EFFECTS OF DENSITY AND SEASON ON THE POPULATION RATE OF CHANGE IN THEMEADOW VOLE

We developed an empirically based model of density-dependent vole popu lation growth based on experimental data on population dynamics of Mic rotus pennsylvanicus in large field enclosures. Statistical analysis o f the data indicated that both density dependent regulation and season al effects were important in influencing vole population growth. Toget her, 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 (lin ear for population density and sine for seasonality) provided an adequ ate description of the data, with more complex functional forms leadin g to al best minimal improvements. The natural rate of population incr ease, averaged over all seasons, was estimated as (mean +/- SE) r(max) = 6.0(+/-0.4) yr(-1). This estimate suggests an impressive power of p opulation increase, implying that each female vole could be replaced b y about 400 daughters a year later (assuming density-independent growt h). A survey of literature, however, indicates that this is by no mean s the largest rate of increase observed in a vole population.