People who live and work along the southern border of the U.S. will tell you that the farther away the political debate occurs from the border, the more detached it becomes from the reality of life in the borderlands. That long-held view has been confirmed yet again as the wrangling in Washington over President Donald Trump’s demand for $5.7 billion to build a wall along the border devolves into an argument over whether such a wall should be concrete slab or steel slats.

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

Here’s a look at some of these potential impacts.

1. Threatening diverse landscapes

The border stretches for 1,954 miles from the Gulf of Mexico in Texas to the Pacific Ocean in California over one of the nation’s most diverse landscapes. It includes six separate eco-regions, ranging from desert scrub to forest woodlands to wetland marshes, both freshwater and salt.

Construction of a border wall will bisect the geographic range of 1,506 native animals and plants, including 62 species that are listed as critically endangered. A team of conservation experts, including Edward O. Wilson, famed biologist and naturalist, argued in a paper published last July in Bioscience that a border wall puts these habitats at risk. A wall increases soil erosion. It will alter natural water flows and the patterns of wildfire, exacerbating the risks of both to people and animals by trapping their escape.

2. Exacerbating flooding

Flooding disasters occurred in Arizona after 700 miles of fencing was constructed during the George W. Bush administration. The barriers acted as dams during rainy season flash floods. In 2008, at the Organ Pipe Cactus National Monument in southwest Arizona, a five-mile-long segment of 15-foot-high wire mesh fence trapped debris flowing through a natural wash during a 90-minute summer thunderstorm, causing water to pool two-to-seven feet high.

The same storm sent torrents into the city of Nogales, Arizona, a border town 66 miles south of Tucson, causing millions of dollars in property damage in Nogales, Sonora on the Mexican side. In 2011, another deluge at Organ Pipe knocked over a segment of fence, and in 2014, the twin cities of Nogales flooded again after border barriers clogged with debris during a rainstorm.

3. Perils to wildlife and plants

The border wall could disconnect a third of 346 native wildlife species from 50 percent or more of their range that lies south of the border, the Bioscience paper concluded. That raises the risk to their survival by shrinking and isolating animal populations and limiting their ability to roam for food, water, and mates. Fencing also traps wildlife from escaping fires, floods, or heat waves. Even the pygmy owl is at risk, because when flying, it’s range is less than five feet off the ground.

Border fencing disrupts seasonal migration, affecting access to water and birthing sites for Peninsular bighorn sheep that roam between California and Mexico. The inability to cross the border has fragmented populations of Sonoran pronghorn, and diminished the chances of re-establishing colonies of the Mexican gray wolf, jaguars, and ocelots in their range in the United States. Jaguars once roamed the banks of the Rio Grande, but have virtually disappeared from Texas.

Limits on migration, in turn, affects plants. The seeds of mesquite trees germinate best after they have passed through the digestive systems of javelinas and coyotes, according to a report by Defenders of Wildlife. (See panoramas of the borderlands.)

4. Dividing a river

The meandering Rio Grande, the official border between the United States and Mexico, was long believed to be a geologic obstacle to construction of a border fence. The river channel shifts course from time to time and floods in the spring. To build a wall north of the river would, in effect, cede control of those lands to Mexico and isolate property and homes owned by U.S. citizens on the Mexican side of the wall.

That thinking has changed. Last spring, Congress approved $1.6 billion to build more wall, mostly in Texas. Plans by Homeland Security call for construction of 25 miles of wall on flood control levees in Hildalgo County, sometimes more than a mile from the border. Another eight miles is planned to go up in neighboring Starr County.

5. Disrupting wildlife refuges and parks

Proposals under consideration would locate the wall through seven Texas wildlife conservation areas, including the Lower Rio Grande Valley National Wildlife Refuge and Big Bend National Park, prized among national parks as a place so remote it is considered to be one of the best place in the Lower 48 to view the night sky.

In Mission, Texas, the National Butterfly Center, where more than 200 butterfly species live near the banks of the Rio Grande, has been notified that the wall will divide the 100-acre sanctuary, placing almost 70 percent of it on the Mexican side. Plans also call for bisecting a wildlife refuge and state park, placing most of the land on the Mexican side.

After ferocious objections, Homeland Security shelved plans to build the wall through the Santa Ana National Wildlife Refuge in Alamo, Texas, where more than 400 species of birds, banded armadillos, and endangered wildcats live.

6. Exemption from environmental oversight laws

Construction of the border wall does not have to meet the requirements of more than 30 of the most sweeping and effective federal environmental laws, such as the Endangered Species Act, National Environmental Policy Act, the Clean Air Act, and the Clean Water Act. That’s due to the REAL ID Act, passed by Congress in 2005 in response to the 9/11 terrorist attacks. It authorizes Homeland Security to waive any laws in the name of national security.