During Donald Trump’s first presidential term, he began an ambitious and costly border militarization program, including the construction of over 450 miles of wall that severed wildlife corridors and fragmented ecosystems in some of the country’s most remote and biodiverse regions. With his second inauguration on Monday, environmentalists are bracing for any new phase of construction that could exacerbate the ecological toll of the border wall.

“It’s an absolute travesty and a disaster for border wildlife,” said Margaret Wilder, a human-environment geographer and political ecologist at the University of Arizona, regarding the environmental impact of the existing border wall and the prospect of renewed construction. She said the wall harmed efforts “after many decades of binational cooperation between the US and Mexico to protect this fragile and biodiverse region. I don’t think Americans realize what is at stake.”

What’s at stake is the historically unparalleled separation of wildlife populations along the more than 635 miles of pedestrian border wall – largely impassable to anything bigger than a jackrabbit – that has been built along the southern border. “This [the border wall] is a massive uncontrolled experiment in the evolutionary history of wildlife species in the borderlands,” said Laiken Jordahl, south-west conservation advocate for the Center for Biological Diversity. “The places that are still unwalled are some of the most remote, rugged, and important habitats for wildlife that we have left.”

New Mexico and Arizona have approximately 391 miles of the pedestrian border wall, 263 miles of which were constructed during the first Trump administration.

A recent Wildlands Network and Sky Islands Alliance study showed the impact of the pedestrian border wall (30ft-high steel pillars 4in apart) on wildlife movement and habitat connectivity in the exceptionally biodiverse Sky Island region of Sonora Mexico and the south-western US. Motion-activated cameras placed along 100 miles of Arizona border showed an 86% decrease in wildlife crossings and a 100% reduction in crossing for large animals such as bears, pronghorns and jaguars.

“The sky islands are a continental crossroads where many different species, including humans, have been passing through for millennia,” said the Sky Island Alliance program director, Emily Burns. “Putting in a continuous wall across the border of Arizona is extremely bad for medium and large wildlife.”

All indications show a continuation of Trump’s hardline approach to immigration and the southern border. Trump has criticized the Biden administration’s auctioning off border wall materials, describing the sales as “almost a criminal act” that would cost taxpayers millions of dollars when Trump resumes border wall construction. “They know we’re going to use it, and if we don’t have it, we’re going to have to rebuild it, and it’ll cost double what it cost years ago,” said Trump.

However, precise plans for the border wall remain unclear.

“No one has any idea what the hell is about to happen,” said Jordahl. “We’re definitely bracing ourselves for the worst.”

The border wall was a signature priority of Trump’s first candidacy, made possible by the expansive powers given to the Department of Homeland Security by Congress. These powers allowed for the bypassing of federal laws to speed up construction and bolster border security through Customs and Border Protection, the nation’s largest law enforcement agency.

Ricky Garza, border policy counsel with the Southern Border Communities Coalition, has witnessed the steady creep of border militarization in the Rio Grande valley.

“That entire area is just being choked off by border patrol presence,” Garza said, referring to the growing number of green-and-white border patrol vehicles, immigration checkpoints, and border infrastructure. “There is this physical occupation structure that increases as you get closer to the border.”

Texas has the most border with Mexico, but the least wall mileage as the border is formed by the Rio Grande and land on the Texas side is mostly privately owned. Border wall construction has been more common on federally owned land not because these are busy migrant crossings but because building on private property is extremely difficult. During the Trump administration, 263 miles of pedestrian fencing and border wall were built in mostly rural New Mexico and Arizona. Today more than 60% of the Arizona border has been walled.

“In those really remote rugged areas, whether it was the mountains in Arizona, et cetera, there was no indication that people were attempting to cross the border, at least by any numbers,” said Gil Kerlikowske, a former commissioner of Customs and Border Protection. “So why try and put up any type of border wall in a place where you really do not need it?”

When I visited the Arizona border in the waning days of Trump’s first term, construction crews were busy dynamiting hilltops and grading up mountainsides to complete disconnected sections of border walls in some of the most remote and impassable parts of the border.

Burns, of Sky Island Alliance Program, fears construction in these areas could resume as the federal laws to construct the barrier remain waived. “There are construction plans for the places where the wall had been canceled,” said Burns. “It seems very possible that old projects will just be dusted off and reactivated.”

Kerlikowske is less sure. “Tom [Homan] made it clear that he wanted to focus on the border more or less as the borders are,” he said of Trump’s incoming “border czar”.

“What’s really troubling is that the sections of the border that they did not wall off in Arizona and New Mexico are some of the most sensitive places and important wildlife corridors for species like black bear, jaguar and other mammals,“ said Myles Traphagen, Borderlands Program Coordinator for the Wildlands Network.

That closure of unwalled sections would be a death knell for the elusive jaguar, which has reappeared in the US after being hunted to extinction in the 1960s.

“More [wall] would definitely cut off jaguars from crossing into the US in the last corridors it has from Sonora to Arizona,” said Burns. “When these animals move it is a lifeline for populations on the brink.”

Border construction is a bipartisan endeavor. The Obama administration built more than 100 miles of new border wall. Biden resumed construction in 2023 after attempts to redirect money appropriated for the wall in 2019 failed “The money was appropriated for the border wall,” Biden said. “I can’t stop that.”

According to a CBP spokesperson, since 2021, the agency has prioritized barrier funding to close gaps and complete gates along the southern border. Of the 163 gap closures and gate projects approved since 2021, 119 have been completed.

Approximately 57 miles of new border barrier gap closures are planned. These projects are still in the environmental planning process and are estimated to start in early 2025 according to CBP.

Border barriers are deadly for wildlife and for people. The border wall – and CBP’s decade-long prevention through deterrence policies have pushed people into more dangerous and hostile terrain.

“The border wall is an engine of death,” said Garza, of the Southern Border Communities Coalition, as the southern border is now the deadliest land migration route in the world.

With Trump re-entering office he worries things will only get worse. “I don’t want my home to be turned into a sacrifice zone, but that’s what we’re moving toward.”