Grizzly bears in and around Yellowstone National Park no longer need Endangered Species Act protection, the federal government said Thursday.

The area had an estimated 136 to 312 grizzlies when the species was listed as threatened in 1975, but has more than 500 of the bears today, the government said.

"The grizzly is a large predator that requires a great deal of space, and conserving such animals is a challenge in today's world," Deputy Interior Secretary Lynn Scarlett said in announcing the decision. "I believe all Americans should be proud that, as a nation, we had the will and the ability to protect and restore this symbol of the wild."

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The Interior Department said in 2005 that it intended to delist grizzly bears around Yellowstone in the states of Idaho, Montana and Wyoming.

The species remains protected in other parts of Idaho, Montana and Washington state; Alaska, where the bear was never threatened, is the only other place in the United States where the species roams.

Stripping the bears of protection could eventually clear the way for limited hunting of the animals. A measure that would allow such hunting has passed the Montana Senate.

Opponents of the delisting, including more than 250 scientists and researchers who sent the government a letter of protest this week, question whether the bear population is large enough to be genetically diverse and withstand outside pressures such as global warming and food scarcity.

"We cannot take a risk with the nation's premier wild land species," said Michael Scott, executive director of the Greater Yellowstone Coalition.

The Natural Resources Defense Council said it plans to pursue either a lawsuit or some form of congressional intervention to stop the delisting.

But some environmental groups have supported the delisting, calling the grizzly bear a success story, and Wyoming's senators called the decision long overdue.

"At long last," said Sen. Mike Enzi, R-Wyo. "Decades passed. The bears increased in number. The federal government stood immobile. Today that is finally changing. Grizzly management will shift to the state where it should be."

The U.S. Fish and Wildlife Service plans to issue a final rule on the delisting March 29, Enzi said. The rule takes effect 30 days after it is published in the Federal Register.

Fish and Wildlife officials said federal and state agencies will continue to work together to manage the bears and their habitat. The total amount spent monitoring the bears will increase by more than $1 million per year over the next two years to a total of $3.7 million annually, officials said.

The federal agency also has recommended delisting gray wolves in the Northern Rockies, but a disagreement with Wyoming's proposed management plan has stalled the process. The service has said it will decide whether to delist the bald eagle by this summer.

About 1,000 plant and animal species are listed by the federal government as endangered or threatened.

About 20 species have been delisted because their populations grew stronger; nine have been removed because they became extinct.