Chapter 1: The Impact of Borders on Wildlife

In the southern expanse of New Mexico, a pivotal moment occurred eight years ago while I was soaring 800 feet above the grasslands. As I peered out of a small Cessna, a brisk March wind swept through the cabin, and I could barely catch the enthusiastic words of wildlife expert Rurik List Sánchez:

“There they are, at the border!”

Bison! After a prolonged search over the preceding days, we spotted them just as they leaped over a barbed-wire fence separating their essential food and water sources between Mexico and the United States. My mission was to document these unique trans-border bison for Wildlife Conservation magazine, and witnessing their migration in real-time felt serendipitous. Research indicated that this herd—one of only six free-ranging bison groups left in North America—had been making this perilous journey daily for decades as they adapted to life in an arid environment. Seeing them at the border reinforced the significance of my long journey from Washington, D.C.

However, my excitement was clouded by the impending threat posed by the construction of the U.S.-Mexico border wall. Plans for this barrier were already underway, and I couldn't help but wonder how soon it would disrupt the bison's habitat, isolating their pond and pasture. While I had followed border policy and wall developments, seeing the bison cross the boundary illuminated the imminent consequences of such a construction. At that moment, I resolved to focus my career on sharing their story and the struggles faced by countless other species affected by this sudden division.

The plight of these bison, along with numerous other wild species in this region, exemplifies the myriad barriers we create in the lives of other creatures. Barriers are pervasive.

A couple of days ago, back in my home in Washington D.C., I was reminded of this reality. After returning from filming a documentary titled "Think Like a Scientist" on barriers, I received a text from a neighbor: “There’s a vulture feeding frenzy in your front yard!” By the time I rushed outside, the vultures had departed, leaving behind a lifeless Virginia opossum that had met its fate on the road. I was saddened by the loss of this familiar creature, but even more so by the connection between this event and the bison.

Both occurrences, though separated by time and space, reveal a fundamental truth: two critical forces govern the lives of all organisms—the necessity to move and the barriers that impede such movement.

Natural barriers, such as climate, mountain ranges, and oceans, have long existed, but their gradual changes allow for adaptation and promote biodiversity. In contrast, human-made obstacles—whether suburban roads or international borders—often produce the opposite effect. These abrupt barriers defy natural processes and, while they may provide short-term benefits for some species, they ultimately diminish biological diversity.

What was once a natural challenge to survival has now been altered by human activity. A deforested area, a fence, or even the invisible boundary between a wildlife refuge and commercial land can create sudden survival challenges.

In border regions, land managers and wildlife biologists are contending with the repercussions of these barriers. For instance, a herd of pronghorn is dwindling on the Arizona border because all of its breeding males have become trapped on the Mexican side of the wall. Similarly, an increase in bobcat fatalities has been observed in South Texas as border wall construction has fragmented their habitat, forcing them to seek food, water, and mates elsewhere.

Reflecting on the life of my local opossum, I consider the trials it faced and the fate that ultimately befell it on a suburban street. My neighborhood is rife with barriers—fences, roads, buildings, and even subtle obstructions like bright streetlights that disrupt the movements of nocturnal animals, alongside monocultures of grass that offer scant food or shelter. Opossums have been relatively successful in navigating our urban labyrinth, which is why the IUCN Red List classifies them as "Least Concern." However, recent studies in Washington D.C. indicate a decline in their populations.

All living beings face a breaking point where the cumulative challenges exceed their ability to adapt. In Washington D.C., centuries of barriers have diminished the region's biodiversity to a few hardy species. In borderlands, political dynamics are affecting wildlife as artificial barriers rise in landscapes that have previously remained relatively untouched. Globally, human-made barriers are constantly emerging and disappearing.

In this age of climate change, what was once a natural factor in the survival of flora and fauna has become entangled with the unpredictable consequences of human industry. The future of wildlife, including the borderland bison and urban opossums, hinges on their capacity to migrate and find suitable habitats as climate disruptions unfold. Ultimately, physical barriers may play a crucial role in a potential extinction crisis.

Moreover, human awareness and concern are the pivotal factors that influence all other outcomes.

Krista Schlyer is a freelance photographer and author based in the Washington D.C. metro area. She has authored three books, including Continental Divide: Wildlife, People and the Border Wall, and Almost Anywhere. Schlyer is also the recipient of the 2013 National Outdoor Book Award and the Sierra Club’s Ansel Adams Award.

Video produced by Day’s Edge Productions and the Howard Hughes Medical Institute as part of its “Think Like a Scientist” series for Nautilus.