Abstract Background The Norwegian lemming (Lemmus lemmus) is a small rodent endemic to the Fennoscandian alpine and arctic tundra. The species is known for cyclic population outbreaks and mass movements during peak years. Previous research based on microsatellites revealed high genetic variation but a weak population structure in the Norwegian lemming. Results In this study, we revisit the population structure of the species using genome-wide data. To do this, we generated a high-quality de novo reference genome for Lemmus lemmus, and resequenced genomes to 2.5–5 × coverage, from 86 lemmings sampled across the species’ entire geographic distribution. Our results reveal that the population is geographically structured into distinct subpopulations, with an overall pattern characterised by isolation-by-distance among subpopulations. Furthermore, our results are consistent with earlier work suggesting that the species survived the last ice age within a northern refugium. Conclusions Together, these findings provide a genome-wide perspective on today’s population structure of the Norwegian lemming. In addition, we provide a de novo reference genome, which we believe will be a valuable resource to the research community.

Humans have relied on animal fur for centuries, yet fur farming only began recently during the mid-19th Century. Little is known about this incipient domestication or the genomic processes involved. Domestication may involve founder effects, population bottlenecks and low population size, which, when combined with intense artificial selection, lead to inbreeding, a limited gene pool and reduced fitness. The arctic fox (Vulpes lagopus) has been farmed intensively since the early 1900s and has been artificially selected for economic phenotypes. We investigated the origin of these lineages and the genomic consequences of intensive farming by comparing the genomes of farmed and wild arctic foxes from across their range. Our research indicates recent inbreeding through long Runs of Homozygosity and reduced genomic variation in farmed foxes relative to their respective wild populations. We identified a coastal ecotype origin for all Fennoscandian farmed arctic foxes, aligning them phylogenetically with the wild Icelandic population, a geographically isolated and phenotypically distinct coastal lineage. The depleted genome-wide heterozygosity and increased recent inbreeding in farmed fox lineages is consistent with a heavy consequence of domestication, shedding light on the demographic history and genomic consequences of human manipulation. We highlight the need for increased genomic investigations into fur farm populations to understand the incipient domestication process and uncover the cost of intense farming. The genomic consequences of domestication must be considered in the management of fur farms, with actionable steps needed to prevent descendants of escaped farmed foxes from polluting the gene pool in the wild through introgression.

Demographic declines have important consequences for population viability, since they can lead to losses in genome diversity, as well as increased inbreeding and expression of deleterious mutations. Scandinavia was colonized by the Arctic fox (Vulpes lagopus) at the Pleistocene/Holocene transition, and the population has since been on the periphery of the global distribution. The Scandinavian population became even more fragmented in the early 1900s due to human persecution, and experienced an additional decline in the 1980s. We generated high-coverage genomes from pre-bottleneck, as well as modern Scandinavian and Russian specimens, and found that genome-wide diversity was lower and inbreeding higher in Scandinavia compared to the Siberian population, even prior to the historical bottleneck, most likely reflecting the long-term partial isolation and recent postglacial origin of the Scandinavian population. The southern subpopulation has the highest inbreeding levels, likely due to having been recently founded and highly isolated. Our results also show that although inbreeding increased substantially over the past century, the amount of total genetic load did not change. Overall, these findings illustrate the utility of a temporal approach to disentangle the genomic consequences of recent declines from ancient biogeographic processes.

