The aim in this thesis was to study the importance of different types of wetlands for the biodiversity in a landscape. The landscapes were both agricultural and urban, and wetlands were in different successional stages. Diving beetles (Dytiscidae) were used as study organisms. They are a suitable group for biodiversity studies in wetlands, as they are a species rich group with wide ecological and geographical distribution. I studied diving beetle assemblages and diversity in wetlands and migration (flying dytiscids) among wetlands, and in different landscapes. The species compositions and assemblage structures in air and in water were compared.

Most species and individuals, both in air and in water, were found in the agricultural landscape, the most heterogeneous landscape studied. The species trapped in air were known to be good migrants and the species in water were characteristic of wetlands in later successional stages. Different species dominated in air and in water. It was not possible to specify and predict wetland types that had rich or poor diving beetle faunas. High and low diversities were found along all the studied environmental gradients. However, permanence and degree of shading of the wetlands were important in structuring the assemblages in water. In the urban, more homogeneous landscapes, fewer and other species were found both in air and in water. Many of the species were characteristic of new wetlands and the same species dominated both in air and water. In landscapes with wetlands of recent origins, species that are good flyers and have the ability of migrating long distances should be the earliest colonisers. Gradually they should diminish in abundance or in proportion as other, slower species, colonise the water. This was supported in the studies, as different species dominated in the air and in the water, depending on the landscape structures.

The results may indicate how wetland faunas change when an agricultural landscape gradually tums into an urban area. Urbanisation would imply a species poorer fauna, and other species than in agricultural landscapes. Accordingly, to obtain a rich wetland insect fauna as exemplified by diving beetles, it seems necessary to take into account more than single wetlands or ponds. A series of wetlands in different environments and successional stages should be recommended on a landscape scale.