Population Genetic Structure of the Northern Idaho Ground Squirrel (Spermophilus brunneus brunneus)

Abstract

Spermophilus brunneus is one of the rarest species of North American ground squirrels. It occurs in just five counties in western Idaho. We sampled blood from 14–64 individuals in 11 populations of the northern subspecies S. b. brunneus. Analyses of allozymes at five polymorphic loci revealed significant population structure (F_st = 0.167). Four populations located within a 2-km radius in Bear Meadow (Adams Co.) were indistinguishable with respect to allelic frequencies (Nei's D = 0.002), but the remaining seven, more distant populations (also in Adams Co.) were differentiated from this group and from each other. F_ST values ranged from 0.034 to 0.124 when the four populations in Bear Meadow were pooled and compared with each of the seven isolated populations. Regression of Slatkin's M against geographic distance among the 55 pairwise comparisons of populations suggested a strong effect of isolation-by-distance, consistent with a one-dimensional stepping-stone model of gene flow. Significant genetic structure in S. b. brunneus populations apparently is due to genetic drift in populations with small N_e (i.e., 20–50), reinforced by lack of gene flow following recent habitat fragmentation. Fragmentation results from the shrinking of meadows due to invasion by conifers, itself the consequence of fire suppression in the past 100 years.