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Competition for Pollination and the Evolution of Flowering Time

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Competition for Pollination and the Evolution of Flowering Time Competition for pollination and the evolution of flowering time Item Type text; Dissertation-Reproduction (electronic) Authors Waser, Nickolas Merritt, 1948- Publisher The University of Arizona. Rights Copyright © is held by the author. Digital access to this material is made possible by the University Libraries, University of Arizona. Further transmission, reproduction or presentation (such as public display or performance) of protected items is prohibited except with permission of the author. Download date 29/09/2021 21:25:56 Link to Item http://hdl.handle.net/10150/565376 COMPETITION FOR POLLINATION AND THE EVOLUTION OF FLOWERING TIME bY Nickolas Merritt Waser A Dissertation Submitted to the Faculty of the DEPARTMENT OF ECOLOGY AND EVOLUTIONARY BIOLOGY In. Partial Fulfillment of the Requirements For the Degree of DOCTOR OF PHILOSOPHY WITH A MAJOR IN BIOLOGY In the Graduate College THE UNIVERSITY OF ARIZONA 19.77 THE UNIVERSITY OF ARIZONA GRADUATE COLLEGE I hereby recommend that this dissertation prepared under my direction by Nicholas Merritt Waser___________________________ entitled COMPETITION FOR POLLINATION AND THE EVOLUTION OF FLOWERING TIME be accepted as fulfilling the dissertation requirement for the degree of Doctor of Philosophy________________________________ . uLrrt f 2 / fy\o-y^ i Dissertation Director Date As members of the Final Examination Committee, we certify that we have read this dissertation and agree that it may be presented for final defense. .2/ , ^ / f z z t fll&YoL ^ ? 4- ________ / f a / ___ ^ C c jJ jlr____ ^ Mn-J____ Final approval and acceptance of this dissertation is contingent on the candidate's adequate performance and defense thereof at the final oral examination. STATEMENT BY AUTHOR This dissertation has been submitted in partial fulfillment of requirements for an advanced degree at The University of Arizona and is deposited in the University Library to be made available to borrowers under rules of the Library. Brief quotations from this dissertation are allow­ able without special permission, provided that accurate acknowledgment of source is made. Requests for permission for extended quotation from or reproduction of this manu­ script in whole or in part may be granted by the head of the major department or the Dean of the Graduate College when in his judgment the proposed use of the material is in the interests of scholarship. In all other instances, however, permission must be obtained from the author. f SIGNED: t U l <r&<> ACKNOWLEDGMENTS My professional growth and the completion of this dissertation have depended on the help of many people. First and foremost, I wish to thank my major advisor, Dr. H. R. Pulliam, who has freely shared his time and energies as well as his insight into all areas of ecology. Among other things, I am indebted to Dr. W. A. Calder for intro­ ducing me to hummingbirds and their pollination of flowers. The other members of my doctoral committee. Dr s. G." G. Ward, W. B. Heed, and A. C. Gibson, have been an essential source of critical input. In addition, Dr. J. H. Brown has served as a very valuable unofficial committee, member during the last two years of my research. Finally, I wish to thank my graduate student colleagues at Arizona, and most es­ pecially M. V. Price, for many stimulating conversations and for constant encouragement. This research was supported in part by grants from the Frank M« Chapman Memorial Fund, Sigma Xi, and The University of Arizona Foundation. TABLE OF CONTENTS Page LIST OF TABLES « . , « . « . , « « . «. « . » . « . vi LIST OF ILLUSTRATIONS . „ . 0 . „ . , „ vii , ^3 'J*'RAC o 0 o 0 O 0 o 0 O 0 O- O O 0 O 0 0.0" O 0 o o o 1 2C INTRODUCTION . » . o . , . « « = . , . » o « . , . , „ I POLLINATOR SHARING, COEXISTENCE, AND COMPETITIVE DIVERGENCE AMONG CO-OCCURRING PLANT SPECIES ........ ' 3 The Basie Model © © © © © © ■ © © © © © © © © © © © © . 4 Rapid Fixation of the Common Species ....©„ 5 - Insight into Possible Mechanisms of Competition © © © © © © © © © © © © © © © © © 8 Computer Simulations . © . © . © . © . © © 9 The Simulation Programs © . © © © . © . © © 10 Pollinator Movements Limiting to Reproduction © © ■© © © © © © © © © © © © © © 13 Movements and Stigmatic Surfaces Both Limiting © 15 Amounts of Pollen Limiting .».©««,»'.. 16 Effects of Refugia . © .=.©..© © ... © 16 Pollen and Seed Flow between Separate Populations ©©©© ©« ©©©© © ©©©©©© 2 2 Discussion © © © ©■ © © © © © © e o o o . © © © © © © 2 7 COMPETITION FOR POLLINATION AND SEQUENTIAL FLOWERING IN COLORADO WILDFLOWERS ..•.©.©.,.© 37 ^^2czz1 d,s © © © © © © © © © © © © © © © © © © © ©*o © 38 Results © © © © © © © © © © © © © © © © © © © © © © 46 Visitation and Pollination of D. nelsoni and I © aggregata ©.