A 15-foot-high, rust-colored steel wall snakes across the scrubby desert landscape, dividing the twin border cities of Nogales, Ariz., and Nogales, Sonora. On the Arizona side, Border Patrol agents sit at the ready while reconnaissance airplanes drone overhead. On the Mexican side, border crossers driven by poverty lie in wait for nightfall. Then they will face a wall that is decorated with graffiti images, remembering the many lives already lost across the line.

The wall stretches out of the ramshackle city, over arid mountains and through verdant river valleys. Whether it's effective in defending the U.S. from low-wage laborers, drug traffickers and the occasional terrorist remains uncertain. But in dividing the borderlands ecosystems its success is undeniable. A study published in July by Conservation Biology reported that the final elements of border fence construction could prevent the migration and population growth of desert bighorn sheep and ferruginous pygmy owls. That's a mere fraction of the diverse plants and animals endangered by the wall.

In the roughly 150 years since it was established, the Southern border has been largely unregulated. Security crackdown began in the 1990s, focused in urban areas, but post-2001 terrorism hype drove the recent increase in border militarization. In 2006, President Bush signed an act mandating the construction of 670 miles of reinforced border fencing by 2009, 630 miles of which are already completed. The Department of Homeland Security, in turn, used its clout to bypass any environmental law that got in the way of the fence.

At-risk wildlife haven't figured out how to get around the fence, but humans are more resourceful: This summer, U.S. Border Patrol agents found a new 83-foot long tunnel under the border in Nogales. It was the 16th discovered in half as many months.