POPULATION ECOLOGY OF THE GRAY-TAILED VOLE, MICROTUS-CANICAUDUS

Voles of the genus Microtus play a major role as primary consumers of vegetation and as a prey base for many predators in grassland communit ies throughout the world. We present demographic and reproductive data from a 2-yr field study of the gray-tailed vole, Microtus canicaudus, a little-known microtine species endemic to the Willamerre Valley of western Oregon, U.S.A. Population data were recorded on two live-trapp ing grids from October 1991 through September 1993. Population densiti es peaked between 1 June and 20 July 1992 at 54 voles/ha. Peak breedin g with approximately 80% of adult females pregnant or lactating occurr ed between May and June with lesser activity in November and December. Very little breeding occurred during late summer and mid-winter month s. Recruitment rate averaged 1.6 voles/litter/adult female. Mean home range sizes were twice as large for males (253 m(2)) as they were in f emales (135 m(2)). We compare the results from this field study with t hose obtained from enclosed populations of Microtus canicandus and wit h those of ''cycling'' Microtus species. We suggest that agricultural practices, fragmentation of habitat, seasonal flooding of grasslands, and predation work in consort to regulate vole population numbers thro ughout the Willamette Valley of Oregon.