Animals have different responses to anthropogenic impact,so population structure,ecological and behavioral parameters of particular species are affected.Our long-term study of small mammal populations in Russian cities revealed some species to be better adapted to urban environments.An analysis of pairs of related species (Microtus arvalis and M.levis: Apodemus uralensis and A.agrarius) showed that one of each pair (semi-commensals) is distinguished by higher reproductive activity,extensive distribution in urban habitats,and absence of population changes compared to non-commensal species.In the laboratory they demonstrate more active exploratory behavior,and more tolerance to conspecifics.Their reproductive success depends on the distance from the downtown,city size etc.Thus one can predict changes in urban rodent communities by assessing a species pre-adaptation to commensalism.During the initial stages of urban development,there is a decrease in the abundance of all native species and a small growth in the abundance of commensal species which does not compensate for an overall decrease in the total abundance of small mammals.During the next stage a new urban environment is formed with numerous habitats which primarily favor semi-commensal species.In the next phase of urbanization a general decrease in the abundance of semi-commensal and non-commensal species occurs,together with increases in the number of commensal species.In the final stages,first the non-commensal and then the semi-commensal species completely disappear.Typical commensal species,such as mice and rats,occur in central downtown areas and are the dominant species in buildings as well as other vacant territories.