A centerpiece of President Donald Trump’s campaign was the promise of a “big, beautiful wall” to halt illegal immigration along the U.S.-Mexico border. Within a week of taking office, he’d signed an executive order directing construction to begin.

Based on past failures, it’s unclear Trump’s “great wall” will be any more effective at stopping human movement across the border. But one thing is certain: Wildlife that have lived along and across these borderlands, far longer than we have, will be cutoff from habitat, food and mates.

Under the National Environmental Policy Act (NEPA), federal agencies must assess the potential environmental impacts of their proposed actions, prior to taking a decision. Agencies must also provide opportunities for public review and input on those assessments. But the REAL ID Act of 2005 gave the Department of Homeland Security unilateral authority to waive NEPA and any other law or treaty it thought impeded construction of border barriers and roads.

In 2008, Michael Chertoff – then the secretary of the Department of Homeland Security – used this waiver to continue border-fence construction without compliance of nearly three dozen state and federal laws, including the Endangered Species Act, the Clean Water and Clean Air Acts, the Migratory Bird Treaty Act and NEPA. As a result, "scientists have no environmental reference data prior to border wall construction: no surveys or inventories to know what species were there and no population baselines to detect effects of the border infrastructure on their numbers," according to Sergio Avila, a conservation scientist with the Arizona Sonora Desert Museum. REAL ID also limited scientific input on avoidance and mitigation measures that could have minimized impacts on species, habitats and watersheds, Avila said.

Trump is expected to use REAL ID to fast track construction of his border wall. Getting out ahead of him, Outside Magazine requested the U.S. Fish & Wildlife evaluate if any endangered species might be impacted by the proposed project. In a provisional report, the agency projected that a solid barrier extending 1,000 feet into the U.S. and running the length of the U.S.-Mexico border would affect 98 endangered species – ranging from jaguars to leatherback sea turtles – as well as 108 migratory bird species, and four wildlife refuges and fish hatcheries.

Besides sharing a border, the U.S. and Mexico share water and wildlife, and scientists have long argued that artificially blocking the natural corridor between the countries could be catastrophic – restricting animal movement and destroying habitat at best; leading to local or global extinctions at worst.

In 2010, University of Arizona researcher Aaron Flesch tracked ferruginous pygmy owls and desert bighorn sheep to determine impacts the man-made border barrier would have on their movements. He and his co-authors concluded the barriers negatively affected both species.

“For sheep it is very simple: a quadruped is not going to climb a fence," he said. "And four meters is a pretty good jump. Bighorn sheep, deer, mountain lions, bear, they are going to be visibly excluded from crossing a solid fence."

The owls simply can’t or won’t fly high enough, and avoid areas of open country, such as that cleared on either size of the fences.

Another study by biologist Jesse Lasky analyzed the impacts of existing and future barriers on species across the entire U.S.-Mexico land border. The 2011 study determined current border infrastructure already increased the risk to four species listed as threatened globally or by both the U.S. and Mexico, plus another 23 with small range sizes, including the Arroyo toad, the California red-legged frog and the jaguarundi, a small wild cat native to Mexico and Central America. Additional border barriers would only increase the number of species at risk.

To persist in these heavily fragmented environments, these and other species with small populations depend on movement among habitat patches to interbreed with other populations. They don’t recognize political boundaries, but do recognize, and often avoid border infrastructure.

“More than just walls and fences”, says Avila. “Vehicle barriers, miles and miles of new roads, high-powered lights and generators, patrolling in sensitive areas, helicopter over-flights and helipads, forward operating bases, checkpoints, heavy machinery and construction and maintenance crews not only block movement corridors and destroy habitat, but diminish watersheds by diverting or blocking water.”

A draft Department of Homeland Security report to the White House defined the government’s highest priority for a border wall as a 34-mile area in the Rio Grande Valley of south Texas. Far from being a lifeless desert, the area is considered an “International Borderland of Concern” for its exceptionally high diversity of plants and animals, some found in few if any other places in the U.S., including endangered species like the northern ocelot and northern aplomado falcon. The U.S. Fish & Wildlife Service manages three national wildlife refuges in the Valley that together make up the South Texas Refuge Complex. The existing border wall has already affected 60 to 75 percent of the complex’s land.

Ecological catastrophe notwithstanding, many local business owners fear potential economic losses as well. The Rio Grande Valley sits at the junction of two migratory bird flyways. Each year, wildlife watchers from across the globe visit the region to catch a glimpse of the 500 bird and 300 butterfly species, contributing upwards of $463 million in county-level economic output annually, not to mention creating more than 6,000 jobs.

In April of 2017, U.S. House Natural Resources Committee ranking member Rep. Raúl Grijalva of Arizona and the Center for Biological Diversity filed a lawsuit against the Trump administration for failing to analyze the environmental impacts of their southern border enhancement program, as required by NEPA. Legal scholars give such lawsuits incredibly long odds, thanks to the potency of the REAL ID Act waivers. However some, like attorney Jenny Neeley, argue the waiver authority is unconstitutional, both in its scope and absence of accountability, and should be vacated by Congress "before more damage is done."