Wolf Response to Increased Moose Vulnerability on Isle Royale

Peterson, Rolf

Ecological studies of wolves and moose in Isle Royale National Park have been conducted on a yearly basis since 1959. Initial investigators found a single pack of wolves which in mid-winter hunted the entire island, preying on old adult moose and calves. In non-winter months, wolves relied principally on moose calves, and beaver were a minor food supplement. Moose productivity was high and was attributed to intensive predation by wolves. Since that period moose productivity has declined and there is increased evidence of winter nutritional stress, apparently resulting from a combination of plant successional trends which reduced browse supply, increased winter severity, and an increase in the moose population during the 1960s. Among wolf-kills the incidence of moose between one and six year old increased since 1970. This major increase in vulnerability of young moose appears to have been caused by malnutrition early in life, apparently in the fetal stage, and consequent adverse effects on growth and development. From the late 1960s to early 1970s kill rates have been generally high with carcasses underutilized, and in 1972 a second large pack of wolves became firmly established, for the first time. In 1975, a third pack appeared and the wolf population reached a historic high 41 wolves. Over the past decade an increased beaver population has become an important source of food for wolves during the open-water season, contriuting to a wolf population increase. These changes have demonstrated the sensitivity of wolf predation to slight changes in moose vulnerability and have augmented the effect of predation in the dynamics of this moose population. Wolf predation continues to be the principal mortality factor, but the degree to which it operates is determined by moose vulnerability, with long-term leves of the moose population governed basically by food availability.