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Peatlands in the Western Balkans are becoming rapidly reduced due to vegetation succession, further enhanced by climate change and abandonment of traditional land management practices. Currently, the peatlands of this region act as habitat islands embedded mainly in a forest landscape matrix. This allows them to host more forest and generalist species of animals and plants than would be expected from such ecosystem. We were interested in testing if a taxon, unspecialized for such extreme habitats, can maintain stable populations in a peatland, and how environmental fluctuations affect its spatio-temporal distributions along a successional gradient Terrestrial isopods were sampled in four habitat types: a fen, two successional habitats and a beech forest, with pitfall traps during two consecutive years. In total, 1069 individuals belonging to five species were recorded. Activity density increased more than four-fold along the successional gradient, from the fen to an older and more complex successional habitat. This pattern was governed by an increase in the structural complexity of the habitat, where succession habitats supported more favourable shelter sites for terrestrial isopods. Soil moisture decreased over 30% along the successional gradient Additionally, the older successional habitat was more nutrient rich than the fen, providing higher substrate quality for decomposers and enabling the survival of forest species. The fen was inhabited by a single species, Ligidium germanicum, while other species were extremely rare. Its seasonal activity started in the spring, reaching the maximum in June and decreasing at the end of summer. Spatial changes in isopod assemblages along the successional gradient imply that the groundwater level in the fen was sufficiently high to supress the invasion of forest species into the fen. Colonisation of the fen by terrestrial isopods will likely progress as environmental conditions become more suitable by progressive vegetation succession and climate change.