Chapter 4: Population Distribution and Trends of California Spotted Owls

Distribution Geographic Range Following Verner et al.’s (1992) technical assessment of the California spotted owl (CASPO), we divided the range of the California spotted owl (Strix occidentalis occidentalis) into two major physiographic provinces: the Sierra Nevada and the mountains of southern California (Tehachapi Pass was the demarcation between the regions). Verner et al. (1992) noted that these provinces are geographically distinct and that movement of owls between them is probably rare, which remains true today (see “Population and Conservation Genetics of California Spotted Owls” section below). The California spotted owl is also found in the coastal mountains north to Monterey Bay, but much less is known about owl numbers and locations along the coast (see figs. 4-1 and 4-2). That portion of the southern Cascade Range that abuts the Sierra Nevada has been considered to encompass the range of the California spotted owl on the east side of California (see chapter 2). Where the ranges of the northern (S. o. caurina) and California spotted owls meet, a hybrid zone occurs in the area of contact near the Pit River (Barrowclough et al. 2011; see chapter 2). Hereafter, we refer to owls occurring south of the Pit River as belonging to the Sierra Nevada population of California spotted owls. Within the Sierra Nevada population, the distribution of owls is relatively contiguous. The majority of owls occur within the mid-elevation, mixed-conifer forests on the west slope of the Sierra Nevada. Some owls also occur at lower elevations in the oak woodlands of the western foothills in the southern Sierra Nevada, at higher elevations in red-fir forests, and in conifer forests on the eastern slope of the mountains (Verner et al. 1992). In contrast, the owl population in central and southern California is more fragmented because owls inhabit major mountain ranges and mountain complexes that are isolated to varying degrees, which limits movement of individuals among these mountain ranges. In this chapter, we focus almost solely on the Sierra Nevada population of owls while deferring discussion of southern California to chapter 9. However, when discussing general properties of