 But first, the effort to bring back Mexican wolves into the environment is a long one and a controversial one. A few decades ago, they were nearly decimated. But as U.S. Fish and Wildlife has worked to reintroduce them in this region of the world, ranchers have spoken out against it, saying they kill their livestock. And they've also faced opposition from environmentalists, who have sued over the program, saying it violates the Endangered Species Act.