NatGeo: 6 ways the border wall could disrupt the environment.

The reason to build a wall is to keep people out. Yet history is replete with examples of walls all over the globe that rarely deterred determined people from getting in. Janet Napolitano, who served as governor of Arizona and President Barack Obama’s secretary of Homeland Security, was famous for her oft-repeated declaration: “Show me a 50-foot wall, and I’ll show you a 51-foot ladder.” The wisdom of building a wall across the length of the U.S.-Mexico border at a time when the number of arrests of illegal border crossers is at a 45-year low is an issue for the debate over immigration law.

What follows here instead is a look at the implications of wall construction itself—beyond talk over concrete-or-slats—and the unintended consequences that erecting such a barrier could pose. (See what existing parts of the wall look like.)

“Whatever they build, it’s going to be destructive to natural habitat,” says Bob Dreher, an attorney who heads Defenders of Wildlife.

"It’s about the physical reality of what a permanent barrier will do in one of the most sensitive landscapes in North America.”