Demographic models and reserve designs for the California spotted owl

The California Spotted Owl (Strix occidentalis occidentalis), like the more well-known Northern Spotted Owl (Strix occidentalis caurina), appears to be in decline over at least a part of its geographic range. Two different types of reserve design have been implemented to protect Spotted Owls. The Spotted Owl Habitat Area (SOHA) system uses a fairly large number of small reserves; each SOHA is capable of supporting 1-3 nesting owl pairs. The Habitat Conservation Area (HCA) plan proposes a smaller number of fairly large reserves, each including 10-20 active Spotted Owl nests. The HCA reserve design strategy has been incorporated into current conservation planning for the Northern Spotted Owl. The Technical Assessment Team of the California Spotted Owl Assessment project has recommended preserving existing SOHAs within a lightly harvested matrix as an interim policy for conservation of the California Spotted Owl. In this paper we present results from two demographic models of the California Spotted Owl. The first model is based on a simple formulation of a birth--death process; the second model is a somewhat more detailed simulation model. The models are intended to provide a comparison of the SOHA and HCA reserve-design strategies. We are particularly interested in the ability of the two reserve designs to withstand recurring environmental catastrophes in the form of forest fires. The HCA strategy always leads to longer persistence times than the SOHA strategy. The essential difference between the two strategies appears to lie in the shape of the function that gives the probability of colonization of an empty nest site. These results have several implications for the conservation of California Spotted Owls and for conservation biology in general. (1) Some variant of the HCA reserve design strategy may be preferable to the interim strategy being proposed for conservation of the California Spotted Owl. (2) Simple, parameter-sparse models like ours can yield results comparable to those of more complex and detailed models. (3) Models that include the effects of catastrophic environmental perturbations have great potential for application in conservation biology.