The 2,000-mile border joining the U.S. and Mexico is lined with lush jungles, sandy deserts, soaring mountains, and roaring rivers. Wildlife and flora effortlessly cross this boundary every day.

But turn that invisible line into a towering wall of metal and concrete, and those fragile ecosystems will quickly erode, scientists warn.

At least 800 animal species would be adversely affected by President Donald Trump's proposed border wall, said Gerardo Ceballos, an ecology professor at the National Autonomous University of Mexico.

Of those, about 180 species are already in danger of extinction, including the jaguar, pronghorn antelope, bald eagle, and grey wolf, he said.

The impassible barrier would not only disrupt migration patterns but also keep animals and plants from spreading seeds and pollen. Soil and water quality would similarly suffer as complex ecosystems break down. More immediately, heavy construction and truck traffic would threaten habitats for miles beyond the wall itself.

"This will have an enormous impact on environmental systems," Ceballos said by phone from Mexico City.

Trump has said a "big, beautiful wall" along the U.S.-Mexico border would help stem the flow of undocumented immigrants into the United States. If completed, the project would cost as much as $21.6 billion and take more than three years to construct, the U.S. Department of Homeland Security said in an internal report seen by Reuters.

Trump's proposal involves putting fences and walls along 1,250 miles of the border. The remaining 654 miles are already fortified -- and provide tangible proof of what can go dangerously wrong when countries construct impenetrable barriers.

Walls blocking easier entry points in California have forced migrants to seek more dangerous routes in recent years. As a result, Arizona's unprotected desert border has witnessed a rapid rise in deaths since the 1990s.

Ecosystems have also suffered from the obstructions.

Existing walls have blocked off or divided the habitats of rare animals, including the jaguar, ocelot, and the jaguarundi, a type of wild cat. Mountain lions, desert big horn sheep, and low-flying pigmy owls have been stopped at the fortified border.

Walls and fences can also act like dams, collecting vast amounts of water and dumping it into neighboring communities. The twin cities of Nogales -- in Arizona and Sonora, Mexico -- have both endured dangerous flooding since a wall went up in 2008.

"It's just been a disaster. The wall is blocking water, and it's blocking wildlife, but it is not blocking people or drug-smuggling vehicles," said Dan Millis, who coordinates the Borderlands program for the Sierra Club's Grand Canyon chapter.

Millis said he was skeptical the Trump administration could complete the wall, given how costly and complicated the first 654 miles have been. He noted that flooding, erosion, and tool-wielding smugglers have all knocked down the walls at various locations, requiring expensive repairs.

"There are amazing species that live in this area, and beautiful landscapes," he added. "That's what's at stake when we talk about bulldozing [the border] and walling it off."

Ceballos and his colleagues, who announced their findings in late March, determined 800 species were at risk by by studying wildlife maps and consulting data collected by NatureServe, a non-profit in Virginia, and the International Union for Conservation of Nature, a global authority on conservation.

The professor also spent the last six months traveling the border, from Tijuana, on the Pacific coast, to the Gulf Coast of Texas, he told Fox News. The 2,000-mile stretch includes eight "co-regions" of immense biological diversity, including pine forests, dense jungles, and mountain ranges.

A separate analysis by the U.S. Fish and Wildlife Service, conducted during the Obama administration, found that Trump's proposed border would "potentially impact" 111 endangered species, as well as 108 species of migratory birds, four wildlife refuges and fish hatcheries, plus an unknown number of protected wetlands.

Beyond the immediate effects, the border wall could also hamper wildlife's ability to adapt to the effects of a warming planet by limiting their range and diminishing their resources.

"It will reduce their ability to adapt to the effects of climate change," Ceballos said by phone.