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We have monitored the number of arctic foxes and microtine rodents in northern Finland for 30 years. Arctic fox densities were estimated by inventories at den sites, and microtine abundance by snap trapping. Time series analyses showed that the arctic fox population fluctuated widely but always close together with the microtines in a five year cycle. However, there was no time lag in the numerical response of foxes on microtines. The strong dependence on microtines was confirmed by analyses of faecal droppings and food remains at dens. In summer time microtines consisted in average of 45% of the diet and reindeer 30%, but during winters reindeer was the most important food source with 45% compared to 15% for microtines. There was a surprising positive correlation between number of voles and reindeer carcasses, suggesting competition or alternatively an external correlation from e.g. weather. Mean litter size of the arctic fox was also highly dependent on microtine abundance but decreased during the study period despite that food resources had not changed. Further, when microtines had high densities during two consecutive years, arctic foxes only responded to the first year. A feeding experiment resulted in an increase in number of red foxes but had no or little effect on arctic foxes. So, it is difficult to single out one explanation to the decline and second year effect. Food was probably not involved and we do not know if diseases and parasites have been involved. However, both competition and predation, primarily from the red fox, may be responsible together with climatic or weather changes.

Foxes originally were absent from most Alaskan islands in the North Pacific. Red foxes (Vulpes vulpes) were indigenous to the Fox Islands in the eastern Aleutians and to some islands off the Alaska Peninsula and in the Gulf of Alaska. The first recorded introduction of foxes was in 1750 when Russians released arctic foxes (Alopex lagopus) from the Commander Islands on Attu, the westernmost island in the Aleutians. Russians released red and arctic foxes on other islands mainly in the early 1800's. By the 1930's, over 450 islands had been stocked mainly with arctic foxes for fur farming. During this period, the first government surveys and concerns about the effects of introduced foxes on insular avifauna arose, although Aleuts had indicated that seabirds were disappearing on some islands with introduced foxes as early as 1811. Besides foxes, Norway rats (Rattus norvegicus), ground squirrels (Spermophilus undulatus), and other mammals were introduced. Foxes eliminated populations of Aleutian Canada geese (Branta canadensis leucopareia) on all except three small islands. Populations of seabirds, particularly burrow-nesters, were also drastically reduced by foxes. Fox trappers regarded seabirds as feed.