The Role of Plant Production in Microtine Cycles in Northern Fennoscandia

The population cycles of microtine rodents in northern Finland have been continuously followed since the late 1940's. Plant reproduction (flowering, berry and seed production, and the subsequent formation of young rhizomes etc.) has been monitored since 1972. Cycles of sympatric microtines were synchronous and were characterized by synchronous changes in the condition of the rodents. These facts cannot be explained by intrinsic factors, competition or predation only. Plants are also characterized by cyclicity in reproduction and growth, which are synchronous among many plant species. The basic rhythm is not modified by rodents. The short growing season is considered to be a primary factor for cyclic reproduction in plants. Plants need to accumulate reserves for several summers before they are able to flower, and this accumulation and exhaustion of resources is reflected in microtine nutrition and population dynamics. There seems to be a threshold level of temperature sums which activates flowering. Changes in food supply (plant rhythms, local overgrazing, defence) are the primary factors triggering the population decline. Other extrinsic factors such as predation and possibly diseases contribute to the depth of the decline.