Manuscript-Vole population dynamics: factors affecting amplitudes of fluctuation (original) (raw)

Factors affecting amplitudes of fluctuation during 39 population cycles of Microtus ochrogaster and 20 cycles of M. pennsylvanicus were studied in alfalfa, bluegrass and tallgrass habitats over a 25-year period. Thirty-two of the 39 M. ochrogaster population cycles peaked in autumn or winter. Variation in peak densities appeared to be related primarily to length of the increase period. Peak densities and amplitudes of fluctuation were not correlated with initial population densities, rate of increase, length of the reproductive period, survival rates, proportion of reproductive females, or body mass during the increase phase. Cessation of growth of M. ochrogaster populations that peaked in autumn-winter resulted from a combination of a densitydependent reduction in survival and a density-independent reduction in reproduction during the winter. Cessation of growth of M. ochrogaster populations peaking during spring-summer resulted from densitydependent reduction in survival; reproduction remained high during the increase through the peak and decline. Density-dependent predation appears to be the primary mortality factor stopping population growth of M. ochrogaster. Nine M. pennsylvanicus cycles peaked during November-February, and 11 peaked during June-September. No single factor was consistently associated with stoppage of population growth for M. pennsylvanicus. A marked decline in reproduction was associated with stoppage of population growth in six of the M. pennsylvanicus cycles that peaked during autumn-winter and in three that peaked during