This story was originally published by Sierra and is republished here by permission.
In May of 2023, a female mountain lion squeezed through a picture-frame-size opening in a steel fence along the U.S.-Mexico border. While hundreds of people tried to cross into the US, she headed south, looking for prey or water or a potential mate.
The 8.5-by-10-inch hole is one of a handful of wildlife passages cut into the fence, thanks in part to the urging of conservation groups. But these concessions fall far short of what’s needed to help mountain lions and other species survive the blockading of their habitat, wildlife advocates say.
The Trump administration completed 458 miles of fence during his first term but never finished the barrier. Now, new segments are going up, further fragmenting wildlife habitat. This time, President Trump has vowed to complete the project by the end of his second term. In April, Customs and Border Protection (CBP) announced new wall projects in Arizona and California, and construction has already begun on barriers in Texas. More barriers could be going up soon: Congress has included $46.5 billion for wall construction in the Department of Homeland Security’s proposed budget.
Wildlife advocates and biologists who study the Borderlands worry that filling these gaps could sever some of the last, best habitat in the region.
“The wall has a large impact on wildlife and habitats along the border,” said Eamon Harrity, wildlife program manager for the Sky Island Alliance, an Arizona-based conservation group. “The border from San Diego to Texas crosses through some extremely biodiverse areas.”
Divided lands
The geographic ranges of about 1,077 native animals straddle the border. The border bisects important habitat for 83 threatened and endangered species, including ocelots in Texas, jaguars and Sonoran pronghorn in Arizona, and Mexican gray wolves in New Mexico and Arizona. The barriers not only cut off animals from habitat but they also separate them from potential mates.
Today, the Borderlands are divided by a total of 636 miles of fence, erected by the past four administrations. Many of the newer segments, which can cost $10 million to $20 million per mile, tower up to 30 feet high and have slender four-inch gaps between the steel bollards, barely wide enough to fit a hand through horizontally. In most places, a dirt road, between 60 and 150 feet wide, is bulldozed alongside the wall to allow for border patrols. These enforcement zones are often illuminated with bright lights that can disorient birds and insects.
Several species, many of which endure drought and extreme heat intensified by climate change, are already struggling to survive. In Arizona, in a matter of decades — or sooner in some cases — “bears might be gone,” Harrity said. “Jaguars in Arizona will probably be gone. Mountain lions may be fewer and farther between. And desert tortoises along the border will be impacted,” he said. “The list of species that will be affected and potentially extirpated is quite long.”
“The list of species that will be affected and potentially extirpated is quite long.”
Waivers permitted by Congress under the Secure Fences Act and the Real ID Act of 2005 mean that DHS has not had to comply with the Endangered Species Act, the Clean Water Act, the National Environmental Policy Act, or many other statutes when building the fence. On April 8, DHS Secretary Kristi Noem issued the first waiver under Trump’s second administration to expedite construction of 2.5 miles of border fence in California’s Smuggler’s Gulch, another biodiversity hotspot, and two nearby areas.
Studies on the fence’s impacts on wildlife and ecosystems suggest that the barrier has fragmented populations, changed behavior, and cut off some animals from crucial water and food sources. One study, by Harrity and others published in the scientific journal Frontiers in Ecology and Evolution last November, found that just 9% of observed wildlife pass through a stretch of fence in Arizona.
“I have seen deer and wild turkey moving along the wall and unable to cross,” Harrity said. “When you see it in person and you see the panting and the running back and forth, looking for a way to cross and ultimately failing, you recognize that wow, this animal is trying to survive, and there’s this giant thing in its way that’s causing a lot of grief and stress.”
Openings that allow small animals to pass through are few and far between, and it’s unclear whether the current administration will include them in the new wall segments. A spokesperson for CBP declined to say whether wildlife openings or other accommodations will be part of the new projects.
Bulldozing underway in Texas
Wildlife advocates in Texas are watching the new construction with trepidation. The Lower Rio Grande Valley is home to more than 500 bird species, 300 types of butterflies, and more than 700 vertebrate species. While CBP declined to provide the exact locations of the new projects in Texas, the agency confirmed that crews are clearing land.
Scott Nichol, a longtime advocate for border wildlife and a former co-chair of the Sierra Club’s Borderlands campaign, said construction has begun in Starr County near Salineño and Roma, a little over 100 miles north of Brownsville. The Salineño tract, which is adjacent to the Lower Rio Grande Valley National Wildlife Refuge, attracts birders from around the world and is important to the local economy.
