Accepted Manuscripts / Survival on the border: A population model to ...
Survival on the border: A population model to evaluate management options for Norway’s wolves
Joseph Bull, Erlend B. Nilsen, Atle Mysterud & E. J. Milner-Gulland
We present an individual-based model of the Norwegian wolf population, which is used to evaluate the effectiveness of current and potential management policies in fulfilling the Norwegian Government’s stated aim of maintaining three breeding packs within a designated wolf zone. The model estimates the ‘functional extinction rate’ of the population, defined as the proportion of years in which breeding wolf packs are absent. Under current conditions according to estimates from Scandinavia, with observed values of natural survival rates (0.903) and unauthorised mortality (0.203) and allowing for immigration from Sweden, the model predicts that the probability of functional extinction is as low as 0.07. This output variable is highly sensitive to the demographic parameters, and if alternative estimates of natural survival rates (0.73 for cubs, 0.83 for adults) reported for wolves elsewhere and higher rates of unauthorised mortality (0.4) are utilised, the functional extinction rate is 0.67±0.15, and the population is dependent on maintenance by immigration from Sweden. The main determinants of functional extinction rate are the unauthorised mortality rate and the immigration rate from Sweden. The Scandinavian population as a whole shows a rapid non-linear increase in probability of extinction at unauthorised mortality rates above 0.10. Varying levels of the current management interventions (increasing the size of the wolf zone and target number of packs) are ineffective; only when unauthorised mortality rate falls below 0.30 is a self-sustaining population in Norway able to establish. An adaptive harvest policy with culls targeted only at dispersing animals, or taking place only when the population exceeded a threshold level, could be sustainable if the unauthorised mortality rate is reduced. The fact that the Norwegian Government has been explicit about its management strategy and objectives has allowed us to test the ability of this strategy to meet the objectives, and we have shown that it is dependant upon maintaining the current circumstances alongside a high adult survival rate to be able to do so. Given the potentially critical role of the Swedish population in sustaining Norway’s wolves, there is a strong case for joint management of the Scandinavian population. These insights are likely to be relevant for the management of other species living across geopolitical boundaries.
Key words: Scandinavia, Canis lupus, demography, harvesting, cull, dispersal