The arctic fox metapopulation in Fennoscandia has been critically endangered over a century, but due to thorough research and intensive conservation measures it has now started to recover. On Varanger Peninsula in northeastern Norway, research about the causes of the arctic fox decline was initiated in 2004. A conceptual model that hypothesized the irregularity of lemming cycles caused by warmer winters and high abundance of the competitively superior red fox as the two main drivers of the critical state of the local population guided the establishment of a monitoring program that included arctic and red foxes, but also other key ecosystem components. Intensive red fox culling in part of the study area was implemented as an experimental management action. This research has subsequently become a part of the Climate-ecological Observatory for Arctic Tundra (COAT), an adaptive monitoring program for long-term research on climate change impacts on terrestrial arctic ecosystems. Following the paradigm of adaptive monitoring, the results of the first 12 years of the study was evaluated in 2017 and led to the conclusion that red fox culling was not sufficient to help arctic foxes recover in the present ecosystem conditions. We hypothesized that absence of lemming peaks and an initially very small and isolated arctic fox population (i.e. demographic stochasticity) contributed to the lack of a response to the culling action. Consequently, two additional management actions decided by the Norwegian Environment Agency were implemented in collaboration with the National Program for Arctic Fox Conservation to address these hypotheses. Supplemental feeding was initiated together with the release of 65 captive bred arctic fox pups from the Norwegian Captive Breeding Program on Varanger over three years (2018-2020). These combined management actions resulted in a strong increase of the local arctic fox population. At the same time, the research conducted to evaluate these management actions has resulted in several now 20-year long time series that are integrated in the research plan of COAT. These data provide unique knowledge about this low arctic ecosystem affected by rapid climate change and increasing human activities. This study shows how on the one hand a monitoring program emerged from a management question – the conservation of arctic foxes – and on the other hand how a thoroughly designed adaptive monitoring program provides crucial information for managing endangered populations.

Reports of fading vole and lemming population cycles and persisting low populations in some parts of the Arctic have raised concerns about the spread of these fundamental changes to tundra food web dynamics. By compiling 24 unique time series of lemming population fluctuations across the circumpolar region, we show that virtually all populations displayed alternating periods of cyclic/non-cyclic fluctuations over the past four decades. Cyclic patterns were detected 55% of the time (n = 649 years pooled across sites) with a median periodicity of 3.7 years, and non-cyclic periods were not more frequent in recent years. Overall, there was an indication for a negative effect of warm spells occurring during the snow onset period of the preceding year on lemming abundance. However, winter duration or early winter climatic conditions did not differ on average between cyclic and non-cyclic periods. Analysis of the time series shows that there is presently no Arctic-wide collapse of lemming cycles, even though cycles have been sporadic at most sites during the last decades. Although non-stationary dynamics appears a common feature of lemming populations also in the past, continued warming in early winter may decrease the frequency of periodic irruptions with negative consequences for tundra ecosystems.

Long-term studies of cyclic rodent populations have contributed fundamentally to the development of population ecology. Pioneering rodent studies have shown macroecological patterns of population dynamics in relation to latitude and have inspired similar studies in several other taxa. Nevertheless, such studies have not been able to disentangle the role of different environmental variables in shaping the macroecological patterns. We collected rodent time-series from 26 locations spanning 10 latitudinal degrees in the tundra biome of Fennoscandia and assessed how population dynamics characteristics of the most prevalent species varied with latitude and environmental variables. While we found no relationship between latitude and population cycle peak interval, other characteristics of population dynamics showed latitudinal patterns. The environmental predictor variables provided insight into causes of these patterns, as 1) increased proportion of optimal habitat in the landscape led to higher density amplitudes in all species and 2) mid-winter climate variability lowered the amplitude in Norwegian lemmings and grey-sided voles. These results indicate that biome-scale climate and landscape change can be expected to have profound impacts on rodent population cycles and that the macro-ecology of such functionally important tundra ecosystem characteristics is likely to be subjected to transient dynamics.

Abstract Thiamine deficiency can result in life‐threatening physiological and neurological complications. While a thiamine‐deficient diet may result in the onset of such symptoms, the presence of thiaminase – an enzyme that breaks down thiamine – is very often the cause. In such instances, thiaminase counteracts the bioavailability and uptake of thiamine, even when food‐thiamine levels are adequate. Here, we report on a case of failed reproduction in seven Arctic fox (Vulpes lagopus) breeding pairs kept at a captive breeding facility, including the presentation of severe thiamine deficiency symptoms in two male foxes. Symptoms included ataxia, obtundation, truncal sway, star‐gazing and visual impairment. Blood tests were inconclusive, yet symptoms resolved following treatment with a series of thiamine hydrochloride injections, thereby verifying the diagnosis. A fish‐dominated feed, which for the first time had been frozen for a prolonged period, was identified as the likely source of thiaminase and subsequent deterioration in the animals’ health. Symptoms in the two males arose during the annual mating period. All seven breeding pairs at the captive breeding station failed to reproduce – a phenomenon never recorded during the captive breeding facility's preceding 17‐year operation. Relating our findings to peer‐reviewed literature, the second part of this case report assesses how thiamine deficiency (due to thiaminase activity) likely resulted in subclinical effects that impaired the production of reproduction hormones, and thereby led to a complete breeding failure. While previous work has highlighted the potentially lethal effects of thiamine deficiency in farmed foxes, this is, to our knowledge the first study showing how subclinical effects in both males and females may inhibit reproduction in foxes in general, but specifically Arctic foxes. The findings from our case report are not only relevant for captive breeding facilities, but for the welfare and management of captive carnivorous animals in general.

