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Great Gray Owls (<I>Strix Nebulosa</I>) in Yosemite National

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Great Gray Owls (<I>Strix Nebulosa</I>) in Yosemite National University of Nebraska - Lincoln DigitalCommons@University of Nebraska - Lincoln Nebraska Cooperative Fish & Wildlife Research Nebraska Cooperative Fish & Wildlife Research Unit -- Staff ubP lications Unit 2013 Great Gray Owls (Strix nebulosa) in Yosemite National Park: on the Importance of Food, Forest Structure, and Human Disturbance Charles van Riper III US Geological Survey Southwest Biological Science Center, [email protected] Joseph J. Fontaine University of Nebraska-Lincoln, [email protected] Jan W. van Wagtendonk US Geological Survey Western Ecological Research Center Follow this and additional works at: http://digitalcommons.unl.edu/ncfwrustaff van Riper, Charles III; Fontaine, Joseph J.; and van Wagtendonk, Jan W., "Great Gray Owls (Strix nebulosa) in Yosemite National Park: on the Importance of Food, Forest Structure, and Human Disturbance" (2013). Nebraska Cooperative Fish & Wildlife Research Unit -- Staff Publications. 134. http://digitalcommons.unl.edu/ncfwrustaff/134 This Article is brought to you for free and open access by the Nebraska Cooperative Fish & Wildlife Research Unit at DigitalCommons@University of Nebraska - Lincoln. It has been accepted for inclusion in Nebraska Cooperative Fish & Wildlife Research Unit -- Staff ubP lications by an authorized administrator of DigitalCommons@University of Nebraska - Lincoln. R E S E A R C H A R T I C L E ABSTRACT: We studied great gray owls (Strix nebulosa Forster) in Yosemite National Park, California, measuring variables that could potentially influence patterns of occurrence and conservation of this state- endangered species. We found that owl presence was closely tied to habitat (red fir (Abies magnifica A. Murray) and the abundance of meadows), prey, and snags across the landscape. We also found that indicators of human recreational activities negatively influenced owl distribution and habitat use. Great • gray owls appear to prefer mid-elevation red fir forest with meadows that are drier and more productive in terms of small mammal populations. That these areas also have the highest human activity presents a paradox, both for individual owls and for the future conservation and management of this California Great Gray Owls endangered species. The extent to which human recreation in natural areas affects animal behavior, spe- cies distribution, and productivity is a growing issue in natural area management. We present information (Strix nebulosa) in that will allow land managers to better understand how existing natural resources, coupled with human Yosemite National recreation, influence the distribution and habitat use of the great gray owl. Index terms: distribution, great gray owl, habitat selection, human disturbance, meadows, prey selection, Park: on the Yosemite National Park Importance of Food, INTRODUCTION owls hunt primarily in open forest areas. Forest Structure, and In Yosemite, open areas are almost exclu- Although great gray owls (Strix nebulosa sively meadows, and owl home ranges are Human Disturbance Forster) are common throughout their centered within those meadow systems Holarctic range (Mikkola 1983), North (van Riper and van Wagtendonk 2006). America’s southern-most population is Therefore, we focused our survey efforts disjunct in the Sierra Nevada of California. in areas surrounding meadows within Yo- Charles van Riper III1,4 This owl population is genetically different semite National Park. Within these sites, 1US Geological Survey Southwest (Hull et al. 2010) and considered endan- we surveyed for the presence of great gray Biological Science Center gered by the state of California (Bull and owls and quantified potential behavioral, Sonoran Desert Research Station Duncan 1993). Yosemite National Park, environmental, and anthropogenic influ- University of Arizona in the central Sierra Nevada of Califor- ences on variations in owl distribution Tucson, Arizona 85721 nia, contains the core population for this patterns. If major influences on owls are southernmost population (Bull and Duncan natural, we predicted that the availability of Joseph J. Fontaine2 1993; van Riper and van Wagtendonk food and nest sites should be the primary predictor of owl distribution (Reid 1989; 3 2006). Yosemite National Park is renowned Jan W. van Wagtendonk Bull and Henjum 1990; Bull and Duncan for its recreational opportunities and has 1993). If owl distribution is influenced by one of the highest visitor use rates of any 2 anthropogenic factors, it was important that US Geological Survey Nebraska Coop- United States national park (Schwartz and we determine whether humans affected erative Fish and Wildlife Research Unit Lin 2006). Great gray owls are generally