To appreciate why birders consider an obscure wildlife refuge on the Texas border to be the premier birding destination in America, you must walk its trails in solitude, either at dusk or at dawn, as the birds emerge from the forest looking for food.

Earlier this year, in March, I was alone on a path, deep in thought, when a plain chachalaca appeared suddenly in the moss-covered tree beside me. I froze as he dropped down into the scrub and the gray morning light revealed his handsome cascade of brown plumage. After a few tentative steps, I joined him in a slow procession, as he plucked insects from the soil and I considered the feast just 10 feet away.

Tiffany Kersten, an environmental activist who settled here in South Texas after working as a biologist in the refuge, lives for these surprise moments. “You go out and you never know what you’re going to see,” she told me as we watched ducks play in a lake. “Here in South Texas, every day you have the possibility that you’ll go out birding and you’ll see a bird that has never been seen in the United States before, that belongs in Mexico and got lost.”

In an unusual concession to the environment, Congress earlier in March exempted the Santa Ana National Wildlife Refuge, a 2,088-acre patch of extraordinary biodiversity on the Rio Grande river, from the new sections of border wall it funded in the spending bill passed last week.

But it’s a slim victory for conservation because Congress also allocated $1.6 billion to build 33 miles of new barriers around the refuge in the Rio Grande Valley. These wall sections — a compromise to assuage President Donald Trump, who wants a wall across the entire border — are expected to disrupt several other protected parcels of land home to rare animals, plants, and birds, including the National Butterfly Center, a state park, and several other tracts in the federal wildlife refuge system.

The wall won’t just fragment the dwindling wildlife habitat on the border — they’ll also create around 6,500 acres of “no man’s land,” cutting off human access to nature and trapping wildlife the next time the Rio Grande floods.

“Pretty much all of the same species in the Santa Ana refuge will be affected in these other tracts of land where the wall will go,” said Kersten.

On Thursday, the administration announced it will waive 28 federal laws to begin construction on 17 miles of new wall segments in the region as soon as February. So what do we know about the wall and how bad it will be for the environment? Let’s break it down.

The spending bill saved a prized national wildlife refuge

When you trace the 2,000-mile border from west to east (as this Story Map project by Krista Schlyer did), you find shrinking pockets of remarkable biological abundance. At the far west is the Tijuana Estuary, a key salt marsh habitat for some 400 species of migrating birds. At the far east, birds and butterflies stop through the Rio Grande Valley, which is also a permanent home for mammals, reptiles, and amphibians, including the endangered ocelot, that depend on the specific subtropical ecosystem to survive.

As Norma Fowler, a biologist at the University of Texas at Austin, and co-authors write in the April issue of the journal Frontiers in Ecology and Environment, much of the remaining natural habitat in the border region of Texas happens to be on federal lands, which means that the Department of Homeland Security can commandeer it in the name of national security if it chooses. And thanks to the Real ID Act of 2005, DHS also has the power the power to waive most environmental reviews when it wants to build border walls.

Over the years, as DHS has built 654 miles of barriers and wall on the border, it has sliced and diced land set aside for shrinking populations of wildlife.

In the Rio Grande Valley, the 70 miles of existing wall go through the Nature Conservancy’s Southmost Preserve, several state Wildlife Management Areas, sections of the Lower Rio Grande Valley and Laguna Atascosa National Wildlife Refuges, and Audubon’s Sabal Palm Sanctuary, Audubon magazine has reported. And natural areas continue to be at risk of border walls being built across them.

So it’s no surprise that when DHS signaled in 2017 it was eyeing the Santa Ana National Wildlife Refuge in Alamo, Texas, for 3 miles of new wall, environmental groups went on defense. Over the past several months, the Sierra Club and Friends of the Wildlife Corridor lobbied and held protests over the significant harm this wall would bring to the wildlife and overall ecology in the refuge.

The Fish and Wildlife Service calls Santa Ana the “crown jewel” of its 560 refuges in the US.

What a visit to the refuge revealed about how sensitive wildlife is to construction

In March, my colleague Christophe Haubursin and I went down to see for ourselves. Our arrival coincided with the very end of winter, which meant we got to see many birds on the move. Santa Ana is located at an east-west and north-south confluence of two major migratory routes of birds flying from as far north as Canada to as far south as Central America. It’s also the northernmost point for birds whose range extends all the way into South America, a Fish and Wildlife Service spokesperson told me.

The abundant berries, insects, and wetlands in the refuge attract around 400 regular bird residents. And more than 500 of the 1,000 bird species known to inhabit the United States have been sighted there. “Its unique geography means birders who go there get the best of all worlds,” Steve Holmer, vice president of policy at the American Bird Conservancy, told me. “It’s a special, special place.”