Dedicated conservation efforts spanning the past two decades have saved the Fennoscandian Arctic fox (Vulpes lagopus) population from local extinction, and extensive resources continue to be invested in the species' conservation and management. Although increasing, populations remain isolated, small and are not yet viable in the longer term. An understanding of causes of mortality are consequently important to optimize ongoing conservation actions. Golden eagles (Aquila chrysaetos) are a predator of Arctic foxes, yet little information on this interaction is available in the literature. We document and detail six confirmed cases of Golden eagle depredation of Arctic foxes at the Norwegian captive breeding facility (2019-2022), where foxes are housed in large open-air enclosures in the species' natural habitat. Here, timely detection of missing/dead foxes was challenging, and new insights have been gained following recently improved enclosure monitoring. Golden eagle predation peaked during the winter months, with no cases reported from June to November. This finding contrasts with that which is reported from the field, both for Arctic and other fox species, where eagle depredation peaked at dens with young (summer). While the seasonality of depredation may be ecosystem specific, documented cases from the field may be biased by higher survey efforts associated with the monitoring of reproductive success during the summer. Both white and blue color morphs were housed at the breeding station, yet only white foxes were preyed upon, and mortality was male biased. Mitigation measures and their effectiveness implemented at the facility are presented. Findings are discussed in the broader Arctic fox population ecology and conservation context.

Large carnivores influence ecosystem dynamics in multiple ways, for example, by suppressing meso-carnivores and providing carrions for smaller scavengers. Loss of large carnivores is suggested to cause meso-carnivore increase and expansion. Moreover, competition between meso-carnivores may be modified by the presence of larger carnivores. In tundra ecosystems, the smallest meso-carnivore, the Arctic fox, has experienced regional declines, whereas its larger and competitively superior congener, the red fox, has increased, potentially due to changes in the abundance of apex predators. We explored if variation in the occurrence of wolverine and golden eagle impacted the occurrence and co-occurrence of the Arctic fox and red fox in relation to varying abundances of small rodents within the Scandinavian tundra. We applied multi-species occupancy models to an extensive wildlife camera dataset from 2011-2020 covering 98 sites. Daily detection/non-detection of each species per camera trap site and study period (late winter; March-May) was stacked across years, and species occupancy was related to small rodent abundance while accounting for time of the year and status of simulated carcass. The Arctic fox was more likely to co-occur with the red fox when the wolverine was present and less likely to co-occur with the red fox when golden eagles were present and the wolverine was absent. Red foxes increased in occupancy when co-occurring with the larger predators. The Arctic fox responded more strongly to small rodent abundance than the red fox and co-occurred more often with the other species at carcasses when rodent abundance was low. Our findings suggest that the interspecific interactions within this tundra predator guild appear to be surprisingly intricate, driven by facets of fear of predation, interspecific mediation and facilitation, and food resource dynamics. These dynamics of intraguild interactions may dictate where and when conservation actions targeted towards the Arctic fox should be implemented.

