The fragmentation of populations is an increasingly important problem in the conservation of endangered species. Under these conditions, rare migration events may have important effects for the rescue of small and inbred populations. However, the relevance of such migration events to genetically depauperate natural populations is not supported by empirical data. We show here that the genetic diversity of the severely bottlenecked and geographically isolated Scandinavian population of grey wolves (Canis lupus), founded by only two individuals, was recovered by the arrival of a single immigrant. Before the arrival of this immigrant, for several generations the population comprised only a single breeding pack, necessarily involving matings between close relatives and resulting in a subsequent decline in individual heterozygosity. With the arrival of just a single immigrant, there is evidence of increased heterozygosity, signi cant outbreeding (inbreeding avoidance), a rapid spread of new alleles and exponential population growth. Our results imply that even rare interpopulation migration can lead to the rescue and recovery of isolated and endangered natural populations.

Predatory behavior of wolves (Canis lupus) was studied in 2 wolf territories in Scandinavia. We used hourly data from Global Positioning System (GPS)‐collared adult wolves in combination with Geographic Information System (GIS) for detailed analyses of movement patterns. We tested the hypothesis that wolves spend 1–2 days close to larger prey such as moose (Alces alces) and reasoned that 1–2 locations per day would be enough to find all larger prey killed by the wolves. In total, the study period comprised 287 days and yielded 6,140 hourly GPS positions, with an average of 21.4±2.4 (SD) daily positions. Depending on the radius used to define clusters, 4,045‐5,023 (65.9–81.8%) positions were included in 622–741 GPS‐clusters. We investigated all positions within clusters in the field, and 244 (22%) single positions. In total, we found 68 moose and 4 roe deer (Capreolus capreolus) and classified them as wolf‐killed within the study period. Another 10–15 moose may have been killed but not found. The GIS analyses indicated the proportion of wolf‐killed ungulates included in GPS clusters to be strongly dependent on both number of positions per day and the radius used for defining a set of spatially aggregated GPS positions as a cluster. A higher proportion (78%) of killed prey in clusters based on nighttime (2000‐0700) than those based on daytime (0800–1900) positions (41%). Simulation of aerial search during daylight hours for killed moose resulted in a serious underestimation (>60%) as compared to the number of wolf‐killed moose found during the study. The average kill rate, corrected for 14% nondetected moose, in the territories was 3.6‐4.0 days per killed moose. We concluded that the feeding behavior of wolves in Scandinavia was either different from wolves preying on moose and living at the same latitude in North America, or that estimates of wolf kill rates on moose may have been seriously underestimated in previous North American studies.

The difficulty of obtaining pedigrees for wild populations has hampered the possibility of demonstrating inbreeding depression in nature. In a small, naturally restored, wild population of grey wolves in Scandinavia, founded in 1983, we constructed a pedigree for 24 of the 28 breeding pairs established in the period 1983–2002. Ancestry for the breeding animals was determined through a combination of field data (snow tracking and radio telemetry) and DNA microsatellite analysis. The population was founded by only three individuals. The inbreeding coefficient F varied between 0.00 and 0.41 for wolves born during the study period. The number of surviving pups per litter during their first winter after birth was strongly correlated with inbreeding coefficients of pups ( R 2 =0.39, p <0.001). This inbreeding depression was recalculated to match standard estimates of lethal equivalents (2B), corresponding to 6.04 (2.58–9.48, 95% CI) litter-size-reducing equivalents in this wolf population.

Inbreeding (mating between relatives) is a major concern for conservation as it decreases individual fitness and can increase the risk of population extinction. We used whole-genome resequencing of 97 grey wolves (Canis lupus) from the highly inbred Scandinavian wolf population to identify 'identical-by-descent' (IBD) chromosome segments as runs of homozygosity (ROH). This gave the high resolution required to precisely measure realized inbreeding as the IBD fraction of the genome in ROH (F ). We found a striking pattern of complete or near-complete homozygosity of entire chromosomes in many individuals. The majority of individual inbreeding was due to long IBD segments (>5 cM) originating from ancestors ≤10 generations ago, with 10 genomic regions showing very few ROH and forming candidate regions for containing loci contributing strongly to inbreeding depression. Inbreeding estimated with an extensive pedigree (F ) was strongly correlated with realized inbreeding measured with the entire genome (r = 0.86). However, inbreeding measured with the whole genome was more strongly correlated with multi-locus heterozygosity estimated with as few as 500 single nucleotide polymorphisms, and with F estimated with as few as 10,000 single nucleotide polymorphisms, than with F. These results document in fine detail the genomic consequences of intensive inbreeding in a population of conservation concern.