Abstract The effects of habitat loss and fragmentation on the genetic structure and vulnerability of populations strongly depend on the behaviour of a particular species. In this thesis, I examined the effects of forest fragmentation on genetic population structure with the aim of identifying and evaluating the different genetic and behavioural factors important for species conservation and management on different geographical scales. The species studied were the mound building red wood ants Formica lugubris and F. aquilonia, and a lekking bird, the capercaillie, Tetrao urogallus. Habitat loss and fragmentation affected the genetic structure in both wood ants and capercaillie. In general, the effects were related to the time since fragmentation and to the level of habitat loss and isolation from the other existing populations. The loss of genetic diversity due to population fragmentation was less observable than the differences in population structure. The response to habitat fragmentation was further dependent on species characteristics such as dispersal and mating behaviour. Sociality affected the genetic vulnerability of wood ant populations by decreasing gene diversity, increasing inbreeding depression and restricting gene flow between subpopulations. The results on the capercaillie in turn suggested that lekking behaviour restricts dispersal of both sexes, thus elevating the occurrence of inbreeding between individuals. The present study provided important information on species conservation and management in terms of better understanding species' biology and behaviour, as well as increased knowledge concerning the genetic issues that should be taken into account when planning conservation actions. By examining the genetic structure of the species it was possible to clarify the conservation status including the effective population size, the question of origin, and the genetic vulnerability (genetic diversity, inbreeding and inbreeding depression) of the populations and/or species. Overall, the results emphasised the importance of preserving the effective population size and the connectivity of habitat patches when planning species specific management strategies. There were great differences in conservation needs among the species, which should be taken into account especially in local management actions.