Through population genetic analyses on several terrestrial and aquatic model organisms (e.g. carabids, amphipods, cladocerans, snails, fish, mammals, angiosperms), we aim at quantifying (1) the extent to which populations with varying degree of isolation are genetically differentiated, (2) the relationship between population size and genetic diversity, and (3) the influence of species- and habitat specific factors on genetic diversity and the influence of habitat fragmentation in natural populations. Importantly, we will integrate data on (quasi) neutral markers as well as on ecologically relevant traits, and we will integrate data on organisms that vary widely in action radius and dispersalability in studies on the same localities.