Population Cycles of Small Rodents in Fennoscandia

The earliest records of small rodents in Fennoscandia date back to the sixteenth century. Ziegler (1532) and Magnus (1555) reported mass occurrences of the Norwegian lemming (Lemmus lemmus), which supposedly descended from the sky, a hypothesis that prevailed for the next 300 years (Henttonen and Kaikusalo 1993)! The first scientific papers on lemmings (Fellman 1848, Ehrstöm 1852) clearly recognized periodicity of lemming dynamics in Finnish Lapland (for a review see Henttonen and Kaikusalo 1993). Collett (1878, 1895, 1911-12) compiled extensive data on lemmings in Norway more than 100 years ago, providing critical material for Elton (1924) to describe the population cycle of small rodents. As these early records suggest, the Norwegian lemming is the most conspicuous member of the small rodent community in northern Fennoscandia, both in appearance and abundance, but apart from mountainous regions, the Fennoscandian small rodent cycle actually refers to Microtus and Clethrionomys voles rather than to lemmings. At present, the small rodent cycle in Fennoscandia is one of the best documented examples of cyclic population dynamics. Several recent papers review the state of knowledge on small rodent population dynamics in Fennoscandia and elsewhere (Norrdahl 1995, Krebs 1996, Boonstra et al. 1998, Stenseth 1999, Henttonen and Hanski 2000, Turchin and Hanski 2001). One might think that the “puzzle” of rodent cycles has been solved a long time ago, and that the Fennoscandian small rodent dynamics might serve as a useful reference for the study of cyclic populations in general. Unfortunately, this is not so, although substantial progress has been made over the past 15 years, so that we now have a well-supported hypothesis to explain the small rodent dynamics in Fennoscandia. There are several reasons why progress has been slow in unraveling the secrets of the small rodent cycle. First, small rodents occur in great abundance throughout the world and there was a tendency to assume that the rodent cycle, especially in northern latitudes, was a universal phenomenon, calling for a universal explanation (Krebs and Myers-1974). However, this is not so.