Effective population size and genetic structure of a Piute ground squirrel(Spermophilus mollis) population

Piute ground squirrels (Spermophilus mollis) are distributed continuously i n habitat dominated by native shrubs and perennial grasses in the Snake Riv er Birds of Prey National Conservation Area in Idaho, U.S.A. This habitat i s being fragmented and replaced by exotic annual plants, changing it to a w ildfire-dominated system that provides poor habitat for ground squirrels. T o assess potential effects of this fragmentation on ground squirrel populat ions, we combined an estimate of effective population size (N-e) based upon a demographic study with a population genetic analysis. The study area inc luded three subpopulations separated from each other by 8-13 km. The ratio of effective population size to census number (N-e/N) was 0.57. Combining N -e/N with dispersal distances from a radio-tracking study, we calculated th at neighborhood size was 62.2 ha, which included between 204 and 480 indivi duals. Our population genetic analysis (based on randomly amplified polymor phic DNA (RAPD) and microsatellite markers) showed relatively low levels of genetic differentiation (Theta (populations) congruent to 0.07-0.10) betwe en subpopulations and no inbreeding within subpopulations (f = 0.0003). The se estimates of population subdivision translate into an effective migratio n rate (N(e)m) of 2.3-3.3 per year, which represents a high level of gene R ow. Invasion by exotics will reduce the overall productivity of the habitat , and will lead to isolation among subpopulations if favorable habitat patc hes become isolated.