Diffuse Knapweed (Centaurea Diffusa): Reproductive Thresholds, Population Ecology and Responses to the Introduction of the Buprestid Beetle Sphenoptera Jugoslavica
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Diffuse Knapweed (Centaurea diffusa): Reproductive Thresholds, Population Ecology and Responses to the Introduction of the Buprestid Beetle Sphenoptera jugoslavica. By Robert David Powell B.Sc, The University of British Columbia, 1975 M.Sc, Dalhousie University, 1979 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY in THE FACULTY OF GRADUATE STUDIES (Department of Plant Science) We accept this thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA December 1988 © Robert David Powell, 1988 In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. Department of PLANT SCIENCE The University of British Columbia Vancouver, Canada Date nFr.FMRFR ?Q, 1988 DE-6 (2/88) Abstract Diffuse knapweed (Centaurea diffusa), like other semelparous perennials, was found to have a minimum size requirement for flowering. This critical size conditions the plant's phenotypic response to variation in growth conditions. Experimentally crowded rosettes grew slowly, failed to reach the critical size and did not flower in the following season, whereas uncrowded rosettes grew rapidly, reached the critical size in a single year and flowered in the next. The proportion of plants that flowered and subsequently died in both field and experimental populations decreased with individual crowding. The effect of interference-related growth reduction was therefore to accentuate gaps in the population's spatial pattern. Interference-related mortality of seedlings and rosettes acted in the opposite direction. The three interference-related processes of recruitment, pre-reproductive and post-reproductive mortality determine the fine scale spatial pattern of the population, and its density. There is no theoretical basis for interpreting a shift toward regularity as evidence for interference. A model, proposed to explain the apparent ubiquity of critical size-dependent switching to reproductive development in semelparous perennials, shows that this adaptation maxi• mizes the intrinsic rate of increase of semelparous species subject to high juvenile mortality followed by low rosette mortality, seasonality, and extensive variability in rosette growth rates. The model suggests that biological control organisms might be chosen strategically to disrupt selection for the critical size. Age-structured field populations exhibit considerable spatial variation in density. To determine whether that variation could account for a significant proportion of the variability in rosette growth rates under field conditions, the growth rates of individual plants were observed and regressed against a series of 'crowding indices'. Two methods of delimiting a sample of neighbours for this purpose were compared. 'Circular sampling' defines neighbours as plants within a specified radius of a focal plant; 'polygonal sampling' defines neighbours as plants that share a boundary when the ground is partitioned into Dirichlet polygons. A simulation study of the sampling characteristics of the two methods in relation to the degree of aggregation in the population showed that polygon samples are statistically preferable ii in aggregated populations, but the two methods performed about equally, accounting for approximately 25% of the variance in the growth rates of rosettes in the field. The buprestid beetle Sphenoptera jugoslavica is the third insect established in Canada as a potential biological control for diffuse knapweed. The beetle population at the release site in British Columbia was monitored, and experiments were conducted to determine the beetle's effects on its host. S. jugoslavica reduces the survivorship of seedlings and rosettes, delays reproduction, and finally reduces seed output. Under favorable conditions the beetle can contribute to a significant reduction in knapweed population growth. Its effectiveness at the release site is limited by a phenological requirement for arrested plant growth during the oviposition period which leads to large fluctuations in the size of the beetle population, and only intermittent damage to the knapweed population. iii Table of Contents Page Abstract ii Table of Contents iv List of Tables vii List of Figures viii Acknowledgements x Introduction 1 Ecological Theory and the Biological Control of Weeds 1 The Biological Control of Diffuse Knapweed in British Columbia 4 Organization of The Thesis 6 Chapter I. The Role of Spatial Pattern in the Population Biology of Diffuse Knapweed Centaurea diffusa 9 Abstract 9 Introduction 9 Materials and Methods 11 Diffuse Knapweed 11 The Study Site 11 Field Data Collection 12 Spatial Analysis 14 A Spacing Experiment 15 Results 16 The Fate of Rosettes in the Field in Relation to Size 16 The Spacing Experiment 16 Spatial Pattern in the Field 19 The Fate of Rosettes in Relation to Nearest Neighbour Distances 22 Discussion and Conclusions 26 Recruitment 26 Rosette Mortality 29 Pre-reproductive Mortality 29 Chapter II. Why Is Flowering in Semelparous Perennials Dependent Upon a Size Threshold? 