By Melissa Gaskill of Nature magazine

The 1,000 kilometers of impenetrable barrier constructed along the Mexico-United States border, with the aim of stemming illegal human immigration, is also hampering the movements of animals, including several endangered species, a recent study finds.

The species most at risk are those with smaller populations and specialized habitats, says Jesse Lasky, a graduate student at the University of Texas, Austin, and an author on the study, published in Diversity and Distributions. Small range size is associated with a higher risk of extinction, and for some species, the barriers reduce range by as much as 75%. According to the study, species most at risk include four listed by the International Union for Conservation of Nature as endangered or threatened--the Arroyo toad (Anaxyrus californicus), the California red-legged frog (Rana draytonii), the black-spotted newt (_Notophthalmus meridionalis_) and the Pacific pond turtle (Clemmys marmorata)-together with the jaguarundi (Puma yagouaroundi), which is endangered in the United States and threatened in Mexico.

The study also identified three border regions where wildlife is most at risk from the barrier: coastal California, coastal Texas and southeastern Arizona's Madrean Sky Island Archipelago.

Fragmented communities

The Rio Grande Valley, in coastal Texas, contains one of two remaining stands of native Mexican sabal palms. The solid metal and concrete fence, around 5.5 meters high, with an 18-metre-wide strip of open land on either side, runs through a preserve created to protect the trees. Rare animals in the valley include the ocelot, of which fewer than 50 remain in the United States. The fence further fragments their habitat and separates the population in Texas from the larger and more genetically diverse population in northern Mexico.

The border barrier affects 60% to 70% of the habitat in the South Texas Wildlife Refuge Complex, which includes the Laguna Atascosa, Lower Rio Grande Valley and Santa Ana National Wildlife Refuges. At the request of the US Fish and Wildlife Service (FWS), about 100 openings for wildlife were incorporated into the fence in this area. But, says FWS wildlife biologist Mitch Sternberg, who was not involved in the study, these openings are too small for larger animals such as coyotes or bobcats, and bobcats follow regular trails and may not find the openings. "Bobcats don't go out looking for holes in fences as they travel back and forth through brushy habitats. Overall, wildlife connectivity does not exist in these sectors anymore," he says.

Sternberg, who monitors the area's bobcat population using cameras and radio collars on around 20 bobcats, says that the border barriers bisected the home ranges of the tracked bobcats. "I tracked some cats for a long time before the wall construction and did see significant shifts in territories," he says. A pair of collared bobcats that lived on the south side of the wall were trapped on the north side, where they were later killed on a highway while searching for new habitat. The presence of this fence-restricted pair may have caused another male bobcat to wander into an urban area--a rare occurrence--where it was killed by a vehicle. Another tracked pair abandoned their home range during construction of the barrier in their area.

Sternberg hopes to collar additional cats to study distribution and movement patterns after barrier construction, but says "I'm afraid most of the previous bobcats are dead due to forced dispersal during construction, so it will be a real challenge to replicate the study in those areas."

Environmental mitigation

The barrier was mandated under the Secure Fence Act, signed into law by George W. Bush in 2006, and Michael Chertoff, the then Secretary of Homeland Security, waived environmental laws for its construction. In January 2009, US Customs and Border Protection (CBP) committed US$50 million to fund mitigation of "unavoidable impacts to natural and cultural resources" from construction of the barriers, to be paid to the Department of the Interior over several years. In September 2010, CBP funded $6.8 million in mitigation projects, but approximately $30 million that was due to be allocated in 2011 was rescinded in the 2011 appropriations bill.

According to CBP spokeswoman Jenny Burke, the agency "remains committed to responsible environmental stewardship", and intends to resume the mitigation projects when funding permits.

The authors of the recent study recommend restoring historic connectivity through additional openings or removal of the barrier in key areas. Habitat restoration, particularly in areas where crossings are possible, could also mitigate some effects, Lasky says. "General land-use planning is important. We can plan new development and preserves with an eye to where we can promote connectivity and preserve areas where connectivity exists."

This article is reproduced with permission from the magazine Nature. The article was first published on August 2, 2011.