Vole population dynamics: Factors affecting peak densities and amplitudes of annual Microtus ochrogaster population fluctuations

Abstract

Abstract We studied factors affecting highest densities achieved (peak densities) and amplitudes of fluctuation during 39 annual population fluctuations of Microtus ochrogaster in alfalfa, bluegrass, and tallgrass habitats over a 25-year period. Thirty-two of the 39 population fluctuations reached a peak in autumn or winter. Length of the increase period and initial population density appeared to have the greatest correlation with peak densities and amplitudes of fluctuation. Cessation of growth of population fluctuations reaching peak densities in autumn–winter resulted from a reduction in survival, perhaps enhanced by winter reduction in reproduction. Cessation of growth of population fluctuations with peak densities during spring–summer appeared to result mainly from reduction in survival. We conclude that increased mortality is the primary factor stopping population growth during population fluctuations of M. ochrogaster. We suggest summation of independent mortality effects from multiple predators may be responsible for high population fluctuations some years and not others.