owls by altering the landscape or through University of Nebraska – Lincoln considered reclusive in Yosemite, often their presence. We predicted that if humans Lincoln, Nebraska 68583 avoiding human contact when possible negatively affected owl distribution by (Wildman 1992), but the extent to which modifying their habitats, then areas associ- 3 US Geological Survey Western resource availability and human activities ated with more development (e.g., roads, Ecological Research Center influence the distribution of owls across the buildings, and campgrounds) should have Yosemite Field Station landscape is unclear. These owls are, there- fewer owls. If human presence is a factor El Portal, California 95318 fore, an ideal species for assessing potential negatively affecting owl presence, then influences of natural resources and human areas with the highest human use should • influences on population distribution and have fewer owls, independent of human habitat use patterns. Whether owls simply development, habitat type, food, or nest respond to resource availability, or respond site availability. 4 Corresponding author: charles_van_riper@ to human influences in their environment, usgs.gov; 520-491-0721 has important implication as to how to best STUDY AREA AND METHODS manage this endangered species within a changing natural landscape. Study area Mikkola (1983) in Europe, Bull and Hen- jum (1990) in eastern Oregon, and Franklin Yosemite National Park is located in the Natural Areas Journal 33:286–295 (1988) on the Idaho-Wyoming border in central Sierra Nevada of California, USA the United States, found that great gray (Figure 1). The park is over 300,000 ha 286 Natural Areas Journal Volume 33 (3), 2013 large California urban areas, experiences heavy recreational use, and to accom- modate human recreation, the park has developed a series of trails, roads, and campgrounds, many near meadows. Study species The great gray owl is large, ranging in weight from 700 g to 1700 g and having a wingspan of 140 cm to 150 cm (Nero 1980). The species is widely distributed, occurring in conifer forests throughout much of northern Asia, North America, and Europe (Mikkola 1983). In Yosemite, great gray owls occur primarily in montane forests, and spend up to 80 % of their time in or near meadows that they require as foraging sites (Winter 1986; Reid 1989; van Riper and van Wagtendonk 2006). The availability of food resources can pro- foundly affect the distribution of a species (Fretwell 1972; Cody 1985; Morris 2003), including the great gray owl (Nero 1980; Mikkola 1983). The great gray owl hunts for food during crepuscular and nocturnal periods, employing a sit-and-wait strategy. The owl forages almost exclusively on small rodents, with the majority of their diet throughout their North American range consisting of pocket gophers (Thomomys spp.) and voles (Microtus spp.) (Reid 1989; Bull and Duncan 1993). Survey locations Within Yosemite National Park, we focused our efforts on montane and subalpine mead- ows, which are the center of owl abundance (van Riper and van Wagtendonk 2006). By limiting our study to open meadow areas, we enhanced our ability of detecting the Figure 1. Great gray owl study areas in Yosemite National Park, CA. All core meadow complexes that were presence of owls because of increased surveyed during 1987-1989 are identified, along with major roads and the Yosemite park boundary. aural and visual detection distances as- sociated with meadow systems. We used digital orthophoto quadrangles to identify in size and varies in elevation from 600 and increases with elevation from 800 mm 211 non-contiguous meadows within the m along the western boundary to more to a maximum of 1200 mm. Vegetation park, based on accessibility and elevation. than 4000 m along the crest of the Sierra within the park also varies with elevation In order to consolidate the 211 meadows Nevada. The climate is predominately from shrub woodlands in the foothills, to into complexes that were more biologi- Mediterranean with temperatures ranging montane forests at mid-elevations, and sub- cally meaningful than single meadows, from a mean minimum of -1 °C in Janu- alpine forests and alpine meadows at higher we used a geographic information system ary at high elevations to 32 °C in July at elevations (see Mayer and Laudenslayer (GIS) to delineate a 275-m buffer around low elevations (Elford 1970). Precipitation 1988, for a detailed description). Yosemite each meadow. We chose a 275-m buffer generally occurs from November to March National Park, because of its proximity to because that distance would circumscribe Volume 33 (3), 2013 Natural Areas Journal 287 the 95% adaptive kernel estimate of great repeat surveys of meadows suspected of plot. Within that plot we counted all trees gray owl home range size in Yosemite, as having owls, to more
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