The California spotted owl: current state of knowledge.
This conservation assessment represents a comprehensive review by scientists of the current scientific knowledge about the ecology, habitat use, population dynamics, and current threats to the viability of the California spotted owl (Strix occidentalis occidentalis). It is based primarily on peer-reviewed published information with an emphasis on new scientific information that has emerged since the first technical assessment for the California spotted owl (CASPO) was conducted in 1992. Substantial new information and insights exist for owls inhabiting the Sierra Nevada, but much less exists for populations inhabiting the central and southern California parts of its range. Spotted owls are habitat specialists that are strongly associated with mature forests that are multistoried or complex in structure, and have high canopy cover, and an abundance of large trees and large coarse woody debris. Most California spotted owl habitat is concentrated in mid-elevation forests of the Sierra Nevada, which consist primarily of ponderosa pine (Pinus ponderosa Lawson and C. Lawson), mixed-conifer, white fir (Abies concolor (Gord. & Glend.) Lindl. ex Hildebr.), and mixed-evergreen forest types. Currently, there are about ∼2 million ha (∼5 million ac) of suitable habitat in the Sierra Nevada, with 75 percent occurring on national forests. These habitat conditions have been demonstrated to have a strong positive association with key vital rates (e.g., occupancy, adult survival, reproductive success), which drive population persistence. All studies published since CASPO have demonstrated that owl populations on national forests in the Sierra Nevada have declined over the past 20 years. A preponderance of evidence suggests that the past century's combination of timber harvest and fire suppression has resulted in forests that have a considerably higher density of trees but a reduced density of large-diameter trees and logs, a greater density of shade-tolerant fire-sensitive tree species, and an increase in forest fuels. These conditions have resulted in reduced habitat quality, increased habitat fragmentation, and increased risk of high-severity fire in the Sierra Nevada. Climate change is projected to have significant effects on Sierra Nevada forests, including exacerbating the risk and impacts from high-severity fires, which in turn is likely to affect spotted owl habitat and populations. The specter of additional threats in the form of competition from the newly invading barred owl (Strix varia) and environmental contaminants, as well as the continuing dearth of information on central and southern California populations, further raises concerns about the fate of California spotted owl populations. Maintenance of a viable population of spotted owls in the Sierra Nevada and throughout its range will depend on effective, long-term owl conservation practices embedded in an overall management strategy aimed at restoring resilient forest structure, composition, and function, including reducing the risk of large-scale high-severity fires while reducing the risk of habitat loss to the owls.