No president has ever abolished a national monument designated by a predecessor. President Trump may try to change that.

Ryan Zinke, the secretary of the interior, is reviewing 27 national monuments to determine if previous administrations exceeded their authority in setting aside craggy vistas, ancient cliff dwellings and other large tracts of land for protection. He is expected to recommend that some be scaled back, or perhaps eliminated entirely and transferred to state ownership.

Democrats and environmental activists see the review as part of a broad effort within the Trump administration to unravel the conservation legacy of President Barack Obama, who under the 1906 Antiquities Act put more land and water under federal protection than any other president. Yet Mr. Zinke’s study, due by Aug. 24, stretches back 21 years to include other national monuments that remain a source of acrimony, particularly in the West.

Not all of the monuments are truly in the administration’s cross hairs, and Mr. Zinke has already declared some of them safe from changes. While any of the others could be altered, critics and supporters of the review say only a handful face significant scrutiny. Here are some of the big ones to watch.

Toadstools Trail in Utah’s Grand Staircase-Escalante National Monument. Education Images/UIG via Getty Images

UTAH

Grand Staircase-Escalante

1.88 million acres | Designated by Bill Clinton, 1996

It’s no accident that the Antiquities Act review goes back to 1996. That’s when President Bill Clinton, standing on the south rim of the Grand Canyon with the actor Robert Redford by his side, declared the Grand Staircase-Escalante National Monument in southern Utah. The announcement took state leaders by surprise. Angered that Washington had ignored local views and quashed a proposed coal mine that had promised hundreds of jobs, residents in the neighboring town of Kanab flew flags at half-staff. Two decades later, opponents are still seeking ways to shrink the monument.

Republicans and other critics have long charged that the 1.9 million-acre monument closes off many areas to development, particularly the coal reserves along the Kaiparowits Plateau in Garfield and Kane counties. Meanwhile, park managers say that Grand Staircase-Escalante has never been adequately funded or staffed to support its original scientific mission.

When Mr. Zinke visited in May, he declined to say if mineral resources would play a role in deciding whether to rescind or shrink the monument designation. “Monuments should never be put in a position to prevent rather than protect, and the president is pro-energy across the board,” he told the St. George News.

LeftTop: The Bears Ears National Monument in southeastern Utah. RightBottom: Ancient granaries, part of the House on Fire ruins in Bears Ears. Francisco Kjolseth/The Salt Lake Tribune, via Associated Press; George Frey/Getty Images

UTAH

Bears Ears

1.35 million acres | Designated by Barack Obama, 2016

Hailed by environmentalists and Native American groups, pilloried by conservatives and many local residents, Bears Ears National Monument has become the poster child for the fight over Mr. Obama’s preservation agenda.

Mr. Zinke has already vowed major changes to this vast expanse of Utah sandstone and red rocks that Mr. Obama designated for protection, along with 300,000 acres of land in Nevada as Gold Butte National Monument, in the waning days of his administration.

Native American groups, for whom the rock formations and canyons are considered sacred ground, have fought to maintain the monument, arguing that archaeological sites have been defaced and that burial sites have been looted. But Utah’s political leaders denounced the decision. Senator Orrin G. Hatch, a Republican, called it an “egregious abuse of executive powers.”

In a preliminary report unveiled in June, Mr. Zinke recommended shrinking Bears Ears’ borders by identifying and separating “areas that have significant objects to be protected.” The early move has already prompted outrage from outdoor recreation companies like Patagonia and REI, and spurred the Outdoor Industry Association to pull their annual trade show out of Utah. They and others have vowed to go to court to fight any proposed changes to the monument.

Workers install a sign at the Organ Mountain-Desert Peaks National Monument in Las Cruces, N.M., in March 2015. Jett Loe/Las Cruces Sun-News via Associated Press

N.M.

Organ Mountains-Desert Peaks

496,330 acres | Designated by Barack Obama, 2014

When Mr. Zinke flew by helicopter in July over the jagged Organ Mountains in southern New Mexico, he proclaimed the beauty of the monument, though he described it as “a little disconnected.”

“The boundaries are difficult to discern — between private, public, state lands,” he said at a news conference.

