Jaguars once ranged across the US southwest, but a campaign of extermination up until the 1960s eliminated them from the region. In recent decades a few individuals crossed from Mexico into the United States and took up residence in Arizona and New Mexico, but the jaguar’s recolonization in the States is up against a literal barrier—the extensive man-made fences and walls along the US-Mexico border.

Besides the burden posed to US jaguar recolonization, border barriers pose an extraordinary threat to other species and entire ecosystems. Barriers have recently been constructed across a huge scale, and are not subject to any environmental regulation. As such, dozens of environmental protection laws that we rely upon to protect us and our surrounding ecosystems are legally nullified when the US government chooses to build barriers. The risk is exceptional because the US-Mexico border passes straight through the most biodiverse...

Border barriers are a particularly high threat to isolated populations, which face an increased risk of extinction. Populations that get wiped out from a natural disaster are less likely to be re-colonized by others of their species if they cannot migrate easily due to barriers.  Additionally, when populations do not exchange migrants, the members of a small isolated population will mate only with each other. This inbreeding results in lower genetic diversity, slowing adaptive evolutionary change and potentially increasing deleterious recessive alleles that depress survival and reproduction. Large mammals like desert bighorn sheep, which are federally protected in the US and Mexico, are particularly under threat. Their populations interconnect across the international border and their migrations will likely be disrupted by fences. Indeed, previous research has found them to be particularly sensitive to interstate highways, for example, that blocked migration and led to a rapid loss of genetic diversity.

US-Mexico border at Tijuana
US-Mexico border at Tijuana

Finally, border fences pose a threat to species like jaguars that are in the process of shifting or expanding their ranges in response to environmental change, or other ecological factors. As the Earth warms, many populations have begun moving towards the North and South poles, or towards higher elevations, tracking their preferred climates. Barriers that run latitudinally, such as those along the US-Mexico border, may block species from expanding their range, causing them to be squeezed against the border.

Tim Keitt (University of Texas at Austin), Walter Jetz (Yale University), and I recently published a study that for the first time assessed the risk posed by border barriers to all species of amphibians, reptiles, and non-flying mammals. We identified which border species are most threatened by barriers across their range, such as species with small ranges that could be at risk of extinction from current barriers. In particular, we identified 23 species that already have more than 50 percent of their range blocked by barriers, three of which are listed as globally threatened by the International Union for Conservation of Nature (IUCN).  According to our analysis, the wildlife of the California Coast and the Gulf Coast face the greatest threats from current barriers.

Clearly, there is a fundamental conflict between wildlife and the border patrol. On one hand, migration is critical to many wildlife species and secretive animals like to move under the cover of vegetation or night. On the other hand, the border patrol wants to increase visibility and decrease the permeability of the border to stop illegal crossing by humans. For example, the border patrol plans to spray herbicide along the Rio Grande to eradicate Arundo donax, an invasive species of reed that grows tall and dense and provides a good hiding place.

One alternative to physical barriers may be improved remote-sensing technology to allow the border patrol to identify illegal border-crossing without disrupting wildlife dispersal. However, the government recently canceled a “virtual fence” system of remote sensors because it became very expensive and often malfunctioned.

Perhaps a more practical way of reducing the conflict between wildlife and the border patrol is to reduce the quantity of illegal border traffic in other manners, such as improvement in economic conditions in Mexico, increased numbers of worker visas, and policies that reduce the flow of illegal guns and drugs across the border. Some politicians continue to focus on barrier construction, however. The Secretary of Homeland Security still has the authority to fence the entire border at any time, unchecked by any regulatory law, and a bill introduced this year by a Utah Republican Representative would extend the unregulated areas to border patrol activities within 100 miles of the Canadian and Mexican borders. The Obama administration has been pursuing immigration reform that would be linked to heightened border security, possibly by means of physical barriers. And lower level governments may also build barriers; the Arizona State Senate, for example, has recently passed legislation authorizing construction of pedestrian fences.

With such efforts underway, it is imperative that we recognize and study environmental impacts of border barriers. With the proper policy changes and conservation actions, we can limit the ecological damage caused by border policies.

Jesse R. Lasky is a PhD student at the University of Texas at Austin who studies how landscapes affect the ecology and evolution of species and communities.

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