For many species, the ability to rapidly adapt to changes in seasonality is essential for long‐term survival. In the Arctic, seasonal moulting is a key life‐history event that provides year‐round camouflage and thermal protection. However, increased climatic variability of seasonal events can lead to phenological mismatch. In this study, we investigated whether winter‐white (white morph) and winter‐brown (blue morph) Arctic foxes ( Vulpes lagopus ) could adjust their winter‐to‐summer moult to match local environmental conditions. We used camera trap images spanning an eight‐year period to quantify the timing and rate of fur change in a polymorphic subpopulation in south‐central Norway. Seasonal snow cover duration and temperature governed the phenology of the spring moult. We observed a later onset and longer moulting duration with decreasing temperature and longer snow season. Additionally, white foxes moulted earlier than blue in years with shorter periods of snow cover and warmer temperatures. These results suggest that phenotypic plasticity allows Arctic foxes to modulate the timing and rate of their spring moult as snow conditions and temperatures fluctuate. With the Arctic warming at an unprecedented rate, understanding the capacity of polar species to physiologically adapt to a changing environment is urgently needed in order to develop adaptive conservation efforts. Moreover, we provide the first evidence for variations in the moulting phenology of blue and white Arctic foxes. Our study underlines the different intraspecific selective pressures that can exist in populations where several morphs co‐occur, and illustrates the importance of integrating morph‐based differences in future management strategies of such polymorphic species.

Abstract Estimates of species abundance are of key importance in population and ecosystem level research but can be hard to obtain. Study designs using camera traps are increasingly being used for large‐scale monitoring of species that are elusive and/or occur naturally at low densities. Golden eagle (Aquila chrysaetos) is one such species, and we investigate whether existing large‐scale monitoring programs using baited camera traps can be used to estimate the abundance of golden eagles, as an alternative to traditional labour‐intensive searches for active territories and nest sites during the breeding period. The camera‐trap data allowed two measures of abundance to be estimated within each of four main study areas in mid and northern Norway; occupancy was measured as the probability of camera site use, and population size was measured as the number of individuals using camera sites within a study area. Spatial and temporal patterns in occupancy and population size were explored and evaluated against independent estimates of breeding pair density in the study areas. Annual estimates of golden eagle occupancy showed low precision, while estimates of population size were more precise in relation to both estimated and anticipated abundance fluctuations. Estimates of population size may therefore be suitable for monitoring within‐study area temporal abundance trends, while estimates of occupancy seem unsuitable for such for golden eagles. Across study areas, patterns in both average occupancy and average population density estimated from population size were consistent with the spatial pattern in average breeding pair densities (r = 0.99, and r = 0.89, respectively). This suggests that camera‐trap‐based estimates of occupancy and population density, both reflect territory density at large spatial scales. In conclusion, our results suggest that baited camera traps can be a cost‐effective strategy for monitoring the abundance of golden eagles.

Translocation of captive-bred animals has become a widespread conservation practice to counteract species extinctions. We analyse and discuss the apparent success and shortcomings of Arctic fox (Vulpes lagopus) rein-troductions in alpine tundra areas of Norway. We followed the fate of 915 foxes between 2007 and 2020 and estimated the apparent survival and reproductive success of captive-bred and released Arctic foxes, compared to wild-born descendants. Relationship to abundance of small rodents, population size, and age were explored. Overall, apparent survival and probability of breeding were similar between captive-bred and wild-born foxes, positively linked to rodent abundance. For wild-born foxes, both breeding propensity and litter size declined with increasing fox population size. This could be a first sign of the limited capacity of single tundra patches to house self-subsistent populations. Thus, facilitating and maintaining connectivity among remnant and re-established Arctic fox populations, creating functional metapopulations, is essential for further improvement and long-term survival. Relying on the combined measures of supplementary feeding and red-fox (Vulpes vulpes) con-trol, the Arctic fox captive-breeding and reintroduction programme has so far been highly successful. However, anthropogenic drivers facilitating red fox invasion into the Arctic fox habitat, along with climate driven irreg-ularities and dampened small rodent cycles, could inhibit the establishment of a self-sustained population. A more holistic ecosystem approach and conservation measures to restore alpine fauna should be considered.

