WWRC 90-11
Viable Populations of Spotted Owls for Management of Old Growth Forests in the Pacific Northwest
Abstract
The Northern Spoiled Owl (Strix occidentalis caurina Xantus 1859) has become a surrogate for efforts to
preserve remaining old growth forests in the Pacific Norlhwesi. We review here a viability analysis prepared
by the U. S. Forest Service which evaluates risks of extinction resulting separately from (a) stochastic
demographic processes, (b) inbreeding and loss of genetic variability, and (c) habitat loss. Stochastic
demography must include density dependence to be realistic, particularly for a territorial
species. Because of high sampling variance, it is unlikely that adequate data can be accumulated for most
rare species to provide demographic projections that differ significantly from a constant population. A
model that integrates all components of the Forest Service viability analysis predicts a low probability
that Spotted Owls will go extinct under the Forest Service's preferred management alternative. However,
we emphasize that such a model is unrealislic because it does not incorporate spatial distribution
of owls, and population fragmentation imposes the greatest risk of their extinction. A metapopulation model
by Lande (1988) estimates acreage of old-growth forest necessary to preserve the Spotted Owl on Spoiled Owl
Habitat Areas (SOHAs). However, such estimates are dependent upon accurate determinations of the proportion
of SOHAs and other habitats that are truly suitable for Spotted Owls. Habitat requirements of the owls
must be carefully documented to justify management based upon the metapopulation model.
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