Limiting factors might obscure population regulation of moose (Cervidae:Alces alces) in unproductive boreal forests

Abstract

(1) In the core of the moose range in North America, moose numbers are regulated at a low equilibrium density (∼ 0.4 animal x km^‐2) by predation when wolves coexist with a bear species. We hypothesized that top‐down dominance of moose could also occur further north in undisturbed boreal forest where densities generally reach less than 0.1 moose x km^‐2. (2) Summer forage production (leaves of shrubs and trees) averaged only 17 kg x ha^‐1 in our 3090 km² study area, whereas it ranged between 144 and 241 kg x ha^‐1 in the core of moose range in the Québec‐Labrador peninsula. Annual production of preferred winter forage (twigs of deciduous shrubs and trees) was also very low in the study area, contrary to that of the evergreen balsam fir, whose availability was comparable to that measured in southern ranges. (3) Low forage production in the study area was due to a high proportion of outcrops (17%) and open black spruce stands (20%), and to the old age of most forest stands. (4) Forage had a patchy distribution: production of leaves in the most productive habitat type, deciduous stands, exceeded by 22 times that measured in the coniferous stands. Forage distribution influenced habitat use by moose, which generally showed year‐round preferences for forage‐rich mixed and deciduous stands and resulted in large home ranges. (5) Balsam fir dominated moose diets in late winter, whereas this plant species represented only 17‐19% of forage consumed further south in the moose range. We hypothesized that heavy consumption of balsam fir negatively affected the energy balance of moose in winter. (6) Monitored females produced only 78 calves: 100 cows, during three consecutive years. This relatively low rate of calf production could be related to suboptimal nutrition in winter and in summer. We found a significant relationship between snow depth, summer temperature, and lactation rate, an index to cow:calf ratios in autumn. Lactation rate was more strongly related to weather of the preceding year than that of the current one, suggesting that weather had an immediate influence on body reserves of dams, but little impact on calf production until the next year. (7) Calves experienced high survival, particularly during their first summer of life (90%), in contrast with southern moose populations exposed to higher predation pressure. Annual survival of adult females (87%) exceeded that of males (74%) because the latter were more vulnerable to hunting. Natural mortality was similar in both sexes, 8‐10%. In adults, predation may have been lighter, and other natural mortality causes higher in the study area than in the core of the moose range. (8) Limiting factors. i.e. climate, drowning, played a greater role in the demography of the study moose population than predation, the suspected regulating factor. Our data set illustrates that regulation may be difficult to detect in unproductive environments, a common situation at the periphery of any species distribution.