From the window of a small plane, Christina Aiello marveled at the chain of rugged mountains below her, in the Jacumba Wilderness, that extends far into Baja California, Mexico.
“Before we fragmented and developed this habitat, that mountain range was one continuous range of habitat for species like bighorn sheep, mountain lion and others,” said Aiello, a wildlife biologist with the Wildlands Network.
The Peninsular Ranges are interrupted by the steel California-Mexico border and Interstate 8.
Animals have managed to cross between both countries in areas where gaps in the border wall have existed. Miles of steel plates along the California-Mexico border come to a sudden halt where high-altitude, desert terrain serves as a natural barrier.
But that access began to shrink late last year.
U.S. Customs and Border Protection, or CBP, added large coils of concertina wire along the near-vertical slopes. It’s part of the federal government’s larger plan to close the last remaining openings in the border wall, including in San Diego and Imperial counties, with “30-foot-high, 6-inch-squared diameter steel bollards” and a secondary barrier wherever possible, as well as surveillance cameras and LED lighting.
Until then, scientists and conservationists from both countries said the wire fencing is already putting wildlife at risk of injury and cutting their home range in half. That’s why they’re asking CBP to remove the concertina wire and create wildlife passages akin to those working in other states.
Without these measures in place, they said, animals like the endangered bighorn sheep will be forced to travel far outside their range in search of food and water, where they’ll eventually reach highways.
“They'll hit that wall, the border wall, and they won't be able to go anywhere,” said Fraser Shilling, director of the Road Ecology Center at the University of California, Davis. “Maybe they'll go back the other way … to the highway, and then they'll just get hit on the highway. It becomes this weird battle between fences, the border wall and the highway wall.”
In an emailed statement, a CBP spokesperson confirmed that it is working to identify locations for small wildlife openings “within existing and new border wall construction.”
“We appreciate and factor all concerns with the placement of infrastructure, including the placement of concertina wire,” the spokesperson added. “However, national security will always take priority, and infrastructure will be placed where it is deemed operationally necessary.”
In recent years, the Jacumba Wilderness became a hot spot for migrant border crossings. In parts where the wall ended, several hundred people would cross into the U.S. on any given day, weathering the desert’s unforgiving temperatures in makeshift camps before turning themselves in to Border Patrol.
While migrant crossings have dropped significantly over the past year in the San Diego and Imperial region, the Trump administration has been waiving federal environmental regulations to expedite new construction of the border wall.
“I think asking for no new border barrier is unrealistic, and we've kind of accepted that,” said Aiello. “But I think that doesn't mean we totally give up the effort for making things better for wildlife. I think we just adjust what we ask for, and that is going to involve compromise.”
Wildlife openings
Unlike sections of the border wall in Arizona, New Mexico and Texas, there are no openings specifically designed for wildlife to cross the California-Mexico border, according to scientists with the Wildlands Network. They’ve spent the past few years monitoring animal migrations in those states.
Now, they’re leading an effort to include California.
Over the past three months, the organization and several other conservationists have proposed to CBP options they believe could prevent an impassable barrier to animals that depend on resources from both countries.
Simply put, animals that live on the border have their food and breeding grounds in the U.S. and several sources of water are in Mexico.
Janene Colby is a retired wildlife biologist who frequently hikes the remote area. She has a ready anecdote about what splitting the species’ home range in half means.
“If you think of a community that you might live in and … somebody just decides one day they're going to build a fence right down the middle,” she said. “And now you can no longer get to the grocery store, or you can't get to the hospital to have your kids. It's sort of the same thing.”
In a January letter to CBP, the Wildlands Network proposed removal of the concertina wire within the Jacumba Wilderness, which is considered the main lambing ground for the bighorn ewe that live there.
Aiello said biologists have put collars on some of the sheep and are tracking “for signs of them being caught in the fence because, right now, Customs and Border Protection are telling us concertina wire is not a threat to wildlife.”
But, she added, wildlife does not recognize the wire fencing as an impassable barrier. They may attempt to cross it and risk getting tangled up and injured.
Similar calls to preserve open space areas from new border barriers are unfolding all across the border region, including most recently in Texas, where construction is threatening a nature reserve. And it’s led to at least one local lawsuit. In January, the city of San Diego filed a lawsuit against the federal government for allegedly trespassing on city property and harming protected wildlife habitats in the Marron Valley after installing concertina wire.
The Network also suggested installing small openings the size of a sheet of paper and larger ones that could accommodate the bighorn sheep in as many panels as possible, arguing that without them, “some species seem unable or less willing to cross the border.”
Footage from camera monitoring the organization has done in neighboring border states between 2022 and 2024 shows that small wildlife passages helped facilitate more crossings at the border wall for animals like coyotes, bobcats and javelinas. While they have led to some successful migrations, most animals that attempt to cross have failed, the organization found.
“Notably, no humans have been observed using such structures during three years of ongoing monitoring that totals over 6,000 camera-days at wildlife passages,” the Network said in its letter.
Aiello said the organization’s findings underscore the need for more frequent and larger openings for bighorn sheep, black bears and wild turkeys, among others.
While CBP has agreed to look into installing them, officials have not publicly stated where and how many they may add along the California-Mexico section, and it has not agreed to bigger openings. Instead, large animals would have to use new floodgates if they are open.
Aiello worries the option is incompatible with how some species migrate. She hopes CBP will be open to other ideas soon, before more wildlife populations head north toward I-8, which is already a hot spot for roadkill.
‘They live right there”
The small plane starts to make a wide turn away from the border wall and toward the snaky I-8.
Aiello points to a section where the east and westbound lanes split, isolating a critical habitat for a subgroup of bighorn sheep that roam there. She’s trekked these parts before and has seen up close paths the species have made.
“These animals actually form multigenerational knowledge where they travel the same paths,” she said. “The mothers share these movement paths with their lambs. You will see these worn paths into the hillsides that last for decades because they've traveled them so often. They're pretty much etched into these landscapes.”
Depending on the season, ewes move across the highway to access food, water and have their lambs.
That migration has proved deadly for the species and drivers.
“That’s the thing about the bighorn sheep,” said Shilling with the Road Ecology Center. “They live right there. If they have to go back and forth, they’re going to get hit.”
According to the Road Ecology Center, more than 30 sheep have been killed by vehicles on the interstate since 2012, or about two to three every year. In California, more than 200,000 roadkill were reported between 2009 and 2023, a report by the center found. Shilling said that’s likely a big undercount.
“An animal the size of the bighorn sheep, when it gets hit, only about a 40% of the time will it stay visible on the side of the highway,” he said. “And, so, part of the time, it's going to an injured part of the time will get knocked off the edge and is not visible.”
UC Davis, as well as several other state and nonprofit agencies, are working to build a wildlife overpass to reduce animal deaths. But with new barriers sealing gaps on the border wall, scientists and conservationists said there’s now a greater urgency to complete projects like this.
Using $6 million in state grant funding, the university is designing the overpass and areas to fence along the highway. They plan to have it ready by 2027.
“The concept is that if you fence between crossing areas, whether they go right or left, they'll eventually find a way across and you hope that that's what happens,” said Shilling.
He said the need for fencing and an overpass is becoming far more important with the growth in international trade and travel in that region.
Caltrans data show traffic there increased from about 14,000 vehicles a day in 2018 to about 22,000 in 2023.
The wildlife crossing project still needs at least $30 million for construction costs.
