Modeling the impact of moose and wolf management on persistence of Woodland Caribou.

ABSTRACT: Limiting factors of caribou (Rangifer tarandus) populations vary regionally. In tundra environments, this species appears to be regulated by food, either because wolves (Canis lupus) are absent or because migration of caribou allows escape from predation during part of the year. In the boreal forest, the main limiting factors are hunting and predation but because of low caribou densities, no regulation mechanism seems to exist between caribou and wolves. Moose (Alces alces) is the primary prey species of wolves and consequently, if moose abundance increases, wolves should also increase, independently of the caribou population. Thus, caribou could experience high predation rates and be eliminated in high wolf densities. Here we attempted to identify the necessary conditions to maintain caribou numbers in the presence of moose. To do so, we built a deterministic model that simulated the relationship between a caribou population regulated by food competition and limited by predation, a moose population regulated by predation, and a wolf population, the abundance of which is determined by moose abundance. At current hunting rates for caribou and moose in the boreal forest, and in the absence of wolf trapping, the model predicted that the caribou population would be extirpated in approximately 100 years. Wolf trapping was not adequate to conserve the caribou population unless very intensive control was undertaken. In the absence of trapping, cessation of caribou hunting allowed a 3-fold increase in caribou numbers over the long term, if the moose population remained low. According to our model, the best management measure for caribou consisted of maintaining a low moose density through appropriate population and habitat management strategies, which prevented expansion of the wolf population and limited predation on caribou.

Key words: caribou, hunting, interactions, moose, predation, simulation, trapping, wolf

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The hypothesis of exploitation ecosystems predicts that the number of trophic levels and population regulating factors depend on the primary productivity of an ecosystem (Oksanen et al. 1981, Oksanen and Oksanen 2000). In very poor environments (e.g., high arctic and deserts, where productivity is

Because regulating factors of caribou (Rangifer tarandus) vary between ecotypes, the exploitation ecosystem model seems appropriate for prediction of caribou population changes. In tundra environments, low productivity and low carrying capacity contribute to the establishment of undiversified biological communities. Predators are even absent on certain Arctic islands (Klein 1968, Ouellet et al. 1996) and in such cases, caribou populations are regulated by competition for food and population changes can be described by a logistic model (Caughley 1977). Net population growth rate varies with density: populations increase exponentially at first and then growth rate gradually slows down as populations approach carrying capacity. In such environments, the carrying capacity seems to be on the order of 60-100 caribou per 100 [km.sup.2], as observed on Coats Island in Hudson Bay where caribou have occurred since at least the beginning of the 20th century (Ouellet et al. 1996). Massive mortalities from starvation sometimes take place overwinter due to food overexploitation or density-independent factors, such as climatic conditions preventing access to food (e.g., ice crust; Klein 1968, Reimers 1982).

In continental tundra, caribou populations also seem to be regulated by food. Caribou may undertake very long migrations that allow escape from predation most of the year, partly because wolf (Canis lupus) packs are confined to an exclusive territory, but mostly because wolf packs cannot move away from a den site when raising pups during summer (Bergerud 1996). In tundra environments, predation can be a significant limiting factor, but predation rate does not increase as a function of caribou abundance. Caribou numbers can therefore increase and, at high density, they call overexploit food available during summer (Bergerud 1996, Crete and Doucet 1998) or winter (Ouellet et al. 1994, 1996, 1997). As resources available per animal diminish with increasing population, a decrease in birth rate and an increase in adult and calf mortality are noted. Densities averaging 60-110 caribou per 100 [km.sup.2] have been observed in this environment (Messier et al. 1988, Seip 1991). Using the annual increase in lichen biomass, which is the primary food source for caribou (Gauthier et al. 1989), along with losses caused by animal trampling, Arsenault et al. (1997) have estimated the carrying capacity of caribou in tundra at approximately 20 animals per 100 [km.sup.2].

In the boreal forest, the carrying capacity for caribou is not precisely known. Based on lichen biomass, Crete and Manseau (1996) estimated that carrying capacity should be at least that observed in tundra, because alternative food sources (such as leaves, twigs, and deciduous shrubs) are abundant and the climate is milder in that environment. For example, in east-central Quebec, carrying capacity based solely on terrestrial lichen has recently been estimated at 4.1-7.7 caribou per 100 [km.sup.2] (Courtois 2003). Despite a relatively high potential carrying capacity, woodland caribou populations experience very low densities of between 1 and 3 individuals per 100 [km.sup.2] (Seip 1991, Courtois 2003) and most are declining in North America (Mallory and Hillis 1998). Those observations suggest that woodland caribou populations are not regulated by intraspecific competition for winter food. Moreover, as caribou food habits are much less restrictive in summer, the main limiting factors seem to be hunting and predation (Stuart-Smith et al. 1997, Rettie and Messier 1998).