzly bears live or die," attorney Doug Honnold said. Since the 2007 decision, he added, "a lot more trees have died and the problem has only been magnified."

State wildlife agencies, the National Wildlife Federation and the Safari Club had intervened in the case on the side of the federal government.

National Wildlife Federation attorney Tom France said he was encouraged the 9th Circuit had sided with the government at least on the issue of regulatory protections.

"We feel like the grizzly bear in the greater Yellowstone is a real conservation success story," France said.