KEY POINTS:
The jaguars are coming home. Until recently the big cats, which evolved in North America, then spread south, were rarely seen in the United States.
The last known female was shot in 1963. But over the past decade at least four jaguars, probably males and identified from photographs by the unique rosette patterns on their fur, have padded north via a wildlife corridor of "sky islands," mountains that straddle the border, into southern Arizona and New Mexico.
But the cats, protected by the US Endangered Species Act, may become victims of America's paranoia about illegal immigration and national security, as the Department of Homeland Security moves to seal the US-Mexico border.
"It's potentially catastrophic for the species' recovery prospects in the northern part of its range," explains Michael Robinson, who monitors jaguars for the Centre for Biological Diversity.
Steel fences, which have begun to appear along part of the 3138km border, would strand existing jaguars in the US, prevent others from increasing the nascent population, and limit the cat's gene pool.
Last June, the American Society of Mammalogists said jaguars could survive in the US only if they could roam across the border. This charismatic animal is one of 30 species - including pronghorn antelope, ocelots, bears and wolves - environmentalists say would suffer from a solid fence.
This barrier would also devastate ecosystems, repaired in schemes costing millions of dollars, and wreck bustling eco-tourism in struggling communities.
The sky islands, where the Chihuahua and Sonoran deserts intersect with the Rocky Mountains and Mexico's Sierra Madre Occidental, are a vital link in a chain of wildlife corridors, stretching from the Arctic into Latin America.
Conservationists hope corridors will protect species, from iconic predators to reptiles, plants and insects, reeling from climate change and habitat loss.
"If there's any place we can point to on this continent to show how fragmentation of habitat can destroy species, this is it," says Kim Vacariu, western director of the Wildlands Project.
The border fence would effectively cut this unique area in two. So far the Bush Administration has been deaf to environmental pleas to substitute "virtual wall" technologies, such as motion sensors, laser barriers and infrared cameras, for impenetrable steel fences.
In 2005 Congress passed the Real ID Act to fight terrorism. It contained a waiver from environmental laws. Proponents claimed it was needed to fast-track 22km of border fence in San Diego, California.
In what critics say was a disingenuous move, this waiver has been applied to the whole border. Last year the Secure Fence Act directed the building of 1126km of fence and 480km of vehicle barrier.
Recently, a 56km fence was erected on a military range south of Tucson, Arizona, undoing efforts by the US Defence Department to create barriers that stopped vehicles but allowed animals to move freely.
"Nobody on the ground wanted this," says Sean Sullivan, a Sierra Club volunteer in Tucson. "Nobody on the ground knew it was happening." The fences may salve US nativism, yet the exodus of illegal migrants, who have fled north since free trade devastated Mexico's rural economy, continues.
Dozens of tunnels have been found. Building fences is like using a Band-Aid on a haemorrhage. There is growing resistance to what many see as an extraordinary concentration of power at the Department of Homeland Security.
The first rumblings came from Arizona, where Congressman Raul Grijalva introduced the Borderlands Conservation and Security Act in June. It would repeal the Real ID waiver, forcing the department to comply with environmental laws.
The bill has almost unanimous support from every member of Congress with a border district, especially those in Texas.
And in a surprise statement last month, Mexico's Environmental Minister, Juan Rafael Elvira, said the fence would threaten shared eco-systems. He advocated "green corridors", without roads, for wildlife and warned Mexico might take the issue to the International Court of Justice.
Efforts to seal the Rio Grande may violate the Migratory Bird Treaty Act - the chain of refuges along the lower river is a twitchers' paradise that helps bring $150 million a year to local communities - and run foul of the International Boundary and Water Commission.
"The commission prohibits any structures in the flood plain that could alter the midpoint of the river," explains Noah Kahn, an expert on Texas refuges. There are also fears that building fences on levees near the hurricane-prone Texas coast will weaken the structures.
In a nation afraid of cross-border terrorism, fence critics face a tough fight. The Real ID waiver's constitutionality may have to be challenged in court.
Meanwhile, as fences creep along the border, conservationists argue security and wildlife can co-exist. "We've gone to a lot of trouble to declare species like the jaguar endangered," says Kim Vacariu, "so why would we just go ahead and kill it off?"
