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The Past and Future Roles of Competition and Habitat in the Range- Wide Occupancy Dynamics of Northern Spotted Owls

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The Past and Future Roles of Competition and Habitat in the Range- Wide Occupancy Dynamics of Northern Spotted Owls Communication Ecological Applications, 29(3), 2019, e01861 © 2019 by the Ecological Society of America The past and future roles of competition and habitat in the range- wide occupancy dynamics of Northern Spotted Owls 1,13 2 3 4 5 CHARLES B. YACKULIC , LARISSA L. BAILEY, KATIE M. DUGGER, RAYMOND J. DAVIS, ALAN B. FRANKLIN, 4 6 6 7 8 ERIC D. FORSMAN, STEVEN H. ACKERS, LAWRENCE S. ANDREWS, LOWELL V. D ILLER, SCOTT A. GREMEL, 7 9 10 11 6 KEITH A. HAMM, DALE R. HERTER, J. M ARK HIGLEY, ROB B. HORN, CHRISTOPHER MCCAFFERTY, 12 2 6 JANICE A. REID, JEREMY T. ROCKWEIT , AND STAN G. SOVERN 1U.S. Geological Survey, Southwest Biological Science Center, Flagstaff, Arizona 86001 USA 2Department of Fish, Wildlife, and Conservation Biology, Graduate Degree Program in Ecology, Colorado State University, Fort Collins, Colorado 80523 USA 3U.S. Geological Survey, Oregon Cooperative Fish and Wildlife Research Unit, Department of Fisheries and Wildlife, Oregon State University, Corvallis, Oregon 97331 USA 4USDA Forest Service, Pacific Northwest Research Station, Forestry Sciences Laboratory, Corvallis, Oregon 97331 USA 5USDA APHIS National Wildlife Research Center, Fort Collins, Colorado 80521 USA 6Oregon Cooperative Fish and Wildlife Research Unit, Department of Fisheries and Wildlife, Oregon State University, Corvallis, Oregon 97331 USA 7Green Diamond Resource Company, Korbel, California 95550 USA 8USDI National Park Service, Olympic National Park, Port Angeles, Washington 98362 USA 9Raedeke Associates, Seattle, Washington 98133 USA 10Hoopa Tribal Forestry, Hoopa, California 95546 USA 11USDI Bureau of Land Management, Roseburg District Office, Roseburg, Oregon 97471 USA 12USDA Forest Service, Pacific Northwest Research Station, Roseburg Field Station, Roseburg, Oregon 97471 USA Citation: Yackulic, C. B., L. L. Bailey, K. M. Dugger, R. J. Davis, A. B. Franklin, E. D. Forsman, S. H. Ackers, L. S. Andrews, L. V. Diller, S. A. Gremel, K. A. Hamm, D. R. Herter, J. M. Higley, R. B. Horn, C. McCafferty, J. A. Reid, J. T. Rockweit, and S. G. Sovern. 2019. The past and future roles of competition and habitat in the range-wide occupancy dynamics of Northern Spotted Owls. Ecological Applications 29(3):e01861. 10.1002/eap.1861 Abstract. Slow ecological processes challenge conservation. Short-term variability can obscure the importance of slower processes that may ultimately determine the state of a sys- tem. Furthermore, management actions with slow responses can be hard to justify. One response to slow processes is to explicitly concentrate analysis on state dynamics. Here, we focus on identifying drivers of Northern Spotted Owl (Strix occidentalis caurina) territorial occupancy dynamics across 11 study areas spanning their geographic range and forecasting response to potential management actions. Competition with Barred Owls (Strix varia) has increased Spotted Owl territory extinction probabilities across all study areas and driven recent declines in Spotted Owl populations. Without management intervention, the Northern Spotted Owl subspecies will be extirpated from parts of its current range within decades. In the short term, Barred Owl removal can be effective. Over longer time spans, however, maintaining or improving habitat conditions can help promote the persistence of northern spotted owl popu- lations. In most study areas, habitat effects on expected Northern Spotted Owl territorial occu- pancy are actually greater than the effects of competition from Barred Owls. This study suggests how intensive management actions (removal of a competitor) with rapid results can complement a slower management action (i.e., promoting forest succession). Key words: competitive exclusion; ecological forecasting; geographic range dynamics; late-successional habitat; old growth forest; temporal scaling. INTRODUCTION Manuscript received 29 August 2018; revised 17 December Many ecological processes relevant to landscape-scale 2018; accepted 7 January 2019. Corresponding Editor: Carolyn H. Sieg. conservation and management are slow to develop and 13 E-mail: [email protected] are not immediately apparent. These processes can Article e01861; page 1 Ecological Applications Article e01861; page 2 CHARLES B. YACKULIC ET AL. Vol. 29, No. 3 manifest themselves over decades to millennia and Washington and British Columbia (Livezey 2009b), it is involve both loss and recovery of species. Slow pro- clear that BO began to invade the northern portion of cesses of loss include extinction debts in fragmented the NSO geographic range about 50 years ago and were landscapes (Tilman et al. 1994, Jones et al. 2018), found throughout the NSO range in low numbers when exclusion of native species by invading species (Gilbert the Northwest Forest Plan was first implemented (Live- and Levine 2013, Yackulic 2017), and population decli- zey 2009a). Over the last decade, BO has continued to nes in response to ongoing climate change (Peery et al. spread to new