Effects of urbanization on population genetic structure of western gray squirrels

Abstract

Habitat loss and fragmentation due to urbanization are key contributors to the decline of biodiversity. The consequence of these factors is small, isolated populations that are more susceptible to deterministic and stochastic threats of extinction. There is an increasing trend in population reductions of the western gray squirrel (Sciurus griseus) in urban areas of Southern California, USA. Griffith Park (GP) contains one of the last urban populations of western gray squirrels (WGS) present in Los Angeles. We used hairtubes to collect hair of WGS at 3 sites within GP and at 5 sites outside of GP. Twelve microsatellite loci and a 550 bp segment of the mitochondrial control region were used to examine the genetic diversity within GP and among all sample sites, and to determine gene flow within GP. Results revealed subpopulations within GP have low levels of allelic richness at microsatellite loci (AR = 2.28–2.53) and low mitochondrial haplotype diversity (HD = 0.000–0.271). We found significant genetic differentiation (FST = 0.109–0.156, p < 0.001), high levels of relatedness within each GP subpopulation (0.399–0.633), and a lack of private alleles (AP = 0.09–0.27) at microsatellite loci. Mode shifts in microsatellite allele frequencies and positive M-ratio tests provide evidence of bottlenecks within a GP subpopulation. The effective population size for GP (Ne = 9.1) highlights the effects of genetic drift on this isolated population. We suggest conservation efforts that could maintain these last extant populations of a native species in urban Los Angeles.