NATIONAL PARK SERVICE Wolf Ecology and Prey Relationships on Isle Royale

Predator-prey relationships between timber wolves (Canis lupus) and moose (Alces alces) were studied from 1970 to 1974 in Isle Royale National Park, a 544-km² island in Lake Superior, as part of a continuing research program begun in 1958.

Initial studies in the late 1950s and early 1960s showed that a single large pack of wolves hunted the entire island, preying in midwinter on old adult moose and calves. In nonwinter months wolves relied heavily on moose calves; beaver were a minor food supplement. Moose productivity was very high and was attributed to intensive predation by wolves.

By 1970, moose productivity had declined markedly and there was increased evidence of winter nutritional stress. Young adult moose and calves became especially vulnerable to wolves in winter due to the effects of malnutrition early in life and to the unusually deep snow. Also, wolves relied heavily on an increased beaver population. The food resources for wolves thus were expanded significantly. Concurrently, the size of the original pack's territory was reduced, allowing a second pack to establish itself in the summer of 1971. As a result, the winter wolf population increased from 20 in 1971 to 31 in 1974.

The two principal wolf packs were tracked by plane in midwinter for 234 "pack-days." The packs traveled an average of 11 km/day and 33 km/kill. Travel for both packs was least when, because of snow conditions, moose were easier to kill.

Food availability for pack members ranged from 4.4 kg to 10.0 kg/wolf/day and declined for both packs between 1971 and 1974. Concurrently, the amount of spatial overlap between the two packs increased, leading in 1974 to direct conflict and the death of one wolf.

A social hierarchy existed in both packs, with dominant, or alpha, males and females leading the pack in daily activities. Alpha wolves did most of the mating and discouraged courtship behavior among subordinate wolves. Leadership in both packs exhibited pronounced year-to-year stability.

Wolf predation accounts for most of the adult moose mortality, which occurs primarily in winter. Normally, losses between the ages of 1 and 7 years are relatively light but moose mortality increases steadily thereafter. Average annual adult mortality was estimated by a life-table to be 13%. Male moose die slightly sooner than females and exhibit a higher incidence of arthritis and malnutrition. The oldest males and females recorded were 15.5 and 19.5 years, respectively.

The incidence of moose aged 1-6 years among wolf kills increased from 13% prior to 1970 to 53% since 1970. This reflected a major increase in vulnerability of young moose which appears to have resulted from increased malnutrition early in life and resultant adverse effects on growth and development. Bone measurements revealed that calf size at birth was correlated with the severity of the previous winter, and generations of calves born after winters of nutritional distress account for almost all of the young adult moose killed by wolves.

Increased nutritional stress among Isle Royale moose in the early 1970s apparently resulted from a combination of plant successional trends which reduced browse supply, increased winter severity because of deep snow, and an increase in the moose population during the 1960s.