In a ruling hailed as historic for wildlife conservation in America, a US judge on Monday ordered that the world-famous grizzly bears living in and around Yellowstone national park be returned to the endangered species list.

The move means that a controversial sport hunt of grizzlies in Wyoming and Idaho – outside the boundaries of the park – will be canceled indefinitely, extending protections against hunting that have lasted 44 years.

Tim Preso of the environmental law firm EarthJustice, who served as lead attorney in a case challenging the removal of protection for bears, was ecstatic when reached at his office in Bozeman, Montana, after word arrived of the decision.

He called the ruling “momentous”, given the high profile of grizzlies and growing threats to their survival in the 21st century, including climate change, which has affected a key grizzly bear food, the seeds inside whitebark pinecones.

Judge Dana Christensen, who sits on a district court in Montana, had “had done a huge amount of homework, as was evident by the questions he asked when we presented our case, and, because of that, we were hopeful”, Preso said. “This is a case that he will be known for and it’s an important part of the legacy of American conservation. He did what a judge does – apply the law without being prejudiced by politics.”

There are four other grizzly populations in the northern Rocky Mountains in the US, only one of which is significant in size. Saving grizzly bears from near-extinction in the Yellowstone area is considered one of the greatest wildlife success stories. Numbers of bears have rebounded to more than 700 from a low of about 135 three decades ago.

As such, the three states of Wyoming, Montana and Idaho have aggressively pushed to have bears removed from federal protections.

Yet questions remain about the recovery.

A key argument made by Preso and his colleagues is that as an island population of grizzlies, isolated and cut off from other clusters of bears, the Yellowstone bruins face ongoing challenges.

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True biological recovery, as spelled out in the Endangered Species Act, the judge said, means reconnecting the population of greater Yellowstone bears with others in the Lower 48. That hasn’t happened. In fact, the judge labeled the rationale behind the government’s case for delisting the bears arbitrary and capricious.

Wildlife officials in the states expressed disappointment. Leaders of the Wyoming game and fish department were eager to authorize the first hunt of bears in two generations, which would have allowed up to 22 to be “taken” this year. That plan was met with fierce public resistance, including some 650,000 comments from people across the country and around the world who were overwhelmingly opposed to the sport killing of creatures that have become synonymous with Yellowstone, America’s first national park.

Preso said he expects that lawyers for the states will appeal against Christensen’s ruling and will probably be joined by pro-hunting groups such as Safari Club International.

The superintendent of Yellowstone national park, Dan Wenk, who is stepping down after 43 years, expressed concern last weekend at his retirement party about the safety of grizzlies that wander beyond park boundaries and could be killed by hunters, like the infamous case of Cecil the lion in Zimbabwe.

In an interview in Wyoming last week, the celebrated naturalist Jane Goodall also spoke out in support of the bears.

“We should not be killing any animals for fun. We should be celebrating grizzlies being alive not rushing to shoot them.”