NICHE EXPANSION AND INCREASED MATURATION RATE OF CLETHRIONOMYS-GLAREOLUS IN THE ABSENCE OF COMPETITORS

Habitat-niche relationships and space use of locally sympatric Clethri onomys glareolus, Clethrionomys rufocanus, and Microtus agrestis were studied in a coniferous forest in northern Sweden. Individual voles we re live-trapped during a 5-year period covering two cyclic density pea ks. During the first peak, the voles lived sympatrically, but, during the second peak, only C. glareolus was present. Allopatric C. glareolu s in the second peak reached twice as high densities as sympatric ones in the first peak. The number of young female C. glareolus attaining maturity was negatively correlated with the number of adult female C. rufocanus and M. agrestis. Generally, C. rufocanus occurred most frequ ently in areas rich in dwarf shrubs, whereas M. agrestis occurred most frequently in areas rich in grass. During sympatry, C. glareolus freq uently were found in areas with intermediate shrub cover, whereas, dur ing allopatry, their habitat niche was wider and most habitats were us ed. For all three species, breadth of the habitat niche increased with increasing density. The habitat niches of both C. rufocanus and M. ag restis overlapped much with that of C. glareolus, but they overlapped little with each other. The overlap in habitat between C. glareolus an d C. rufocanus decreased with increasing density. In contrast, the hab itat overlap between C. glareolus and M. agrestis and that between C. rufocanus and M. agrestis tended to increase with density. Interspecif ic spacing occurred between adult females of C. glareolus and C. rufoc anus and between C. rufocanus and M. agrestis at high densities. In co ntrast, the adult males of all three species were randomly distributed with respect to use of space. Data supported the general theory for i nterference competition between C. rufocanus and C. glareolus and betw een C. rufocanus and M. agrestis, but not between C. glareolus and M. agrestis.