Date of this Version
Published in Conservation Biology (2004) 18(5):1347-1357.
Recent population expansion of Barred Owls (Strix varia) into western North America has led to concern that they may compete with and further harm the Northern Spotted Owl (S. occidentalis caurina), which is already listed as threatened under the U.S. Endangered Species Act (ESA). Because they hybridize, there is a legal need under the ESA for forensic identification of both species and their hybrids. We used mitochondrial control-region DNA and amplified fragment-length polymorphism (AFLP) analyses to assess maternal and biparental gene flow in this hybridization process. Mitochondrial DNA sequences (524 base pairs) indicated large divergence between Barred and Spotted Owls (13.9%). Further, the species formed two distinct clades with no signs of previous introgression. Fourteen diagnostic AFLP bands also indicated extensive divergence between the species, including markers differentiating them. Principal coordinate analyses and assignment tests clearly supported this differentiation. We found that hybrids had unique genetic combinations, including AFLP markers from both parental species, and identified known hybrids as well as potential hybrids with unclear taxonomic status. Our analyses corroborated the findings of extensive field studies that most hybrids genetically sampled resulted from crosses between female Barred Owls and male Spotted Owls. These genetic markers make it possible to clearly identify these species as well as hybrids and can now be used for research, conservation, and law enforcement. Several legal avenues may facilitate future conservation of Spotted Owls and other ESA-listed species that hybridize, including the ESA similarity-of-appearance clause (section 4[e]) and the Migratory Bird Treaty Act. The Migratory Bird Treaty Act appears to be the most useful route at this time.