Bears and humans have been in conflict in Texas since, well, pretty much as long as we’ve kept track. By the time the U.S. Department of Agriculture conducted the first organized survey of Texas mammals in the 1890s, black bears were in decline after having been persecuted for more than half a century. They were trapped, poisoned, hunted, and pushed out of their habitat by development and agriculture. By the mid-20th century, they were nearly extirpated throughout West Texas.

They weren’t doing much better in Mexico, where they were also being killed to protect people, property, and livestock, and hunted for food, hides, and sport. But ranchers in northern Mexico were generally more tolerant of bears. (Some even viewed a resident bear as a status symbol.) Thanks to that tolerance, relatively lower hunting pressure, and the conservation efforts of the Mexican government and private landowners in the 1970s and 1980s, viable populations of bears remained in the states of Chihuahua and Coahuila, and bear numbers in northern Coahuila, around Sierra del Burro, were some of the highest in North America in the late 20th century. Some of those bears wandered north, crossed the border into the U.S. and recolonized areas in West Texas. In the late 1980s, some of them arrived in Big Bend National Park, where bears had not been for 40 years. Within a decade, a breeding population was reestablished, and their numbers climbed from two bears in 1988 to 29 animals in 2000.

There are still black bears in West Texas today, but as we try to control human migration at the border, we may be damning the species whose scientific name is Ursus americanus to isolation, extirpation, and extinction. When we talk about the border and securing it, the conversation almost always excludes wildlife, even though animals have lived along and across international borders well before the concept of borders or even nations existed. The U.S. Fish and Wildlife Service predicts that a solid barrier running along the entire U.S.-Mexico land border, like the “great, great wall” that Donald Trump wants to build, would affect 111 endangered species, 108 migratory bird species, and four wildlife refuges and fish hatcheries. That would be an ecological disaster, but the barriers that are currently in place—a piecemeal system of 600+ miles of fences, walls, and vehicle barriers that cover about one-third of the border—are already problematic.

By militarizing the border—building walls and fences, Border Patrol outposts, spotlights and service roads and clearing vegetation—we sever the connections that wildlife relies on. We cut animals off from habitats, resources, and breeding partners and block migration routes, just as climate change and habitat loss makes every bit of land ever more important. We know how bad this kind of fragmentation is ecologically—scientists have seen the consequences of constructed barriers on farms, ranches, and national parks, as well as across international borders in Europe and Central Asia. They also know that once the barriers are built, it becomes even harder for conservationists to work to protect animals just when they need it the most.

In 2011, biologist Jesse Lasky conducted a risk analysis for species along the length of the U.S.-Mexico border and identified that the current border blocks put 49 species at risk of connectivity and population loss, including four listed as threatened by the International Union for Conservation of Nature or by both the U.S. and Mexico. In 2014, another study compared the activity and movement of select species in areas along the border with barriers to those without. It found that some animals were much more active on the barrier-free stretches, most likely because the populations in the fenced areas had collapsed. Even birds can’t escape the problem: Some simply can’t fly high enough—take the ferruginous pygmy-owl, which usually moves just 4.5 feet above the ground and depends on interbreeding between between populations in the Mexican state of Sonora and Arizona to persist. Trump’s great wall also promises to threaten the bald eagle, as the wall construction would destroy critical habitat.

In order to get a better sense of the problem, we can look to Europe, where there are roughly 18,000 miles of border fences and walls, many hundreds of which have gone up just since the start of the refugee crisis last year. In Slovenia, for example, new security fences erected late last year cut through range of one of Europe’s largest populations of gray wolves, isolating packs on either side of the Slovenia-Croatia border. The same fence goes through the habitat of a small, threatened population of Eurasian lynx, and a recent study led by ecologist John Linnell argues it may be “last push for the population to spiral down the extinction vortex.”

Europe should know better. Over the last few decades, many Cold War–era border fences have been removed, habitat connections between different countries and regions have been restored, and international and transboundary cooperation on conservation projects has blossomed. All of that has led to an amazing recovery of large carnivore and herbivore populations. Linnell’s team, an international group from all over Eurasia, worries that those gains may quickly be undone as the continent fortifies itself.

Why do animals need these connections? Imagine a landscape as a jumble of Christmas lights. Each light represents a habitat. The wires that connect them are corridors that animals move through, says Aaron Flesch, an ecologist who studies wildlife in the U.S.-Mexico border region. When a light fizzles out because the animals there have been lost, others move along the connecting lines to refill the habitat and relight the bulb. By constructing fences, we’re cutting the cords that run between those Christmas lights, so when they blink out, there’s fewer connections from which they can be relit. When enough of those lights go dim at the same time, Flesch says, the entire network gets destabilized and wildlife populations go extinct.

And border fences and walls don’t just affect wildlife. They also affect the scientists who study them. Smaller, isolated populations may not be sustainable and require constant maintenance from scientists to survive. Conservationists might need to supplement the resources that animals have lost access to and secure new habitat and corridors to move along or provide sources of food and water. In some cases, they’d have to ferry animals across borders to replenish populations or let them interbreed. That kind of transboundary conservation is more difficult with barriers in place. Linnell’s study notes that such opportunities are already shriveling up. Even the metaphorical wall thrown up by the Brexit vote hampers international cooperation and collaboration on research.

It’s possible to make barriers less problematic for animals. You can build wildlife crossing structures or gaps for smaller critters into the fences or temporarily take some sections down during migration or breeding seasons. But policy makers don’t necessarily have to hear any of these ideas out. Legislation exempts border barrier construction from environmental impact assessments and allows the Department of Homeland Security to waive compliance with dozens of federal laws like the National Environmental Policy Act and the Endangered Species Act. In the meantime, there’s one surprising upshot: Flesch says his conversations with researchers in both the U.S. and Mexico has led him to believe that, if anything, the talk of the wall has galvanized proactive collaborations now that wouldn’t have happened otherwise.

That’s a promising sign. But while the barriers that are up now and the ones we’ll almost certainly build in the future are bad for wildlife and for conservation efforts, they also highlight a bigger problem. The gravest problems we as a global society are about to face are equally global in nature—pollution, extinctions, and climate change. Barriers only make tackling them more difficult. In the jumbled network of Christmas lights, cutting the wires just makes it easier and easier for the lights to go out entirely.