JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org. . Wiley-Blackwell and Nordic Society Oikos are collaborating with JSTOR to digitize, preserve and extend access to Oikos. . 1987. How much do weasels shape microtine cycles in the northern Fennoscandian taiga? -Oikos 50: 353-365.Microtine cycles in northern Fennoscandia show a statistical periodicity, and are characterized by a high degree of interspecific synchrony among sympatric species. The deepest crash phase is particularly synchronous. These patterns have been often explained by plantmicrotine interactions combined with heavy predation by specialist small mustelids during the later stages of the decline. The last cycle in the northern Finnish taiga was, however, rather atypical. An initially normal peak was followed by an extra year with high density and thereafter by a low decline with a distinct asynchrony among the species. Moreover, Microtus agrestis, normally one of the dominant species, did not follow the cyclic increase and peak. Our estimates of the numbers of least weasels Mustela nivalis show that also this species failed to follow the cycle in the normal way. We suggest that the virtual absence of least weasels was the main reason for the extended peak and asynchronous, slow decline. During this slow decline vole species bred well. This, in addition to the extension of the peak, could imply that neither food (resource depletion or changes in quality), nor social factors, are enough to trigger the steep crash in a typical four-year cycle. Food manipulations increased microtine densities in a habitat, but did not change the pattern of the decline. During the slow, asynchronous decline vole species disappeared in the order of decreasing body size. This might suggest that, in spite of a low density of least weasels, predation by other predators (mainly stoats) contributed to the gradual decline.