Viability of the Northern Spotted Owl Metapopulation in the Northwestern U.S.H. Resit Akcakaya
(This project was funded by the U.S. Forest Service)
This project focused on factors affecting the viability of the Northern Spotted Owl Strix occidentalis caurina throughout its range in the United States. The study used RAMAS GIS to incorporate two sources of variability in determining the threat the species faces. One of the goals of the project was to demonstrate the effect of uncertainty (resulting from lack of information or measurement error) on the assessment of human impact.
Methods: Variability vs. Uncertainty
The study used RAMAS GIS to develop a spatially-explicit, stage-structured, stochastic metapopulation model of the Northern Spotted Owl throughout its range in the United States. The model was used to evaluate the viability of the metapopulation using measures such as risk of decline and time to extinction. The model incorporated natural variation (resulting from temporal fluctuations in environmental factors) in the form of randomly distributed vital rates (survivals and fecundities). In addition, demographic stochasticity was modeled to describe chance variations in reproduction, survival and dispersal. These types of natural variation (environmental and demographic) were used to express the model results in probabilistic terms such as the viability of the species (for example in terms of the chance of survival or risk of extinction).
Uncertainties that result from a lack of knowledge were incorporated in the form of parameter ranges, and were used to estimate upper and lower bounds on the estimated viability of the species. The effects of this type of uncertainty on the assessment of human impact was analyzed by comparing the species' viability under current conditions, and under an assumed loss of spotted owl habitat in the next 100 years.
Based on the habitat maps provided by the Forest Service, RAMAS GIS found 18 habitat patches. The size distribution of the patches was very skewed, with the 4 largest patches making up about 96% of the total area of all patches, and the seven largest making up about 98%. Because of the large differences in sizes of neighboring populations, the viability results (risk of decline) were not very sensitive to the rate of inter-patch dispersal of juvenile spotted owls.
Results: Effect of uncertainty
The model predicted a large difference between lower and upper bounds on the viability of the northern spotted owl, based on the best-case and worst-case scenarios which were parameter combinations that resulted in best and worst chance for survival. According to sensitivity analyses, the viability of the metapopulation was most sensitive to the set of vital rates used (the dependence of fecundities and survival rates on habitat), and also sensitive to the degree of spatial correlation among vital rates of the populations, and to the carrying capacities of the populations. In addition, metapopulation occupancy was sensitive to dispersal and Allee effects.
Thus, the ranges of parameters were quite large, and resulted in wide range of risks of extinction. Despite this uncertainty, the results were sensitive to parameters related to habitat loss: under all assumptions and combinations of parameters, the model predicted that habitat loss results in substantially higher risks of metapopulation decline. This result demonstrated that even with relatively large uncertainties, risk-based model results can be used to reliably assess human impact.
The results of the study are summarized in the following paper:
Akçakaya, H.R. and M.G. Raphael. 1998. Assessing human impact despite uncertainty: viability of the northern spotted owl metapopulation in the northwestern USA. Biodiversity and Conservation 7:875-894.
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