Two years ago, along the southern border west of El Paso, a Mexican gray wolf loped north through the Chihuahuan Desert and into the United States. A few days later, unable to find a mate, he returned to Mexico.Today, an 18-foot-high steel barrier could block his path. Sections of President Trump’s border wall built in recent weeks slice through 20 miles of this remote New Mexico desert, where a creature’s ability to traverse vast distances can be a matter of life and death.Mexican wolves are one of the most endangered mammals on the continent, with just 114 in New Mexico and Arizona and a few dozen across the border in Sonora, Mexico. With a narrow gene pool, their long-term survival may hinge on crossing the border to find mates, just as they did for thousands of years.Wolves are hardly the only wildlife threatened by the border wall. The new bollard-style barriers in New Mexico also obstruct the movements of kit foxes, cougars and ringtail cats. The walls fragment their populations and increase the risks of inbreeding.