Global Short Term Trend: Decline of 10-30%
Comments: Many populations are declining, especially in the northern parts of the range (USFWS 2007). Recent demographic data suggest that populations in 13 long-term demographic study areas decreased by about 3.7 percent per year from 1985 to 2003 (Anthony et al. 2006, USFWS 2007).
Range-wide monitoring of northern spotted owl populations and habitat during the first 10 years of implementation of the Northwest Forest Plan (1994-2003) yielded estimated annual population declines of 0 to 10 percent across study areas (weighted average = 3.4 percent); the average annual rate of decline for the four demographic areas in Washington was 7.1 percent (Anthony et al. 2004, Lint 2005).
In British Columbia, Blackburn et al. (2002) monitored the occupancy of spotted owls within 40 survey areas between 1992 and 2001. Spotted owl occupancy declined by 49% between 1992 and 2001. The average annual rate of decline was -7.2%. The substantial decline between 1992 and 2001 occurred despite protection of suitable habitat during a large portion of the 10-year study within 39 of the 40 survey areas and Special Resource Management Zones (SRMZs). Results suggested that the decline may have slowed in recent years, possibly in response to habitat protection measures. Assuming that the study area is representative of the entire spotted owl population in B.C., then the number of spotted owls in B.C. is not stable, but has declined sharply over the past 10 years. Blackburn et al. (2002) concluded that current suitable habitat levels protected in SRMZs under the Spotted Owl Management Plan are inadequate to stabilize the owl population; the population is expected to decline further over the next few decades. Accordingly, Chuttrer et al. (2004) estimated that breeding populations in British Columbia may be declining by as much as 35 percent per year (Chutter et al. 2004), and more recent studies indicate that the decline is continuing (USFWS 2007). The amount of interaction between spotted owls in Canada and the United States is unknown (Chutter et al. 2004).
Global Long Term Trend: Decline of 30-70%
Comments: Area of occupancy, number of subpopulations, and population size have undergone a significant long-term decline, but the precise level of decline is uncertain.
As of 1990, habitat reduction was continuing at a rangewide rate of 1-2% per year, and the owl was threatened by extirpation in significant portions of its range (Thomas et al. 1990). Thomas et al. (1993) concluded that habitat loss was occurring at a rate of about 1-5% per year. Territorial female population declined by an estimated average of 7.5% each year during 1985-1991 (Thomas et al. 1993). Overall population on 12 study areas declined at a rate of 3.2% per year during 1985-1992 (Thomas et al. 1993). Populations in the Klamath Mountains of California and in the Coast Range of Oregon were declining in the late 1980s (Thomas et al. 1990). In northwestern California, a 60-83% reduction in population was recorded on at least one National Forest under current management plans by the U.S. Forest Service (Franklin et al. 1990). USFWS (1990) categorized the status as "declining." Though one demographic analysis concluded that the northern spotted owl population was stable, Taylor and Gerrodette (1993) showed that this conclusion was not statistically justified.
An analysis of all capture-recapture data showed a declining annual survival rate for adults on 11 large study areas during 1985-1993 (Burnham et al. 1996). Burnham et al. (1996) concluded that there was a plausible inference of a declining population during the study period and that the rate of population decline was accelerating.
In a range-wide analysis based on estimates of demographic parameters averaged across multiple studies and juvenile survival corrected for emigration, Franklin et al. (1999) estimated that the population of territorial females declined at a annual rate of 3.9% udring the 1980s and 1990s. Although the overall analysis indicated a declining population, some individual study areas appeared to have stationary (i.e., stable) populations whereas other studies suggested substantial declines. Another model that only capture-recapture data from selected studies suggested that most of the selected study areas had stationary populations. However, three study areas exhibited evidence indicating that populations were declining on these areas. The authors concluded that "there is still uncertainty regarding the health of the spotted owl population."
In northwestern California, among basic demographic parameters, annual survival varied the least over time whereas recruitment rate varied the most (Franklin et al. 2000). A forecast of annual rates of population change, estimated from life history traits, suggested that populations may change solely due to climate influences (e.g., annual variations in temperature and precipitation), even with unchanging habitat conditions (Franklin et al. 2000).