Abstract Climate change is altering the world's ecosystems through direct effects of climate warming and precipitation changes but also indirectly through changes in biotic interactions. For instance, climate‐driven changes in plant and/or insect communities may alter plant–pollinator interactions, thereby influencing plant reproductive success and ultimately population dynamics of insect‐pollinated plants. To better understand how the importance of insect pollination for plant fruit set varies with climate, we experimentally excluded pollinators from the partly selfing keystone species Vaccinium myrtillus along elevational gradients in the forest‐tundra ecotone in central Norway. The study comprised three mountain areas, seven elevational gradients spanning from the climatically relatively benign birch forest to the colder alpine areas above the tree line, and 180 plots of 1 × 1 m, with experimental treatments allocated randomly to plots within sites. Within the experimental plots, we counted the number of flowers of V. myrtillus and counted and weighed all fruits, as well as seeds for a selection of fruits. Excluding pollinators resulted in lower fruit production, as well as reduced fruit and seed mass of V. myrtillus. In the alpine sites pollinator exclusion resulted in 84% fewer fruits, 50% lower fruit weight, and 50% lower seed weight compared to control conditions. Contrary to our expectations, the negative effect of pollinator exclusion was less pronounced in the forest compared to alpine sites, suggesting that the importance of insect pollination for seed production is lower at low elevations. Our findings indicate that the keystone species V. myrtillus is relatively robust to changes in the pollinator community in a warmer climate, thereby making it less vulnerable to climate‐driven changes in plant–pollinator interactions.

Accelerating climate change is causing severe habitat fragmentation in the Arctic, threatening the persistence of many cold-adapted species. The Scandinavian arctic fox (Vulpes lagopus) is highly fragmented, with a once continuous, circumpolar distribution, it struggled to recover from a demographic bottleneck in the late 19th century. The future persistence of the entire Scandinavian population is highly dependent on the northernmost Fennoscandian subpopulations (Scandinavia and the Kola Peninsula), to provide a link to the viable Siberian population. By analyzing 43 arctic fox genomes, we quantified genomic variation and inbreeding in these populations. Signatures of genome erosion increased from Siberia to northern Sweden indicating a stepping-stone model of connectivity. In northern Fennoscandia, runs of homozygosity (ROH) were on average ~1.47-fold longer than ROH found in Siberia, stretching almost entire scaffolds. Moreover, consistent with recent inbreeding, northern Fennoscandia harbored more homozygous deleterious mutations, whereas Siberia had more in heterozygous state. This study underlines the value of documenting genome erosion following population fragmentation to identify areas requiring conservation priority. With the increasing fragmentation and isolation of Arctic habitats due to global warming, understanding the genomic and demographic consequences is vital for maintaining evolutionary potential and preventing local extinctions.

Spatial capture-recapture modelling (SCR) is a powerful tool for estimating density, population size, and space use of elusive animals. Here, we applied SCR modelling to non-invasive genetic sampling (NGS) data to estimate red fox (Vulpes vulpes) densities in two areas of boreal forest in central (2016-2018) and southern Norway (2017-2018). Estimated densities were overall lower in the central study area (mean = 0.04 foxes per km2 in 2016, 0.10 in 2017, and 0.06 in 2018) compared to the southern study area (0.16 in 2017 and 0.09 in 2018). We found a positive effect of forest cover on density in the central, but not the southern study area. The absence of an effect in the southern area may reflect a paucity of evidence caused by low variation in forest cover. Estimated mean home-range size in the central study area was 45 km2 [95%CI 34-60] for females and 88 km2 [69-113] for males. Mean home-range sizes were smaller in the southern study area (26 km2 [16-42] for females and 56 km2 [35-91] for males). In both study areas, detection probability was session-dependent and affected by sampling effort. This study highlights how SCR modelling in combination with NGS can be used to efficiently monitor red fox populations, and simultaneously incorporate ecological factors and estimate their effects on population density and space use.

