Yellowstone National Park's internationally famous wolf population has stabilized at about 100 wolves over the last two years as their main food source, elk, has declined.
According to data from the park, at the end of 2011 there were 98 wolves in 10 packs with two roaming loners occupying portions of the vast and mountainous ancient caldera.
That compares with 97 wolves at the end of 2010, and represents a 60 percent decline from 2007 numbers.
Mollie's was the largest pack last year, at 19 members. It traditionally occupies the Pelican Valley in the interior of the park, but this winter has staged in the Lamar Valley farther north. They migrated when a lack of snowfall made it harder for them to hunt bison, their traditional winter food staple.
The movement by Mollie's has caused more rivalry between the other packs in the area -- the Agate, Blacktail and Lamar Valley packs -- for the limited food source. Amid the rivalry, the Agate pack has been whittled down to only three members. In all, 10 collared wolves were killed in 2011, six of them in pack rivalries. One other wolf was killed after it lost its fear of people.
Studies of wolf kills confirm that elk are the packs' main source of protein. Out of 343 wolf kills found by park staff last year, 267 were elk (78 percent), compared to only 15 bison (4 percent).
Out of those elk kills, 44 percent were cow elk, followed by calves at 27 percent; bulls, 18 percent; and yearlings, 3 percent. The bison kills included five calves, one yearling, two cows and six bulls. Some of the kills could not be identified.
An estimated 25,000 visitors observed wolves last year, mainly in the Slough Creek and Lamar Valley areas.
Visitors wouldn't have seen two park packs, though, because they disappeared in 2011, although one was replaced by a pack from outside the park. The Quadrant Mountain pack was lost, but was replaced by the 8-mile pack. The Grayling pack was lost without replacement.
Pup production for Yellowstone's wolves was stable in 2011, with eight breeding pairs producing an average of 4.1 pups. Thirty-four pups survived to the year's end.