Natural areas are continuously disappearing. Surviving patches resemble islands in terms of limited area, isolation and distance from each other. Road construction and agricultural activities contribute to habitat isolation. Field studies suggest that roads represent barriers and cut off the gene flow by dividing animal populations into fractions on either side of the road. Several mobility diagrams show significant isolation effects of roads on populations of forest-dwelling mice ( Apodemus flavicollis) and carabid beetles. Small habitat islands tend to hold more animal species than expected according to the island biogeographic theory. The N s N i ratio is highest in small isolates, indicating continuous movement of individual animals from surrounding agricultural areas resulting in unstable species composition.