areas and increase in local abundances 2012). Slow processes of recovery include the recovery once they are present (Fig. 1; Yackulic et al. 2012, Ross- of late-successional habitats degraded by humans (Aide man et al. 2016, Zipkin et al. 2017). Removal of BO was et al. 1996) and the range expansion of tree species lim- an effective management action in one study area at the ited by dispersal (Svenning and Skov 2007), with impli- leading edge of the invasion, halting declines in NSO ter- cations for animal species that depend on these plant ritorial occupancy (Diller et al. 2016); however, it is communities (Yackulic and Ginsberg 2016). Slow pro- unclear whether BO removal will be as effective in areas cesses create challenges for conservation efforts as with different forest conditions, or where BO are more short-term drivers of trends may be more obvious than established. forces acting over longer time periods. Furthermore, the In this paper, we quantify the relative importance of optimal management strategy may involve staggering habitat and BO competition in past and future NSO ter- different management actions over time. Slow rates of ritorial occupancy dynamics. We rely on long-term stud- change necessitate a focus on the processes driving loss ies of NSO populations at eleven study areas spanning or recovery (e.g., extinction and colonization) and what the United States portion of the NSO historic geo- they imply for future states (e.g., extent and frequency graphic range. We begin by estimating current trends in of occurrence), in addition to the current state of the NSO habitat covariates and territorial colonization and system (Yackulic et al. 2015). extinction rates for BO and NSO. We then quantify the Here, we show how processes of colonization and influences of habitat and competition with BO on extinction can be analyzed to elucidate the relative expected NSO territory occupancy. Lastly, we forecast importance of slower and faster processes and forecast future interactions between BO and NSO under current future occupancy under different management scenarios. conditions and under scenarios with various levels of While our approach is general, we focus our analysis on BO removal or changes in habitat condition. the past and potential future drivers of the population dynamics of Northern Spotted Owls (Strix occidentalis METHODS caurina; hereafter NSO) in the Pacific Northwest region of the United States of America. NSO populations have Territorial colonization and extinction probabilities for been declining for over three decades, with initial decli- NSO and BO nes driven primarily by habitat loss (Lande 1988). Older forest, the preferred habitat of NSO throughout most of NSO and BO data were previously analyzed by Dug- their geographic range, was harvested from the late 19th ger et al. (2016) using two-species occupancy models, century through the late 20th century, when declines in however that study was focused on NSO demography ommunication NSO populations led to the listing of the species as and did not present BO vital rates or consider the impli- “threatened” under the U.S. Endangered Species Act in cations of the joint dynamics of the two owl species. In C 1990 and the development of the Northwest Forest Plan both studies, the spatial unit of the analysis (hereafter in 1994 (Davis et al. 2017). Since 1994, the Northwest the “site”) is NSO territories that were delineated by gen- Forest Plan has significantly lessened the rate of habitat erating Thiessen polygons based on annual NSO activity loss on federal lands from timber harvesting, but a centers (see Dugger et al. 2016). Surveys focused on recent increase in large, high-severity wildfires has led to identifying NSO breeding pairs, and thus, occupancy for reduced NSO survival and significant habitat losses in this species was defined in terms of pairs. In a few some regions (Davis et al. 2016, Rockweit et al. 2017). instances, surveyors were unable to confirm a mate and Even without further loss, recruitment of old forest was these detections of unpaired individuals were treated as expected to take many decades (Lint et al. 1999) and non-detections. For BO, exact assessments through fol- declines in NSO were expected as populations equili- low-up surveys were usually not made, so we defined brated with past habitat loss (Thomas et al. 1993). occupancy to include both paired and unpaired individ- Nonetheless, the degree and ubiquity of ongoing decli- uals. In 10 of 11 study areas, we analyzed data from nes in NSO populations (Fig. 1) have outpaced forecasts 1995 to 2013, while in one study area (Green Diamond), linked solely to habitat loss and there is now strong evi- only data from 1998 to 2013 were available for analysis. dence linking recent declines to competition with invad- We base inferences for each study area on the model ing Barred Owls (Strix varia; hereafter BO; Yackulic with the lowest Akaike information criterion corrected et al. 2014, Dugger et al. 2016). for sample size (AICc; Burnham and Anderson 1998). While there is some debate regarding the changes that For more details on two-species occupancy modeling, allowed BO to expand their distribution westward into see Appendix S1.
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