Each spring, I load a couple of vans full of students and drive south in search of birds. The Lower Rio Grande Valley, near the border with Mexico, offers a world of difference biologically. It’s home to flashily hued green jays, great kiskadees and Altamira orioles. Many of these tropical interlopers can be found nowhere else in the United States but on the southern border.

A wide range of spectacular species — not just birds but mammals, wildflowers, butterflies and more — sit squarely in the path of the proposed U.S.-Mexico border wall.

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Thanks to the efforts of regional conservationists several parks, nature preserves and refuges currently dot the riverside. The National Butterfly Center, Salineño Wildlife Sanctuary, Estero Llano Grande State Park and Bentsen-Rio Grande Valley State Park are among the places that have plants and animals whose distinctive habitats occur only along this unique river flood plain.

Each of these places could be irreparably harmed by Congress’ recently approved plan to build new barriers at the border.

Whereas elsewhere on the border, large swaths of native habitats were long ago put to the plow, swallowed by urban development or abandoned to barriers — such as the fences along about 650 miles of borderland in New Mexico, Arizona, Texas and California — here, protected areas help generate $344 million in ecotourism benefits for the Rio Grande Valley economy per year.

A monolithic wall to separate the U.S. and Mexico in the only places where biologists have found certain rare and endangered plants and animals means both direct habitat destruction and new barriers for already limited populations. This would harm animals such as ocelots in South Texas and desert bighorn sheep in Arizona, along with endangered plants that need the free movement of pollinating insects and seed-dispersing birds.

When species become extinct locally or globally, it can lead to cascading environmental crises, ranging from soil erosion to flooding. If Tamaulipan thornscrub — a once abundant but now disappearing ecosystem — vanishes, it may take with it the plants, microbes and wildlife that have evolved to depend on it. In both predictable and unpredictable ways, that will affect the economy and quality of life for people in its native South Texas region.

Erecting a wall would also block crucial migration pathways that knit together a range of species distributed on both sides of the border. In 2011, one study estimated that border barriers already bisect the geographic distributions of and threaten 50 unique species. A barrier along the entire U.S.-Mexico border would endanger many more.

Wall-building also means consuming materials, deforestation, pollution, increased carbon emissions and noise that disturbs wildlife — all in a unique river ecosystem containing biodiversity found nowhere else on Earth. Despite this, federal agencies have exempted this barrier’s construction from normal environmental impact reviews.

That should concern everyone.

The evidence that a wall will bring significant environmental costs far outweighs evidence of any benefit in terms of border security. What’s more, environmental review could bring more attention to alternatives that would lessen the ecological upheaval. Having electronic sensors, for example, would be a way to detect human border-crossers without hindering the free movement of other living things in the same space.

We can’t pretend we can build a wall without tremendous environmental impact, not to mention great financial cost. Even small segments of new wall on federal lands in places such as Big Bend National Park or the Lower Rio Grande Valley will devastate habitats and local recreation and ecotourism. Although recent legislation forbids, at least temporarily, a wall across Santa Ana National Wildlife Refuge, the much larger Lower Rio Grande Valley National Wildlife Refuge has no such exemption. I hope never to take my ornithology students birding along the river only to discover a wall. When that happens, what gets severed — more than border crossing — is our own American connection to a rich natural heritage found only at our border to the south.

Tim Keitt is a professor of integrative biology at The University of Texas at Austin.