A study published in Frontiers of Ecology examined grey wolf pack stability over 33 years from five National Parks in the United States. The parks were located in Alaska, Wyoming, and Minnesota. Many different population management methods have been used throughout American history, ranging from complete eradication to full protection under the Endangered Species Act. The study examined the survival of packs based on mortality levels of pack members.

The largest average pack sizes were found in Yellowstone and Grand Teton National Parks, where sizes ranged from 6 to 8 in the early spring, to 10 to 12 in the fall. The smallest were found in Voyageurs National park, with a range of 4 to 5 members.

The authors found that human caused deaths tended to have much more drastic effects than natural deaths among pack members. Human caused deaths are more likely to involve pack leaders, breeding females, or other healthy members of the pack. When breeding females were lost, the pack was very unlikely to produce pups the next spring. Most human caused deaths were from legal hunting or illegal poaching. Most accidental deaths were caused by vehicle collisions. Packs that experienced no human caused deaths had an 80% or greater chance of persisting into the next year. Human caused deaths reduced that chance to as low as 60%.

The authors noted that the dissolution or collapse of a pack doesn’t necessarily imply the death of all the pack members. Pack dissolution and formation are a natural part of the lives of grey wolves. Overall populations tend to persist even when packs come apart.

Voyageurs National Park Wolf Project

History of Wolves at Yellowstone Video