Mr. Zinke has pledged to remain open-minded about the review. But environmentalists at the Natural Resources Defense Council and the Center for Biological Diversity are concerned that the large Western monument designated by the Obama administration could be altered.

In designating Organ Mountains-Desert Peaks, Mr. Obama claimed the monument would spur $7.4 million in new economic activity. Ranchers, however, have worried that the monument will hurt their business. They also argue Mr. Obama did not follow the spirit of the Antiquities Act, which calls for protected lands to be “confined to the smallest area compatible with proper care and management.”

A rain storm passing over Mt. Katahdin in the Katahdin Woods and Waters National Monument in northern Maine. Robert F. Bukaty/Associated Press

Me.

Katahdin Woods and Waters

87,563 acres | Designated by Barack Obama, 2016

The governor of Maine has denounced this sweeping monument in forestland that once was the heart of large-scale logging operations in the state.

Mr. Obama designated the monument in Maine’s North Woods in August 2016 on land donated by Roxanne Quimby, a co-founder of Burt’s Bees cosmetics. The announcement came a day after a nonprofit foundation run by Ms. Quimby, a longtime conservationist, transferred the land to federal ownership and pledged a $40 million endowment.

Gov. Paul R. LePage, a Republican, has called the monument an “ego play” by Ms. Quimby, dismissed the protected land as a “mosquito area” that won’t attract tourists, and claimed that closing off the area to timber harvesting hurts the state’s economy.

Mr. Zinke toured the area in June. He said he was “comfortable” with the land remaining public, according to The Associated Press. “I’m confident there’s a path forward here that will work,” Mr. Zinke said.

LeftTop: Sea turtles on Turtle at Midway Atoll in the Papahānaumokuākea Marine National Monument. RightBottom: Landing at Henderson Field, Midway Atoll. Saul Loeb/Agence France-Presse/Getty Images; Jonathan Ernst/Reuters

Expansion Original HAWAII

Papahanaumokuakea

89.5 million acres | Designated by George W. Bush, 2006

283.4 million acres | Added by Barack Obama, 2016

President George W. Bush created the Papahanaumokuakea Marine National Monument (pronounced “Papa-ha-now-moh-koo-ah-kay-ah”) off Hawaii in 2006. Last year, Mr. Obama quadrupled it.

Now the ocean reserve more than twice the size of Texas is the largest protected area anywhere on Earth. Commercial fishing of tuna and other species is prohibited, though recreational fishing is allowed with a permit. New mining and oil and gas drilling are also forbidden.

Scientists and environmentalists hailed the decision as critical to helping the diverse ecosystem withstand the threat of climate change. “The oceans are the untold story when it comes to climate change, and we have to feel a sense of urgency when it comes to protecting the ocean that sustains us,” said Senator Brian Schatz, Democrat of Hawaii, who proposed the monument.

But the decision wasn’t without detractors. The Western Pacific Regional Fishery Management Council, a quasi-government agency that helps manage fisheries in the Pacific Islands, opposed the monument, arguing that Hawaii’s fishermen are already being hurt by the protections because they now must fish in the high seas in competition with other countries.

Republicans in Washington argue that the Antiquities Act was designed to protect “lands,” not waters. Representative Rob Bishop, Republican of Utah and chairman of the House Natural Resources Committee, called the monument “unjustified.”