© © © © © © © © © © © © © 46 Pollen-Transfer by Hummingbirds in Situations of Flowering Overlap between D© nelsoni and I© aggregata ..... 56 IV, V TABLE OF CONTENTS (Continued) Page Seed Set Depression in Natural and Synthetic Situations of Flowering Overlap * o @ « « o «. ©« ® © © © 67 Discussion © © © © © © © © © © © © © © 75 APPENDIX A: FORTRAN PROGRAMS FOR SIMULATING THE POLLINATION OF TWO CO-OCCURRING PLANT SPECIES © © © '. © . © © © © © . © © , 86 APPENDIX Bs TESTS OF NORMALITY AND HOMOGENEITY OF VARIANCES OF SELECTED SEED SET DATA . © 100 APPENDIX C; COMPARISONS OF MEAN NUMBERS OF FLOWERS PER PLANT IN SYNTHETIC POPULATIONS, 1976 © © © © © © © © © © © © © © © © © . 103 LITERATURE CITED © 105 LIST OF TABLES Table Page 1. Flower species visited by broad-tailed hummingbirds at RMBL, 1975-1976 . ........ 49. 2. Summary of pollen collected from humming­ birds and bees, 1975-1976 ............. 51 3. Visitors to flowers of D „ nelsoni and I. aggregata at RMBL, 1975-1976 . 55 4. Seed sets per flower from pollinator exclusion experiments, 1975 ............. 57 5. Nectar productions and standing crops in flowers of D. nelsoni and I_. aggregata in the- study meadow at RMBL, 1975-1976 . „ . 62 6. Seed sets as a function of flowering time in the natural study meadow at RMBL, 1975—1976 ." . « . .. « . '. = = . 68 7. Seed sets in synthetic populations, 1975—1976 o o o o « e o o o e o e o o o e o o o e o 71 8. Seed sets from the hand pollination experiment, 1976 , . , . « « . 74 vi LIST OF ILLUSTRATIONS Figure Page 1. The frequency of an initially common flower species, pn# as a function of the number of generations’, n ......... 7 2 . The. effect of varying the number of polli­ nator movements per generation on the number of generations to. fixation of one plant species ................. 14 3.o The effect of varying the initial pollen supply in anthers on the number of genera­ tions to fixation of one plant species ..... 17 4. The effect of size of refugial areas on the number of generations to fixation of one plant species, with one (A) and two (B) seed choices per refugial loca­ tion * .... ...... ... ..... 20 5 o The effect of size of refugial areas on the number of generations to fixation of one plant species, with two (A) and four (B) seed choices per refugial location ..... 21 6. The effect of pollen flow on coexistence .... 24 7- The effect of combined seed and pollen flow on coexistence .............. 26 8. A diagram of synthetic populations of D. nelsoni and I_. a g g r e g a t a .......... 42 9. Plants and flowers of D. nelsoni and ]C. aggregata . ......... .... 47 10. Cross—sectional drawings of flowers, with a broad-tailed hummingbird shown to the same scale ............... 53 11. Flowering phenology of D. nelsoni and I. aggregata in the study meadow at RMBL, 1975-1976 ..... ........ ...... 59 vii viii LIST OF ILLUSTRATIONS (Continued) Figure, Page 12. Foraging movements of hummingbirds in meadows, 1976 . .... .... ....... 60 13. Foraging movements of hummingbirds in synthetic populations, 1976 .......... 64 14. Stigmas and pollen grains of D. nelsoni and I. aggregata drawn to common scales . 66 15. The sexual composition of the D. nelsoni population in the study meadow in 1976 .... 70 ABSTRACT Separation of flowering time among morphologically similar flower types represents one form of evolutionary divergence which may result from competition for a shared pollinatoro In this dissertation, I use computer simula­ tions based on a deterministic model of pollination of two plant species to examine mechanisms and consequences of competition for pollination. Competitive exclusion of one of the two species occurs in all simulations except those which introduce partial barriers to gene flow between dif­ ferent sections of a mixed plant population. From this general result, I predict that separation in flowering time or habitat will commonly allow coexistence between po­ tential competitors. I next describe empirical research designed to explore whether competition for hummingbird pollination is sufficient to explain the sequential flowering of two wildflowers in Colorado. Experiments and observations on natural and synthetic populations of Delphinium nelsoni and Ipomopsis aggregata indicate that individuals of each species suffer a reproductive loss when they flower synchronously in mixed stands. This reduction in fitness can be attributed to interspecific pollen trans­ fer by hummingbirds. These findings support the ix . X interpretation that patterns of flowering time as well as those of floral morphology often represent character displacement as a result of competition for pollination in which pollen, pollinators, stigmatic surfaces or some combination of these limit reproduction. V INTRODUCTION Because of their central role in the sexual repro­ duction
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