When Brendan Lenihan was in the Border Patrol between 2006 and 2011, he spent a lot of time in a mountainous basin and range landscape that envelops borderland communities in southern Arizona. On both sides of the border, twisted mounds of creosote and tall saguaro cactus dominate the flat, dusty expanses that stretch between the jagged lines of mountains that jut up dramatically against the sky. It’s dry, hot, and drab, and seemingly everything is covered in thorns. But the desert is always good for a surprise, like the bright green slashes of willow and cottonwood-lined creeks that cut across the landscape, or the amber pockets of grassland meadows that yield firework displays of orange and yellow wildflower blooms in the spring.

“I just stepped over the wires laying on the ground, and I was in Mexico.”

Once, while Lenihan was out on a patrol near Sasabe, Arizona, a tiny town next to the Buenos Aires National Wildlife Refuge, “I walked across a four- or five-strand cattle fence that I thought was just from some ranch in the US,” he said. The fence was knocked down, as Lenihan recalled, “so I just kind of stepped over the wires laying on the ground, and I was in Mexico.” He didn’t realize it until his faulty GPS device rebooted, and he stepped back over the line.

Unlike in South Texas, where the international border follows the Rio Grande River, there’s no natural feature along Arizona’s 370-mile southern edge to define where the US ends and Mexico begins. Pedestrian and vehicle fencing built during the border-construction boom of the mid-2000s cuts through the scrub along some stretches. Currently, about 700 miles of the 2,000-mile-long border is marked with some kind of barrier.

That may change. With the executive order signed on January 25th, President Donald Trump took the first steps toward making good on his oft-repeated campaign promise to build a wall across the entire 2,000-mile-long border. The Department of Homeland Security will first focus on about 100 miles of “high priority” stretches of the border that are near cities, including areas south of Tucson.

Lenihan’s accidental crossing into Mexico is understandable: The land on either side of the border is the Sonoran Desert. A biodiversity hot spot, the desert is indifferent to human divisions. So Trump’s proposed wall will cut across numerous mountain ranges, arroyos, rivers, and communities, including the Tohono O’odham Nation, which abuts 62 miles of US-Mexico border, and includes about 2,000 tribal members who live in Mexico.

Trump’s proposed wall will cut across numerous mountain ranges, arroyos, rivers, and communities

All told, public and tribal lands comprise nearly 80 percent of the state’s border, and there are 4.3 million acres of designated wilderness in this part of the state. The area is home to a host of endangered species, from the handful of American jaguars living at the far northern end of the big cat’s range, to the diminutive cactus ferruginous pygmy owl. But even public land and animals protected by the federal government are not shielded from the wall.

In April, the Center for Biological Diversity and Arizona Congressman Raúl M. Grijalva, a Democrat, sued the Trump administration over the executive order. The suit argues that DHS hasn’t conducted the requisite environmental analysis for its enforcement activities required under the National Environmental Policy Act. But unlike other federal lawsuits brought against the administration over the president’s executive orders, like the travel ban, the CBD case is likely to fail.

Thanks to a 2005 law, the DHS has the authority to waive any and all laws, except for the Constitution, in the name of border barrier construction. The Trump administration can disturb human remains, desecrate sacred sites, destroy wildlife habitat, foul air and water, and bring new development to the second largest tract of wilderness in the lower 48 — and all with near complete impunity.

Before he joined the Border Patrol, Lenihan started his professional life in what is essentially the family business: the National Park Service. His archaeologist father, Daniel Lenihan, started NPS’s Submerged Resources Center, leading teams of scuba divers who map shipwrecks in national parks around the country; his mother also worked for NPS in Santa Fe before she had her two sons.

Lenihan grew up in Santa Fe, New Mexico, but the family spent time bouncing around from park to park, staying in rangers’ houses or cabins around the country, playing with the kids of other Parks Service families. Later, both brothers spent time on their dad’s team, and Lenihan dove to the sunken hull of the USS Arizona in Pearl Harbor, and the wreckage of one of the ferries that carried immigrants to and from Ellis Island. During the spring and summer of 2006, he served as a law enforcement ranger at Lassen Volcanic National Park in Northern California, doing search and rescue operations in the backcountry and making sure no one was growing weed on public land.

“Illegal immigration fascinated me.”

Lenihan saw a recruiting ad for the Border Patrol, which went on a hiring spree in the mid-2000s. Between growing up in New Mexico and a love of Cormac McCarthy novels, Lenihan knew the draw of the desert, and he thought that serving in CBP would be a great adventure.

“Illegal immigration fascinated me,” Lenihan said, “and I wanted to know what was really going on in the deserts of Arizona and New Mexico and Texas. I wanted to see it for myself.”

Being out on a patrol in the mountainous desert — where you might stumble upon massive golden eagles nesting in the craggy peaks, and herds of hog-like javelina regularly come rushing out from the scrub — “can be very quiet and very desolate, and it was a very nice place to commune with nature,” Lenihan said. But there’s no mistaking the Sonora of southern Arizona with the desert national parks Lenihan grew up visiting. Border Patrol has carved these public lands with 12,000 miles of illegal off-road tracks. (The roads are not permitted and located in wilderness areas, where development is restricted.) Both smugglers and migrants leave more than a trace, too, with an estimated 2,000 tons of trash pulled out of the parks every year. Looking out at the rugged mountains, Lenihan said you can sometimes see bits of broken glass shimmer like tinsel in the harsh light.

