JACKSON - Grizzly bears in the greater Yellowstone ecosystem suffered a tough year in 2007, as more than twice the number of bears died this year than last.

In 2007, an estimated 27 bears have died, with 24 of those deaths caused by humans. Last year, 12 bears died, with seven of those determined as human-caused.

The estimates are based on formulas for dead bears. For example, a road-killed bear may count as three, as managers calculate there are likely two additional dead bears not found. A radio-collared bear counts as one.

In Wyoming, where the majority of grizzlies live outside Yellowstone National Park, there were 10 deaths this year, all human caused. Last year, there were four.

The ecosystem-wide death tally pushes grizzlies near the so-called "mortality threshold," a percentage of the population managers use to determine if a review of management is needed. That number is 9 percent of female grizzlies.

Reports indicate there have been 19 female grizzly deaths in a population of 240 females over 2 years of age, so-called independent-age females, according to The Associated Press. That number equals 7.9 percent of independent-age females.

But Chuck Schwartz, Interagency Grizzly Bear Study Team leader, said the high number of grizzly deaths this year is not out of whack with normal fluctuations in the number of bear deaths for a given year.

"We've had them in the past," Schwartz said of the high-mortality year. "They jump up and down. Periodically we can have what you would call a bad year. It's not outside the norm to see a periodic bad year."

Still, Schwartz said if the mortality threshold is exceeded this year and next, that will trigger a "management review" to see if something needs to be done to stem the mortality problems for bears. If the high death rates continue, it could prompt the grizzly to be relisted under the Endangered Species Act.

The bear was removed from federal protection earlier this year, but there have not been any significant changes to bear management. No hunting seasons have begun, and no significant development has taken place in grizzly habitat.

Schwartz said the deaths are possibly attributed to drought conditions with fewer food sources. There is also a near-record number of females with cubs this year, which means there may have been more conflicts caused by mothers defending their cubs.

In Wyoming, five bears were killed in hunting-related incidents and 10 bears killed total this year. Last year, there were four bear deaths in Wyoming: one an agency removal, one roadkill, one after it charged a hunter, and one in a case of mistaken identity for a black bear.

Mark Bruscino, bear management officer with the Wyoming Game and Fish Department, echoed Schwartz and said one high death year for grizzlies does not mean the population is in trouble.

"With this many bears and this many people sharing the land, there's going to be some problems," Bruscino said.

In Wyoming, there were no maulings by grizzlies this year outside the national parks. The 10-year average of grizzly attacks on humans is two per year.

In Montana, there were three grizzly attacks. Bruscino said it was likely just a "blip" in the long-term trend, and there may be four maulings next year in Wyoming.

"I don't think anything special is happening," he said.

Still, the high number of grizzly deaths this year is worrisome to some who say for bear deaths to spike the first year after delisting represents a problematic future for bears.

There are an estimated 600 grizzlies in the greater Yellowstone area. Managers estimate 60 percent of the population is female.

With bears going into hibernation now, the number of deaths should slow or stop for the year.

Environmental reporter Whitney Royster can be reached at (307) 734-0260 or at royster@tribcsp.com.