Small Rodent Communities on Mires: Implications for Population Performance in Other Habitats

Herbivorous small rodents were rare on South and maritime Scandinavian mires. In North Scandinavia they were as common on mires as in abandoned agricultural land and reforestation areas. The number of vole species on mires increased also towards northeast. Mires are original environments for the species, while abandoned fields and reforestations are new (secondary) habitats. Food composition of at least Microtus agrestis and Clethrionomys glareolus caught on mires differed substantially from that in other habitats. The diet of Microtus oeconomus and M. agrestis was partly affected by the minerotrophic status of a particular mire. These two species overlapped strongly in the food niche as well as in habitat and time niches. C. glareolus showed almost no overlap in the food niche with the other species. Reproduction and individual development in M. agrestis did not vary between types of mires and not between mires and secondary habitats in the same region. Environmental conditions in secondary habitats did not affect population performance in North Scandinavia but caused an increase in numbers in South Scandinavia. The large original niche overlap between M. agrestis and M. oeconomus explains the inverse distribution of these species in Scandinavia. There seems to exist a dynamic zone of competition in northern Scandinavia with occasional or temporary sites of coexistence.