“The decimation of forest cover, use of floodlights, and restriction of access has the potential to end birding there,” Nichol said in an email. The locations also include critical habitat for federally endangered plant species, including the Zapata bladderpod and prostrate milkweed.
According to a 2024 DHS environmental stewardship plan, 17 miles of new wall in Starr County will consist of 18-foot-high segments with four-inch spacing between bollards. CBP will also construct roads in a 150-foot-wide enforcement zone, though in sensitive areas the zone will be confined to 50 feet, according to the document.
When Congress allocated $1.3 billion in funding for the wall in 2019 and almost as much in 2020, it barred construction in Bentsen State Park, the National Butterfly Center, La Lomita Mission, and Santa Ana National Wildlife Refuge after a public outcry. What’s left in this area are short stretches of unfenced border totaling seven miles — some of them within the Lower Rio Grande Valley National Wildlife Refuge, which Congress did not spare.
The refuge’s Madero Tract, part of the Lower Rio Grande wildlife corridor, is of particular concern for conservation groups. “As that corridor is repeatedly fragmented, the bits of habitat, like Madero, that become isolated are less and less likely to be able to remain viable, and their biodiversity decreases,” Nichol said.
Researchers at Texas A&M and Sul Ross Universities are studying how two species, mountain lions and black bears, are contending with existing barriers in South Texas. Their preliminary results suggest the fences are interfering with mountain lions’ normal behavior. A cat the team is tracking “either bounced away from the barrier system or traced alongside it until it could find a place to go around,” said Chloe Nouzille, a Ph.D. student at Texas A&M University-Kingsville who co-leads the small mountain lion study.
Jaguar movement at risk
In Arizona, sections of wall built during Trump’s first term cut through the Cabeza Prieta National Wildlife Preserve and Organ Pipe Cactus National Monument, as well as tribal lands. Wildlife advocates expect much of the new construction to occur on federal lands targeted under previous plans.
Harrity is particularly worried about the San Rafael Valley, a large desert grassland between the Patagonia Mountains and the Huachuca Mountains in southeastern Arizona. Male jaguars have used this corridor in recent years, but a plan to build 25 miles of new fence in the valley, unveiled in mid-April, could block their path. Harrity’s study, which looked at an area where 13 wildlife openings were punched into border walls, shows that small-to-medium-size animals like javelinas and coyotes are using the wildlife openings — a video even shows a female mountain lion and her kittens squeezing under the wall. Large animals like jaguars, however, are out of luck.
Harrity said that DHS should include wildlife openings to allow at least some animals to pass through. “They’re too small to be used by humans, but they do make a big difference for some animals,” he said. Expanding the four-inch gaps between bollards would also help, Harrity added.
Settlement in question
As workers clear land to build new walls, environmental groups also wonder about the fate of hard-won remediation measures aimed at correcting some of the harms caused by the existing fence. Under a 2023 settlement agreement with the Sierra Club and the Southern Border Communities Coalition, CBP should have added more wildlife passages and installed erosion controls, among other concessions. But little of the work has been done, said Erick Meza, the Sierra Club’s Borderlands coordinator, who is based in Tucson. Last fall, a judge in Texas issued an injunction that halted almost all of the projects. “Right now, all of those funds are frozen,” he said. “So they cannot do a lot of that work.”
Sierra Club attorney Gloria D. Smith said the settlement itself is still in effect, but “any mitigation or non-construction-related work is prohibited.” The case is now before the Fifth Circuit Court, which she described as “very conservative.”
As more native vegetation is cleared for new steel fences, Harrity worries not just about the wildlife and plants at risk but also how a blockaded border will affect the people who live alongside it.
“Sadly, I think the true cost of these walls will be felt by generations to come,” Harrity said. “The continent has never seen a barrier of this magnitude in ecological history.”
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April Reese is an American freelance science writer and editor based in Aveiro, Portugal. She lived in Santa Fe, New Mexico, for 17 years. Her reporting from the American Southwest, Portugal, and around the world has appeared in Scientific American, Discover, bioGraphic, Science, Nature, New Scientist, and other outlets. She holds a master’s degree in Environmental Studies from the Yale School of the Environment.
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