And it’s not just the birds that make it special — rare plants and reptiles like the endangered indigo snake can find safe harbor in the refuge too. (The Rio Grande Valley region is native habitat for the endangered ocelot cat, and there are about 50 left in the entire US. Most are in the Laguna Atascosa National Wildlife Refuge; the last official sighting of one in the Santa Ana refuge was in 1992.)

Though most birds could theoretically fly over a wall, the big concern is that the wall would destroy some of their habitat and put other species at risk of drowning the next time the Rio Grande floods. (Right now the reptiles and mammals that can’t fly can crawl over the levee that runs through the refuge when the river floods.)

But to many activists’ surprise, the pushback against building the wall in the refuge paid off: Congress included special language in the spending bill to exempt it.

“It was something that I fought for,” Rep. Henry Cuellar (D-TX) said in an interview. “Thanks to the Democrats on the Appropriations Committee, this has been a priority to protect the environmentally sensitive areas.”

Except Congress allocated $1.6 billion for more border wall — $445 million for 25 miles in Hidalgo County and $196 million for 8 miles in Starr County — which are slated to impact several other critical protected areas for wildlife. (An additional $251 million was allocated to replace 14 miles of existing secondary wall in the San Diego sector and $445 million was included to replace an unspecified amount of other wall.)

“I was worried about this: that Santa Ana would be saved and everyone else gets wall,” said Jim Chapman, a Texas environmentalist who helped organize against the Santa Ana wall. “We never wanted to just save Santa Ana. It’s kind of bittersweet.”

The spending bill authorizes money for a wall that would go through other protected areas

The picture that has emerged from plans shared with the local press is that the new 33 miles of wall will connect existing sections in Hidalgo and Starr counties. And it turns out a lot of it will go through land that’s already been protected for wildlife in one form or another.

Kersten, the local activist, has been looking at all the Rio Grande Valley National Wildlife Refuge tracts, state properties, county parks, and private nonprofit nature sites that would be affected by the proposed, funded border walls for Hidalgo County. And she’s compiled an astonishing list that include, among other sites:

La Parida Banco Refuge Tract (447 acres)

Bentsen-Rio Grande Valley State Park (797 acres)

El Morillo Banco Refuge Tract (654 acres)

National Butterfly Center (100 acres)

Cottam Refuge Tract (1,037 acres)

Pharr Settling Basin (720 acres)

Milagro Refuge Tract (846 acres)

Marinoff Refuge Tract (432 acres)

That means the total nature space affected by the proposed, funded walls would be more than 6,500 acres in Hidalgo County, according to Kersten’s calculations. (The location of the 8 miles of wall in Starr County is still undetermined.)

Among the most visited natural areas in Hidalgo County is the National Butterfly Center in Mission, Texas. It’s a 100-acre wildlife center and botanical garden run by the North American Butterfly Association.

Plans suggest the proposed wall would go right through it, though Jeffrey Glassberg, the president of the center, says he hasn’t heard a word from the government about its intention to build through the center.

“This is complicated — it is not clear to me what this legislation authorizes or requires,” he said. “But they would have to exercise eminent domain to build a wall through it.”

A wall would certainly destroy the little remaining butterfly habitat at the center, he said. And it would further erode the region’s ecotourism. Look no further than what happened at Audubon’s Sabal Palm Sanctuary once a border wall was built through it: Visits fell by half because the wall made it a much less pleasant place to be, according to Glassberg.

Border fences have been terrible for wildlife and plants in other areas of the border

The 654 miles of existing barriers and walls — built with materials ranging from barbed wire to steel, bollard to wire mesh, and chain link — have already been a disaster for the unique and sensitive ecosystems and wildlife on the border.

Not many scientists have measured the full impact of fences on biodiversity on the border. One of the few studies to tackle these questions was written by Jesse Lasky, a biologist at Penn State who has studied the impact of border fences on border species, with his co-authors in 2011. They estimated that 134 mammal, 178 reptile, and 57 amphibian species live within about 30 miles of the line. Of those, 50 species and three subspecies are globally or federally threatened in Mexico or the United States. And they survive only because people on both sides have worked hard to conserve them.

With Thursday’s announcement of the waivers in the Federal Register — a move environmental groups like Defenders of Wildlife and Sierra Club plan to challenge in court — DHS is signaling is plans to move quickly to begin construction on the first 17 miles of new wall in Texas.

For humans who enjoy the protected areas and the local plant and animal species dependent on this ecosystem, it’s bad news. “[They] would lose some of their last remaining US habitat,” as the Texas biologists wrote in Frontiers in Ecology and Environment.