The Trump administration took its final step Monday to weaken the Endangered Species Act, a bedrock law that brought the bald eagle, the American alligator, the California condor, the humpback whale and the grizzly bear back from the brink of extinction.

New rules will allow the administration to reduce the amount of habitat set aside for wildlife and remove tools that officials use to predict future harm to species as a result of climate change. It would also reveal for the first time in the law's 45-year history the financial costs of protecting them.

The long-anticipated changes, jointly announced by the Interior and Commerce departments, were undertaken as part of President Trump's mandate to scale back government regulations on corporations, including the oil and gas industry, that want to drill on protected land.

"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," Commerce Secretary Wilbur Ross said in a statement.

"These changes were subject to a robust, transparent public process, during which we received significant public input that helped us finalize these rules."

The rules would affect listing decisions in the future, officials said, and are not retroactive. They will go into effect 30 days after being published in the Federal Register.

Within hours of Monday's announcement, the state attorneys general of California and Massachusetts joined a conservation group, Defenders of Wildlife, in declaring the changes illegal and vowing to challenge them in court.

"You can anticipate that we will see many states join this action," said Maura Healey, attorney general of Massachusetts. "The way this was done was illegal under federal laws and this is an administration that needs to be held accountable."

Jamie Rappaport Clark, who led the Fish and Wildlife Service during the Clinton administration, said the people who have spent careers trying to recover and protect the nation's endangered species find the Trump administration's move "devastating".

"There is nothing biologically positive about the rules," she said. "We will argue that they are illegal."

In May, a UN report on world biodiversity found that 1 million plant and animal species are on the verge of extinction, with alarming implications for human survival.

The report, written by seven experts from universities around the world, directly linked the loss of species to human activity and showed how those losses are undermining food and water security, along with human health.

More plants and animals are threatened with extinction now than at any other period in human history, the report said.

Only Congress can change an act, but rule revisions reflect an administration's interpretation of a law and how it should be applied. The 1973 Endangered Species Act passed unanimously in the Senate and by an overwhelming margin in the House.

Under the administration's new rules, it would have been nearly impossible to designate the polar bear as threatened in 2010 because of the loss of sea ice in the Arctic, one of the fastest-warming areas in the world. Nearly 200,000 square miles of barrier islands in Alaska were listed as critical habitat.

Officials relied on climate models to predict how warming would impact polar bear habitat more than 80 years into the future. The new rules called such predictions into doubt and said officials can now only determine impacts in what it described, vaguely, as the "foreseeable future".

"When you start reaching out to 70 or 80 years" to project climate impacts on the planet and wildlife, the amount of certainty about what could happen "starts to degrade significantly," said Gary Frazer, assistant director for endangered species at the US Fish and Wildlife Service, a division of Interior.

In responses to comments submitted to the proposed rule changes, administration officials argued that they were not discounting climate science, only asserting that climate projections were uncertain.

"Our regulatory framework for the foreseeable future does not undermine these requirements," officials said.

The new rules would also limit the area of land that can be protected to help species recover and survive. Currently, land that plants and animals occupy is set aside for their protection, in addition to areas that they once occupied or might need in the future.

Now, critical habitat that is not occupied might not be protected, opening it up for oil and gas exploration or other forms of development.

Another rule change stripped away language that said a secretary "shall make a [listing] determination solely on the basis of the best scientific and commercial information regarding a species' status," regardless of its costs.

By removing that sentence, the administration allowed the interior and commerce secretaries to consider the economic impact of a listing. Potential threats to business opportunities and other costs can now be factored by the government and shared with the public.

Administration officials said those considerations would not affect listing decisions, but lawmakers and conservationists noted that it could inflame public opposition to proposals to rescue fragile populations.

"There is no reason to do this except to deliberately inflame public opposition to the species or to the act as a whole," said Bob Dreher, a vice president for conservation at Defenders of Wildlife.

A Republican senator who had fought in the past to change the act applauded the administration's efforts.

"The Trump administration is taking important steps to make the Endangered Species Act work better for people and wildlife," said Senator John Barrasso from Wyoming. "These final rules are a good start, but the administration is limited by an existing law that needs to be updated."

Republicans argue that recovering listed species takes too long. Of more than 1,900 listed species, only 47 have been recovered. There are about 1,200 animals and 700 plants currently listed, according to Fish and Wildlife. Eighteen species, including the American wolverine and Taiwanese humpback dolphin, are under consideration for an endangered or threatened listing. Another 18 are being considered for delisting after their populations grew.

"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.

Conservationists took the opposite view, decrying the changes as a major rollback of the strongest wildlife conservation law in the world, credited with saving dozens of species of flowers, insects and larger, more iconic animals.

"They're trying to make it difficult, if not impossible, to protect unoccupied habitat," said Rebecca Riley, legal director for nature programs at the Natural Resources Defense Council. "We know climate change is going to force animals to move to new habitats."

For example, she said, the western snowy plover that nests on the coast is being affected by sea level rise. "As the sea rises, it will need to move inland. If we can't protect those areas because it's not occupied today, that habitat might not be there when they need it," Riley said.

Some lawmakers also opposed the changes.

"Today, the Trump administration issued regulations that take a wrecking ball to one of our oldest and most effective environmental laws, the Endangered Species Act," Senator Tom Udall from New Mexico said in a statement. "As we have seen time and time again, no environmental protection - no matter how effective or popular - is safe from this administration."

Speaking during a telephone news conference, Healey said the new rules "continue the Trump administration's agenda to benefit oil and gas … and allow the federal government to ignore the devastating impacts of climate change, ignore the UN biodiversity report and the alarming crisis facing a million species.

"These changes violate the purpose of the Endangered Species Act," Healey added, "and sounds like the plan of a cartoon villain and not the president of the United States."

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