As birds sing and lizards scuttle in the lush vegetation of the Tijuana River Valley, helicopters circle overhead and Border Patrol agents on all-terrain vehicles comb the area looking to stop illegal border crossers.
Two big metal fences and stadium lighting divide homes in Mexico from the largest intact coastal wetland in Southern California.
Over the decades, fencing construction and associated roadwork have affected wildlife habitat along a 14-mile stretch between the Pacific Ocean and the Otay Mesa Port of Entry. The project has included filling in canyons, cutting down mesas and paving over coastal sage scrub in territory that's home to several endangered or threatened species.
"In most cases, the lands that were used were very high-quality natural habitat," said Christopher Peregrin, reserve manager for the Tijuana River National Estuarine Research Reserve.
But, he said, "the more significant environmental issue that we deal with is not the fence or the road itself.... It's the border environment."
The prime environmental threat to the wetland is water pollution from the Tijuana River and its tributaries. Trash, sewage and sediment from south of the border routinely flood the reserve during and after storms. Those challenges have continued to varying degrees since the fencing and related projects were undertaken.
It's unclear how much the efforts to curb unauthorized immigration have affected the area's long-term ecology.
"The infrastructure has greatly improved the areas north of the border, especially in the estuary area where trails had been made by hundreds of migrants passing through on a daily basis. Trash was left behind on the trails," said James Nielsen, a spokesman for the Border Patrol's San Diego sector.
"Having the infrastructure put in place has created an environment where a lot of the native plants have been able to grow back in areas where they were completely trampled," he said.
Still, environmental groups sued in 2004 to block the Border Patrol from filling in several canyons in the research reserve. The legal challenges focused on an area called Smuggler's Gulch, a waterway that connects to the reserve through a series of culverts.
Environmental organizations also opposed construction of a border fence in the Otay Mountain Wilderness Area. To counter such efforts, Congress inserted a provision into the Real ID Act of 2005 that gave the Department of Homeland Security the ability to waive as many as 37 federal laws — including the Clean Water Act, the Endangered Species Act and the Solid Waste Disposal Act — when constructing border fencing.
Eventually, the wilderness area was subject to blasting and leveling to accommodate 3.6 miles of fencing and 5 miles of access roads.
Border-enforcement hawks like Rep. Duncan Hunter (R-Alpine) have hailed the waiver provision as a win for both national security and the environment.
"For San Diego, between the trash that piled up and the degradation to the land from countless crossings, including vehicle drive-throughs, the fence has been a net positive for the environment," said Joe Kasper, spokesman for Hunter.
"The environmental impact of the fence has been nothing but positive, truly," he added.
Critics of that perspective include Dan Millis of the Sierra Club's Borderlands program.
He and others have voiced concerns about the fencing restricting animal migration and causing habitat fragmentation that harms endangered species such as jaguars, Sonoran pronghorns and ocelots in Arizona, New Mexico, Texas and elsewhere.
In California, concerns have been raised about the potential of similar impacts on populations of mountain lions, deer and bighorn sheep. Fencing has yet to be constructed in the sheep's main travel corridors, according to San Diego Zoo Global.
"Right now, we're looking at 653 miles of barriers and walls built [across the U.S.-Mexico border] without regard for environmental protections that cause a lot of damage and have been for a decade," Millis said.
"It means that all the protections that every American can depend on to protect public health, clean water and environment — people and wildlife in the borderlands, we're not afforded that same freedom."
Beyond the issue of habitat displacement, the health of the Tijuana River Valley seems to have had a mixed record when it comes to the Border Patrol's activities.
The project resulted in erosion that brought in unwanted sediment to the area, but the Border Patrol and U.S. Army Corps of Engineers have spent millions of dollars on restoration efforts and other projects to address such impacts.
"Certain members of Congress just got [mad] because environmentalists and conservationists were quite effective in delaying the whole thing," said Paul Ganster, a professor with San Diego State University's Institute for Regional Studies of the Californias. "When it had to move forward, it moved forward without much care to what the impact was."
He added: "The Border Patrol had to do a lot of work after the fence was installed to control erosion, to re-vegetate. And gradually, they've more or less done a reasonable job of taking care of some of that."
Others said the legacy of the border fence's repercussions on water quality has been more intractable.
When Smuggler's Gulch was filled in, the associated culverts were poorly designed, said Oscar Romo, a researcher with UC San Diego who coordinated the Tijuana River National Estuarine Research Reserve's coastal training program for about a decade.
"In Smugglers, the [water quality] problem has increased dramatically," he said. "It's created a new problem, which is that the water flows faster. There's just a great amount of trash and sediment that's very difficult to remove."
While the culverts do speed up water and increase erosion and trash downstream in the wetlands, the project's contribution to the area's overall water-quality issues is difficult to quantify, Peregrin said.
"It's always hard for me to point the finger at one particular project, because the issue is so huge," he said.
Smith writes for the San Diego Union-Tribune.