Abstract Climate change has different and sometimes divergent effects on terrestrial and marine food webs, and in coastal ecosystems, these effects are tightly interlinked. Responses of opportunistic coastal predators and scavengers to climate change may thus be complex and potentially highly flexible, and can simultaneously serve as indicators of, and have profound impacts on, lower trophic levels. Gaining mechanistic understanding of these responses is therefore important, but often not feasible due to lack of long‐term data from marked individuals. Here, we used a Bayesian integrated population model (IPM) to elucidate the effects of arctic warming and concurrent changes in terrestrial and marine resource availability on population dynamics of the opportunistic arctic fox (Vulpes lagopus) in Svalbard. Joint analysis of four types of data (den survey, age‐at‐harvest, placental scars, mark‐recovery) revealed relatively stable population size and age structure over the last 22 yr (1997–2019) despite rapid environmental change linked to climate warming. This was related to the fact that terrestrial resources (reindeer carcasses, geese) became more abundant while the availability of marine resources (seal pups/carrion) decreased, and was driven by divergent trends in different vital rates (e.g., increased pregnancy rate but decreased pup survival). Balanced contributions of survival vs. reproduction and of immigration vs. local demography further stabilized population size. Our study thus sheds light on the mechanisms underlying population dynamics of opportunistic carnivores exploiting terrestrial and marine resources and suggests that exploitation of resources across different ecosystems can buffer predators against climate change. Additionally, it highlights the large potential of IPMs as tools to understand and predict the effects of environmental change on wildlife populations, even when data on marked individuals are sparse.

Selection for crypsis has been recognized as an important ecological driver of animal colouration, whereas the relative importance of thermoregulation is more contentious with mixed empirical support. A potential thermal advantage of darker individuals has been observed in a wide range of animal species. Arctic animals that exhibit colour polymorphisms and undergo seasonal colour moults are interesting study subjects for testing the two alternative hypotheses: demographic performance of different colour morphs might be differentially affected by snow cover with a cryptic advantage for lighter morphs, or conversely by winter temperature with a thermal advantage for darker morphs. In this study, we explored whether camouflage and thermoregulation might explain differences in reproduction and survival between the white and blue colour morphs of the Arctic fox Vulpes lagopus under natural conditions. Juvenile and adult survival, breeding propensity and litter size were measured for 798 captive-bred and released or wild-born Arctic foxes monitored during an 11-year period (2007-2017) in two subpopulations in south-central Norway. We investigated the proportion of the two colour morphs and compared their demographic performance in relation to spatial variation in duration of snow cover, onset of snow season and winter temperatures. After population re-establishment, a higher proportion of blue individuals was observed among wild-born Arctic foxes compared to the proportion of blue foxes released from the captive population. Our field study provides the first evidence for an effect of colour morph on the reproductive performance of Arctic foxes under natural conditions, with a higher breeding propensity of the blue morph compared to the white one. Performance of the two colour morphs was not differentially affected by the climatic variables, except for juvenile survival. Blue morph juveniles showed a tendency for higher survival under colder winter temperatures but lower survival under warmer temperatures compared to white morph juveniles. Overall, our findings do not consistently support predictions of the camouflage or the thermoregulation hypotheses. The higher success of blue foxes suggests an advantage of the dark morph not directly related to disruptive selection by crypsis or thermoregulation. Our results rather point to physiological adaptations and behavioural traits not necessarily connected to thermoregulation, such as stress response, immune function, sexual behaviour and aggressiveness. Our findings highlight the need to explore the potential role of genetic linkage or pleiotropy in influencing the fitness of white and blue Arctic foxes as well as other species with colour polymorphisms.

Genome-wide association studies provide good opportunities for studying the genetic basis of adaptive traits in wild populations. Yet, previous studies often failed to identify major effect genes. In this study, we used high-density single nucleotide polymorphism and individual fitness data from a wild non-model species. Using a whole-genome approach, we identified the MC1R gene as the sole causal gene underlying Arctic fox Vulpes lagopus fur colour. Further, we showed the adaptive importance of fur colour genotypes through measures of fitness that link ecological and evolutionary processes. We found a tendency for blue foxes that are heterozygous at the fur colour locus to have higher fitness than homozygous white foxes. The effect of genotype on fitness was independent of winter duration but varied with prey availability, with the strongest effect in years of increasing rodent populations. MC1R is located in a genomic region with high gene density, and we discuss the potential for indirect selection through linkage and pleiotropy. Our study shows that whole-genome analyses can be successfully applied to wild species and identify major effect genes underlying adaptive traits. Furthermore, we show how this approach can be used to identify knowledge gaps in our understanding of interactions between ecology and evolution.