Anursus: A Population Analysis System for Polar Bears (Ursus maritimus)

ANURSUS estimates the mean and standard error of polar bear (Ursus maritimus) population parameters (i.e., cub survival rate, litter survival rate, subadult and adult survival rates, litter production rate, litter size, and mating and reproductive intervals) from age specific observations of litter size and family group status. The parameterization for recruitment estimates is unconventional so that the 3-year reproduction cycle of polar bears may be correctly described (Taylor et al. 1987). Previous estimates of annual polar bear cub survival rate considered only the loss of individual cubs; they did not consider abandonment of single cub litters or loss of entire litters. We provide an estimation procedure that accommodates all sources of cub mortality for arctic polar bear populations. Data required for the procedure include the female age structure; number of females with offspring; presence or absence of cubs-of-the-year, yearlings, or 2-year-olds; and observed litter size. The average age of 1st reproduction may be calculated by weighting each age by its probability of 1st reproduction and determining the weighted average. The probability of 1st reproduction at age x is determined from age specific litter production rates and the standing age distribution. The average age is for all females in the population during the census period. The mean interval between producing litters (litter recruitment interval) and the mean interval between mating availability (mating interval) are different for polar bears. The reciprocals of the 2 intervals, mean litter recruitment rate and mean mating rate, are useful to compare populations but should not be used for population projections. In addition to litter recruitment interval and mating interval, 2 measures of the expected number of litter recruitment events are also defined.