Wolf and Moose Dynamics on Isle Royale

Abstract

Moose ( Alces alces ) arrived on Isle Royale in the early 1900s (Mech 1966) . For 50 years moose interacted with the forest without being exposed to predation or significant human harvest. By the late-1920s the impact of moose on the forest had become noticeable and the population probably comprised between 2,000 and 3,000 moose (Murie 1934) . By the mid-1930s many moose had died of malnutrition and the population declined to probably a few hundred animals (Hickie 1936) . Although there were suggestions and one attempt to introduce gray wolves ( Canis lupus ) to Isle Royale in the 1940s and 1950s, the attempt failed in 1952 (Mech 1966) . While humans were trying to reintroduce wolves, they arrived on their own in the late 1940s by crossing an ice bridge connecting Isle Royale and Canada. Analysis of mitochondrial deoxyribonucleic acid (mtDNA) indicated that the population of wolves on Isle Royale was founded by a single female (Wayne et al. 1991) . Since the founding event, the wolf population on Isle Royale has, to our knowledge, remained genetically isolated. Humans do not harvest the wolves, moose, or forest on Isle Royale. Although present on the nearby mainland, white-tailed deer ( Odocoileus virginianus ), coyotes ( Canis latrans ), and black bears ( Ursus americanus ) are absent from Isle Royale. The diet of Isle Royale wolves is ~95% moose during winter, and the diet in summer is >85% moose. Most of the remaining diet consists of beavers ( Castor canadensis ). The only significant causes of death for moose on Isle Royale are wolf predation and malnutrition, both of which are sometimes exacerbated by severe winters and winter ticks ( Dermacentor albipictus ). Between 40% and 60% of the diet of moose in winter is a single species, balsam fir ( Abies balsamea ). Although the wolf–moose system on Isle Royale is commonly characterized as a single-prey/single-predator system, this characterization may not be entirely justified. The importance of other factors such as canine parvovirus (Wilmers et al. 2006) , moose ticks (Peterson and Vucetich 2006) , and winter severity (Vucetich and Peterson 2004a) have been clearly documented. Nevertheless, compared with many communities of large vertebrates, the wolf–moose system on Isle Royale seems simple (Smith et al. 2003) .