Distinctiveness of the northern spotted owl

The northern Spotted Owl is one of three recognized subspecies of Spotted Owl Strix caurina. Populations of the California Spotted Owl S. occidentalis occidentalis are geographically adjacent to those of the northern Spotted Owl, occurring in mature forests at mid-elevations in the Sierra Nevada and southern Coast Range of California. Populations of the Mexican Spotted Owl S. occidentalis lucida are geographically disjunct, occurring in the mountains of southern Utah, southern Colorado, western New Mexico, eastern Arizona, and north-central Mexico (Figure 1). The three subspecies have been recognized as distinct by ornithologists for at least forty years.

Figure 1: Distribution of spotted owl subspecies in western North America (from [1]).

Barrowclough et al. [1] collected small amounts of blood from five or ten individuals in each of thirteen widely separated geographical localities that span the geographical range of the species as a whole. Nucleotide sequences of the mitochondrial control region were used to assess patterns of genetic divergence within and among the subspecies.²

They identified 37 distinct haplotypes, but phylogenetic analysis shows that they fall naturally into three distinct clades corresponding with the northern, the Californian, and the Mexican Spotted Owl. In only one case was a haplotype belonging to one of these clades collected in the ``wrong'' population.³ This suggests that populations of the three subspecies have been evolutionarily independent for some time.

Within each subspecies, however, there is no apparent relationship between geographical location and the phylogeny of haplotypes present within populations. This suggests that there has been considerable movement among populations within subspecies, i.e., that there are probably not evolutionarily independent entities below the level of taxonomically recognized subspecies.

The combination of these two pieces of evidence suggests that it is reasonable to treat each subspecies as an evolutionarily significant unit and that it is reasonable for conservationists to be concerned about protecting any of these subspecies if they are endangered.⁴