Abstract:
The Double-crested Cormorant (Phalacrocorax auritus) is a widespread, colonial, North American waterbird with bicoastal and inland distributions. Four subspecies have been described within North America corresponding to five geographic breeding regions: Interior and North Atlantic cormorants (P. a. auritus); Southeastern cormorants (P. a. floridanus); Alaskan cormorants (P. a. cincinnatus); and Pacific cormorants (P. a. albociliatus). Management strategies vary widely across the species' range according to local perceptions rather than relative population status. An understanding of population genetic structure is necessary for delineating appropriate management units.
We examined the genetic structure of Double-crested Cormorants across their range in the United States and Canada to quantify variation within and among breeding sites and to assess the status of traditional geographically defined subspecies. Sequences (700bp) from domains I and II of the mitochondrial control region were analyzed for 234 Double-crested Cormorants from 23 breeding sites. Variation was also examined at 8 microsatellite loci for 395 cormorants from the same 23 breeding sites. The mtDNA and
microsatellite data provided strong evidence that the Alaskan subspecies is genetically divergent from other populations in North America (net sequence divergence = 6.72%; ΦST for mtDNA control region = 0.738; FST for microsatellite loci = 0.05). Our data also suggested strong genetic divergence in the southwestern U.S.; southern California may represent a zone of introgression resulting from a northward expansion of a unique lineage from the species' range in northwestern Mexico. In contrast, there was little support for recognition of subspecies within the conterminous U.S. and Canada, outside of Alaska. Rather than genetically distinct regions corresponding to the putative subspecies, we observed a distribution of genetic variation consistent with a pattern of gradual isolation by distance. This pattern implies that genetic differences across the range are due to geographic distance rather than discrete subspecific breaks. Although three of the four subspecies were not genetically distinct, potential demographic separation, habitat differences, and recent declines at some colonies within the regions, suggests that the Pacific and possibly the North Atlantic breeding regions may still warrant consideration as distinct populations.
This thesis provides the first species-wide assessment of the phylogeography and population genetic structure of the Double-crested Cormorant. It further resulted in the first microsatellite markers developed specifically for a North American pelecaniform. The mitochondrial and microsatellite data provide a comprehensive assessment of the four putative subspecies described for the species. Given the highly varying conservation status of Double-crested Cormorants throughout their range, results of this study provide guidance for conservation and management practices on their behalf in North America.
