WASHINGTON (Reuters) - The Trump Administration took steps on Monday to significantly weaken the U.S. Endangered Species Act, prompting state attorneys general and conservation groups to threaten legal action to protect at-risk species.

The 1970s-era Act is credited with bringing back from the brink of extinction species such as bald eagles, gray whales and grizzly bears, but the law has long been a source of frustration for drilling and mining companies, and other industries because new listings can put vast areas of land off-limits to development.

The weakening of the Act’s protections is one of many moves by U.S. President Donald Trump, a Republican, to roll back existing regulations to hasten oil, gas and coal production, as well as grazing, ranching and logging on federal land.

“These changes crash a bulldozer through the Endangered Species Act’s lifesaving protections for America’s most vulnerable wildlife,” Noah Greenwald, the Center for Biological Diversity’s endangered species director, said in a statement. “For animals like wolverines and monarch butterflies, this could be the beginning of the end.”

The changes would end a practice that automatically conveys the same protections for threatened species as for endangered species, and would strike language that guides officials to ignore economic impacts of how animals should be safeguarded.

The original Act protected species regardless of the economic considerations.

“The revisions finalized with this rulemaking fit squarely within the President’s mandate of easing the regulatory burden on the American public, without sacrificing our species’ protection and recovery goals,” U.S. Secretary of Commerce Wilbur Ross said in a statement.

The changes were announced by the Interior Department’s U.S. Fish and Wildlife Service (USFWS) and the Commerce Department’s National Marine Fisheries Service (NMFS).

Massachusetts and California will lead a multi-state lawsuit joined by conservation groups once the final rule is published in the Federal Register in the coming weeks, challenging what they say was an “illegal” process to revise it.

FILE PHOTO: A grizzly bear and her two cubs approach the carcass of a bison in Yellowstone National Park in Wyoming, United States, July 6, 2015. REUTERS/Jim Urquhart

“By gutting key components of the Endangered Species Act, one of our country’s most successful environmental laws, the Trump Administration is putting our most imperiled species and our vibrant local tourism and recreation industries at risk,” said Massachusetts Attorney General Maura Healey.

“We will be taking the Administration to court to defend federal law and protect our rare animals, plants, and the environment,” she added on a call with reporters.

According to the revision, the Fish and Wildlife Service would need to write separate rules for each threatened species, slowing their protection until conditions worsen. Previously, threatened species, which account for 20% of listed species under the Act, would receive the same automatic protections as endangered species, according to the liberal Center for American Progress policy research organization.

“Ending this practice ... would strain the resources of USFWS and NMFS, costing managers valuable time before they can take action to protect a species,” said Kate Kelly, the organization’s public lands director.

The revised rules will also prohibit designation of critical habitat for species threatened by climate change, the impacts of which tend to be felt in the future, the Center for Biological Diversity said.

Trump rejects mainstream climate science and agencies such as the Interior Department have stopped weighing climate impacts in their regulations.

Some lawmakers from Western states and free market conservation groups applauded the changes, seeing them as helping states and landowners. Wyoming Republican Senator John Barrasso said the revision was a good first step but Congress should also reform the Endangered Species Act.

“We must modernize the Endangered Species Act in a way that empowers states, promotes the recovery of species, and allows local economies to thrive,” Barrasso said.

But environmental groups said the overhaul comes at time when U.N. scientists are warning that up to 1 million plant and animal species are facing an “imminent risk” of extinction because of human activity.

“Instead of undercutting the Endangered Species Act and other bedrock environmental laws, we should be strengthening these laws to improve their effectiveness for people and wildlife,” said Jamie Rappaport Clark, president and CEO of Defenders of Wildlife.