MISSOULA, Mont. -

Federal wildlife officials are back to the drawing board in their effort to take Yellowstone grizzly bears off the endangered species list.

US Fish and Wildlife Grizzly Bear Recovery Coordinator Chris Servheen said the Yellowstone ecosystem is home to over 600 grizzlies, which he said is almost triple the number of bears there thirty years ago.

"We have a population that is expanding in numbers and range throughout the Yellowstone system,? said Servheen. ?We have a very sound and detailed management plan in place, to manage the bears, and we have the commitment of all the state and federal agencies in the Yellowstone ecosystem to the future of the grizzly bear."

Servheen?s agency removed the population from the endangered species list in 2007, but conservation groups fought the delisting in court, and last fall they were granted a victory in the 9th District Court of Appeals. Last week officials announced that Yellowstone grizzlies will remain protected until 2014.

Wildlife officials say the Yellowstone grizzly population is growing and stable, but Bonnie Rice, a senior representative for the Sierra Club?s Bozeman office, said they are not confident the numbers are strong enough to support delisting.

"I think there's a lot of uncertainty, in terms of the decline in some key food sources for the grizzly bear,? said Rice. ?White bark pine is certainly right up there, in terms of seeing a huge decline, and we are not sure what the impacts of that are going to be on bears."

Rice said the Sierra Club is currently supporting research that will further their understanding of the strength of the Yellowstone grizzly population and their food sources, and she says her group will continue to oppose the delisting until the research is completed.

"I think until we really understand the impacts, it's premature to delist the grizzly when they are so dependent on those foods, and there's a lot of uncertainty in terms of what other foods they might turn to."

Servheen said US Fish and Wildlife will continue to push for the population to be delisted.

"I think there's general feeling that the grizzly bear is recovered, and the question is how we get to move forward, because delisting is what we're supposed to do under the endangered species act, so we're still in the process of making a decision on how we're going to move forward in response to that 9th circuit ruling," said Servheen.