Stability Properties of the Spotted Owl Metapopulation in Southern California

Spotted owls in the Southern California Province have an insular population structure characterized by large (about 200 pair sites) to small (about 2-4 pair sites) local populations distributed among discrete mountain ranges (fig. 9A, table 9A). The distribution of habitat "islands" is discontinuous across the landscape, reflecting natural discontinuities in vegetation structure and composition, in topographic conditions, and in the effects of extensive human-induced habitat disturbance and fragmentation. Lowland areas surrounding these mountain ranges are primarily desert scrub and chaparral habitats that are unsuitable for spotted owls. Relatively narrow gaps between populations, like the 6-mile separation at Cajon Pass, between the San Gabriel and San Bernardino populations (fig. 9A), are probably not complete barriers to dispersing owls. Longer separations, however, such as that of about 30 miles through the Los Angeles Basin, between populations in the San Bernardino and Santa Ana Mountains (fig. 9A), may present significant survival risks to dispersing owls such that successful colonization is very unlikely. The degree to which these gaps act to severely reduce or eliminate demographic rescue between populations is unknown. To date, however, no banded spotted owl has been located within any population outside that of its origin (LaHaye pers. comm.). The most significant gaps between owl populations are discussed in table 3K and shown on a distribution map in that chapter (fig. 3A). Even within many of the mountain ranges, the distributions of habitat and owl sites are discontinuous. For example, in the Cleveland and Los Padres National Forests (NFs), most suitable spotted owl habitat is patchily distributed because it is resticted to deeply dissected canyons dominated by oaks and surrounded