Recovery Plan for the Northern Spotted Owl - DRAFT

AbstractThis appendix discusses the use of information on habitat suitability in therecovery program. Information on habitat suitability is divided into six categories:structural features of sites used by owls, amount of habitat in homeranges, habitat selection for roosting and foraging, abundance of owls indifferent habitats, demographic rates in different habits, and functional studiesof resources needed by owls. Our current knowledge about each of these areasis summarized, emphasizing knowledge obtained since the review by Thomaset al. (1990). New information provides a clearer description of habitats usedby owls and suggests that, as predicted by Thomas et al. (1990), a wider rangeof habitats may be suitable for owls in California than in Oregon and Washingtonwest of the Cascade crest. New evidence also suggests that adult survivalrates decline with decreasing amount of old-growth near to the nest. Thisfinding is important because of the extreme sensitivity of population viability toadult survival rates. Recommendations for future work include standardizinghabitat measurements, preparing habitat maps for the demographic studyareas, and emphasizing new research in several specific areas or habitats.This appendix summarizes information from habitat studies on northernspotted owls. Thomas et al. (1990) should be consulted for a comprehensivereview of this topic: this appendix emphasizes studies which have appearedsince Thomas et al. (1990). We discuss how habitat information will be used inthe recovery program, review the state of our knowledge about suitable habitatfor northern spotted owls, and offer tentative suggestions about priorities forfuture research on habitat requirements of owls.We were assisted in the preparation of this report by Gary Benson, RandyDettmers, Jeff Grenier, David Johnson, Jo Ellen Richards, Kristin Schmidt,and David Solis, each of whom analyzed studies and data bases describinghabitat relationship of spotted owls in a portion of the owl's range. Reports byBenson (1991a,b) and Grenier (1991) are included in the administrative recordof the Recovery Team. Bruce Bingham, Mark Boyce, Jennifer Blakesley,Charlie Brown, Joe Buchanan, Lowell Diller, Lee Folliard, Eric Forsman, AlanFranklin, Rocky Gutierrez, Tom Hamer, Larry Irwin, Steve Kerns, KevinMcKelvey, Joe Meyer, Barry Noon, Malcolm Pious, Steve Self, and Cindy Zabelprovided us original summaries and analyses of their data or pre-publicationmanuscripts. Previous drafts of the manuscript were reviewed by Bob Anthony,Edie Asrow, Gary Benson, Bruce Bingham, Lowell Diller, Alan Franklin,Rocky Gutierrez, Richard Holthausen, Bruce Marcot, and Steve Self.Studies of Habitat SuitabilityHabitat suitability might be defined qualitatively as the degree to which a givenhabitat provides the resources owls need to survive, reproduce, and dispersesuccessfully. No single definition captures all of the ways of measuring habitatsuitability. For example, consider a study to determine the suitability of aparticular habitat for foraging. Suitability might be assessed as the amountthat foraging birds use the habitat, by capture rates per unit of time spenthunting in the habitat, by susceptibility to predators of owls foraging in thehabitat, or by time required for the prey population to rebound after beingdepleted. Each of these measures would provide useful information and couldlegitimately be considered an aspect of habitat suitability. Habitat suitability isthus best considered an area of study, rather than a specific parameter. In aspecific context, however, more precise definitions of habitat suitability may bereasonable. Furthermore, it is certainly possible to measure habitat featuresor attributes that have little or no relationship to the habitat's suitability for285