WASHINGTON — Geographic and physical challenges – including the Rio Grande and threatened wildlife – will make it difficult to build the “big, beautiful wall” that President Trump has promised on the U.S.-Mexico border, Interior Secretary Ryan Zinke said Wednesday.

Building a wall “is complex in some areas,” including Big Bend National Park and along the river, which twists through nearly half of the 2,000-mile border, he said.

Hundreds of species live within 30 miles of the border, including threatened jaguars and Mexican gray wolves. The Trump administration is poised to relax protections for the jaguars, which live in northern Mexico and parts of the southwestern United States, to make it easier to build the wall.

Throughout the campaign, Trump energized his crowds with his insistence that a wall will be constructed along the border and that Mexico will pay for it. Zinke’s comments, and the administration’s budget proposal seeking billions in U.S. taxpayer dollars to finance the project, offer a reality check and a possible sign the president is moving away from his initial plan.

The complications Zinke highlighted were the same faced by Trump’s predecessors, George W. Bush and Barack Obama, as they sought to build or complete hundreds of miles of fencing.

Fencing that is already in place is a mixture of various designs, including towering steel bollards designed to keep both people and vehicles from moving north and shorter steel posts aimed only at blocking cars. In parts of Texas’ Rio Grande Valley, some stretches of fencing are nearly a mile away from the border in part to accommodate flood plains and an international treaty.

And in Texas, almost all of the land along the border is privately owned. When Bush tried to build border fencing starting in 2006, he faced stiff opposition from local ranchers and farmers, many of whom took the government to court on plans to use their land.

The Department of Homeland Security is responsible for the border wall, but Zinke said the Interior Department will play a critical support role. According to the Government Accountability Office, federal and tribal lands make up about 632 miles or roughly 1/3 of the nearly 2,000 miles of border. “At the end of the day, what’s important is American security and to make sure we have a border,” Zinke told reporters on a conference call. “Without a border a nation cannot exist.”

An internal report prepared for Homeland Security Secretary John Kelly estimated that a wall along the entire border would cost about $21 billion.