Recovery Plan for the Northern Spotted Owl - DRAFT

uted across the range of ecological conditions in the province and thesignificant reduction of linkages and demographic support to adjacentprovinces.Moderate: The threat is not severe at the present time but would beexpected to become severe within the next 10 generations if correctivemeasures are not undertaken. In most cases, these corrective measureswill have to include actions to reverse present conditions and trends.Low: The threat to the population is currently low and is expected toremain low as long as conservation measures are undertaken.Unknown: Inadequate information currently exists to assess the threat.Not all threats are equally important, and no attempt was made to assignthem weights. Comparisons between provinces cannot be based simplyon the number of threats that fall in specific categories, e.g., the numberof threats rated severe or moderate.Low Populations. Small populations are vulnerable to extinction from anumber of causes. Random fluctuations in environmental conditions (environmentalstochasticity) and age and sex structure of populations (demographicstochasticity), along with potential loss of genetic variability (geneticstochasticity) are most likely to influence small populations.Declining Populations. Population trends for northern spotted owls havebeen difficult to estimate because many of the adult and subadult birds areprobably nonterritorial and difficult to detect on surveys. These "floaters" maywait for several years for a territory to become available before they pair andbegin reproducing. If a population is declining, the number of territorial birdsis likely to remain nearly constant as long as floaters remain, because territorialbirds that die are replaced rapidly from the pool of floaters. Thus, territorialbirds are the only segment of the population that can be monitored effectively,but trends in this segment of the population do not necessarily providean accurate estimate of trends in the overall population.One way to solve this problem is by analyzing birth and death rates. Theserates then can be used to calculate whether the population is declining. Theanalyses, because they depend on how birth and death rates vary with age, areoften complex. The underlying principle, however, is simply that the birth rateequals the death rate in a stable population. If the birth rate is less than thedeath rate, then population size declines.The 1990 Status Review (USDI 1990) provided estimates of the rate of populationchange for two populations, one in northern California and one in southernOregon. Both populations were shown to be declining. By the fall of 1991,data from 2 more years were available from these areas, and data were alsoavailable from three other study areas (Figure 2.3).At the request of the Recovery Team, a group of 12 researchers was convenedat Colorado State University to analyze this new information. The resultsindicated that all five populations declined from 1985 to 1991 (Table 2.5). Theestimated rates of decline varied from 7 to 16 percent and averaged about 10percent. The analyses also suggested that the rate of decline may be increasing.Details of the analysis are summarized in Appendix C. These new estimatesreinforce the widely held belief that populations of spotted owls are decliningthroughout all or most of their range.Limited Habitat. Throughout much of the range of the northern spotted owl,habitat is highly fragmented and is resulting in decreased owl productivity and35