Nature knows no borders. Border security can take a heavy toll on endangered wildlife

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• JUDY WOODRUFF:

  President Trump has long said he wants to build a hardened wall across the U.S.-Mexico border to reduce the flow of drugs and illegal immigration.

  But what would a continuous wall from California to Texas mean for wildlife in the area?

  William Brangham is back again with a report from Arizona. It is part of our weekly series examining the leading edge of science.

• WILLIAM BRANGHAM:

  Tens of thousands of migrants risk their lives each year crossing this dangerous, remote desert between the U.S. and Mexico.

  Some have found creative ways to get over or around the steel fences built to keep them out. But many of the wild plants and animals here in the Sonoran Desert can't do that. These miles of fencing divide their natural habitat and threaten their survival.

  At least 50 species near the border are already endangered, like the Sonoran pronghorn, the gray wolf and the ocelot.

• SERGIO AVILA-VILLEGAS, Arizona-Sonora Desert Museum:

  Nature has no borders. Nature knows no political boundaries.

• WILLIAM BRANGHAM:

  Sergio Avila-Villegas is a wildlife biologist with the Arizona-Sonora Desert Museum in Tucson. He's spent the last 16 years studying this area.

  He took us to the Coronado National Memorial in the Huachuca Mountains on the Arizona-Mexico border. He says the border fence — you can see it there in the distance — means animals have to range farther afield to find food, water and mates.

  There's this one, what seems to my eye a very thin fence that runs across the border, but you're saying this really does have a major impact on the species that live here?

• SERGIO AVILA-VILLEGAS:

  From a sparrow perspective, from a grasshopper perspective, and from even plant perspective, this is a very difficult thing to overcome, and this blocks reproduction of plants and animals.

• WILLIAM BRANGHAM:

  In one study from 2011, biologists found border fences increased the risk of population decline and extinction, especially for endangered species.

  Another study from the same year found border security infrastructure could interfere with black bear breeding. Before the border fence between the U.S. and Mexico went up about 10 years ago, conservationists tried to stop it, but ultimately lost that fight.

  Eighty percent of Arizona's border with Mexico has some kind of barrier. Gaps do occasionally exist where wildlife can pass, but finding those places isn't easy.

• PRESIDENT DONALD TRUMP:

  I would build a great wall.

• WILLIAM BRANGHAM:

  Now that President Trump plans to build what may be a continuous wall from California to Texas, Avila-Villegas and other scientists worry this will only accelerate the extinction of some animals.

• SERGIO AVILA-VILLEGAS:

  Long-term, this could be a division of genetical populations, where a group of animals from one side cannot reproduce with another group of animals, breaking the connectivity, creating some genetical problems. The survival of the species is at risk.

• WILLIAM BRANGHAM:

  Species like this jaguar, which was spotted several times in Arizona over the last few years.

  Historically, these jaguars roamed from the Southwest United States down through the Amazon Basin, all the way to Argentina. Scientists estimate this big cat now occupies less than half of its original range because of habitat loss and poaching. There may be fewer than 50,000 breeding adults left.

• SERGIO AVILA-VILLEGAS:

  If there's an impermeable barrier, we are losing the opportunity to have the third largest cat in the world to recover its populations in the United States. Jaguars deserve an opportunity to live in this place too.

• WILLIAM BRANGHAM:

  The Coronado National Memorial is considered a critical habitat zone, meaning it would normally be protected by the Endangered Species Act.

  But Congress wanted the fence built quickly. In 2005, it allowed the Department of Homeland Security to bypass all environmental laws during construction, including the requirement to study what this fence would do to wildlife.

  Congressman Rob Bishop, Republican from Utah, thinks those exemptions aren't enough. He's introduced legislation that would extend those legal waivers to Border Patrol agents, who do have to obey environmental laws.

• REP. ROB BISHOP, R-Utah:

  So, everything from California to Texas is almost all federal property, and over half of that is in a wilderness designation, which has specific requirements for what can and cannot be done.

  That's where the Border Patrol is prohibited from doing their job, and that's the mistake, because that becomes the avenue for most of the illegal entrants into this country. And I think it has a direct correlation to the amount of federal land and the amount of restrictions the Border Patrol has on how they can do their jobs on that corridor.

• WILLIAM BRANGHAM:

  Border Patrol can't build bases, towers, or roads without permission from other federal agencies, like the Fish and Wildlife Service. They're also not allowed to drive over protected lands, a rule smugglers don't have to follow.

• CYNDI TUELL, Environmental Lawyer:

  We have one of the largest wilderness areas in a national wildlife refuge in Southern Arizona, and Border Patrol has been driving over it for close to a decade.

• WILLIAM BRANGHAM:

  Cyndi Tuell is an environmental lawyer and border lands conservation advocate who says she's seen Border Patrol in places they're not supposed to be.

• CYNDI TUELL:

  The main focus of my work as a conservation attorney is to try to get Border Patrol, Department of Homeland Security to follow the law. If they're one of the largest law enforcement agencies in the country, they very much need to be complying with the law themselves.

• WILLIAM BRANGHAM:

  She spends part of her time travelling the backcountry, looking for signs of Border Patrol's impact on public lands. She says their vehicles can crush vegetation and erode soil, and that habitat is destroyed when they build bases and surveillance towers.

• CYNDI TUELL:

  Every time I'm out here, though, I interact with a Border Patrol agent, or I see the signs of militarization. I see tanks. I see heavily armed men. I see vehicle tracks two or three miles into a wilderness area that I know shouldn't be there.

  GIL KERLIKOWSKE, Former Commissioner, U.S. Customs and Border Protection: Customs and Border Protection has to be able to be on the borders, whether it's in environmentally sensitive land or not.

• WILLIAM BRANGHAM:

  Gil Kerlikowske served as the commissioner for customs and border protection from 2014 until the new administration took over. He's now a fellow at Harvard University.

• GIL KERLIKOWSKE:

  I think it's very difficult for those agents and their vehicles and other types of equipment to be in an area and not to leave some type of footprint.

  But I would mention that, as often as hard as they work to try and reduce their environmental impact, I think there's always going to be, as a result of that human intervention, some type of an effect on the environment.

• SERGIO AVILA-VILLEGAS:

  The infrastructure seems to be a one-solution-fits-all for many different problems, and I don't think it's addressing the root causes. And I really think that the environment is paying the ultimate price for this.

• WILLIAM BRANGHAM:

  Have you and the conservation community made those concerns known to the Border Patrol and the Customs and Border Protection?

• SERGIO AVILA-VILLEGAS:

  A lot of our concerns have been voiced through the conservation and science community to the Department of Homeland Security. They don't go anywhere. The Border Patrol and the Department of Homeland Security have no mandate to listen to these concerns.

• WILLIAM BRANGHAM:

  We asked the Department of Homeland Security if they have required any passageways for wildlife in the solicitations they have put out to contractors who want to build the new wall.

  They have told us that the current plans — quote — "will not result in significant environmental impacts. As a result, for this particular project, DHS is not planning for mitigation."

  President Trump ordered 5,000 more agents be hired and deployed along the border, and plans for his wall are now being drafted. The few wildlife corridors that do remain could soon be closed for good.

  From the Sonoran Desert in Arizona, I'm William Brangham for the PBS NewsHour.