An Experimental Analysis of Dispersal in Fluctuating Vole Populations: Demographic Parameters

In the removal—grid and fenced—enclosure experiments, more dispersal occurred when resident populations were increasing or at peak densities than during phases of declining density.

Abstract

Demographic attributes of dispersers from Microtus ochrogaster populations in eastern Kansas, USA, were characterized by three experimental methods: (1) voles were trapped and removed from two grids every 2 wk from 1973—1977 and voles that colonized these vacuums were compared to residents on neighboring control grids; (2) two grids were burned in the springs of 1976 and 1977; voles colonizing grids during a 6—wk period after the fire were compared to resident voles before the fire; (3) ten virgin male and female voles from a laboratory colony were introduced in 1975—1976 and again in 1976—1977 into three fenced enclosures equipped with exit doors. After breeding populations were established, voles emigrating through exit doors were compared with voles remaining inside the enclosures. In the removal—grid and fenced—enclosure experiments, more dispersal occurred when resident populations were increasing or at peak densities than during phases of declining density. The loss from control grids attributable to dispersal onto removal grids was 22% higher during phases of increasing density than during phases of declining density. In all experiments, fewer dispersing adult females were in reproductive condition than residents, and subadults showed the greatest dispersal rates of all age classes. Subadult female dispersers were in reproductive condition more frequently than residents in the removal experiment but less frequently than residents in the burning experiment. In all experiments there were no consistent differences between residents and dispersers in breeding activity of adult or subadult males. More males than females dispersed in the removal experiment but not in the burning or enclosure experiments. Density data for populations in fenced enclosures with exit doors indicated that immigration as well as emigration are necessary for multiannual cycles in numbers.