Harvesting strategies for conserving minimum viable populations based on World Conservation Union criteria: brown bears in Norway

The World Conservation Union (IUCN) defines populations as vulnerable if the probability of extinction is larger than 10/% within the next 100 years. With the objective of minimizing problems with predation on domestic livestock and, at the same time, conserving a viable population, we consider different threshold harvesting strategies for a small population of brown bear, based on a population dynamics model with growth rate and demographic and environmental variances estimated from the present Swedish population. Taking into account uncertainties in present estimates of the demographic parameters and in population size, we show that the population can be harvested when the population size exceeds 34 female bears,aged one year and older, if the entire population exceeding the threshold is harvested. To minimize the expected long-term population size, however, we show that it is optimal to harvest only a proportion equal to 35% of the population exceeding a lower threshold of 12 female bears. This strategy gives an expected long-term population size of around 20 female bears. If the growth rate of the population is reduced by ca. 3%, the threshold must, under some conditions, be doubled. We argue that the small thresholds are mainly a result of the high intrinsic growth rate of the population considered in the present paper. However, the analysis also suggests that IUCN's criterion might allow a rate of extinction that is too high.