Last week, for the first time in decades, a radio-collared Mexican gray wolf crossed from New Mexico into Mexico. It was a rare and remarkable moment. It may also be the last time a wolf ever makes that crossing.

The Mexican gray wolf, known in Spanish as the lobo, was nearly wiped out entirely in the 1970s. Today, every single wild Mexican wolf alive is descended from just seven animals rescued through a captive breeding program. That is not a healthy gene pool. It is a crisis. Inbreeding has already caused lower survival rates for pups, cancers, and birth defects. The one thing that could help is what the border wall is about to eliminate: the ability for wolves on opposite sides of the border to find each other, mate, and rebuild a population with enough genetic diversity to survive.

The Department of Homeland Security has used legal waivers to override the Endangered Species Act and other environmental protections in order to accelerate border wall construction across New Mexico, Arizona, Texas, and California. The result is a steel barrier up to 30 feet high, with openings designed only for small animals like reptiles and rodents. A wolf cannot fit through those gaps. Neither can a jaguar.

We are calling on Congress and the Department of Homeland Security to act on four fronts. First, pause construction in the New Mexico Bootheel until an independent environmental review assesses the impact on the Mexican wolf's last remaining migration corridor. Second, require wildlife crossings large enough for wolves, jaguars, and other large mammals in all current and future border wall construction. Third, pass legislation preventing DHS from waiving the Endangered Species Act for construction projects. And fourth, fully fund a binational Mexican wolf recovery plan that protects habitat on both sides of the border and gives this species a real chance.

As Michael Robinson of the Center for Biological Diversity put it, "Sealing off the Bootheel would isolate wolves and other rare mammals like jaguars and ultimately make them all less likely to survive." Border security and species survival are not mutually exclusive. We can have both, if we choose to.

With only 319 wild Mexican wolves in the United States and 36 in Mexico, there is no margin for error. The door between their worlds is closing. We are asking Congress and the administration to stop it before it shuts for good.