The aim of this report, commissioned by the Swedish Environment Protection Agency (SEPA), was to evaluate if and under which conditions a Favourable Reference Value (FRV) of 170 to 270 wolves (Canis lupus) represents a viable population in Sweden. To address this question, I performed a Population Viability Analysis using forward-in-time genome-informed simulations implemented in SLiM. I modelled a large population in Karelia and a smaller population in Scandinavia (i.e., including wolves from both Sweden and Norway) and examined the demographic and genetic viability of the population. I first modelled the effects of survival, reproductive output, population size and migration rates on the probability of extinction of the population. Secondly, using field-based empirical estimates for demographic and life-history traits, I examined the effect of varying population size and migration rates on genome-wide diversity (e.g., nucleotide diversity, inbreeding and two components of genetic load) as proxy for viability. 

Simulations indicate that with reduced survival rate and female reproductive output, the risk of extinction would range between 22 and 32% for a population size of 50 and between 1 and 10% for a population size of 100. However, when using higher survival rates and female reproductive output values based on field-based estimates, the risk of extinction was close to 1% for a population size of 50 and no extinction was reported for a population size ≥100.

Furthermore, for a population size of 170 to 270 wolves in Sweden (i.e., 210 and 310 for the whole Scandinavian population), between 1 to 3 effective (i.e., reproducing) immigrants per decade would be needed remain within a 5% window of loss in nucleotide diversity and increase in inbreeding. However, while migration rates above a threshold of 1-3 effective immigrants per decade would increase genetic diversity and would potentially induce a genetic rescue effect, it would also represent a risk of introducing new deleterious variation, especially for lower population sizes. Moreover, the simulations showed that larger population sizes would be more immune to loss of diversity. Yet, there would also be a non-negligible risk of introduction of new deleterious variation with ≥8 migrants per decade. Consequently, the trade-off between genetic rescue and introduction of deleterious variation needs to be taken into account when determining a target population size for management.