LIMITING FACTORS FOR NORTheRN FLYING SQUIRReLS (Glaucomys sabrinus) IN The pACIFIC NORThWeST: A SpATIO-TeMpORAL ANALYSIS Submitted in Partial Fulfillment Of the Requirements for the Degree of Doctor of Philosophy in Interdisciplinary Studies With a Concentration in Arts and Sciences And a Specialization in Forest Ecology and Management At the Union Institute & University Cincinnati, Ohio Todd M. Wilson Program Review Meeting, August 7, 2008 Graduation Date, May 31, 2010 First Core: Joseph Meeker, Ph.D. Abstract Northern flying squirrels may be important indicators of the long-term health and management of Pacific Northwest forests, including development of successful regional strategies for managing northern spotted owl habitat under the Northwest Forest Plan. I used live-trapping, radio-telemetry, and within-stand measures of squirrel habitat to (1) assess mid-term effects of variable-density thinning on squirrel populations in young, managed forest, (2) quantify multi-dimensional space use by individual squirrels, (3) contrast squirrel populations across 33 natural and managed stands that were representative of wide gradients in within-stand structural complexity found in Pacific Northwest forests, and (4) test hypotheses evaluating the relative importance of four primary limiting factors for flying squirrel populations— food, competition, predation, and dens. Squirrel populations remained low 12 years after stands were treated with variable-density thinning. At the same time, high-density populations were found elsewhere across western Washington. Most space use by individual squirrels occurred above the forest floor and there were marked differences in movement patterns and behaviors between the breeding and non-breeding seasons and between forest supporting low- and high-density squirrel populations. Forests that supported high squirrel abundances generally exhibited high amounts of multi-dimensional structure in the midstory and overstory layers, low to moderate amounts of understory, and few canopy gaps. Three variables, variance in overstory tree d.b.h., area intercept at 10-m above ground, and amount of canopy gaps ≥100m2 could correctly classify 97% of the stands as supporting either high or low squirrel abundances. The structural complexity i of a forest and how individual structural components are apportioned within multi-dimensional space may determine the capacity of a forest to support abundant squirrel populations. A predation-structural complexity hypothesis is consistent with results reported from this and several past studies in the region and suggests that predation may be a primary limiting factor for squirrels, with food, dens, and competition playing hierarchically less important roles in most regional forests. Efforts to restore structural complexity across a dynamic forest landscape may need to consider the spatial and temporal permeability of forests for flying squirrels, especially before long-term ecological benefits of management activities like variable- density thinning are realized. Key Words: northern flying squirrel, northern spotted owl, strix occidentalis caurina, pacific Northwest, radio telemetry, forest structure, livetrapping, variable-density thinning, structural complexity, landscape permeability, predator-prey ecology. ii Acknowledgments I was directly involved in the collection of data for this study from the onset in 1991, first as field crew leader, then as a supervisory biologist, and concluding as principal investigator in 2004. However, this research would not have been even remotely possible were it not for the substantial efforts of numerous individuals who contributed in various ways to components of this research and there are several individuals who deserve special recognition for their contributions. First, as the original principal investigator for the Forest Ecosystem Study (FES), A. Carey developed the overall FES design, and, as a mentor and team leader, contributed significantly to the thought processes that led me to conduct the radio-telemetry studies, finish the mid-term sampling of squirrels on the FES, and expand the team’s research to evaluate a broad range of forest types across the region. Second, considering that the amount of trapping effort to conduct this study required walking the equivalent distance between Seattle, WA and New York, NY over 3.5 times, several people deserve formal recognition for their trapping efforts: G. Braden, S. Brewster, C. Knox, B. Haveri, D. Johnson, D. Liguiri, A. Lovato, S. Hirst, N. Palazzotto, D. Schwab, M. Stidham, L. Villa, A. Wilson, and S. Wilson. The following people spent hundreds of hours braving cold, rain, heat, and stinging insects while waiting for their squirrel to leave the den each night: