Resources and predation: drivers of sociality in a cyclic mesopredator.

In socially flexible species, the tendency to live in groups is expected to vary through a trade-off between costs and benefits, determined by ecological conditions. The Resource Dispersion Hypothesis predicts that group size changes in response to patterns in resource availability. An additional dimension is described in Hersteinsson's model positing that sociality is further affected by a cost-benefit trade-off related to predation pressure. In the arctic fox (Vulpes lagopus), group-living follows a regional trade-off in resources' availability and intra-guild predation pressure. However, the effect of local fluctuations is poorly known, but offers an unusual opportunity to test predictions that differ between the two hypotheses in systems where prey availability is linked to intra-guild predation. Based on 17-year monitoring of arctic fox and cyclic rodent prey populations, we addressed the Resource Dispersion Hypothesis and discuss the results in relation to the impact of predation in Hersteinsson's model. Group-living increased with prey density, from 7.7% (low density) to 28% (high density). However, it remained high (44%) despite a rodent crash and this could be explained by increased benefits from cooperative defence against prey switching by top predators. We conclude that both resource abundance and predation pressure are factors underpinning the formation of social groups in fluctuating ecosystems.

Genetic rescue in an inbred Arctic fox (Vulpes lagopus) population.

Isolation of small populations can reduce fitness through inbreeding depression and impede population growth. Outcrossing with only a few unrelated individuals can increase demographic and genetic viability substantially, but few studies have documented such genetic rescue in natural mammal populations. We investigate the effects of immigration in a subpopulation of the endangered Scandinavian arctic fox (Vulpes lagopus), founded by six individuals and isolated for 9 years at an extremely small population size. Based on a long-term pedigree (105 litters, 543 individuals) combined with individual fitness traits, we found evidence for genetic rescue. Natural immigration and gene flow of three outbred males in 2010 resulted in a reduction in population average inbreeding coefficient (f), from 0.14 to 0.08 within 5 years. Genetic rescue was further supported by 1.9 times higher juvenile survival and 1.3 times higher breeding success in immigrant first-generation offspring compared with inbred offspring. Five years after immigration, the population had more than doubled in size and allelic richness increased by 41%. This is one of few studies that has documented genetic rescue in a natural mammal population suffering from inbreeding depression and contributes to a growing body of data demonstrating the vital connection between genetics and individual fitness.

Synergies and trade-offs between provisioning and climate-regulating ecosystem services in reindeer herding ecosystems.

Reindeer (Rangifer tarandus) pastoralism utilizes vast boreo-arctic taiga and tundra as grazing land. Highly fluctuating population sizes pose major challenges to the economy and livelihood of indigenous herder communities. In this study we investigated the effect of population fluctuations on core provisioning and regulating ecosystem services in two Sámi reindeer herding districts with contrasting fluctuation trends. We compared 50-year long time series on herd size, meat production, forage productivity, carbon footprint, and CO2-equivalence metrics for surface albedo change based on the radiative forcing concept. Our results show, for both districts, that the economic benefits from the provisioning services were higher than the costs from the regulating services. Still, there were major contrasts; the district with moderate and stable reindeer density gained nearly the double on provisioning services per unit area. The costs from increasing heat absorption due to reduction in surface albedo caused by replacement of high-reflective lichens with low-reflective woody plants, was 10.5 times higher per unit area in the district with large fluctuations. Overall, the net economic benefits per unit area were 237 % higher in the district with stable reindeer density. These results demonstrate that it is possible to minimize trade-offs between economic benefits from reindeer herding locally and global economic costs in terms of climate-regulating services by minimizing fluctuations in herds that are managed at sustainable densities.