“We’ve always had a lot of appreciation for America's best idea,” said Lenihan, who just finished his last semester of law school at the University of Arizona. He strongly believes in protecting public lands for future generations to enjoy. But when it comes to the border, his Parks Service and Border Patrol work has left Lenihan torn between their philosophies of conservation and enforcement.

“The border wall is fundamentally a human rights issue.”

Once, he was walking down a trail about 15 miles north of the border when he came across the starkest reminder of the harsh environment and the risk migrants take trying to cross it: a human skull, with the rest of the body sitting under a mesquite tree nearby. “I was just enjoying myself, and I was kicking rocks, and I almost went up to kick it — I thought it was just a white stone,” he said. Last year, there were 170 migrant deaths in the Arizona desert, according to Humane Borders. Many of the deceased are unidentified, and only skeletal remains were found.

“The border wall is fundamentally a human rights issue — it restricts the free flow of people,” said Tucson resident Mike Wilson, a Tohono O’odham tribal member. What makes the ecology of the desert so singular, with its host of plants and animals adapted to the heat and drought, can make that restriction lethal for humans. For nearly a decade, Wilson put water out for people trekking north from the border through the Baboquivari Valley — one of the deadliest areas in southern Arizona. Despite the risk, the route is frequented by migrants; there’s even a Mexican bottled water brand named for it. The label shows the craggy silhouette of the nearly 8,000-foot Baboquivari peak jutting up above the surrounding range, the granite mass a beacon guiding migrants toward the north.

The north–south valley, which runs along the reservation boundary and marks the western edge of the Tucson CBP’s patrol area, has its own pockets of natural water, where creeks run under the bent boughs of sycamores and live oaks. The cool shade of places like Brown Canyon, on the east side of the mountain range, are a respite for desert wildlife, attracting big cats like mountain lions and even the occasional jaguar. Both species range widely, and are among the many animals that regularly cross back and forth across the border — behavior that Trump’s wall would end. All told, border-wall construction threatens the habitat of 111 endangered species, according to a survey conducted by Outside last year.

Border-wall construction threatens the habitat of 111 endangered species

“We don't even really have the science to make sure what the overall impacts are from a biological perspective,” says Randy Serraglio, a southwest conservation advocate at the Center for Biological Diversity. One study found that the endangered cactus ferruginous pygmy owl — which bears a striking resemblance to The Sword in the Stone’s fussy cartoon owl, Archimedes — won’t fly up over the 30-foot wall. (The owl rarely flies higher than 4.5 feet.)

The wall also adds to the existing divisions of the landscape — which can trap animals in areas with little food or few mates. Mexico’s Highway 2, which runs just south of the border, already presents a significant barrier to Sonoran pronghorn, a diminutive antelope subspecies. The pronghorn were nearly wiped out a decade ago, and were preserved through a captive breeding program. The wall, while not as deadly for them as the road, will further isolate the Arizona herd from animals south of the border.

If the wall proves disastrous for native species, it’s worth remembering there are other ways of looking at the border. “The border should be treated as a zone to manage and steward — not seal,” Lenihan wrote in a legal paper published last year in The Arizona Journal of Environmental Law and Policy. “A well-managed, low-risk environment on the border is a realistic and respectable objective.” Managing the border differently would make it possible to bring some of the Park Service mentality to the area: “That goal leaves room for another American value that defines this country — the stewardship and

preservation of our nation’s most precious natural treasures.”

CBP can’t help but leave a trace out in the desert

“I think a lot of these land-management agencies understand that the border patrol isn't intentionally trying to damage the natural environment,” Lenihan said, while acknowledging that CBP can’t help but leave a trace out in the desert. So for him, it’s about figuring ways to limit that impact — and the best way to do so, in Lenihan’s opinion, is through staffing and technology, not a wall.

Environmental and human rights organizations agree that a wall will only exacerbate the border’s problems, but they see an increased technological footprint along the border — a “virtual fence” approach built around surveillance towers, remote sensors, and other monitoring systems — as part of the larger problem of militarization. “People are part of the environment, so human rights problems on the border and environmental problems on the border have the same source,” said Dan Millis, the Sierra Club’s borderlands program coordinator: militarization, wall construction, and border patrol operations that are not sensitive to human rights or the environment.

If jaguars and bighorn sheep and mountain lions and pronghorn likely won’t cross the wall, it’s clear to Lenihan who will. “The people who I have met, who I have arrested, when I see the willpower that they have to want to reunite with family members, or when I see the willpower to find a job or to escape something horrible in their home country — that kind of willpower isn't going to be…” Lenihan paused. “A wall isn't going to stop that.”

Correction: The original version of this story mis-identified Mike Wilson, the Tohono O’odham tribal member. His first name is, indeed, Mike. We regret the error.