九重

The relations between population change of the Japanese field vole Microtus montebelli and extrinsic factors (predators, foods and weather) were discussed on the basis of the results of investigation at near the summit of Mt. Hitome for fice years (from December 1975 to December 1980). We refer also to whether the vole's outbreaks recorded so far are related to the weather in the south district of Mt. Aso. The results obtained are summarized as follows: 1. The vole population at Mt. Hitome reached to a peak density (outbreak) in June 1976, afterwards declining until December 1980. 2. It appeared that predators and weather had no direct influence on the population change there. 3. Several patches of withered plants which consist mainly of eulalia (Miscanthus sinensis) and bamboo grasses (Sasa nipponica and Arundinaria pygmaea) were caused by the vole's damage. Although the population might have declined due to shortage of food supply in such an area, it did not necessarily seem to follow that a food supply has always a direct influence on the population change. 4. The vole's outbreaks appeared to be concerned with scanty precipitation in the south district of Mt. Aso, whereas did not with it at Mt. Hitome. Thus, there seemed to be some regional differences in this respect. 5. Up to the present there seems to be no single extrinsic factor that, by itself, causes population changes. It seems possible that the vole must be exposed to several factors, acting together, in order to experience an increase in population size.