Mortality Patterns in the Isle Royale Moose Population

Abstract

Sex, age and temporal distribution of 439 Isle Royale moose (Alces alces), dying of natural causes from approximately 1950 to 1969, are analyzed. At least 45% of the animals examined had been killed by wolves (Canis lupus). Calves and yearlings comprised 29.3% and 3.5%, respectively, of the wolf-kills, but were underrepresented in relation to their occurrence in the population. A pronounced selectivity for animals was demonstrated with age classes 12-17 years comprising 29.3% of the wolf-kills. As a group, the wolf-kills also showed a significant preponderance of females. Age distribution of moose dying of unknown causes (34.9% of the total sample) differed significantly from that of wolf-kills. Young adults comprised 24.2% of the unknown mortality remains, while middle-aged (7-11 years) and old animals accounted for 37.9% and 12.4%. Moose dying of unknown causes showed a highly significant preponderance of males, but age distribution between sexes did not differ statistically. Age distribution among winter wolf-killed moose showed a significant increase of younger animals in the kill during the latter half of the period 1959-69. The role of wolf pr?dation in the regulation of this population is discussed. Coactions of the timber wolf (Canis lupus) and its principal prey, moose (Alces alces), at Isle Royale National Park have been under investigation since 1958. Various aspects of the predator-prey relationships in this insular community are described in Mech (1966, 1970), Shelton (1966), Jordan et al. (1967) and Jordan et al. (1971). These studies demonstrate a marked selectivity for juvenile and old animals. Elsewhere in North America, investigations of wolf pr?dation on other ungulates indicate similar patterns: Murie (1944) for Dall sheep (Ovis dalli)', Pimlott et al. (1969) and Mech and Frenzel (1971) for white-tailed deer (Odocoileus virginianus) ; Banfield (1954) for caribou (Rangifer tarandus) ; Tener (1954) for muskoxen (Ovibos moschatus) ; and Fuller (1966) for bison (Bison bison). Other authors (Pimlott, 1967; Krantz, 1970, and Schaller, 1972) have emphasized the importance of the age distribution of prey taken to the predator's effect on the prey population. The purpose of this article is. to define age specific mortality patterns in the Isle Royale moose population and assess the role of wolf pr?dation as a mortality factor. Study Area and Methods Vegetation and faunal history of the 544-km2 island study area in northern Lake Superior have been described by Linn (1957) and Mech (1966). Moose became established on Isle Royale in the decade after 1900 (Hickie, undated). Prior to the advent of the timber wolf about 1948 (Hakala, 1954), the population underwent severe fluctuations (Krefting, 1951, 1974). The present population of at least 1200 animals sustains no exploitation other than wolf pr?dation. Deaths