University of Nebraska - Lincoln DigitalCommons@University of Nebraska - Lincoln Dissertations and Theses in Biological Sciences Biological Sciences, School of 2-2012 Herbivory affects patterns of plant reproductive effort and seed production Natalie M. West University of Nebraska-Lincoln, [email protected] Follow this and additional works at: https://digitalcommons.unl.edu/bioscidiss Part of the Biology Commons, and the Ecology and Evolutionary Biology Commons West, Natalie M., "Herbivory affects patterns of plant reproductive effort and seed production" (2012). Dissertations and Theses in Biological Sciences. 41. https://digitalcommons.unl.edu/bioscidiss/41 This Article is brought to you for free and open access by the Biological Sciences, School of at DigitalCommons@University of Nebraska - Lincoln. It has been accepted for inclusion in Dissertations and Theses in Biological Sciences by an authorized administrator of DigitalCommons@University of Nebraska - Lincoln. HERBIVORY AFFECTS PATTERNS OF PLANT REPRODUCTIVE EFFORT AND SEED PRODUCTION by Natalie M. West A DISSERTATION Presented to the Faculty of The Graduate College at the University of Nebraska In Partial Fulfillment of Requirements For the Degree of Doctor of Philosophy Major: Biological Sciences Under the Supervision of Professors Svata M. Louda and Brigitte Tenhumberg Lincoln, Nebraska February, 2012 HERBIVORY AFFECTS PATTERNS OF PLANT REPRODUCTIVE EFFORT AND SEED PRODUCTION Natalie M. West, Ph.D. University of Nebraska, 2012 Advisors: Svata Louda & Brigitte Tenhumberg Insect herbivory can have a major influence on plant reproduction, and potentially drive selection for strategies that reduce or resist herbivore effects. I used a combination of field experiments and ecological modeling to examine how modifications in the patterns and timing of reproductive investment might ameliorate the consequences of herbivore damage for plant reproduction. I performed experiments to examine how changes in reproductive effort after apical damage and reduction of insect herbivory affected seed production in two thistles native to Nebraska. I then used field data to parameterize a life history model predicting the resource allocation among buds and size and timing of flowering that would optimize fitness under a continual risk of herbivory. In monocarpic Cirsium canescens, insect herbivores had a severe impact on plant seed production. Plants did modify reproductive effort in response to apical damage. High seed production from a large apical head, as well as increased flowering and seed production with apical damage from later flower heads, played a role in improving seed production. However, changes in flowering and investment patterns were insufficient to compensate for high insect damage; plants had lower seed set under ambient herbivory. We found similar effects in the iterocarpic Cirsium undulatum, although plant responses were not consistent between years. The combination of these two experiments allowed us to quantify the influence of plant reproductive response on the consequences of insect damage, and how it varies between plants with different life history strategies. To better understand how the risk of insect herbivory might shape optimal plant allocation patterns, I constructed a stochastic dynamic programming model (SDP) to examine the optimal allocation between flower heads through time, and the size and time at which buds should flower to maximize fitness. The model predicts optimal allocation patterns should vary with survival risk, and plants should favor strategies that reduce the duration of risk. Both the model and experiments demonstrate the pressure insect herbivores can exert over plant reproductive strategies, and broaden our understanding of how ecological interactions can affect influence basic life history decisions. Copyright by Natalie M. West © 2012 i Acknowledgements I would like to thank the many people who have given help, time, resources, and encouragement throughout this process. My advisors, Svata Louda and Brigitte Tenhumberg, have provided me with unending advice and support in my project as well as my development as a scientist. Jutta Burger, Laura Campbell, Claire Hafdahl, Deirdra Jacobsen, Amy Johannes, Deb Knight, Joshua Knight, Michael Mellon, Allison Mettler, Rachel Paseka, Joseph Rose, Caitlin Spilinek, Andy West, Veronica West, and Adam Yarina provided assistance in the field and lab. The Delwin and Dusty Wilson and Duane and Kelly Wilson families of Arthur County, NE, provided help, encouragement and use of their land and resources. I also thank the staff and researchers at Cedar Point Biological Station for their logistical support and encouragement, as well as my committee members, Ann Antlfinger, Chad Brassil, Jean