Genetics and Connectivity

​​Population genetics

The characteristics of individual and groups of brown bears are influenced by the environment they live in and thus they can differ across a range of scales, e.g., both within and between populations due to varied background conditions and population history. Studying these differences between or among groups or populations of organisms at the DNA-level constitutes the field of population genetics. Various molecular methods assess the genetic characteristics and variation between individuals (i.e., genetic diversity) to identify similarities and differences among groups. Every bear has a unique DNA-profile, with the exception of identical twins, which means individuals can be identified, counted, and their sex established, and populations can be grouped based on how genetically similar they are. Comparing the genetic profiles of bears, both at the individual and population level, provides us with knowledge about their history, status, health, and relatedness, as well as the long-term effect of management actions. This is important because high genetic diversity increases the ability of bear populations to cope with their current environment and any subsequent future changes.

Population history and phylogeography: Initial genetic studies on the Scandinavian brown bear assessed the timing and recolonization routes of brown bears after the last glacial maximum. Studies using maternally inherited mitochondrial DNA (mtDNA) showed that bears recolonized from central Europe to southern Scandinavia, and eastern Europe and northwestern Eurasia into northern Scandinavia (1-4). So far, Ersmark et al. (2019) is the most comprehensive study on the phylogeography of the Scandinavian brown bear population and their results provided new evidence that recolonization after the LGM may had it source in central Europe (today’s Belgium and France) rather than the Iberian Peninsula, as previously assumed (2, 3). The most recent study suggested that the current genetic substructure of the bear population in Scandinavia is similar to the subgrouping found in historic samples Xenikoudakis, Ersmark (1), indicating that the current subpopulation structure is likely the product of historical ecological processes. Xenikoudakis et al. (2015) further identified numerous mtDNA-haplotypes among historic brown bear samples from Sweden, that are not present anymore, illustrating that once lost haplotypes but also alleles, will be lost forever. Like these maternal patterns, the y-chromosomal haplotypes are inherited from the father to the male offspring. Here, the Scandinavian brown bear population can be characterized with substantially low Y-haplotype diversity, compared to the neighboring Finnish-Russian population. Assessments found six different y-haplotypes in Scandinavia while in individuals from the Karelian population (northern Norway, Finland, and north-western Russia) 44 haplotypes could be identified leading to the assumption that the y-haplotype diversity in north-eastern Europe might be even higher; highlighting again the consequences of the past bottleneck of the Scandinavian brown bear population.

Current genetic structure of the Scandinavian brown bear: Genetic assessments showed that the Scandinavian brown bear population is structured into three genetic units, or clusters (5-8), partly shaped due to isolation-by-distance, especially in the northern parts (9). However, reasons for the persisting, historic substructure are also linked to current conditions and characteristics of the Scandinavian population. A similar genetic structure as today was identified also using historical samples, suggesting that the distinct subdivision, although overlapping, cannot be explained by the genetic bottleneck or by overhunting and anthropogenic fragmentation in recent times (1). Areas with high brown bear densities inversely affect natal dispersal as males do not have to disperse far to find mating partners unrelated to them (10). A combination of availability of suitable habitat, individual bear density, and differences in relatedness among bears in multiple areas can affect dispersal as well as influence successful reproduction of individuals outside the area they were born. It seems that current anthropogenic pressures are helping to further this population subdivision (11). On a more local scale, brown bears in Scandinavia can display fine-scale population structure and make these individuals more prone to spatial and temporal changes in the environment and landscape; especially caused by human settlements and infrastructure inbreeding (11, 12). Such individuals can show higher levels of relatedness among each other that further increases the risk of inbreeding (11-13).

Current genetic diversity: Genetic diversity appears to be lower in brown bears from the Scandinavian population compared to the neighboring Karelian brown bear population in Finland and Russia. However, it is not at a critically low level. The populations in Finland and Russia likely consist of a much larger and more diverse gene pool and thus have greater genetic variation than the bear population on the Scandinavian Peninsula (14-16). The genetic characteristics, including genetic diversity, contain important knowledge as this information is usually strongly linked to previous, current, and perhaps ongoing processes of population structure, that again, can have large effects on a population, especially when (sub) populations are fragmented and the focal population is isolated. Genetic drift, the random shuffling of genes within a population, can further lead to homogenization and, in the worst case, inbreeding. Anthropogenic pressure can amplify these effects and thus be detectable in the genetic make-up. For instance, genetic analyses combined with data on hunting pressure showed that different levels of harvest intensity can influence the genetic variation and subsequent structure of the Scandinavian bear population (11). Genetic diversity should be carefully monitored because a substantial reduction in genetic diversity, nuclear as well as sex-specific DNA and as partly reported for the Scandinavian brown bear population, can have severe consequences to the viability of a population (1, 4, 15, 16). Low levels of genetic diversity can lead to severe inbreeding effects and extinction on the long-term, especially when a population is isolated and subject to drastic environmental changes (17, 18). Conservation and management should therefore aim to not only conserve the number of individuals, but also their adaptive potential.

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​​Population connectivity

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The number of brown bears in Scandinavia has increased over the last few decades and bears have re-expanded their presence into areas in which they were wiped-out decades ago (19, 20). Simultaneously, brown bears also recovered in the neighboring country of Finland, helped by a strong influx of migrating brown bears from Russia (15). A recent assessment comparing genetics among male brown bears in Norway, Sweden, and Finland, revealed asymmetric, or unequal, gene flow between bears from Scandinavia and Finland. In brown bears, males are usually the sex that shows the highest rates of long-distance dispersal and thus is representative for inter-population connectivity. In other words, bears seem to move more often from Scandinavia to Finland, and less often from Finland to Scandinavia, i.e., more bears with genetic characteristics from Scandinavia have been detected in Finland than vice versa (14). That assessment indicates that both expansion fronts appear to have met, and connectivity may increase in the future. However, DNA-based monitoring data from all three countries should be harmonized and analyzed together to monitor this process (21). Any halt of migration and gene flow may affect the newly gained connectivity and may increase isolation of the Scandinavian brown bear population.