The long-term dynamics of small mammal communities inhabiting a mosaic of agricultural and forest habitats, that is, mixed forest, lakeside alderwood forest and arable land, set aside in the second period of the study, was analysed. No cyclicity was recorded in the numbers of the dominating species, that is, the bank vole (Myodes glareolus) and the yellow-necked mouse (Apodemus flavicollis). Fallowing of the arable land resulted in a decline in diversity and species richness of small mammals in the habitat mosaic under study. The greatest changes were recorded in the community inhabiting a narrow belt of spatially heterogeneous alderwood along the lakeshore. In this community, a considerable increase in the number of yellow-necked mice and a general increase in the dominance of forest species took place. We suggest that the fallowing of extensive arable lands, taking place in Poland for the last two decades, constitute an attractive source of food for small mammals and may lead to a considerable decline in small mammal species diversity within the landscape or even cause local extinctions in some species.