AN ABSTRACT OF THE DISSERTATION OF Shon Schooler for the degree of Doctor of Philosophy in Entomology presented on December 1, 2003. Title: Negative Effect of Purple Loosestrife and Reed Canary Grass on the Diversity of Wetland Plant and Moth Communities. Abstract approved: _________________________________ Peter B. McEvoy Invasive plants have the potential to reduce the diversity of species in plant and animal communities. I examined the negative effect of two invasive wetland plants, purple loosestrife and reed canary grass, on the species richness and diversity of plant and moth communities within 24 wetland study sites in the Pacific Northwest. I hypothesized that as the cover of the invasive species increased, the diversity of the local plant and moth community would decrease. Increasing cover of purple loosestrife and reed canary grass was associated with reduction in the diversity of wetland plant communities irrespective of the diversity measure examined. Moth species richness was positively correlated with plant species richness, but I found no detectable direct negative association between loosestrife and canary grass cover and moth community diversity. Wetland hydrology, soil characteristics, and topography were measured to control for potentially covarying and confounding influences on plant diversity. Temperature, ambient light, and surrounding land-use were measured to control for potentially covarying and confounding influences on moth sampling and diversity. None of these variables was significantly associated with invasive species abundance. This strengthens the conclusion that the invasive species are the cause of the decline in biotic diversity. Understanding the mechanisms that influence plant invasions will lead to more effective management strategies. I examined the role of soil nutrients in the invasive potential of purple loosestrife. I hypothesized that nitrogen was the primary nutrient limiting plant growth and that higher soil nitrogen concentrations would increase the growth of purple loosestrife within 13 wetland sites in the Willamette Valley, Oregon. Using greenhouse experiments and field studies I found that nitrogen was the primary resource limiting both plant community biomass and purple loosestrife growth. Purple loosestrife grew well in soils taken from nine wetlands currently not colonized by loosestrife. Given their similar hydroperiods, this suggests that these wetlands will be susceptible to invasion should loosestrife colonize. Plant species richness was negatively associated with soil nitrate and ammonium concentrations. This trend included invaded and non-invaded sites. Therefore, to prevent repeated invasions, management strategies should consider methods for reducing soil nutrient concentrations, particularly nitrogen. ©Copyright by Shon S. Schooler December 1, 2003 All Rights Reserved Negative Effect of Purple Loosestrife and Reed Canary Grass on the Diversity of Wetland Plant and Moth Communities. by Shon S. Schooler A DISSERTATION submitted to Oregon State University in partial fulfillment of the requirements for the degree of Doctor of Philosophy Presented December 1, 2003 Commencement June 2004 Doctor of Philosophy dissertation of Shon S. Schooler presented on December 1, 2003. APPROVED: _______________________________________________ Major Professor, representing Entomology ________________________________________________ Director of the Graduate Program in Entomology ________________________________________________ Dean of Graduate School I understand that my dissertation will become part of the permanent collection of Oregon State University Libraries. My signature below authorizes release of my dissertation to any reader upon request. Shon S. Schooler, Author ACKNOWLEDGEMENTS Many people have helped make this project a success. First and foremost, thanks to Peter McEvoy who provided the ecological and academic guidance to build this study from concept to manuscript. Eric Coombs was always there to temper theory with application and deepen our environmental awareness through his intimate knowledge of natural history and experience with weed biological control programs . Thanks once again to Tony Ives and Dan Young for showing me the wonders of insects, ecology, and scientific investigation. I could not have done the labor intensive moth sampling without the aid of many stalwart field assistants; Kumari Sivam, Marjo Schat, Jason Fuller, Kirby Winter, Laurie Hewitt, Christine Buhl, and Colin Meston. Thanks to all my committee members; Peter McEvoy, Andy Brower, Eric Coombs, Ralph Garono, Paul Hammo nd, Mary Kentula, Virginia Lesser, and Phil Rossignol for their time and patience in helping design and critique this work. Thanks also to the watershed coordinators, Jay Mower and Margaret Magruder, who graciously gave me a place to sleep while sampling far from home. Gene Korienek and Deanne Hudson helped my sanity immensely by letting me stay in a wonderful apartment while finishing this dissertation. A special thanks to Doug Johnson for always finding me a GPS receiver when I needed it. A firm handshake to all my colleagues in the former OSU Department of Entomology for their warmth and friendship. Finally, thanks to the guidance and support of my parents who have given me the universe, and my brother who reminds me what it means to live within it. There are so many others who have helped over the years; Gary Brown, Emily Roth, Joel David, Jessica Gonzales, Nancy Hendrikson, Elaine Stewart, Troy Clark, Dave Ambrose, Al Clarke, Tom Kollasch, Ralph Cook, Dennis Wiley, Carrie Stevenson, Geoff Dorsey, Mart Hughes, Bruce Sutherland, Rich Guadagno, Jim Houck, and Scott Carter, to name just a few. TABLE OF CONTENTS Page General Introduction ................................................................................................................ 1 Ecological Effect of Invasive Plants ......................................................................... 2 The Study Organisms ................................................................................................. 5 Summary ..................................................................................................................... 8 Literature Cited .......................................................................................................... 9 Effect of purple loosestrife and reed canary grass on wetland plant and herbivore diversity ... 13 Introduction ................................................................................................................ 13 Materials and Methods ............................................................................................... 20 Results ........................................................................................................................ 34 Discussion .................................................................................................................. 58 Literature Cited............................................................................................................ 66 The growth of purple loosestrife is positively associated with soil nitrogen concentrations ... 73 Introduction ................................................................................................................ 73 Materials and Methods ............................................................................................... 76 Results ........................................................................................................................ 79 Discussion .................................................................................................................. 89 Literature Cited............................................................................................................ 92 General Conclusion.................................................................................................................... 93 Bibliography .............................................................................................................................. 96 Appendices ................................................................................................................................ 105 LIST OF FIGURES Figure Page 2.1 Map of wetland study site locations in the Pacific Northwest ........................................................ 21 2.2 Percent of moths recaptured decreases with increasing distance from the blacklight trap.............. 26 2.3 Assessment of the accuracy of water level and area inundated measurements .............................. 37 2.4 Study site hydrology ........................................................................................................................ 38 2.5 Comparison of temperatures between reference and invaded wetlands during sampling events .... 40 2.6 Reference site night-time illumination circumscribes the ambient light of the invaded sites .......... 40 2.7 Estimation of plant species sampling accuracy ............................................................................... 42 2.8 Rank-abundance curves for 24 wetlands in the Pacific Northwest................................................... 43 2.9 All measures of diversity decrease with increasing abundance of the invasive plant species ......... 45 2.10 The number of plant families per site decreases with increasing invasive cover ........................... 46 2.11 Effect of invasive species on native plant species and abundance ................................................