Hansson Lennart, Henttonen Heikki
Department of Wildlife Ecology, Swedish University of Agricultural Scienes, S-750 07, Uppsala, Sweden.
Department of Zoology and Kilpisjärvi Biological Station, University of Helsinki, P. Rautatiekatu 13, SF-00100, Helsinki 10, Finland.
Oecologia. 1985 Oct;67(3):394-402. doi: 10.1007/BF00384946.
Microtine rodents are known to show extreme population variations (cycles) but non-cyclic populations have also been recognized during recent years. The cyclic populations have been widely thought to be regulated by intrinsic mechanisms. However, such predictions for cyclic populations are usually not applicable to non-cyclic ones and extrinsic factors may have to be included in any explanation.A hypothesis that the degree of fluctuations in small rodent numbers is related to the sustainable number of generalist predators was tested on mainly literature data by computing "indices of cyclicity" for local populations. These indices were related to latitude and snow cover (two measures) as these variables will affect the amount of alternative prey available for these generalists. Within Fennoscandia such indices for Clethrionomys glareolus and Microtus agrestis were clearly positively related to latitude and snow cover. The fraction of populations with summer declines in numbers, characterizing highly cyclic populations, increased in the same way. Cyclicity indices in Great Britain were similar to those in southern Fennoscandia, both areas being poor in snow, but were higher at the same latitudes in eastern Europe with more snow. Indices of density variations were generally low in North American Clethrionomys species and very variable in Microtus species.The gradients observed and differences between continents are interpreted as due to microtine-vegetation interactions in northern European areas poor in generalist predators but with important small mustelid predation, and to similar snowshoe hare-vegetation interactions in mainly Canada-Alaska, where small rodents may serve as alternative prey for numerically fluctuating hare predators, at least in the forests. Western European microtine populations, and probably many others, seem to be regulated by generalist predators.
已知田鼠类啮齿动物会表现出极端的种群波动（周期变化），但近年来也识别出了非周期性种群。周期性种群一直被广泛认为是由内在机制调节的。然而，这种对周期性种群的预测通常不适用于非周期性种群，任何解释都可能必须纳入外部因素。一个关于小型啮齿动物数量波动程度与广食性捕食者可持续数量相关的假说，主要通过计算当地种群的“周期性指数”，依据文献数据进行了检验。这些指数与纬度和积雪覆盖（两种测量指标）相关，因为这些变量会影响这些广食性动物可获得的替代猎物数量。在芬诺斯堪的亚地区，棕背䶄和普通田鼠的此类指数与纬度和积雪覆盖明显呈正相关。数量在夏季下降的种群比例，这是高度周期性种群的特征，也以同样的方式增加。英国的周期性指数与芬诺斯堪的亚南部地区相似，这两个地区积雪都较少，但在积雪更多的东欧相同纬度地区，指数更高。北美棕背䶄物种的密度变化指数总体较低，而田鼠物种的指数变化很大。观察到的梯度以及各大洲之间的差异被解释为，在北欧地区，由于广食性捕食者较少，但小型鼬科动物捕食作用重要，存在田鼠与植被的相互作用；而在主要是加拿大 - 阿拉斯加地区，存在类似的雪兔与植被的相互作用，在那里小型啮齿动物可能至少在森林中作为数量波动的雪兔捕食者的替代猎物。西欧的田鼠种群，可能还有许多其他种群，似乎是由广食性捕食者调节的。
