The rapid construction of border fences across Europe is aimed at stemming the influx of refugees, affecting millions of lives. But those fences are leading to a crisis for an unintended target—wildlife.

A new study evaluates the impact of the 15,000 to 19,000 miles of fences in place between Western Europe and Central Asia as well as the consequences from hundreds of miles of additional border fencing erected since 2015. That construction was done hastily, without assessing the threats the barriers could pose to gray wolves, brown bears, Eurasian lynx, and other native species, according to the researchers.

Immediate threats include entanglement in razor- and barbed-wire fencing. The barriers can also halt seasonal migration and limit species’ ability to move into new territory in response to climate change. Over time, fences can lead to population fragmentation and inbreeding, the study found.

John Linnell, a senior research scientist at the Norwegian Institute for Nature Research and lead author of the paper, which was published in the journal PLOS Biology, said the research was prompted by the construction of fencing in 2015 along the 450-mile border between Slovenia and Croatia, a region he and his colleagues knew to be a large continuous block of habitat vital to many species for migration through the Balkan states.

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For instance, of the 10 wolf packs living in Slovenia, five spend time roaming both sides of the border with Croatia to hunt and den. If the corridor is shut off, the wolves may struggle to survive in their smaller range, the researchers said.

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“We then started asking how widespread this issue was—and then we started digging and discovered that it is in fact very widespread,” Linnell said. “The [previous] geopolitical literature had summarized some of this information, but they had focused more on what the fences mean, why they are constructed, and what it says about the state of global politics. They had never really focused on what type of fence, how long it was, where it was actually built, and what impacts they had on wildlife.”

Linnell said fences aren’t inherently bad for wildlife—as wildlife corridors can be integrated into designs—but barriers built for the sole purpose of stopping people will inhibit large animal dispersal as well.

In Mongolia, for instance, more than 5,300 Mongolian gazelles died last year along the Trans-Mongolian railway, a train route from China to Russia. The railway is lined on both sides by barbed-wire fencing that cuts off the 600-mile migration route the animals use to access greener pastures each season. Extreme winter weather combined with the fences resulted in thousands of gazelles freezing or starving. Others were hit by trains or became entangled in the fencing.

Linnell says the study is a warning call as Estonia, Latvia, and Lithuania fence borders with Russia and Belarus.

“There is a clear trend to a greater degree of focus on national interest and a reduction of commitment to global processes—ironically and sadly at a time when there has never been more of a need for large-scale cooperation to address global issues,” Linnell said. “This does not only concern wildlife but politics in general.”

With the end of the Cold War in the early ’90s, fences came down as border restrictions eased, and international cooperation on environmental issues helped the recovery of wolves and other large animals.

But after 9/11, fences appeared again.

On the border between the United States and Mexico, more than 600 miles of barriers have been built since 2006 under legislation that allowed the building of fences with minimal environmental review.

According to a 2011 study, the fences pose significant risk to hundreds of species, among them five critically endangered animals with habitat within 30 miles of the border, including the jaguarundi—a small feline species—the Pacific pond turtle, and the California red-legged frog.

“When you’ve got these small populations of animals already, and you squeeze them out of a large part of their habitat, it makes it that much harder for these species to survive,” said Jesse Lasky, an assistant professor of biology at Penn State University and lead author of the study. “If a single lake gets a disease and these amphibians and reptiles in this dry region can’t disperse, you can wipe out whole subpopulations at once.”

Presumptive Republican presidential nominee Donald Trump has proposed completely closing off the 1,989-mile-long land border between the U.S. and Mexico with a 30- to 50-foot-high concrete barrier.

An analysis published in Outside magazine found such a project could harm 111 endangered species, 108 species of migratory birds, and four wildlife refuges.

“Any impacts we studied in 2011 would be substantially worse under Trump’s plan,” Lasky said. “A lot of tropical animals end up migrating to the northern parts of Mexico, and as suitable habitat opens up north of the border, a lot of those animals could get left on the wrong side of the wall.”