Conservation groups have sued the U.S. Fish and Wildlife Service to protect the Mexican gray wolf, a subspecies with a small population in Arizona and New Mexico, according to a report in the Los Angeles Times. The groups want the courts to force the agency to abide by rules it adopted almost 40 years ago that call for the creation of a sustainable Mexican gray wolf population.

The agency reintroduced the current population of Mexican gray wolves to the Southwest in 1998, with the goal of creating a stable population of 100 wolves and 18 breeding pairs. The population is now at 83 wolves with just six breeding pairs, significantly under threshold of viability, according to the agency’s assessment.

Not everyone supports the expansion of the Mexican gray wolf population. Ranchers fear wolves will threaten their livestock, and hunters fear wolves will kill game animals. These objections are similar to those over the gray wolf, which has seen sustained population growth in the northern Rockies after a population of wolves was introduced to Yellowstone.

According to the Los Angeles Times report, the U.S. Fish and Wildlife Service has drawn up several plans to create a robust population in the years since it committed to protecting the Mexican gray wolf. The most recent, from 2012, calls for reintroducing two more populations—in the Grand Canyon and New Mexico’s Rocky Mountains—for a total of 750 wolves. But the proposal was never adopted due to political pressure, according to the lawsuit. Now the U.S. District Court in Arizona will have to decide whether to compel the agency to act.