Effects of Density and Season on the Population Rate of Change in the Meadow Vole

We developed an empirically based model of density-dependent vole population growth based on experimental data on population dynamics of Microtus pennsylvanicus in large field enclosures. Statistical analysis of the data indicated that both density dependent regulation and seasonal effects were important in influencing vole population growth. Together, these two factors explained approximately one-half of variance in the realized per capita rate of change exhibited by experimental vole populations. A population model assuming simple functional forms (linear for population density and sine for seasonality) provided an adequate description of the data, with more complex functional forms leading to at best minimal improvements. The natural rate of population increase, averaged over all seasons, was estimated as (mean ± SE) r max = 6.0(±0.4) yr -1 . This estimate suggests an impressive power of population increase, implying that each female vole could be replaced by about 400 daughters a year later (assuming density-independent growth). A survey of literature, however, indicates that this is by no means the largest rate of increase observed in a vole population.