It's an unexpected twist that has caught both conservationists and federal wildlife experts by surprise:

The Northern Rockies gray wolf population has apparently shrunk for the first time in over a decade, and officials say they're not sure why.

At the start of 2008, there were an estimated 1,513 gray wolves living in Wyoming, Montana and Idaho, and biologists with the U.S. Fish and Wildlife Service had predicted by mid-year there would be more than 2,000.

The population has been growing for years by a fairly reliable 24 percent annually.

But the mid-year census numbers are in, and instead of growth, Fish and Wildlife officials say wolf numbers have declined to an estimated 1,455, or about 350 fewer than had been anticipated.

Ed Bangs, the federal gray wolf recovery coordinator, cautioned the mid-year counts represent just a "best guess" number, and are not as reliable as the end-of-year figures.

Regardless, the Fish and Wildlife Service has now revised downward its expected year-end total. Instead of growing, the agency says, by the start of '09 the wolf population should be about the same as it was at the start of '08.

This mid-year decline is most likely part of a natural process, Bangs said, but biologists aren't sure yet if it's the result of a typical disease cycle, or something else entirely.

"I've been predicting for a number of years that wolf populations can't keep growing," Bangs said. "They regulate their own population and sooner or later, once they've filled up the suitable habitat, the wolf population will flatten out."

Another possible explanation is that wolves are going through a "disease-outbreak year," Bangs said, when pups haven't survived because of an epidemic, as happened during past outbreaks of canine dysentery, which temporarily caused the wolf populations to dip.

It looks as if the wolves in Yellowstone National Park, at least, could have suffered just such an outbreak this year, Bangs said, although it hasn't been confirmed yet.

"It could be happening in the larger population. It's a normal cycle, but we don't know that yet," he said.

More wolves have also been killed for livestock depredation this year as compared to last, Bangs said.

By mid-year 2007, federal agents had killed or authorized to be killed 134 wolves. By the same date this year, 172 wolves have been "lethally controlled," according to federal statistics.

Louisa Willcox, wildlife advocate with the Natural Resources Defense Council, said her group is also surprised by this population dip.

"We're scratching our heads about this, too," Willcox said. "Bangs and the state had predicted 535 wolf pups would be born this year. When you subtract the known mortality that's occurred thus far, there's an unaccounted-for 350 wolves, if you agree with their predictions."

Either pup production was way down, more pups died from disease, or a great deal more wolves were killed than were detected or reported, Willcox said.

Contact environment reporter Chris Merrill at (307) 267-6722 or chris.merrill@trib.com.