Dynamics of Colonization and Abundance in Central Alaskan Populations of the Northern Red-Backed Vole, Clethrionomys rutilus

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Population dynamics of Clethrionomys rutilus and Microtus oeconomus were examined during colonization of a severely burned (July 1971) area of black spruce forest, located 40 km north of Fairbanks, Alaska. Livetrapping from June 1972 to September 1976 on two trapping grids, one in burned and the other in unburned forest, revealed that colonization of the burned area by Clethrionomys was gradual, characterized by midwinter abandonment until food and cover resources were sufficient to allow overwintering in 1975, four years after the fire. Reproduction did not occur on the burned area during the first two post-burn years. Despite establishment of a resident population in 1975, recruitment was dependent upon immigrant voles, particularly pregnant females. A high rate of immigration into recently burned areas, and hence successful colonization, is possible for Clethrionomys because source populations exist in all areas of mature forest. Microtus oeconomus , captured only twice on the unburned area, arrived on the burned area in 1974, three years after the fire. However, Microtus density was low, and the population consisted primarily of transient voles. Clethrionomys density underwent a simple sequence of annual fluctuations. In unburned forest, peak density, variable from year to year, was reached in the fall, and low densities occurred yearly in early summer. Density was high in 1974 and 1975, low in 1972, 1973, and 1976. High 1974 and 1975 densities were due to increased reproductive output, especially of first generation (spring-born) females. Stomach content analysis indicated that Clethrionomys relied heavily upon the fruits of several berry-producing plants in all seasons. The phenology of central Alaska fruit production results in fruit scarcity in early summer; it is hypothesized that this variable strongly influences population density each year and perhaps the amplitude of the annual cycle of abundance.