President Trump may have just dramatically slashed the size of two national monuments in Utah, but America's 59 national parks — as well as over 270 other national park sites — cannot be unilaterally downsized by Trump or any other president.

The legal reason is simple and inarguable:

"National parks can only be established by Congress, and everyone agrees they can only be undone, if at all, by Congress," Holly Doremus, a natural resources law expert at University of California, Berkeley, told Mashable in an email.

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National monuments — like Bears Ears and Grand Staircase-Escalante — are a different story. Combined, Trump reduced both protected lands by nearly 2 million acres combined. Bears Ears was reduced in size by over 80 percent.

Unlike national parks or other national park sites — places like national rivers, national preserves, and national seashores — national monuments are established by the president, who wields the unilateral power to protect places of historic or scientific interest from resource extraction and development. This presidential power is granted by the 1906 Antiquities Act and was first employed by the conservation-minded president Theodore Roosevelt to establish Devils Tower National Monument in Wyoming, also in 1906.

But while presidents can establish a national monument, it's doubtful — though still arguable — they can change monument boundaries, like Congress can. Because the Trump administration just did this, they have now entered murky legal territory.

"Congress can certainly undo a national monument designation," said Doremus. "It is disputed whether the president can unilaterally do so. Most natural resource scholars think not."

Theodore Roosevelt used the Antiquities Act to protect Devil's Tower National Monument in 1906. Image: national park service

A review of the matter, published earlier this year by two natural resource lawyers in The Virginia Law Review, supports this belief. They argue that only Congress has the power to "downsize" a national monument, just like only Congress can nix or dissolve a national monument. They write:

If the President cannot abolish a national monument because Congress did not delegate that authority to the President, it follows that the President also lacks the power to downsize or loosen the protections afforded to a monument.

Still, no president has ever cut the boundaries of a national monument to such an extreme degree and then been challenged in court. The authors add:

Importantly though, no Presidential decision to reduce the size of a national monument has ever been tested in court, and so no court has ever ruled on the legality of such an action.

Until this gets settled, it's possible the Trump administration will alter or downsize other national monuments. In a once-secret memo given to the president in August, Interior Secretary Ryan Zinke recommended that four national monuments be downsized and six others altered.

National monuments like Bears Ears are not protected on a president's whim: They are typically reviewed for years and seek public input to confirm their natural and cultural significance, as President Obama articulated in December 2016:

"Following years of public input and various proposals to protect both of these areas, including legislation and a proposal from tribal governments in and around Utah, these monuments will protect places that a wide range of stakeholders all agree are worthy of protection."