Population dynamic and genetic consequences of spatial density-dependent dispersal in patchy populations

Predictions about sex-specific, spatial density-dependent dispersal and the ir demographic and genetic consequences were rested in experimental populat ions of root voles (Microtus oeconomus). Each population consisted of two d emes inhabiting equal-sized habitat patches imbedded in a barren matrix are a. We used a neutral two-allele allozyme marker to monitor gene flow. Initi ally, the two demes were genetically distinct and had different densities s o that the size of a high-density deme (genotype bb) was four rimes larger than that of a low-density deme (genotype aa). The sex-specific dispersal p attern was in accordance with our prediction. Male dispersal was unconditio nal on deme-specific densities, and the majority of the first-generation ma les became dispersed from both demes, whereas female dispersal was strongly density dependent, so that dispersal took place exclusively from the high- density to the low-density deme. The demographic implication of this disper sal pattern was that the initial density difference between the demes was q uickly canceled out. We built a mathematical model that predicted that the initially rare allele (a) would increase in frequency given the dispersal p attern, and this was supported by our experimental data. This result relies mostly on the density-independent male-dispersal strategy, which presumabl y stems from inbreeding avoidance. Our study highlights the importance of i ncorporating sex-specific dispersal strategies in population genetic models . Sex-biased dispersal may act as a deterministic force counteracting the t endency for stochastic loss of alleles in small and fragmented populations.