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Antagonists and Mixed Mating: Consequences for the Demography of Impatiens Capensis (Balsaminaceae)

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Antagonists and Mixed Mating: Consequences for the Demography of Impatiens Capensis (Balsaminaceae) ANTAGONISTS AND MIXED MATING: CONSEQUENCES FOR THE DEMOGRAPHY OF IMPATIENS CAPENSIS (BALSAMINACEAE) by Janette Ann Steets B.S., Muhlenberg College, 2000 Submitted to the Graduate Faculty of Arts and Sciences in partial fulfillment of the requirements for the degree of Doctor of Philosophy University of Pittsburgh 2005 UNIVERSITY OF PITTSBURGH FACULTY OF ARTS AND SCIENCES This dissertation was presented by Janette Ann Steets It was defended on June 24, 2005 and approved by Dr. David Carr Dr. Susan Kalisz Dr. Jeffrey Lawrence Dr. Stephen Tonsor Dr. Tia-Lynn Ashman Dissertation Director ii ANTAGONISTS AND MIXED MATING: CONSEQUENCES FOR THE DEMOGRAPHY OF IMPATIENS CAPENSIS (BALSAMINACEAE) Janette Ann Steets University of Pittsburgh, 2005 Given the prevalence of intermediate levels of outcrossing among angiosperms, a general mechanism explaining the evolution and maintenance of this condition is needed. Although numerous theoretical models predict mixed mating to be evolutionarily stable, the conditions favoring intermediate selfing are often very stringent and have limited applicability. Here I investigate the role of two plant antagonists, vegetative herbivores and intraspecific competitors, in influencing the mixed mating system of Impatiens capensis (Balsaminaceae). This species exhibits an obligate mixed mating system by producing heteromorphic flowers (i.e., individuals produce both selfing, cleistogamous and facultatively-outcrossing, chasmogamous flowers). Thus, these antagonisms may affect mating system at the level of relative heteromorphic flower production, outcrossing within chasmogamous flowers and whole-plant outcrossing. In a comparative study exploring how herbivory and intraspecific competition jointly affect mating system expression, I found that these antagonisms affect plant growth and mating system traits differently, and thus the mating system response could not be accurately predicted from plant growth response. Using surveys of wild populations and experiments manipulating herbivory under field conditions, I found that herbivory reduced outcrossing by increasing proportional cleistogamous reproduction. In the field, I found that herbivory increased outcrossing among iii chasmogamous flowers due to effects on flower display, pollinator visitation rate and pollinator fauna composition. Overall, herbivory slightly lowered whole-plant outcrossing. To understand further the consequences of mixed mating, I manipulated herbivory in two wild I. capensis populations to explore the demographic consequences of mixed mating, herbivory, and the interactive effects of mating system and herbivory. I found that selfed individuals had higher rates of germination and survival and lower fecundity than did their outcrossed counterparts. Herbivory also had demographic consequences as it reduced population growth rate due to its effect on vital rates of selfed individuals. Overall, the results presented in this dissertation offer important insight to the ecological factors that cause variation in mating system as well as the long-term consequences of variation in mating patterns. Furthermore, these findings have implications for population genetic diversity and structure and point to the role of natural enemies in contributing to the maintenance of a mixed mating system. iv TABLE OF CONTENTS PREFACE............................................................................................................................... xii 1. INTRODUCTION .................................................................................................................. 1 2. HERBIVORY ALTERS THE EXPRESSION OF A MIXED MATING SYSTEM ............. 4 2.1. Abstract............................................................................................................................. 4 2.2. Introduction....................................................................................................................... 4 2.3. Materials and Method ....................................................................................................... 7 2.3.1. Study system ........................................................................................................... 7 2.3.2. Survey of populations ............................................................................................. 7 2.3.3. Experimental manipulation of leaf damage ............................................................ 8 2.3.3.1. Effect of leaf damage on plant size and mating system.................................. 9 2.3.3.2. Effect of leaf damage on floral display and geitonogamy ............................ 10 2.3.3.3. Effect of leaf damage on floral traits ............................................................ 10 2.3.3.4. Effect of leaf damage on pollinator abundance and composition................. 11 2.3.3.5. Effect of leaf damage on progeny................................................................. 11 2.3.3.6. Effect of leaf damage on cumulative female fitness..................................... 12 2.4. Results............................................................................................................................. 13 2.4.1. Survey of populations ........................................................................................... 13 2.4.2. Experimental manipulation................................................................................... 13 2.4.2.1. Effect of leaf damage on plant size and mating system................................ 13 2.4.2.2. Effect of leaf damage on floral display and geitonogamy ............................ 14 2.4.2.3. Effect of leaf damage on floral traits ............................................................ 14 2.4.2.4. Effect of leaf damage on pollinator abundance and composition................. 14 2.4.2.5. Effect of leaf damage on progeny................................................................. 15 2.4.2.6. Effect of leaf damage on cumulative female fitness..................................... 15 2.5. Discussion....................................................................................................................... 15 2.5.1. Direct effects of leaf damage ................................................................................ 15 2.5.2. Indirect effects of leaf damage.............................................................................. 17 2.5.3. Consequences for mating system evolution.......................................................... 19 3. COMPETITION-DEPENDENT EFFECTS OF HERBIVORY FOR MATING SYSTEM EXPRESSION IN IMPATIENS CAPENSIS................................................................................. 25 3.1. Abstract........................................................................................................................... 25 3.2. Introduction..................................................................................................................... 25 3.3. Materials and Methods.................................................................................................... 28 3.3.1. Study system ......................................................................................................... 28 v 3.3.2. Experimental design and data collection .............................................................. 28 3.3.3. Statistical analyses ................................................................................................ 30 3.4. Results............................................................................................................................. 32 3.4.1. Do plants respond to multiple antagonistic interactions additively or non- additively?......................................................................................................................... 32 3.4.2. Do growth, reproduction and mating system traits respond to multiple antagonists similarly?........................................................................................................................... 32 3.4.3. Is mating system response to multiple antagonists predictable from growth response?........................................................................................................................... 33 3.5. Discussion....................................................................................................................... 33 3.5.1. Combined effects of herbivory and competition .................................................. 33 3.5.2. Allocation to flower size vs. flower number......................................................... 35 3.5.3. Consequences of antagonists for population genetic diversity............................. 36 3.5.4. Consequences of plant antagonists for mating system evolution ......................... 37 4. THE CONSEQUENCES OF VEGETATIVE HERBIVORY FOR THE MAINTENANCE OF INTERMEDIATE OUTCROSSING IN AN ANNUAL PLANT.......................................... 42 4.1. Abstract........................................................................................................................... 42 4.2. Introduction..................................................................................................................... 43 4.3. Materials and Methods.................................................................................................... 45 4.3.1. Study system ......................................................................................................... 45 4.3.2. Consequences of herbivory
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