The Ecology and Evolution of Rare, Soil Specialist Astragalus Plants in the Arid Western U.S
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University of Denver Digital Commons @ DU Electronic Theses and Dissertations Graduate Studies 1-1-2016 The Ecology and Evolution of Rare, Soil Specialist Astragalus Plants in the Arid Western U.S. Joseph M. Statwick University of Denver Follow this and additional works at: https://digitalcommons.du.edu/etd Part of the Botany Commons, Developmental Biology Commons, and the Evolution Commons Recommended Citation Statwick, Joseph M., "The Ecology and Evolution of Rare, Soil Specialist Astragalus Plants in the Arid Western U.S." (2016). Electronic Theses and Dissertations. 1161. https://digitalcommons.du.edu/etd/1161 This Dissertation is brought to you for free and open access by the Graduate Studies at Digital Commons @ DU. It has been accepted for inclusion in Electronic Theses and Dissertations by an authorized administrator of Digital Commons @ DU. For more information, please contact [email protected],[email protected]. THE ECOLOGY AND EVOLUTION OF RARE, SOIL SPECIALIST ASTRAGALUS PLANTS IN THE ARID WESTERN U.S. __________ A Dissertation Presented to the Faculty of Natural Sciences and Mathematics University of Denver __________ In Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy __________ by Joseph M. Statwick June 2016 Advisor: Anna A. Sher ©Copyright by Joseph M. Statwick 2016 All Rights Reserved Author: Joseph M. Statwick Title: THE ECOLOGY AND EVOLUTION OF RARE, SOIL SPECIALIST ASTRAGALUS PLANTS IN THE ARID WESTERN U.S. Advisor: Anna A. Sher Degree Date: June 2016 ABSTRACT Organisms that specialize in uncommon habitats are, by their very nature, inherently uncommon. Specialization has its advantages, namely reduced competition and predation, but it also incurs costs. Specialists often have small population sizes, narrow ranges, and fragmented habitat, all of which engender negative consequences on an evolutionary timescale. Herein, I examine benefits and costs of specialization in selenium-hyperaccumulating plants in the genus Astragalus (Fabaceae). These plants are disproportionately likely to be rare and of conservation concern. Thus, I optimized germination pretreatments for Astragalus species such that seed loss can be minimized during ex situ cultivation, and found that physical scarification is most effective in breaking hard-seed dormancy. Through analysis of soil in seleniferous habitats, I found that soil hydrology can rapidly deplete bioavailable selenium, potentially further reducing the habitat available for accumulators. To better understand the relationship between soil bioavailable selenium and plant performance, I subjected Astragalus species to a gradient of selenium concentrations in the greenhouse. Both non-accumulators and hyperaccumulators had less herbivory with increasing selenium concentrations, and also grew larger, despite the energetic cost of selenium uptake. One potential explanation for their larger growth is that selenium reduced inadvertent drought stress during the experiment, so I tested that hypothesis using a full factorial experiment of drought stress ii and selenium dosage. Although drought stress reduced lifespan and selenium extended it, there was no evidence that selenium ameliorated drought stress. As a case study of the potential population genetic consequences of specialization, I examined the genetic structure and diversity of two allopatric cryptic sister species of Astragalus. Despite known low pollen and seed dispersal and strong genetic isolation by distance, populations were relatively diverse and not substantially inbred. Additionally, the genetic data did not support a two-species arrangement, so I recommend the species be consolidated, although several populations are somewhat isolated and merit special conservation attention. In summary, hyperaccumulators derive ecological benefits from their specialization that outweigh its metabolic cost, but may suffer low connectivity between populations, if not necessarily inbreeding depression. Conservation efforts should thus focus primarily on minimizing threats to and preserving connectivity of specialist habitats. iii ACKNOWLEDGEMENTS I would like to thank my committee: Anna Sher, Jennifer Ramp Neale, Scott Nichols, and Robin Tinghetella, as well as my committee chair, Michael Daniels. I would also like to acknowledge the former members of my committee, Martin Quigley and Tom Quinn. I thank my additional co-authors, Brian Majestic and Anna Williams; the members of the Denver Botanic Gardens Research and Conservation Department; and the DU Organismal Biologists group. I thank the journal editorial staffs and anonymous peer reviewers to whom I submitted these chapters and my additional research assistants Alex Dvorak, Graham Frank, Margarita Salazar, Jullie Ngo, Nathan Kingsley, Lisa Clark, Grace Johnson, and Christina Szabados. I thank the other contributors who provided advice, support, supplies, samples, permits, etc., including but not limited to Jennifer Ackerfield, Mary Barkworth, Paul Bedard, Scott Bingham, Ronald Bolander, Mike Bone, Benton Cartledge, Roger Clark, Carol Dawson, George Ferguson, Robert Fitts, Jill Handwerk, Anna Lincoln, Sarah Lindsey, Mitchell McGlaughlin, Ernie Nelson, Michael Piep, Elizabeth Pilon-Smits, Dana Truman, Stanley Welsh, and Katy Wilcox Wieczorek. This work was supported by funding from the University of Denver Graduate Studies Doctoral Fellowship, Pustmeuller Fellowship, Summer Undergraduate Fellowship, and Graduate Studies Dissertation Fellowship, and also by Denver Botanic Gardens, the Bureau of Land Management, the Colorado Native Plant Society, and the Colorado Mountain Club. Finally, I would like to thank my family and friends, particularly Malee Statwick, Patricia Statwick, James Statwick, Barbara Alexander, and Somkit Rasameetham, for their personal and professional support. iv TABLE OF CONTENTS Chapter One: Germination pretreatments to break hard-seed dormancy in Astragalus (Fabaceae).......................................................................................................................1 Summary .............................................................................................................1 Introduction .........................................................................................................1 Methods...............................................................................................................3 Results.................................................................................................................5 Discussion ...........................................................................................................7 Chapter Two: Soil selenium in seleniferous environments of the arid west ......................9 Summary .............................................................................................................9 Introduction .......................................................................................................10 Methods.............................................................................................................13 Soil Collection .......................................................................................13 Soil Analysis..........................................................................................15 Statistical Analysis .................................................................................16 Results...............................................................................................................16 Discussion .........................................................................................................21 Chapter Three: Characterization and benefits of selenium uptake by an Astragalus hyperaccumulator and a non-accumulator......................................................................26 Summary ...........................................................................................................26 Introduction .......................................................................................................27 Methods.............................................................................................................32 Study Species.........................................................................................32 Greenhouse setup ...................................................................................33 Tissue Concentration via ICP-MS ..........................................................34 Plant performance ..................................................................................35 Herbivory...............................................................................................35 Statistical Analysis .................................................................................36 Results...............................................................................................................38 Soil dosage versus tissue concentration ..................................................38 Tissue concentration versus herbivory....................................................40 Plant performance ..................................................................................41 Herbivory versus plant performance.......................................................42 Soil dosage versus plant performance.....................................................42 Leaf selenium and bioconcentration factor versus plant performance......45 Discussion .........................................................................................................46