The approval of a new federal plan to dramatically boost populations of the Mexican wolf does little to quell the longstanding scientific debate over how the embattled animal should be managed.

On Wednesday, the U.S. Fish and Wildlife Service signed off on a $178.4 million recovery plan that aims to increase the wolf’s total population from 144 today over the next 35 years to about 520 in Arizona, New Mexico and two northern Mexican states, Sonora and Chihuahua.

Service officials say the plan will improve the wolf’s genetic diversity enough to ultimately take it off the endangered species list, where it’s classified as a subspecies of the gray wolf. The plan will ultimately establish at least two “resilient, genetically diverse Mexican wolf populations,” the service said.

But the same day, the plan was greeted by formal threats of lawsuits from two teams of environmental groups who see the plan as a path to extinction. The groups, including the Tucson-based Center for Biological Diversity, Western Watersheds Project, Wildlife Guardians and Defenders of Wildlife, want far more wolves released into a much bigger area.

“The plan reads like something that wolves’ most virulent opponents would have written in their wildest dreams,” said Christopher Smith, a Wildlife Guardians’ wildlife advocate. “Clearly, the U.S. Fish and Wildlife Service is catering to a very narrow set of interests that want to see this amazing species banished from their native Southwestern home.”

Ranchers have countered that the plan is still so sweeping that it could lead to scientifically unwarranted wolf population increases that they fear would trigger more livestock deaths than have occurred to date. They want far fewer wolves living in a far smaller area than the plan calls for.

On Thursday, Sen. Jeff Flake, R-Ariz., who has supported ranchers’ efforts to hold down the plan’s scope, issued a news release calling the plan “yet another federal regulatory nightmare for ranchers and Arizona’s rural communities.”

Here’s a guide to key issues:

How many wolves?

The new recovery plan calls for boosting wolf populations to an average of 320 in southeast Arizona and southwest New Mexico and 200 in Mexico over eight years. Current estimates are 113 in the U.S. and 31 in Mexico.

While this population far exceeds that of a few years back, biologists generally agree that the wolves’ future is hobbled by a lack of genetic diversity in the wild today. The 281 Mexican wolves living in captivity in 55 institutions have a higher genetic diversity than wild populations, the recovery plan said.

Releasing more wolves should ensure that wild wolf populations have about 90 percent of the gene diversity of captive wolves, the service said. The service used computer models to develop its strategies.

But in commenting on the service’s draft recovery plan, biologist David Parsons, the service’s first Mexican wolf recovery coordinator in the 1990s, said “it is more likely to cause the second extinction of Mexican gray wolves in the wild than to secure their recovery.”

He, environmental groups and several other outside scientists fault this plan for having abandoned the population goal of 750 from a 2012 recovery plan draft. It was fashioned by a different team of scientists than this plan.

The wildlife service never released the 2012 plan, saying it wanted more information about wolf habitat and prey in Mexico, and needed to focus on getting the Mexican wolf subspecies listed separately as endangered from the larger gray wolf species. Critics charged that service officials dropped this plan because state officials in four Southwestern states opposed it.

Two scientists, Carlos Carroll and Rich Fredrickson, both involved in the 2012 wolf plan, did projections last summer that concluded wolf populations would decrease over time if the new plan was adopted.

Carroll went so far as to say that the existing U.S.-based wolf population faced a 42 percent chance of extinction. His and Fredrickson’s predictions employed more pessimistic assumptions than the wildlife service’s about factors such as the number of female wolves available for breeding, adult wolf mortality and disease outbreaks.

The Arizona Cattle Growers Association fears that the wolf recovery program will have the opposite effect. In the 1990s when the wildlife service adopted gray wolf recovery programs for the northern Rockies and the western Great Lakes, wolf populations within 15 years far exceeded the programs’ original goals, wrote Norm James, a Phoenix attorney representing cattle growers.

James has argued that the Mexican wolf’s U.S. population goal should be trimmed to an average of 100 over four years. Mexico, which James asserted contains 90 percent of the wolf’s historic range, should have 400 wolves, he said.

“The population goal for the southwestern United States should be consistent with the limited historical range and the numbers of wolves that occupied southern Arizona and New Mexico,” James wrote, adding that the U.S. population deserves less attention because it’s classified as a “non-essential, experimental species.”

Where to put them?

The new plan would keep the U.S.-based wolf population where it currently lives, south of Interstate 40 including the Mogollon Rim area of north-central Arizona and western New Mexico. The Mexican population is expected to live in large sections of the northern and southern Sierra Madre Occidental range.

Environmentalists and scientific critics again favor the 2012 plan’s blueprint, calling for establishing three U.S. wolf populations including the existing one.

One new population would live in the Grand Canyon area, stretching north into southern Utah. The second would be in the southern Rocky Mountains in northern New Mexico and southern Colorado. The third would be the existing population south of I-40.

Some of these differing opinions can be explained by varying scientific views on where the Mexican wolf lived before it was extirpated from the wild during the 20th century.

On one side, a recent paper whose co-authors include Arizona Game and Fish biologist Jim Heffelfinger argued that the wolf’s historic range extends north from Mexico only into southeast Arizona and southwest New Mexico. That’s based partly on historical accounts by scientists from the early to middle 20th century, and partly on more recent comparisons of skull sizes and other body parts of the Mexican wolf versus other subspecies.

But seven other scientists led by Sarah Hendricks at the University of Idaho have argued that genetic testing shows the historic range could be much larger. They cited a handful of historic DNA samples of other wolves from California and from as far away as Nebraska and Utah that matched or were similar to DNA samples of Mexican wolves.

Heffelfinger and his colleagues warned in their 2017 paper that “an artificially inflated historical range based on fragmented sampling of molecular markings” could lead to a wolf recovery plan that’s legally untenable and ecologically unsound.

But Hendricks and her colleagues argued that given the difficulties of reintroducing Mexican wolves so far, a bigger range with more wolf habitat is needed for the animal to survive.

Most likely, however, this debate will be settled not by scientists, but by the courts.

Contact reporter Tony Davis at tdavis@tucson.com or 806-7746. On Twitter@tonydavis987