Recovery Plan for the Northern Spotted Owl - DRAFT

isolated. In these provinces, small clusters of owls are separated widely byhabitat unsuitable for dispersal, and populations are vulnerable to extinctionfrom random demographic, genetic, and environmental events (Shaffer 1987).As distance increases beyond 12 miles, the probability of interchange amongclusters decreases rapidly (Thomas et al. 1990). Within provinces wherespacing among patches of suitable habitat commonly exceeds this distance,persistence of clusters is threatened severely.Isolation of Provinces. Small, isolated populations are vulnerable to adversedemographic and genetic effects, as well as the effects of large-scale disturbance.Adverse effects of isolation not only threaten subpopulations, but mayexclude isolated populations from genetic interchange with a larger, interactivepopulation.Provinces can be isolated from some or all of the surrounding provinces byphysical barriers (e.g., the Columbia River) and/or by distance between areas ofsuitable habitat. Immigration of only a few individuals per generation may beadequate to prevent deleterious genetic effects from inbreeding. However, immigrationof a larger number of individuals may be needed for demographic rescue.Predation and Competition. From 1975 to 1991, some 344 adult or subadult,and 85 juvenile spotted owls have been radio-tagged within the range ofthe subspecies (Johnson pers. comm.). Among these, 91 adults or subadultsand 60 juveniles are known to have died. Forty percent of the adults orsubadults, and 25 percent of the juveniles died from predation by other birds.Key avian predators of spotted owls are the great homed owl (Bubovirgfntanus), goshawk (Accipitergentalis), and red-tailed hawk (Buteojamaicens s). The common raven (Corvus corax also is considered a predator,more likely preying on juvenile than adult spotted owls.The great homed owl is the most commonly documented predator of spottedowls (Miller 1989). Great homed owls have become more abundant throughoutmuch of the range of the northern spotted owl, although severity of thisthreat is difficult to measure. Relative densities of the two species in undisturbedlandscapes are unknown, however ratios of great homed owl density tonorthern spotted owl density are highest in more fragmented portions of thenorthern spotted owl's range (Table 2.5). Johnson and Meslow (unpubl. data)found that great horned owls occupied areas that contained significantly lessmature/old-growth forest and interior habitat; had greater edge-to-area ratios:had more shrub/forb, sapling, and shelterwood stands; and were more fragmentedthan those occupied by northern spotted owls.Barred owls are expanding into areas occupied by northern spotted owls. Forthe period of 1980 to 1991, barred owls were reported from 17 locations inCalifornia, 260 locations in Oregon, and several hundred locations in Washington.Most of these observations occurred since 1985, and were made incidentalto spotted owl surveys. Relative density of barred owls is high in manyareas of the spotted owl's range (Table 2.5) and they are displacing spottedowls in some areas (Taylor and Forsman 1976, USDA 1988). Hybridizationbetween the two species also has been documented.Lack of Coordinated Conservation Measures. At the time of listing, the lackof effective regulatory provisions and conservation measures was judged to beone of the most significant threats to the northern spotted owl. Since thattime, various conservation measures have been applied to federal andnonfederal lands. The Endangered Species Act prohibits actions that willresult in taking owls, regardless of the land ownership on which the taking40