Knops, and Ellen Paparozzi for advice as the project developed. Thank you also to Travis Hinkelman, Ben Nolting, Holly Prendeville, and Tomomi Suwa for all the great discussions and commiseration. I also wish to particularly acknowledge Laura Campbell and Michael Mellon. Thank you for your friendship, laughs, nerdy conversations, and philosophical musings. To my husband, Joshua, and my in-laws, Deb and Dave, thank you for your support, encouragement, and tolerance. Finally, I would like to thank my parents, Andy and Veronica, for their unwavering support, assistance, encouragement, and sense of humor. You have sustained me through this process, as well as you have everything else. I am truly blessed. ii Table of Contents Chapter 1 ……………………………………………………………1 Introduction ............................................................................3 Methods ……………………………………………………..7 Results ………………………………………………………16 Discussion …………………………………………………. .20 Tables ……………………………………………………… .31 Figures ……………………………………………………....32 Appendices ………………………………………………….40 Chapter 2 …………………………………………………………... .53 Introduction ………………………………………………....54 Methods ……………………………………………………..57 Results ……………………………………………………….64 Discussion ..………………………………………………….67 Tables ………………………………………………………..78 Figures ……………………………………………………….79 Appendices …………………………………………………..83 Chapter 3 …………………………………………………………….95 Introduction…………………………………………………..97 Methods ……………………………………………………..102 Results ………………………………………………………112 Discussion …………………………………………………...114 Figures ………………………………………………………123 Appendices ………………………………………………….133 iii List of Tables and Figures Table 1.1. Cirsium canescens end of season plant performance measurements 31 Figure 1.1. Experimental predictions. 35 Figure 1.2. Average damage score per flower head for individual plants 36 Figure 1.3. Cirsium canescens whole plant seed production 37 Figure 1.4. Cirsium canescens seed production per flower head 38 Figure 1.5. Cirsium canescens seed production per flower head position 39 Figure A1.1:1 Seeds per apical flower head in the apical damage treatment 42 Figure A1.1:2 Seed production with low versus severe apical damage 43 Table A1.2:1 Initial size measures for experimental C. canescens plants 44 Table A1.3 Analysis of C. canescens structure and reproductive effort 45 Table A1.4 Analysis of C. canescens potential versus realized seed production 48 Table 2.1. Cirsium undulatum end of season plant performance measures 78 Figure 2.1. Cirsium undulatum whole plant seed production 80 Figure 2.2 Cirsium undulatum average seed production by flower head position 81 Figure 2.3. Cirsium undulatum seed contribution from apical versus subsequent flower heads 82 Table A2.1:1 Analysis of treatment effects on C. undulatum 83 Table A2.2:1 Initial C. undulatum measurements 89 Table A2.2:2 Comparison of C. undulatum and C. canescens results 90 Figure A2.2:1. Flower head damage per individual C. undulatum plant 93 Figure A2.2:2. Variation in C. undulatum apical damage treatment 94 Figure 3.1. Effect of resource tradeoff on flower head growth 126 iv Figure 3.2. % Individuals remaining over time 127 Figure 3.3. Expected future fitness over time 128 Figure 3.4. Optimal allocation strategies with equal survival between flower heads 129 Figure 3.5. Average flower head diameter resulting from optimal allocation strategies 130 Figure 3.6. Average fitness resulting from optimal allocation strategies 131 Figure 3.7. Optimal allocation strategies with unequal survival between flower heads 132 Figure A3.1:1. Size change per time step by flower head size 135 Figure A3.1:2. Seeds produced by flower head size 136 Figure A3.2:1 Effects of variation in s-curve parameters 142 Figure A3.2:2. Different functions for flower head growth and fitness 143 Figure A3.2:3 SDP model results with linear growth and fitness functions 144 Figure A3.2:4 Linear fitness and growth effects on flowering results 145 Figure A3.3:1 Population allocation averages over time 147 1 Chapter 1. Variation in cumulative insect floral herbivory affects expression of plant tolerance Abstract. Insect floral herbivory can dramatically reduce plant reproductive success. Thus, plants should have evolved mechanisms that minimize the effect of insect herbivores, particularly in monocarpic species that must maximize fitness in a single flowering year. Tolerance is one such mechanism; however, few experiments to date evaluate underlying mechanisms of plant tolerance under natural conditions. We compared plant seed production by the monocarpic Cirsium canescens (Platte thistle) in control (undamaged) plants versus plants with damage imposed upon the apical, flower head. We hypothesized that C. canescens would tolerate damage to its large, early, apical flower head by increasing reproductive