Nancy Brown drives the government truck slowly past mossy ponds, thick shrouds of beard-like Spanish moss and majestic ebony trees, gleefully identifying the song of the kiskadee and the gurgling call of the chachalaca.
As the truck rounds a bend near the greenish-brown Rio Grande, a bobcat scampers ahead, disappearing into the lush subtropical foliage. Lizards dart about. A tortoise lazes in the sun.
Somewhere in the forest, well-camouflaged by evolution, are ocelots and jaguarundi, both of them endangered species of cats.
These are some of the natural wonders in the Rio Grande Valley that Brown and other wildlife enthusiasts fear could be spoiled by the fences and adjacent roads the U.S. government plans to erect along the Mexican border to keep out illegal immigrants and smugglers.
Environmentalists have spent decades acquiring and preserving 90,000 riverfront acres of Texas scrub and forest and protecting their wildlife.
Now they fear the hundreds of miles of border fences will undo their work and kill some land animals by cutting them off from the Rio Grande, the only source of fresh water.
A fence could also prevent the ocelots and other animals from swimming across the water to mate with partners on the other side.
"If you have a fence that runs several miles long, if you are a tortoise or any animal that can't fly over or go through it, then you have a pretty long distance that you have to go to get water," said Brown, an outreach manager at the Santa Ana National Wildlife Refuge, 225 miles south of San Antonio. Also, "any destruction of any brush is very damaging."
In addition, some worry that the barrier — described in some plans as triple-layer metal fencing — will damage the tourism industry along the Rio Grande.
The wild cats, reptiles and at least 500 species of birds attract visitors from around the world who bring the impoverished region $150 million a year.
Depending on how far inland the fence is built, it could create a no man's land north of the river, hurting tourism.
While the Department of Homeland Security said it has not made any final decisions on where the fence will go, meetings this week with the Border Patrol have wildlife officials convinced that some of the 70 miles planned for the Rio Grande Valley will be erected on the string of wildlife refuges along the border.
Homeland Security spokesman Russ Knocke said environmental concerns will be taken into account in the final decisions.
But Homeland Security Secretary Michael Chertoff has used his authority to waive environmental regulations for security reasons in other states, and Knocke said he would do so in the Rio Grande Valley if necessary.
"We do have to be mindful of the fact that we are remedying a problem that has been more than two decades in the making," he said.
The refuges show signs of immigrant activity — food wrappers and water jugs, discarded wet clothing, the plastic bags used to carry a change of clothing across the river.
Similar evidence is found up and down the river, despite the presence of Border Patrol agents and the sensors and cameras that make up the current "virtual" fence.
The fence idea "is wholly incongruous with a 30-plus year investment by the federal government, the citizens and the landowners of the Rio Grande Valley who have worked hard to protect their special land and waters," said Carter Smith of the Nature Conservancy.
The organization said the government should instead use more border agents, sensors and cameras.
President Bush called for about 700 miles of fence along the U.S.-Mexico border, and Homeland Security is committed to completing 370 miles by the end of 2008. Congress has budgeted $1.2 million for the fences.
Close to $100 million has been spent creating, restoring and maintaining the refuges, wildlife officials said.
"The bottom line is the wildlife corridor took us many years to put together," said Karen Chapman of Environmental Defense. "It represents work, hard work, by a number of federal, state and local agencies and citizens of the Valley. And when we were working to put that wildlife corridor together, nobody was doing it with the thought that someday it was going to be stuck behind a wall."
The four-county Rio Grande Valley contains 11 distinct ecosystems, Brown said.
"From a biological standpoint this area is really, really impressive," she said. "You have a coastal climate meeting a desert climate meeting the temperate and the tropical."