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A Demographic and Genetic Analysis of Minimum Viable Population Size to Inform the Population Reference Value for Wolves in Sweden: Final report
IUCN SSC Conservation Planning Specialist Group.
Responsible organisation
2024 (English)Report (Other academic)
Abstract [en]

In May 2022, the Swedish Government commissioned the Swedish Environmental Protection Agency (SEPA) to investigate, based on the best available knowledge and scientific expertise if, and under what circumstances, the population reference value for the wolf (Canis lupus) as defined for favorable conservation status according to the European Union Habitats Directive, could be between 170 and 270 individuals in Sweden as stated in the parliament proposition from 2012 (prop. 2012/13:191). This report details an independent analysis requested by SEPA to inform future decision-making for wolf conservation in Sweden. Based on the distinction between minimum viable population (MVP) and favorable reference population (FRP) value as described in the Habitats Directive guidance documentation, the analyses described in this report specifically address the identification of a minimum viable population size for wolves in Sweden. Translation of this MVP value to a population abundance incorporating larger-scale ecosystem functionality, representation evolutionary genetic considerations – the FRP value – requires a process of “upscaling” to a larger population abundance. The translational process is outside the scope of this analysis and is instead to be conducted by SEPA after receipt of this PVA. 

Another important issue governing the interpretation of this PVA concerns the ambiguity around the explicit definition of population viability in the Habitats Directive and supporting documents. To be fully operational, a definition of viability for a specific population should be quantitative and reflect an acceptable level of risk tolerance over a defined time frame. Because this quantitative definition was provided by neither the EU nor SEPA, it is not possible to provide a definitive interpretation of the PVA results in terms of what combinations of characteristics constitute a viable wolf population in Sweden. The process of setting quantitative thresholds for acceptable risk is a complex normative process that must be conducted by policy makers and not within the species research community. In the absence of such a definition, simulation model results can be viewed on the basis of alternative definitions of viability in order to provide guidance to policy makers in their exploration of attitudes on acceptable risk. 

This analysis was conducted using the simulation software Vortex, an individual-based demographic modeling package used around the world for exploring threats to endangered species and evaluating alternative management strategies. The wolf population in Scandinavia, distributed across south-central Sweden and southeast Norway, was considered to be a single population for purposes of simulating population dynamics. In addition, the population of wolves in Finland/Russia was included as a separate demographic unit to simulate occasional immigration of wolves into the Scandinavian population from this source. The core model structure featured two timesteps per year (each six months in duration) in order to more precisely account for reproduction in the spring and the population census to take place in the winter. The dataset of known living wolves in Scandinavia as of 1 October 2022 (N = 463) was used to initialize the predictive models, with the full pedigree of these individuals and their ancestry used to establish the starting population genetic structure. This valuable information influences the rate of retention of genetic variability (gene diversity) into the future as a function of relatedness among individuals and the inbreeding that can occur as adults form pairs in order to reproduce. Average rates of reproduction and survival, including both natural sources of mortality and anthropogenic mortality in the form of legal and illegal culling, were assembled from the literature and used to generate a population dynamics model with an expected realized annual population growth rate of approximately 2% which has been observed in the wild over the past decade of detailed census counts. The model explicitly counts population abundance at a point in the simulated annual cycle that generally corresponds to the actual wild population census taken as of 1 October. 

Because the current population of wolves in Scandinavia is larger than the range of population reference values (170 to 340) tested in the analysis, the simulations feature gradual removal (culling) of wolves over the first five to seven years in order to reduce the population to an abundance consistent with a given minimum population abundance threshold value. After that point in time, the population is maintained at or above the abundance threshold through the use of legal harvest when necessary (i.e., culling is not performed if the population is assessed to be less than the stated threshold). Wolves identified as valuable to the genetic viability of the population, especially immigrants from the Finland/Russia population, are exempt from removal. This selection process works to minimize the genetic costs of the removal program. Occasional immigration of wolves from the Finland/Russia population is simulated using random dispersal mechanics, with average immigration rates ranging from no immigration (an isolated Scandinavian population receiving no more wolves) to, on average, one wolf immigrating into the Scandinavian population every three years (a time interval that is roughly similar to the average generation length for this population). Immigration rates considered in this analysis are the actual rates, with each new migrant being at risk of dying before they successfully reproduce and, therefore, incorporate their genetic variation into the local population. The impact of their immigration on local population genetic viability, however, is observed through their reproductive success (determined by defined probabilities in the stochastic modeling environment) before mortality removes them from the population. 

