Recovery Plan for the Northern Spotted Owl - DRAFT

Use of the informationAssessments and predictions about the suitability of habitats for owls generallyinvolve three steps:1. Define habitats and habitat categories.2. Determine suitability of each habitat category.3. Evaluate basis for extrapolating findings to other populations.The six types of information identified earlier play different roles in these threesteps (Table B. 1). Descriptive information about the structure of utilized sitesand amounts of habitat within home ranges are particularly helpful in defininghabitat categories. Knowing the amount of habitat present in home rangesalso provides some indication of which landscapes provide suitable conditions.Correlational information provided by studies of which habitats are preferredfor roosting and foraging, and by studies of density and demographic rates, aremost useful in measuring the suitability of existing habitats. Correlationalstudies also provide some assistance in defining new habitats and decidinghow much the results can be extrapolated to other populations of interest.Functional studies, in which causes are identified and distinguished fromcorrelates, provide a good basis for measuring suitability and an excellent basisboth for defining new habitat categories and for evaluating whether, or howwell, the results of studies can be extrapolated to other populations. A fewexamples illustrating these generalizations and showing how studies of habitatsuitability complement and reinforce each other are given later. More detaileddiscussions of how each type of information can be used in the recoveryprogram are provided in subsequent sections.Early surveys for owls revealed that density was high in areas with plentifulold-growth and low elsewhere. These descriptive and correlational results weresupplemented by functional studies that sought to identify the specific resourcesneeded by owls that were present in old-growth and absent in otherareas. The results of the correlative and functional studies convinced mostinvestigators that densities would remain low outside of old-growth. Since oldgrowthwas expected to decline in abundance, this led to the prediction thatoverall populations would decline. In this example, the parameter indicatinghabitat suitability was abundance, and the habitat categories were old-growthand other. The critical assumption on which the prediction was based wasthat density would remain low outside of old-growth areas.The landscape model discussed in Appendix A permits the definition of up tosix habitat types, and the specification of various "rules" that define behaviorand success of owls within each type. These rules are assumed to remain thesame within each type through time. The parameters are thus the demographicrates and other rules followed by the model, and the habitats aredefined by the investigator. Both correlative and functional studies are neededto define the habitat types, estimate parameter values within them, andevaluate the assumption that parameter values will be similar in study areasand areas about which inferences are made using the landscape model.Owls are sometimes present in areas where most of the trees are fairly young(e.g., 50 to 80 years old), but some older and larger trees are present (e.g.,Thomas 1990:184). This leads to the hypothesis that if some large trees wereleft during harvest, density of owls might increase substantially when theregenerating stand reached an age of 50 to 80 years. In this prediction thehabitat is 50 to 80-year-old stands with remnant large trees, the parameterindicating suitability is density, and the primary assumption is that density infuture habitats of this type will be similar to the existing habitats that havebeen studied. Descriptive and correlational studies were useful in developing288