Population Ecology of the Japanese Field Vole (Microtus montebelli) in Kyushu (2) : Relation between Population Change and Extrinsic Factors

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\u27s 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\u27s 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\u27s 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.九重の一目山におけるハタネズミ Microtus montebelli について，5年間（1975年12月～1980年12月）に及ぶ個体群変動と外的要因（捕食者，食物および天候）との関連，更に記録に残されている阿蘇地域における本種の大発生と天候との関係について検討した．1）一目山の個体群は1976年6月に大発生し，その後に若干の増減を示したものの全体的に漸減傾向を辿り，調査期間中には再び大発生レベルの個体数には達しなかった．2）九重におけるこのようなハタネズミの個体数変動と捕食者および天候との間には明確な関係を見出し得なかった．3）一目山における本種の分布は均一でなく，ススキやササの繁茂地を中心としてパッチ状を呈した．このような場所では食物条件は非常に悪化し，これが個体数変動に影響を及ぼした可能性もあるが，逆に食物条件は個体群の変動に必ずしも影響するものではないことも知られた．4）阿蘇における本種の大発生と年降水量の過小との間にはかなり明瞭な相関が認められた．しかし，九重ではこのような一致は見られず，相関のあり方には地域差のあることが推察された．5）本種の個体群変動に必然的に作用する外的要因を見出し得なかった．個体数の増加はこれらの外的要因が相互に絡み合って生じるものと思われる