President Donald Trump’s proposed wall along the US-Mexico border is meant to stop human crossings, but it could also disrupt the fragile migratory habits of animals.

Plans that are currently being developed have the wall running through the Santa Ana Wildlife Refuge in South Texas, which is home to hundreds of animal species living in a delicately managed ecosystem.

If the wall is built, it would “essentially destroy the refuge,” an official from the refuge told the Texas Observer.

Construction of the wall, which could ultimately span more than 1,000 miles and cost an estimated $21.6 billion to build, could begin as early as winter 2018, if funding for its construction gets approved in the budget.

The US Customs and Border Protection agency, which will oversee the wall, has about $20 million in reserves, which is hardly enough for construction. A bill in the House of Representatives has earmarked $1.6 billion in the upcoming budget, which may or may not survive future negotiations, so the ultimate construction of the wall could get delayed indefinitely.

Despite these uncertainties, the CBP is moving ahead with the preliminary phases of project. The agency put out two proposals to private contractors back in March — one solid concrete, and the other open-ended — and received hundreds of bids.

Private contractors have begun scoping out the wildlife refuge, according to the Observer.

The part that goes through the Santa Ana Wildlife Refuge will be three miles long, 18 feet tall and made with concrete, officials involved with the process told the Texas Observer.

If the project happens, land on both sides will have to be cleared to make way for surveillance, cameras and light towers.

The federal government owns the land involved in this plan, so officials at the US Customs and Border Protection determined it would make economic sense to build on it rather than buy out landowners elsewhere, they told the Observer. Plus, fights over “eminent domain,” the difficult process by which government can seize private land for various projects, does not have be carried out in this case.

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The Santa Ana Wildlife Refuge was established as a sanctuary for migrating birds in 1943. It hosts ”approximately 400 bird species, 450 types of plants, half of all butterfly species found in North America, and such rarities as the indigo snake and altamira oriole,” according to the US Fish and Wildlife Service.

The refuge is an “ecological crossroads,” allowing birds and animals across North America to take a breather, get food and water, and safely go about their lives.

Wildlife advocates hope to block the wall’s construction in the refuge by calling for an environmental review of the area to determine how animals would be affected.

An earlier review of a proposed border wall found that more than 100 species that are either endangered, threatened, or a candidate for protection would be harmed by the wall.

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The Trump administration has sought to roll back environmental protections in other ways. It’s currently reviewing dozens of national monuments to potentially strip them of protections, ending protections for animals, opening public lands to fossil fuel extraction, and removing regulations for wetlands.   

The refuge is the latest ecosystem that’s under review and many environmentalists are determined to protect it.

As Congressman Filemon Vela, a Democrat from Brownsville, Texas told the Observer:

“These refuges are sacred from an environmental standpoint. There’s so little protected land left and we need to do whatever we can to save it.”