Negative Effect of Purple Loosestrife and Reed Canary Grass on the Diversity of Wetland Plant and Moth Communities
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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. Redacted for Privacy 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 ammoniurn 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: Redacted for Privacy Maj or Professor, representing Entomol Redacted for Privacy Director of the Graduate Program in Entomology Redacted for Privacy Dean of G chool 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. Redacted for Privacy 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 Cohn Meston. Thanks to all my committee members; Peter McEvoy, Andy Brower, Eric Coombs, Ralph Garono, Paul Hammond, 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 2.5Comparison 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 2.7Estimation of plant species sampling accuracy 42 2.8 Rank-abundance curves for 24 wetlands in the Pacific Northwest 43 2.9All 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 48 2.12 Guild species richness declines with the increase of invasive species cover 50 2.13 Effect of invasive species at the 1m2 spatial scale 51 2.14 Moth species accumulation curves continue to rise with increasing sampling effort 52 2.15 Moth species richness is positively correlated with plant species richness 53 2.16 Rank abundance plots of the 20 moth communities 54 2.17 Diversity measures of the moth community 56 2.18 Effect of invasive species on moth family richness 58 LIST OF FIGURES (Continued) Figure Page 3.1 Wetland study site locations in the Willamette Valley, Oregon 75 3.2Wetland site hydrology 78 3.3N:P ratios in dominant plant tissue was less than 14:1 in all cases 79 3.4Relationship between soil nitrogen concentration and plant biomass 80 3.5 Relationship between soil phosphorus concentration and plant biornass 81 3.6Relationship between soil nitrogen concentration and loosestrife biomass 82 3.7Relationship between soil phosphorus concentration and loosestrife biomass 83 3.8Relationship between soil nitrogen concentration and loosestrife growth 84 3.9Relationship between soil phosphorus concentration and loosestrife growth 85 3.10 Relationship between soil nitrogen concentration and plant species richness 86 3.11 Relationship between soil phosphorus concentration and plant species richness 87 3.12 Potential for purple loosestrife invasion of uncolonized sites 89 LIST OF TABLES Table Page 2.1Factors influencing the species richness and diversity of wetland plant communities 18 2.2Environmental factors affecting light trap catch 19 2.3Environmental variables that may influence plant and moth diversity 22 2.4Site characteristics measured by site and location 23 2.5 Formulas used to compute diversity indices 33 2.6 Wetland study site classification 35 2.7 Dominant plant species of wetland field sites 36 2.8Plant species correlations with purple loosestrife and reed canary grass cover 47 3.1Multiple linear regression results for soil nutrient and loosestrife biomass analysis 86 3.2 Multiple linear regression results for soil nutrient and plant species richness analysis 88 LIST OF APPENDICES Appendix Page A Study site location and contact information 106 B Plant species information 107 C Moth species information 112 D Purple loosestrife and reed canary grass impact study collaborators 117 E Mean percent cover of vascular plant species within the 24 wetland field sites 118 F Abundance of moths sampled at 20 wetland field sites 126 Negative Effect of Purple Loosestrife