Reliable and abundant resources are likely to favour specialization, while
unpredictable environmental variation should favour a generalist strategy.
The rodent population cycles of northern latitudes can be seen as both pred
ictable and unpredictable, depending on the scale in time and space. The ar
ctic fox Alopex lagopus is an opportunistic carnivore, but paradoxically, i
t seems to function as a specialist on fluctuating rodent Arvicolinae popul
ations in most inland areas. We have studied the dietary response of arctic
foxes in Sweden during 5 years of varying abundance of Norwegian lemming L
emmus lemmus, and how these changes influenced the reproductive success of
the foxes. The arctic fox population on mainland Fennoscandia is threatened
by extinction and the situation has deteriorated during the 1980s and 1990
s because of an absence of lemming peaks. Our results showed that in all ye
ars, lemming was the main prey for arctic foxes, with 85% frequency of occu
rrence in summer faeces (scats). Bird remains (mainly Passeriformes) were p
resent in 34% of the seats, reindeer Rangifer tarandus in 21%, voles and sh
rews in 4% and hares Lepus timidus in 2% of the seats. The occurrences of l
emming, bird and larger mammal (reindeer and hare) remains in the seats var
ied significantly between years. Temporal Variations within summer seasons
and dietary differences between sub-areas, indicated that arctic foxes fed
opportunistically on the alternative prey types. Den occupancy rates were p
ositively correlated with lemming population densities during the previous
winter, indicating a strong numerical response. We conclude that from a fun
ctional aspect, the arctic fox in Sweden is a lemming specialist, since lem
ming is the main prey and their abundance is the best predictor of arctic f
ox reproductive success. Other prey are used opportunistically in relation
to their availability.