The administration's review has alarmed conservationists worried that protections could be lost for areas that include ancient cliff dwellings, towering sequoia trees, deep canyons and ocean habitats.

President Trump acted on complaints that a century-old law had been misused to create oversized monuments that hinder energy development, logging and other uses.

In the West, the federal government owns and manages almost half of all land. Public land in the whole country totals about 640 million acres. The federal government owns nearly 62 percent of the land in Idaho, just over 63 percent of the land in Utah, and almost 80 percent in Nevada.

Conservation and tribal groups recently bought TV ads and sent letters to Mr. Trump in a last-ditch effort to stop Zinke from reducing national monument areas. Groups that support the reduction had been pleading their case on social media and lobbying federal officials behind the scenes.