Overruling the strong recommendations of its own scientists, the U.S. Fish and Wildlife Service is withdrawing a proposal to list the North American wolverine, Gulo gulo, as Threatened under the U.S. Endangered Species Act.

USFWS scientists had urged the wolverine be listed due to the likely effect of climate change on the rare predatory weasel's reproductive habits. The animals depend on deep, persistent snow for their breeding, and an increasingly warm planet means that winter breeding habitat will likely become unsuitable for the wolverine.

Nonetheless, USFWS Director Dan Ashe announced Tuesday that the agency will back a call by one of the agency's regional directors to withdraw the listing proposal, citing "uncertainty" over the actual effect of disappearing snowfields on the wolverine's survival.

Female wolverines dig multi-chambered birthing dens in snowbanks at least five feet deep, and rely on those dens until May. Unless deep snow remains in the spot until that late in the year, the area is unsuitable for wolverine breeding.

Snow melt in the northern Rocky Mountains now happens an average of two weeks earlier than it did in 1960, and a similar or greater change in melt timing is expected over the next half century. In California's Sierra Nevada, where a small population of wolverines is thought to exist since a sighting in the Tahoe National Forest in 2008, snowmelt is peaking as much as 20 days earlier than in the mid-20th Century.

USFWS scientists had concluded, during the agency's evaluation of the wolverine's status, that the species absolutely depends on deep snow to reproduce -- and thus to survive as a species. But in a May 30 memo from USFWS Region 6 Regional Director Noreen Walsh that was leaked to environmental protection groups, Walsh ordered that the effort to list the wolverine be abandoned, claiming that climate models were insufficient to determine whether very specific sites in North America would become unsuitable breeding habitat for wolverines.

USFWS Director Ashe echoed Walsh's argument in a Tuesday interview with the Associated Press. "Climate change is a reality," Ashe told AP reporter Matthew Brown. "What we don't know with reliability is what does climate change mean for denning habitat that wolverines prefer."

Wildlife protection groups, which had blasted Walsh's order in June, are reacting similarly to USFWS' final withdrawal of the listing proposal.

"Absolute certainty has never been the standard when it comes to deciding whether or not to protect endangered species. That's like withholding medicine until a patient is taking their last breath," said Noah Greenwald, endangered species director at the Center for Biological Diversity. "These wolverines face a very real, clear and present danger from global warming. We know it, federal scientists know it. The only people denying that grim reality are those making this decision to call off protections for one of the rarest, most threatened mammals in the country."