Recovery Plan for the Northern Spotted Owl - DRAFT

Recent LiteratureWe might begin by examining the data from utilized habitats. Finding thatutilized habitats frequently met our definition of Type A habitat, and that homeranges consistently contained large amounts of Type A habitat, would supportthe hypothesis. We would also examine data from correlational analyses. Thehabitat categories would be Type A and other. Strong preference for Type Astands would tend to support our hypothesis, as would a finding that densityand demographic rates were higher in circles, landscapes, and territories withhigh amounts of Type A habitat. Results from functional studies would beuseful in determining whether the specific features of importance to owls werehighly correlated with the elements in our definition of Type A habitat and ifthey were, whether this was likely to be true in other environments to which wemight wish to extrapolate our findings. We would not expect that all these testsand analyses to support the predictions, but if few of them did, then we probablywould discard or substantially modify the definition of Type A habitat andstart again. If most of the predictions were verified, then we would tend to feelthat a good definition of suitable habitat had been developed.A process very much like that above has been carried out in evaluating thehypothesis that older (i.e., mature and old-growth) forests provide suitablehabitat for owls. Descriptions of utilized habitat have shown that nesting,roosting, and foraging owls usually are found in older stands and the structuralfeatures of these stands has been described in quantitative terms. Comparisonof stands have shown that owls exhibit a strong preference for olderstands for roosting and foraging. Reproductive success has been shown to behigher for pairs having more older forest in or near their home ranges. Densityand adult turnover rates also have been shown to be positively correlated withamount of older forest in the landscape. Several hypotheses about the possiblefunctions of older forest in supporting owls have been generated, and evidencesupporting some of them has been reported. Thus, while no one piece ofevidence would (or should) provide high confidence that older forest providessuitable habitat for owl populations, taken together these studies do providesuch confidence. This of course does not mean that older forest is the onlyhabitat that provides suitable habitat, but hypotheses predicting that otherhabitats also provide suitable habitat must be evaluated in the same comprehensiveway that the hypothesis about older forest has been evaluated.As the previous examples indicate, the results from descriptive, correlational,and functional studies are best used in combination, rather than in isolation.Descriptive information is most useful for defining habitat categories; correlationalinformation at the stand, home range, and landscape level is most usefulfor measuring suitability of specific habitats; functional information is particularlyuseful for identifying the specific resources of value to owls and thusproviding insights about how widely results can be extrapolated to otherpopulations. Information from any single type of study would provide a poorbasis for assessments or predictions about habitat suitability: taken together,however, they provide an excellent basis for such analyses. Subsequentsections explore in more detail the ways that each type of information contributesto analyses of habitat suitability.Thomas et al. (1990) provided a thorough review of the literature, includingmany pre-publication reports, available at the time of their report. Our discussionof habitat suitability is based largely on their review. Several reports haveappeared since their study, and we have conducted new analyses of a fewtopics. The new studies, or studies from which we have extracted originaldata, are described briefly in the following section.290