30 Abstract 30 Introduction 30 Explanations for the Critical Size Threshold in Semelparous Perennials 31 Why Have Semelparous Perennials Evolved Critical Size Thresholds? 34 Methods 35 The Life History of Centaurea diffusa 35 The Model 36 The Intrinsic Rate of Increase 39 Results 40 Conditions Selecting for Pure Size Dependence 43 Selection for Other Phenotypic Responses: Patterns of Rosette Survivorship .. 44 The Fitness of Genotypes in Relation to Variability in Individual Growth Rates 47 iv Discussion and Conclusions 51 Intrinsic Rate as a Measure of Fitness 51 Adaptations of Semelparous Perennials 53 An Hypothesis to Explain Size Threshold Dependent Switching 54 Phenotypic Thresholds and Reaction Norms, and Seasonality 54 Approaches to Testing the Hypothesis 55 Chapter III. Circular Neighborhoods, Polygonal Neighborhoods, and The Growth of Centaurea diffusa in Relation to Individual Crowding 58 Abstract 58 Introduction 58 Methods 60 Simulation Methods 60 Field Materials and Methods 63 Descriptions of Crowding 64 I: Circular Crowding Indices 64 II: Polygonal Crowding Indices 66 The Size of the Focal Plant 67 Results: Simulation Experiment 67 Numbers of Neighbors 68 Angular Dispersion of Neighbors 68 Field Results 70 Discussion 74 Predictions of Plant Performance Based on Indices of Crowding 74 The Sample Must Permit the Detection of Gaps and Edges 74 The Distinction Between Adjacent and Once Removed Neighbors 76 Circular and Polygonal Indices in Population Dynamic Models 77 Crowding and Rosette Growth in Diffuse Knapweed 79 Chapter IV. The Effect of Sphenoptera jugoslavica Obenb. (Col. Buprestidae) on Its Host Plant Centaurea diffusa Lam. (Compositae) 80 Abstract 80 Introduction 80 Efforts to Control Diffuse Knapweed 80 Life History and Phenology of the Beetle in Relation to its Host 81 Oviposition 83 Methods 83 The Distribution of Beetle Eggs in the Field 83 Oviposition Preference Tests 83 The Effect of Temperature on Oviposition 84 Results 85 Oviposition in the Field 85 Oviposition Preferences 85 The Effect of Temperature on Oviposition 89 Discussion 90 Larval Feeding 91 Methods 93 The Effect of Sphenoptera Root Damage on Rosette Growth 93 The Effect of Plant Size on Larval Survival 93 The Effect of Sphenoptera Damage on Plant Size and Seed Production 94 v Mortality of Rosettes Caused by Larvae 94 Results 95 Tie Effect of Sphenoptera Root Damage on Rosette Growth 95 The Effect of Plant Size on Larval Survival 97 The Effect of Sphenoptera Damage on Plant Size and Seed Production 98 Mortality of Rosettes Caused by Larvae 100 Discussion 102 Adult Feeding 104 Methods, 104 Feeding Preference Tests 104 The Effect of Temperature on Adult Feeding Rate 105 Seedling Mortality in Response to Drought and Herbivory 105 Results 106 Adult Feeding Preference Tests 106 The Effect of Temperature on Adult Feeding Rate 106 Seedling Mortality in Response to Drought and Herbivory 106 Discussion 107 How Much Damage Can Adult Feeding on Foliage Do? 107 Discussion and Conclusions 108 Conclusion 114 Literature Cited 119 Appendix I: Conditions for Pure Size Dependence 131 Appendix II: Critical rm Values 132 Appendix III: Fitness of Genotypes in Relation to Variation in Individual Growth Rates 132 Appendix IV: Limits of Integration to Calculate the Fitness of Size Depen• dent and Age Size Dependent Types 133 vi List of Tables Chapter I Table I The Relative Growth Rates of Rosettes Subject to Varying Degrees of Crowding 19 Table II The Fates of Seedlings and Established Plants in Relation to the Nearest Estab• lished Plant 25 Chapter IV Table III Contingency Analysis of Beetle Egg Presence, Leaf Angle, and Number of Leaves for Five Sites in the White Lake Basin 88 Table IV Oviposition Preference Tests 89 Table V The Effect of Root Damage on Rosette Growth 96 Table VI Growth as a Function of Size and Root Damage 96 Table VII The Effect of Root Damage on the Bolting Size 97 Table VIII The Effect of Sphenoptera Attack on Plant Size and Seed Production 100 Table IX Effects of Larvae on Rosette Mortality 101 Table X Root Crown Diameters