THE ECOLOGY AND EVOLUTION OF ARMAMENT DEFENSE VARIATION IN AFRICAN ACACIAS By MEGAN CATHLEEN GITTINGER A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY UNIVERSITY OF FLORIDA 2013 1 © 2013 Megan Cathleen Gittinger 2 To my mom, who always supports my endeavors with healthy skepticism 3 ACKNOWLEDGMENTS I thank James Ekiru, James Olingnengero, John Lemboi, and Moso for research assistance in the field, without Ekiru I may have been trampled by elephants; Mike Littlewood for logistical support at Mpala Research Centre, cheery disposition and endless stories from the earlier days of Mpala and Kenya; Kathleen Rudolph for commiseration in the field research and for moral support along the way; Todd Palmer for feedback and support throughout my dissertation and for the Penicillins; Craig Osenberg for including me in lab meetings; the Research Reviews group for valuable feedback on experimental design; Dr. David Augustine for insightful feedback on herbivore foraging and for allowing me access to long-term exclosure plots at Mpala; Bernard Hauser for insights in plant development; Heather McAuslane for comments and discussions on inducible plant defense; Pamela Soltis for lab-support and guidance on molecular phylogenetics; Julian Resasco and Schuyler van Montfrans for being there from the beginning; Matthew Smith and Adrian Stier for adopting me as friend; Ashley Seifert for being my husband and for always finding what I have to say interesting; and my entire family for providing endless support and for being amazing role models for personal and professional life. 4 TABLE OF CONTENTS page ACKNOWLEDGMENTS .................................................................................................. 4 LIST OF TABLES ............................................................................................................ 8 LIST OF FIGURES .......................................................................................................... 9 ABSTRACT ................................................................................................................... 10 CHAPTER 1 GENERAL INTRODUCTION .................................................................................. 12 Why Study Structural Defense Traits? .................................................................... 12 Why Study Acacia Spines? ..................................................................................... 12 General Overview ................................................................................................... 14 2 HERBIVORES AND PLANT SIZE AS DETERMINANTS OF SPINE DEFENSE MORPHOLOGY IN AN AFRICAN ACACIA ............................................................ 15 Background ............................................................................................................. 15 Methods .................................................................................................................. 17 Study Site ......................................................................................................... 17 Natural Variation in Spine Defenses ................................................................. 18 Herbivory and Spine Defenses ......................................................................... 18 Analysis ............................................................................................................ 19 Results .................................................................................................................... 19 Natural Variation in Spine Defenses ................................................................. 19 Herbivory and Spine Defenses ......................................................................... 20 Herbivory, Spine Defenses, and Plant Size ...................................................... 21 Discussion .............................................................................................................. 22 Relaxation of Spine Defenses following Herbivore Exclusion ........................... 22 Size-based Relationships in Spine Defenses ................................................... 23 3 SPINE VARIATION DETERMINES DEFENSIVE EFFICACY AGAINST BROWSERS OF DIFFERENT SIZES AND FEEDING STRATEGIES ................... 37 Background ............................................................................................................. 37 Methods .................................................................................................................. 40 Study Site and Feeding Trials .......................................................................... 40 Plant Selection and Branch Treatments ........................................................... 41 Biomass Estimates ........................................................................................... 41 Patterns of Biomass Removal .......................................................................... 43 Branch Type Characteristics ............................................................................ 44 Results .................................................................................................................... 44 5 Defense Efficacy of Spines Against Goats and Camels ................................... 44 Patterns of Biomass Removal .......................................................................... 45 Branch Type Characteristics ............................................................................ 46 Discussion .............................................................................................................. 48 Ecological Function and Consequences of Spine Form and Investment .......... 48 Mechanistic Links to Spine Variation ................................................................ 51 Evolutionary Significance of Armament Variation ............................................. 53 4 EVOLUTION OF SPINE FORM AND DOMATIA IN AFRICAN VACHELLIA .......... 59 Background ............................................................................................................. 59 Methods .................................................................................................................. 61 Species Sampling ............................................................................................. 61 DNA Extraction, Amplification and Sequencing ................................................ 62 Phylogenetic Estimation ................................................................................... 62 Trait Evolution .................................................................................................. 63 Results .................................................................................................................... 63 Phylogenetic Estimation ................................................................................... 63 Evolution of Spine Morphology ......................................................................... 64 Discussion .............................................................................................................. 65 Phylogenetic Relationships of African Vachellia ............................................... 65 Independent Origins of Spine Form and Domatia in Vachellia ......................... 66 Evolutionary Convergence of Structural Defense Traits ................................... 68 5 SPINES, PRICKLES AND THORNS, OH, MY! PHYSICAL ARMAMENTS AS STAND UP TRAITS FOR STUDYING PLANT DEFENSES ................................... 80 Background ............................................................................................................. 80 Armament Morphology and Development............................................................... 83 Prickles ............................................................................................................. 84 Spines .............................................................................................................. 85 Thorns .............................................................................................................. 86 Armament as an Effective Form of Resistance ....................................................... 88 Defensive Benefits of Armament ...................................................................... 88 Effects of Armament on Herbivore Feeding Behavior ...................................... 90 Costs of Defensive Armament ................................................................................ 94 Allocation Costs ................................................................................................ 94 Opportunity and Ecological Costs .................................................................... 97 Armament Plasticity ................................................................................................ 99 Responses as Induced Defenses ................................................................... 100 Ontogenetic Shifts in Armament ..................................................................... 102 Physiological Changes and Resource Allocation ........................................... 104 Future Directions .................................................................................................. 106 6 GENERAL CONCLUSIONS ................................................................................. 112 Determinants of Spine Defense