Mexican conservationists are alarmed over Trump’s wall, with the loss of connectivity threatening already stressed bison, pronghorn, bighorn sheep, bears and other animals.

About one-third of the border, roughly 700 miles, already has fencing; President Trump has been pushing a controversial plan to fence the remainder.

A wall running the entire nearly 2,000-mile frontier from the Pacific Ocean to the Gulf of Mexico, conservationists warn, would be catastrophic for borderland ecosystems and many wildlife species, undoing years of environmental cooperation between the two countries to protect animals that must move freely or die.

The wall is currently a key bargaining chip, and a sticking point, in ongoing immigration legislation negotiations taking place this week in Congress. Also expected this week: a federal court ruling on whether the administration can legally waive environmental laws to expedite border wall construction.

Rurik List began studying wildlife south of the U.S.-Mexico border in 1994, doing research in the expansive grasslands of the Janos Valley in the Mexican state of Chihuahua. Back then, the international boundary was pretty porous there, marked only by barbed wire fencing. Most animals could easily pass over, under, or if they were bison, bust right through it, in their quest for food, water, mates, or suitable habitat as they moved between Chihuahua and New Mexico.

In late 2008, things changed.

Flying over the Janos Valley a few months later, List, a Mexican ecologist now at Universidad Autónoma Metropolitana-Lerma, saw that the U.S. government had erected stout, neck-high crisscrossed steel struts called Normandy barrier across about half of the six-mile (10-kilometer) wide valley, bisecting the grasslands.

From that point forward, Mexico’s only bison herd (Bison bison), numbering around 100 animals, could no longer just muscle their way into the country in search of greener pastures wherever they liked. They’d have to walk the long way around the new barrier, or just stay put. So would pronghorn (Antilocapra Americana). And bighorn sheep (Ovis canadensis). List believes both species rely on the bison holes, allowing their small U.S. populations to intermix with even smaller Mexican ones — herds with a tenuous foothold after near annihilation by hunters.

Still, most of the valley’s smaller wildlife — foxes, rodents, lizards, birds, and more — weren’t impeded by the new Normandy barrier. However, on the same flight List found a far greater obstruction dividing north from south at another of his border zone study sites near the town of Agua Prieta, where fencing twelve or more feet high (3.5 meters) now ran the line. “That type of barrier, it cannot be crossed by most animals and also some birds,” List told Mongabay.

A porcupine in the Janos Valley of Chihuahua, Mexico, where the species is listed as in danger of extinction. Photo by Rurik List courtesy of CONABIO.

A badger (Taxidea taxus), considered threatened in Mexico, in the grasslands of the Janos Valley. Photo by Rurik List courtesy of CONABIO.

Grasslands near the U.S. border in the Janos Valley of Chihuahua, Mexico. Photo by Rurik List.

Pronghorn, North America’s fastest species, in New Mexico. The species is abundant in the U.S. but listed as in danger of extinction in Mexico. Photo by mnchilemom via Flickr (CC BY 2.0).

A double row of pedestrian fencing, impassable to non-flying wildlife, and vehicle barrier mark the border in El Pinacate y Gran Desierto de Altar Biosphere Reserve in western Sonora, Mexico. Photo by Rurik List.

Pedestrian fencing stretches for miles into the rolling landscape on the outskirts of Nogales, Arizona. This kind of fencing is impassable to most wingless wildlife. Photo by Rebecca Kessler for Mongabay.

Border marker 198 overlooks miles of fencing between Sonora, Mexico and Arizona, U.S. Photo by Rurik List.

Border wall in Tijuana, Mexico. Photo by Rurik List.

The border fence in Tijuana, Mexico. Photo by Rurik List.

Border infrastructure ends in the Pacific Ocean, between Tijuana, Mexico, and San Diego County, California. Photo by Rurik List.

Now looms a far bigger threat: President Trump’s promised “big, beautiful wall” aimed at stopping undocumented human migration and drug trafficking along the entire U.S.-Mexico border. That much-debated structure is currently a key bargaining chip, and sticking point, in immigration negotiations taking place this week between Republicans and Democrats in the U.S. Congress. By many accounts, the negotiations are wide open, but the pressure is on to reach an agreement by March 5. That’s when Trump is terminating the Deferred Action for Childhood Arrivals program (DACA), which has been protecting unauthorized immigrants brought to the U.S. as children from deportation. But the president has said he is willing to reinstate the program if he receives $25 billion for his wall in exchange, to be spent over ten years.

As recently as February 5 Trump tweeted: “Any deal on DACA that does not include STRONG border security and the desperately needed WALL is a total waste of time. March 5th is rapidly approaching and the Dems seem not to care about DACA. Make a deal!”

List and other Mexican scientists and conservationists are very concerned about whether or not the U.S. will decide to build its mega-wall. Small picture: a nearly impermeable border wall in the Janos Valley will stop most wildlife in their tracks and gravely threaten Mexican populations of bison, pronghorn, and bighorn sheep. Big picture: a wall running the entire nearly 2,000-mile frontier from the Pacific Ocean to the Gulf of Mexico, List and other conservationists warn, would be catastrophic for borderland ecosystems and many wildlife species, undoing years of environmental cooperation between the two countries to protect animals that must move freely or die.

If Trump’s wall is built, “this barrier will rewrite the biological history of North America. A history that for millennia allowed animals to travel along the grasslands and forests from Mexico to Canada,” List wrote in an issue of Jornada Ecologica. “The future of the bison and many other species that the two countries share is at stake at the border.”