Abstract
Three 0.8—ha fenced enclosures (X, Y, and Z) were used to determine the consequences of dispersal on the fitness of individual prairie voles, Microtus ochrogaster. Dispersers from enclosure X were marked and were either placed in a vacant enclosure (Z) or returned to their original enclosure (X) (frustrated dispersers). Enclosure Y contained a population of residents where all dispersers from that population were allowed to leave. The survival and reproduction of animals in all enclosures were monitored for 10 wk. Nine replicates of the experiment were performed over a 2—yr period. There was a greater proportion of subadults among dispersers than among residents. There were no differences in sex ratio or reproductive condition between dispersers and residents. Dispersers had the highest survival, followed by residents on enclosure X, and finally, frustrated dispersers. The reproductive activity of dispersers and frustrated dispersers was equal, and both were greater than that of residents on enclosure X. When survival and reproductive activity were combined into a general fitness index, dispersers had the highest relative fitness. Residents on enclosure X and frustrated dispersers had lower and approximately equal fitness. A comparison of the survival and reproductive activity of animals on enclosures X (where one—half of the dispersers were returned to the enclosure) and Y (where all dispersers were allowed to leave), indicated that only the survival of adult females on enclosure Y increased as a result of the reduced density on that enclosure. It was concluded that (1) dispersal into optimal habitat resulted in high survival and reproduction, (2) frustrated dispersal resulted in low survival, but once these animals became established, they showed high reproductive activity, and (3) females appeared to benefit most from living in a population when density was reduced by dispersal.
