CASE STUDY 4: Risk Assessment Methods in Animal Populations: The Northern Spotted Owl as an Example
D. R. Anderson, U.S. Fish and Wildlife Service
This paper described an analysis of northern spotted owl population dynamics performed to support ongoing studies of the impacts of clear-cutting of old-growth forest on the prospects for future survival of this endangered species (Salwasser, 1986). The paper summarized a method for estimating rates of population increase or decrease based on capture-recapture techniques and illustrates the methods with data on the northern spotted owl. The method proceeds in three steps: use of capture-recapture data to estimate age-specific survival or fecundity rates, estimation of the finite rate of population change (Leslie's parameter λ), and experiments on samples of marked animals in natural environments. Mathematical models for estimating population parameters, including λ, have been developed extensively, and computer programs are available (Burnham et al., 1987). Experimental studies are desirable to test hypotheses about relationships between population parameters and risk factors.
The case study was of a population of northern spotted owls in California studied for 6 years (Franklin et al., 1990). Capture-recapture data yielded estimates of age-specific survival and fecundity for females, as well as estimates of mean population size (37 females) and annual recruitment (0 to 19 females; mean, 8). On the average, the eight females entering the population each year would have included six immigrants from outside the study area and only two locally raised recruits. The calculated value of λ was 0.952 ± 0.028, which indicated a decreasing population.
In this case, the risk factor was clearance of the old-growth forest on which the species is believed to depend. Although the study area contained much suitable habitat, the population appeared not to be self-sustaining, but to be maintained by immigration from remaining areas of old-growth. It was suggested that the study population is temporarily above the long-term carrying capacity because of the drastic loss of