Role of Landscape Use in the Regulation of Large Herbivore Populations --Lessons from Caribou and Muskoxen.

Robert G. White, Jan E. Rowell, Karen L. Gerhart, Raymond D. Cameron, Don E. Russell, and Brad Griffith
Institute of Arctic Biology, Univ. of Alaska Fairbanks, Fairbanks, AK 99775-7000; Canadian Wildlife Service, Box 6010, Whitehorse, Yukon Territory, YIA 5L7; Arkansas Cooperative Wildlife Research Unit, Univ. of Arkansas, Fayetteville, AR 72701.

Only two large arctic herbivores have persisted since the Pleistocene era -- muskoxen (Ovibos moschatus sub. sp.) and caribou (Rangifer tarandus sub. sp.). These species use the landscape very differently even though their summer and winter ranges may overlap. In extreme cases, muskoxen spend their whole lives within a confined home range of a few hundred hectares, whereas caribou have distinctly different and widely separated seasonal ranges. We hypothesize that muskoxen have evolved mechanisms that tightly match reproduction rate to resource availability and quality, which minimizes chances of local extinction; caribou protect against extinction by occupying extensive, diverse landscapes. For muskoxen, the proximate control over reproduction is the quality of late summer-autumn range (i.e., during the rutting season), specifically, the food resource that controls the acquisition of body reserves supporting gestation. When these reserves are insufficient, female muskoxen fail to conceive and undergo a breeding pause. Although elements of this same mechanism apply to caribou, food resources during lactation and fetal development also are important to net productivity. Muskoxen appear to adjust reproduction rate in response to short-term changes in habitat quality, a means of protecting spatially-limited forage resources. In contrast, by making low intensity, short-term use of spatially disjunct resources, caribou maximize productivity and, therefore, population size while denying resident predator populations the opportunity to exert regulatory effects.