In this thesis, I describe and explain drastic population fluctuations in the arctic fox Alopex lagopus (L.). I proceed to examine evolutionary consequences of unpredictable fluctuations in terms of life history strategies. In fluctuating populations, the risks and benefits of different behaviours vary with changes in population density. Animals must adapt behaviourally or genetically to such changes.
The majority of data in this thesis come from a long-term field study in Sweden, den inventories during 24 years (1974-1997) and an expedition along the north coast of Siberia. Breeding success and juvenile survival are the most important and variable factors in arctic fox population dynamics and experimental feeding showed their relation to food availability. Time series analyses confirmed a four-year cyclicity in both arctic fox numbers and litter size in Sweden. However, geographical regions were asynchronous with more pronounced regularity in the south. The total number of arctic foxes in Sweden is low; the estimate for 1994, about 60 adults, remains valid in 1997. The future of the Fennoscandian arctic fox is a matter of public concern, and I have highlighted some results and conclusions which have a direct bearing on conservation. The functional response of the arctic fox in Siberia to different prey species varied, with a type II response for the Siberian lemming Lemmus sibiricus and a type III response for the collared lemming Dicrostonyx torquatus. Also the numerical response differed, and the results suggest that Arctic foxes and Lemmus fluctuated in limit cycles, the cyclicity of the foxes being driven by their lemming prey.
The reproductive strategy in stable arctic fox populations is fundamentally different from that in fluctuating populations, which have larger litter sizes both at birth and at weaning. I present the jackpot hypothesis, suggesting that selection shapes reaction norms of reproductive output according to the degree of population fluctuations. Reproductive output is subsequently adjusted to resource levels by fine-tuned mechanisms, reducing the cost of reproduction. Competition for resources determines dispersal strategies, and long-range dispersal is performed by the best individuals. Population fluctuations seem to favour a risk-prone strategy, especially in high quality animals.
Stockholm: Department of Zoology, Stockholm University , 1997. , 39 p.