Ecological correlates of regional variation in life history of the moose Alces alces

The food limitation hypothesis of population regulation states that a stable equilibrium will exist between a population and its food resources due to a density-dependent decrease in fecundity and/or increase in mortality. This hypothesis was tested for the moose (Alces alces) by comparing regional variation in life history characteristics in four Norwegian study areas, chosen to represent a gradient both in summer and winter range conditions. The rate of body growth was most rapid in the northern study area with the best summer ranges. Lowest body growth occurred in the population living under the poorest winter conditions. After snow-free winters the rate of body growth increased substantially, leading to large annual variations in yearling body masses. These differences in growth lead to significant age-specific differences between populations in female carcass masses in autumn. Probability of maturing as a yearling increased with body mass in the three southern populations. In contrast, in the northern study area no such relationship was found, leading to a delayed age at maturity in the population with the largest females. This shows that the relationship between age at maturity and body mass differs regionally, suggesting different selective regimes. The peak timing of ovulation of old females in the autumn showed a latitudinal delay. Females in the alpine population, with the poorest winter conditions, had significantly later mean calving dates and produced fewest calves per year. Gestation length appears to be dependent on nutritional condition of females during pregnancy. Mortality was highest in the northern study area where most of the deaths occurred during the summer. Very few calves died during the winter. These results suggest that a stable high-density equilibrium between moose and their food resources as expected from the food limitation hypothesis is unlikely. The decrease in fecundity and the increase in mortality under poor nutritional conditions during the winter has only a small effect on the population growth rate and is therefore unlikely to have a strong regulatory effect. In the absence of large predators, this will lead to large fluctuations in population size that will overshoot the carrying capacity.