House Democrats representing districts along the U.S.-Mexico border expressed alarm on Friday that the Trump administration’s plans to waive certain environmental laws to build a wall could hurt wildlife.

The Department of Homeland Security (DHS) announced earlier this week that it will use its authority under a 2005 law to bypass laws and regulations when necessary for enhancing infrastructure along the border.

The waiver made public this week applies only to a 15-mile segment around San Diego. It exempts projects to build infrastructure like fencing or a wall from laws like the Endangered Species Act, Clean Water Act, National Environmental Policy Act and Safe Drinking Water Act.

In a letter to Acting Secretary of Homeland Security Elaine Duke, Texas Democratic Reps. Vicente González and Beto O’Rourke asked whether DHS would give itself the authority to waive laws for other sections of the southwestern border.

Specifically, the lawmakers worried that the Trump administration might seek to build a wall cutting through the Santa Ana Wildlife Refuge, which is located along the Rio Grande River.

They described it as “one of the top bird-watching sites in the nation with more than 400 species of birds.” The refuge also serves as a habitat for half of all butterfly species in North America as well as the endangered ocelot and jaguarundi, according to its website.

“It generates hundreds of millions of dollars through ecotourism every year, and is an irreplaceable treasure to the region. A wall cutting through the refuge could do serious environmental and economic damage, and the American public deserves transparency for what could be billions of taxpayers’ dollars spent on a wall along the U.S.-Mexico border,” the lawmakers wrote.

Three other Texas Democrats signed onto the letter: Reps. Henry Cuellar, Filemon Vela, Lloyd Doggett and Joaquin Castro. Cuellar and Vela also represent parts of the U.S.-Mexico border.

Building a wall along the border with Mexico was one of Trump’s central campaign promises and favorite chant of supporters at his rallies.

Trump pledged that Mexico would pay for the wall, a suggestion its leaders have vehemently rejected.

The House last week passed a government spending package that included $1.6 billion to begin construction of the wall. But Senate Democrats are expected to block it from passing in the upper chamber.

The Washington Post this week published a transcript of a January phone call that showed Trump acknowledging to Mexican President Enrique Peña Nieto that he had a “political problem” in pledging the U.S. wouldn’t pay for the wall.

“They are going to say, ‘who is going to pay for the wall, Mr. President?’ to both of us, and we should both say, ‘we will work it out.’ It will work out in the formula somehow. As opposed to you saying, ‘we will not pay’ and me saying, ‘we will not pay,’ ” Trump said, according to the transcript.