A total of 30 model scenarios, defined by unique combinations of population reference value and mean expected immigration rate, formed the core of the analysis. Demographically, the simulation models performed as expected, with long-term wolf abundance in the Scandinavian population governed by the expected mean rate of population growth and reaching a type of equilibrium after approximately ten years near the appropriate population reference value. This stable abundance was about 20% - 25% larger than the scenario-specific population reference value, owing to the production of new pups in early spring preceding the 1 October census. The simulated populations would decline to a number much closer to that value after the October – April timestep when winter mortality and, if necessary, removal of wolves occur. Because of the relatively larger starting abundance combined with the mean positive long-term population growth rate, extinction risk across the range of scenarios tested here was quite low, exceeding 0.01 over 100 years in just three of the 30 scenarios making up the analysis and never exceeding 0.02. 

As expected from theoretical principles of conservation genetics, simulated populations maintained at smaller population abundance threshold values would show a more rapid rate of loss of genetic variation (gene diversity) over time, particularly if future immigration did not occur. Across the range of values tested here, immigration of wolves from the Finland/Russia population improved gene diversity retention over time. More frequent immigration (one wolf every three to six years) resulted in the Scandinavian population retaining at least 95% of the gene diversity present at the start of the simulation over the full duration of the simulation (100 years) across nearly all tested population reference values. When immigration averaged one wolf every three years, this retention increased to 99% to 100.5% of the original value, owing to the infusion of new genetic variation into the Scandinavian population from the Finland/Russia source. The process of retaining high levels of gene diversity in a population is influenced by stochastic (random) variability, however, resulting in a risk that these particular genetic goals may not be achieved even under favorable conditions. Therefore, choosing a genetic criterion for population viability should not only specify the desired level of gene diversity retention, but also the degree of confidence with which that desired level of retention is likely to be achieved.

Given the nature of the current models discussed in this report, and acknowledging the assumptions built into these simulations as described above, the analysis suggests that the wolf population in Scandinavia (south-central Sweden and southeast Norway) can potentially be considered viable within the interval of 170 to 270 individuals in accordance with the broad definitions presented in the European Union’s Habitats Directive. However, this condition requires the following processes to be maintained through time: 

  • The Scandinavian wolf population must have the demographic characteristics to, at a minimum, sustain a positive population growth rate, ideally similar to or greater than what has been observed over the past decade of detailed observations of reproduction and survival (annual growth lambda λ ≥ 1.02, with the possibility of considerably higher growth rates in the absence of legal harvesting of wolves); and 
  • Immigration of wolves from Finland/Russia into Scandinavia should be, on average, no less than one individual every three years. 

The above discussion defines conditions for maintaining a viable population of wolves in Scandinavia. A similarly viable wolf population in Sweden would also require the same general demographic conditions: reproductive and survival parameters that result in a capacity for sustained population growth, and consistent immigration of wolves from the recognized source population in Finland/Russia. However, because the Swedish population represents only a portion of the total wolf population in Scandinavia, any specification of a minimum viable population for the purposes of setting a favorable reference population in Sweden would require proper scaling of the larger regional population. In addition, it is critically important to recognize that the precise demographic characteristics of a viable population in Sweden or elsewhere cannot be specified until a clear demographic and genetic definition of wolf population viability is presented by the appropriate national or regional authorities.

Place, publisher, year, edition, pages
2024. , p. 50
National Category
Environmental Sciences
Research subject
Hunting; Species protection
Identifiers
URN: urn:nbn:se:naturvardsverket:diva-10982OAI: oai:DiVA.org:naturvardsverket-10982DiVA, id: diva2:1848046
Available from: 2024-04-02 Created: 2024-04-02 Last updated: 2024-04-02Bibliographically approved

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