Nevertheless, one stretch of fencing has already gotten a boost from the security-related REAL ID Act passed by Congress last year.
The new law gives the secretary of homeland security the go-ahead to waive any other law in order to tighten border control.
DHS Secretary Michael Chertoff did just that in September so that construction could continue on a 14-mile (22.5-kilometer) stretch of fence near San Diego, California (see map).
The decision forced a federal district court to drop a lawsuit by the Sierra Club and the San Diego Audubon Society. The suit charged that the government had not fully disclosed the fence's environmental impact.
The nonprofit groups said the project failed to consider endangered species and rare habitatincluding the Tijuana River National Estuarine Research Reserve, which lies just inland on the U.S. side of the border.
At risk there, the groups say, are 350 species of birds, 20 fish species, and a variety of endangered plants and animals.
The DHS override also squelched opposition to the San Diego fence by the California Coastal Management Program, a state agency that reviews federal activities affecting the coast.
DHS's Agen said his agency takes environmental regulation "pretty seriously."
"But at the same time," he said, "the secretary's priority is securing the homeland. If [Chertoff] feels that some of these environmental regulations are preventing national security, then he'll use his authority to waive those [laws]."
Neeley, of the Defenders of Wildlife, said the California decision doesn't bode well for Arizona (see map), which boasts a smorgasbord of federally protected lands along its boundary with Mexico.
A fence along the Arizona-Mexico border would be especially destructive for jaguars, she said. The big cats have recently been pushing their range north into the U.S. again after being eradicated from the nation about 50 years ago. (Read "Are Wild Jaguars Moving Back Into the U.S.?")
"The fence would end any chance of natural recovery of that species in the U.S.," she said.
Environmentalists are also pointing to the imperiled Sonoran pronghorn, an antelope species that lives in small herds on both sides of the Arizona-Mexico border.
And conservationists worry about how fences will impact the Sky Islands, a mountain region containing 40 isolated peaks in southeastern Arizona and northern Mexico. The mountains are separated by a sea of grasslands and desert.
These habitats host native high-desert plants and provide additional migration routes for border-crossing creatures.
Beyond impeding animal migration, the infrastructure associated with a fence would impact even species that can get around it, Neeley says. For example, high-voltage lighting along the fence would pose problems for bats.
Lower-impact moves to reduce vehicle travels through Arizona have shown mixed success.
Organ Pipe Cactus National Monument, for example, has benefited from a two-foot-high (half-a-meter-high) vehicle barrier.
But now surrounding landsincluding the Cabeza Prieta National Wildlife Refuge run by the U.S. Fish and Wildlife Serviceneed protection, as traffic gets diverted through them, park managers say.
Curt McCasland, assistant manager at the Cabeza Prieta refuge, wonders whether a fence would be effective along the remote 56 miles (90 kilometers) between the refuge and Mexico.
Cars and trucks pose the worst hazards to the Cabeza wilderness, he says, and an existing fence has done little good.
"Throughout the refuge there are areas where the fence has been cut, and now vehicle tracks riddle virtually the entire refuge," he said.
Neeley, of the Defenders of Wildlife, says any barriers are like Band-Aids. She says the real solution lies in immigration reform, so she's hopeful that the federal government will see progress on the issue soon.
"The bottom line is we need to get the people who are crossing the border out of the desert," she said. "If we can get the people out of the desert, we can ratchet down the need for Border Patrol to construct so much infrastructure."
Meanwhile, if she has to choose, she'd rather fight the environmental effects of illegal immigrationdiscarded water bottles, abandoned cars, human-caused wildfiresthan what she considers the increasingly destructive efforts to control it.
"You can pick up the trash, revegetate a trail," she said. "It's nothing like a 15-foot [4.5-meter-tall] steel wall."
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