Human actions impact biodiversity worldwide. Specialist species are particularly sensitive to environmental degradation whereas generalists may even benefit from habitat changes. As a consequence, specialists are declining while generalists are increasing and, therefore, communities become more similar. This phenomenon is known as biotic homogenization. In boreal forests logging and forest management is the most important factor changing the native habitat. In this thesis I studied if human actions are homogenizing boreal forest bird communities. I further studied if protected areas can maintain their diversity and, thus, prevent homogenization. Through the individual studies, biodiversity was measured using various metrics such as species richness, taxonomic, functional and phylogenetic diversities and Species Specialization Index. My results showed that human actions have diverse impacts on diversity in boreal forests. Intensive forest management has negative impacts on forest bird assemblages, and communities in protected areas are not sheltered from habitat changes in the surrounding areas. These negative impacts are particularly strong on forest specialists. The most important message of this thesis was that to really understand biodiversity, various diversity measures in all spatial scales should be considered. It is worth remembering that in biodiversity more is not necessarily better, but each area and patch has its own natural state of biodiversity. Human-induced changes to this baseline denote unwanted impacts on the communities and the whole ecosystems. With the growing demand and exploitation of natural resources, these complex relations in various scales and especially acknowledging them in conservation set growing demand of co-operation with scientists and conservation planners.