All National Monuments Under Review Land Monument Marine Monument Name Location Established/expanded Current size (acres) Ironwood Forest Arizona 2000 128.9k Sonoran Desert Arizona 2001 486.1k Vermilion Cliffs Arizona 2000 279.6k Berryessa Snow Mountain California 2015 330.8k Carrizo Plain California 2001 204.1k Giant Sequoia California 2000 327.8k Mojave Trails California 2016 1.6m San Gabriel Mountains California 2014 346.2k Katahdin Woods and Waters Maine 2016 87.6k Basin and Range Nevada 2015 703.6k Gold Butte Nevada 2016 296.9k Organ Mountains-Desert Peaks New Mexico 2014 496.3k Rio Grande del Norte New Mexico 2013 242.6k Cascade Siskiyou Oregon/California 2000/2017 112.0k Bears Ears Utah 2016 1.4m Grand Staircase-Escalante Utah 1996 1.9m Rose Atoll Pacific Ocean (Amer. Samoa) 2009 8.6m Northeast Canyons and Seamounts Atlantic Ocean 2016 3.1m Marianas Trench Pacific Ocean (N. Mariana Isl.) 2009 60.9m Pacific Remote Islands Pacific Ocean 2009/2014 316.9m Papahanaumokuakea Pacific Ocean (Hawaii) 2006/2016 372.8m Ironwood Forest, Arizona Established: 2000 | Current size: 128.9k acres Sonoran Desert, Arizona Established: 2001 | Current size: 486.1k acres Vermilion Cliffs, Arizona Established: 2000 | Current size: 279.6k acres Berryessa Snow Mountain, California Established: 2015 | Current size: 330.8k acres Carrizo Plain, California Established: 2001 | Current size: 204.1k acres Giant Sequoia, California Established: 2000 | Current size: 327.8k acres Mojave Trails, California Established: 2016 | Current size: 1.6m acres San Gabriel Mountains, California Established: 2014 | Current size: 346.2k acres Katahdin Woods and Waters, Maine Established: 2016 | Current size: 87.6k acres Basin and Range, Nevada Established: 2015 | Current size: 703.6k acres Gold Butte, Nevada Established: 2016 | Current size: 296.9k acres Organ Mountains-Desert Peaks, New Mexico Established: 2014 | Current size: 496.3k acres Rio Grande del Norte, New Mexico Established: 2013 | Current size: 242.6k acres Cascade Siskiyou, Oregon/California Established/Enlarged: 2000/2017 | Current size: 112.0k acres Bears Ears, Utah Established: 2016 | Current size: 1.4m acres Grand Staircase-Escalante, Utah Established: 1996 | Current size: 1.9m acres Rose Atoll, Pacific Ocean (Amer. Samoa) Established: 2009 | Current size: 8.6m acres Northeast Canyons and Seamounts, Atlantic Ocean Established: 2016 | Current size: 3.1m acres Marianas Trench, Pacific Ocean (N. Mariana Isl.) Established: 2009 | Current size: 60.9m acres Pacific Remote Islands, Pacific Ocean Established/Enlarged: 2009/2014 | Current size: 316.9m acres Papahanaumokuakea, Pacific Ocean (Hawaii) Established/Enlarged: 2006/2016 | Current size: 372.8m acres

No Longer Under Review, No Changes Recommended Name Location Established/expanded Current size (acres) Grand Canyon-Parashant Arizona 2000 1.0m Sand to Snow California 2016 154.0k Canyons of the Ancients Colorado 2000 178.0k Craters of the Moon Idaho 1924/2000 737.5k Upper Missouri River Breaks Montana 2001 377.3k Hanford Reach Washington 2000 194.5k Grand Canyon-Parashant, Arizona Established: 2000 | Current size: 1.0m acres Sand to Snow, California Established: 2016 | Current size: 154.0k acres Canyons of the Ancients, Colorado Established: 2000 | Current size: 178.0k acres Craters of the Moon, Idaho Established/Enlarged: 1924/2000 | Current size: 737.5k acres Upper Missouri River Breaks, Montana Established: 2001 | Current size: 377.3k acres Hanford Reach, Washington Established: 2000 | Current size: 194.5k acres

No matter what Mr. Zinke’s review finds, changes to federal monuments are not imminent. Environmental groups have vowed to sue if President Trump tries to eradicate or reduce protected lands.

They say they are not heartened that Mr. Zinke has declared a handful of monuments safe from changes. “These pardons have been so random,” said Jenny Rowland, a public lands research manager for the Center for American Progress, a liberal advocacy group.

The Interior Department did not respond to requests for comment.

No president has reduced the size of a federal monument since 1963, when President John F. Kennedy revised the boundaries of the Bandelier National Monument in New Mexico. Scholars are split over whether the Antiquities Act even allows a president to do so, or whether Congress has the sole power to undo a designation.

Todd F. Gaziano, a senior fellow in constitutional law at the Pacific Legal Foundation, a group that advocates for limited government, has said environmental groups are engaged in “magical thinking” when they argue that a president cannot revoke a designation. “There is no court case that holds the president doesn’t have significant reduction authority,” he said.

The Interior Department has received more than 1.4 million public comments, which several outside organizations have said swung overwhelmingly toward preserving the national monuments.