G. Braden, B. Haveri, A. Lovato, C. Knox, D. Mickelson, A. Morehouse, N. Palazzotto, D. Tigner, L. Villa, and S. Wilson. L. Villa was instrumental in developing the night telemetry protocol used for collecting movement pattern data. D. Thysell, S. Brewster, and C. Cray provided vegetation survey support for squirrel activity sites. J. Benjamin, J. Brann, iii E. Flores, M. Gerhardstein, J. Grant, E. Krier, A. Morehouse, A. Nelson, M. Norris, N. Palazzotto, H. Purdom, D. Tigner, and B. Ulvog helped collect forest structure data used in the final analyses. N. Palazzotto provided critical leadership in managing field crews during the 2004-2006 field seasons. K. Peeler and B. Galleher provided GIS support for the radio-telemetry studies. Several people contributed to data entry and management, including E. Hamilton, T. Nguyen, H. Thompsen, Y. Sands, and L. Villa. Funding for this long-term research was provided by the Fort Lewis Forestry Division, U.S. Army; Ecosystem Processes Research Program and Olympic National Forest, USDA Forest Service; and a grant from the USDA CREES Research Initiative to A. Carey in 1994. Additional live traps loaned by J. Lehmkuhl, T. Manning, and S. West helped expand the scope of this study in 2005. Logistical support was provided by the Washington Department of Natural Resources, the Olympic National Forest, Fort Lewis Forestry Division, and the Olympic National Park. Several individuals within those agencies helped with study site selection, thinning treatments, or funding, including J. Foster, S. Gremel, S. Horton, G. McCausland, J. Rohdhe, K. O’Halloran, S. Piper, and J. Stephenson. Special thanks goes to my committee members including J. Meeker, J. Koprowski, E. Grumbine, R. Van de Poll, A. Carey, and one external reviewer for the Dean’s office. Each one of these members added unique insight and perspectives that vastly improved earlier drafts of this dissertation. In addition, J. Tallmadge provided critical mentoring that helped shape the iv overall direction of my doctoral program at the onset. Finally, special thanks also goes to my wife who had to tolerate years of my often single-minded focus on the studies included herein, and to my daughter who came along just in time to greatly improve my multi-tasking capacity during the last 3 years of this work. v Table of Contents List of Tables .......................................................................vii List of Figures .......................................................................ix ChApTeR ONe—INTRODUCTION ...................................... 1 Background ......................................................................... 1 Literature Review .................................................................. 4 Limiting Factors for Flying Squirrels ....................................... 10 Problem Statement ............................................................. 18 Study Objectives and Hypotheses .......................................... 28 ChApTeR TWO—STUDY AReA AND MeThODS ................ 36 Study Areas ....................................................................... 36 Field Methods ..................................................................... 41 Population Estimation .......................................................... 56 Descriptive Statistics and Univariate Analyses ........................ 60 Multivariate Analyses ........................................................... 60 ChApTeR ThRee—ReSULTS ........................................... 70 Squirrel Populations ............................................................. 70 Movement Patterns and Behaviors ......................................... 76 Univariate Habitat Predictors................................................. 88 Multivariate SEM Relationships .............................................. 97 ChApTeR FOUR—DISCUSSION ..................................... 108 Caveats ............................................................................108 Flying Squirrel Populations in Western Washington ..................110 Habitat Predictors for Squirrel Populations .............................112 Limiting Factors .................................................................114 Spatio-temporal Movement Patterns .....................................133 Implications for Live-Trapping Studies ...................................135 ChApTeR FIVe—MANAGeMeNT IMpLICATIONS ........... 139 Variable-Density Thinning ....................................................139 Flying Squirrel and Spotted Owl Habitat ................................140 Future Direction ................................................................146 LITeRATURe CITeD ..................................................... 150 AppeNDIX—ANeCDOTAL