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Issues in Risk Assessment (1993)

Chapter: CASE STUDY 4: RISK ASSESSMENT METHODS IN ANIMAL POPULATIONS: THE NORTHERN SPOTTED OWL AS AN EXAMPLE

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Suggested Citation:"CASE STUDY 4: RISK ASSESSMENT METHODS IN ANIMAL POPULATIONS: THE NORTHERN SPOTTED OWL AS AN EXAMPLE." National Research Council. 1993. Issues in Risk Assessment. Washington, DC: The National Academies Press. doi: 10.17226/2078.
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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

Suggested Citation:"CASE STUDY 4: RISK ASSESSMENT METHODS IN ANIMAL POPULATIONS: THE NORTHERN SPOTTED OWL AS AN EXAMPLE." National Research Council. 1993. Issues in Risk Assessment. Washington, DC: The National Academies Press. doi: 10.17226/2078.
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The scientific basis, inference assumptions, regulatory uses, and research needs in risk assessment are considered in this two-part volume.

The first part, Use of Maximum Tolerated Dose in Animal Bioassays for Carcinogenicity, focuses on whether the maximum tolerated dose should continue to be used in carcinogenesis bioassays. The committee considers several options for modifying current bioassay procedures.

The second part, Two-Stage Models of Carcinogenesis, stems from efforts to identify improved means of cancer risk assessment that have resulted in the development of a mathematical dose-response model based on a paradigm for the biologic phenomena thought to be associated with carcinogenesis.

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