A characteristic based on long-term (1958-2018) field and stationary studies is given for the abundance and the demographic structure of the population, the course of reproduction, nutrition, and other ecological features common to twin species of the common vole (Microtus arvalis sensu lato). It has been found that the northern boundary of their distribution in the past sixty years first moved north and reached 63 degrees 40'N, and then it also quickly retreated to the south. A sharp reduction in sown areas, primarily cereals caused by the reorientation of agriculture from grain to dairy and vegetables is the reason for the latter. This, as well as the replacement of common voles with native species of rodents (tundra, water, short-tailed field, and bank voles), caused a sharp decrease in the number and narrowing of the range of species in the eastern regions of Fennoscandia. The main place in the diet of twin species is occupied by the green mass of herbaceous plants (the occurrence rate is 72%), the smaller one is occupied by seeds (25%), and the bark of trees is a very rare component (3%). Reproduction in the conditions of the northern periphery of the range proceeds only during one rather limited spring-summer season. The age and sex composition of the population is heterogeneous and varies seasonally.
