Density and Reproduction in Native and Invasive Linaria Vulgaris
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Density and reproduction in native and invasive Linaria vulgaris populations at multiple spatial scales. Laura Jane Harrison Submitted in accordance with the requirements for the degree of Doctor of Philosophy The University of Leeds School of Biology September 2013 ii The candidate confirms that the work submitted is her own and that appropriate credit has been given where reference has been made to the work of others. This copy has been supplied on the understanding that it is copyright material and that no quotation from the thesis may be published without proper acknowledgement. © 2013 The University of Leeds and Laura Jane Harrison. The right of Laura Jane Harrison to be identified as Author of this work has been asserted by her in accordance with the Copyright, Designs and Patents Act 1988. iii Acknowledgements I am in awe of the apparently endless supply of encouragement, enthusiasm and knowledge from my supervisor Bill Kunin, who has been a true mentor. Many thanks also for supportive supervision (and apologies about the Globeflower) to Stephen Compton, who fully deserves his escape to sunnier climes. I have enjoyed and benefited greatly from support and conversations over cake with members of the various incarnations of Bill’s group. Discussions with Richard Gunton have been particularly helpful. Tim Benton’s advice to learn R early was a great time-saver. I am also grateful for technical help and advice from the late David Blakeley, Anna Clough, Roger Key, Martin Lappage at the University of Leeds Experimental Gardens, Lance Penketh at the Plant Growth Suite and Chris Wright and Fiona Reynolds at the Farm. Dan Chapman, Peter Harrison and Ralf Ohlemüller helped to extract climate information for analysis. Thanks to Rob Colautti and John Wiley and Sons for permission to reproduce figure 1. This PhD was funded by NERC and I have also received travel and education grants from the British Ecological Society. I am grateful to staff at NERC and in the Faculty of Biological Sciences (particularly Chris Ashman, Keith Ray and Caroline Murphy) who solved various administrative tangles involved in needing sick leave and switching to part-time study. Paul Ashton helped me to locate sites for my pilot fieldwork, while James Rosindel assisted with pilot fieldwork and Ailsa Gibson and Simon Stevens provided accommodation. I am very grateful to the following landowners and managers for their permission to carry out research: Essex County Council, Essex Wildlife Trust, Haines Borough Council, Haines iv Harbour Master, Lancashire Wildlife Trust, London Borough of Havering, Tony Partridge, Lord John Petre, Nancy Sewright, Thurrock County Council and Justin Rushworth. My research has depended on records collected and maintained by volunteers and staff at the Botanical Society of the British Isles, particularly by Ken Adams and Kevin Walker. Melinda Lamb assisted with finding records in North America, while Pam Randles and Emily Cowles at Takshanuk Watershed Council helped when planning fieldwork in Haines. I am also grateful to Gino Graziano and Steven Seefeldt for information and for their invitation to speak at the Alaskan Committee for Noxious and Invasive Plant Management conference. Thanks to the people of Haines for the warm welcome I received to their beautiful borough: I also appreciated their warm and well-resourced library! Thanks to Mike and Lavina Smith for hospitality and the bicycle loan. I received fantastic health support during a headache of a PhD from James Anderson, Grant Ragsdale, James Taylor, Robert Whittaker and the South East Alaska Regional Health Consortium. I simply could not have completed the PhD without all I learned from Breathworks and Vidyamala Burch. Thanks also to all my friends at Triratna Leeds Buddhist Centre; particularly fellow scientists Dh. Samānārtha and Uddyotani Docherty. My love and apologies to Oli Larkin for the time I have spent on the thesis instead of with him. Finally, I cannot sufficiently express my gratitude to my extraordinary parents Susan and Eric Harrison, who were my UK field assistants, and who can now spot Toadflax at fifty paces. They have supported both their offspring through biology PhDs and are perhaps wishing they had not taken us on as many inspiring nature walks as children. v Abstract Comparing invasive plants in their native and invasive ranges can answer questions about invasion mechanisms and inform management options. However, few studies have considered how density varies with spatial scale or how individual fitness is affected by conspecific density at different spatial scales. A census was carried out of 15 native UK populations of the perennial herb Linaria vulgaris Miller (Plantaginaceae), and of seven invasive populations from a climatically matched area of Alaska. There was no difference in density between native and invasive populations when compared at spatial scales of 0.0625, 0.5, 1 or 4 m2, or when density was measured as a mean field of each population. However, invasive populations covered a larger area, so density was greater at broad spatial scales. The effect of conspecific density on the height and reproduction of ramets varied with the spatial scale and between ranges. Invasive ramets were shorter than native ramets, and therefore produced fewer mature fruit. However, this was more than compensated for by the greater number of viable black seed in invasive fruit than native fruit. One of the reasons for this was the presence of the seed feeding weevil Rhinusa antirrhini in over half of the native, but none of the invasive, fruit. The majority of seed was estimated to fall within 1 m of maternal plants when surrounded with vegetation, but seed travelled further in an unvegetated area. Germination rates were very low in both the field and laboratory. The thesis ends with a description of the biology of L. vulgaris. This work demonstrates that invasion and escape from natural enemies can occur at a broad spatial scale, without increased density and vigour at a fine spatial scale. vi Contents Acknowledgements .................................................................................... iii Abstract ....................................................................................................... v List of figures .............................................................................................. ix List of tables ............................................................................................... xi Chapter 1. Spatial scaling, density and density effects in plant invasions ....................................................................................... 1 1.1 ABSTRACT ........................................................................................... 1 1.2 INTRODUCTION .................................................................................... 2 1.3 SPATIAL SCALE AND DENSITY ............................................................. 3 1.3.1 Spatial scale in ecology .......................................................................... 3 1.3.2 Density measurement and density effects .............................................. 4 1.3.3 Spatial scale and density dependence .................................................. 11 1.4 SPATIAL SCALE, DENSITY AND DENSITY EFFECTS IN PLANT INVASIONS ........................................................................................ 12 1.4.1 Definitions, stages and scales of invasion ............................................. 12 1.4.2 Comparing native with invasive species and the native with the invasive range ..................................................................................................... 17 1.4.3 Invasion mechanisms at different stages .............................................. 24 1.4.4. Density effects and density dependence in plant invasions .................. 32 1.5 STUDY SPECIES AND THESIS STRUCTURE ....................................... 35 Chapter 2. Comparison of the density of native UK and invasive Alaskan Linaria vulgaris Miller populations ............................ 40 2.1 ABSTRACT .......................................................................................... 40 2.2 INTRODUCTION ................................................................................... 41 2.3 METHODS ............................................................................................ 46 2.3.1 Fieldwork ..................................................................................... 46 2.3.2 Statistical analysis ....................................................................... 57 2.4 RESULTS ............................................................................................. 61 2.4.1 Population characteristics ..................................................................... 61 2.4.2 Density comparison .............................................................................. 67 2.5 DISCUSSION ........................................................................................ 84 2.5.1 Density across spatial scales and density measurement ...................... 84 2.5.2 Stages, definitions and impacts of invasion........................................... 90 2.5.3 Implications for invasion mechanisms ................................................... 92 vii Chapter 3. Comparison of the height and sexual reproduction of UK native and Alaskan invasive L. vulgaris populations ........ 95 3.1 ABSTRACT ........................................................................................