In California, during the fence-building frenzy of 2009, I met a Border Patrol agent and project engineer at Smuggler’s Gulch — a deep canyon near the Pacific Ocean where the Department of Homeland Security had dumped in an estimated 2 million cubic yards of dirt and then fenced in the landfill to prevent access from Mexico. The canyon stream still flowed across the border, so engineers built a tunnel under the landfill to permit the water’s passage. The tunnel entrance and exit were gated, but the agent predicted that migrants would soon use the tunnel to cross. “Ninety percent of this is politics,” said the engineer.

Borderlanders have done their best to adjust to this increasingly bizarre world. Crossing times at the border first doubled, then tripled, from what they had been before the fences, but people learned to factor the holdups into their commute times. Television news programs even began reporting crossing delays along with the weather report.

Such adjustments, though, barely mitigate the outrage that many Americans feel toward the encroachment of hideous barriers into their once open land.

Nor do these daily adjustments change the fact that the border fence is harming an ancient human ecosystem. Mutual interdependence has always been a hallmark of cross-border lives. Residents on both sides of the line regard parts of Mexico and the United States as their home. For them, the border is a connective membrane, not a line of demarcation. Often describing themselves as “transborder citizens,” they have more in common with one another than with their host nations.

Besides sentiments of belonging and shared destiny, this “third nation,” as it is known, is bolstered by synergistic local economies. Border states are among the fastest-growing regions in both countries. Ciudad Juárez, once a city of 1.5 million, lost about a quarter-million inhabitants who fled from drug cartel-related violence to various destinations across Mexico. Yet the city’s industries continue to add jobs, fostering some $80 billion in trade between Juárez and neighboring El Paso in 2011, a $10 billion increase from the previous year. In El Paso, the arrival of 30,000 sanctuary seekers from Juárez created a boom in real estate and restaurant businesses.

Cross-border institutions also reinforce binational ties. For more than a century, the International Boundary and Water Commission, with representatives from the United States and Mexico, has overseen water issues along the border and supported joint development projects.

The border fence, however, undermines this cooperation and cohesion, as it splinters lives and scars landscapes. Likewise, to many who live here, the border patrols, with their ever watching drones, feel more like an occupying army.