What Causes Extended Lows in Microtine Cycles? Analysis of Fluctuations in Sympatric Shrew and Microtine Populations in Fennoscandia

Synchronous declines of sympatric shrew and microtine populations in boreal regions of Fennoscandia have been claimed as evidence that predation deepens and extends the low phase of the microtine population cycles. Assuming that shrews and microtines have the same maximum intrinsic rate of increase, and that the predators kill the shrews and microtines that they encounter unselectively, this hypothesis predicts that sympatric shrew and microtine populations will decline and increase simultaneously. In this study conducted at 60°56′N in southeast Norway, as well as in 13 of 15 other studies at sites in Fennoscandia, the shrew populations declined simultaneously with the sympatric microtine populations, but recovered sooner. The shrew-microtine ratio among animals trapped peaked about 2 years after the peak in the microtine population. The same pattern was found in the diet of raptors in Fennoscandia. Since the maximum intrinsic rate of increase does not seem to be higher in shrews than in microtines, the pattern suggests that either predation is not causing the extended lows in the microtine populations, or that the proportion of predators that selectively kill microtines fluctuates during the microtine cycle. The most likely candidates for selective killers are weasels.