Copyright is owned by the Author of the thesis. Permission is given for a copy to be downloaded by an individual for the purpose of research and private study only. The thesis may not be reproduced elsewhere without the permission of the Author. Impacts of invasion on community structure: habitat and invertebrate assemblage responses to Gal/una vulgaris (L.) Hull invasion, in Tongariro National Park, New Zealand Vaughan Francis Keesing 1995 A thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Zoology at Massey University, Palmerston North. Acknowledgments If I were to blame anyone but myself for this endeavour it would have to be Dr David Steven, who first incited and ignited my interest in invertebrates, and in higher education. I trust he sees this as praise. I acknowledge with appreciation the following organisations for their financial assistance in furthering my education and creating the monetary support without which this thesis would have been impossible: Department of Conservation (Scholarship) Miss E.L. Hellaby Indigenous Grasslands Research Trust British Council Higher Education Link Scheme Graduate Research Fund, Massey University I am also indebted to the staff of the Department of Ecology (all of them) at Massey University for logistic and intangible support. Special thanks goes to Barbara Just, who has an incredible talent for finding resources so badly needed by so many students. I also wish to thank Petra van Kan and Erica Reid for proofing the final draft, a task I did not relish. Special thanks also go to Steve Pilkington, Jean Garman, Cathy Lake, and Kerry Sanders for their help in field work, even when it was snowing. Of course no thesis passes the starter's gate or gets to the finish line without a hardy bunch of patient people, my supervisors. am very grateful to Dr Jill Rapson (chief I supervisor) for her immense patience, guidance, motivation (a fortune on string and carrots) and her ability to encourage, seeing the positive rather than the negative. To Dr Ian Henderson for sharing his knowledge, suggesting avenues (or side tracks ?) and supplying much needed statistical support. And To Dr Clare Veltman for her creative ideas, suggesting directions, and positive attitude. The friendly help I received from Harry Keys (DoC), Dr Peter McGregor, Dr Ian Andrews, Dr Pauline Syrett, and Dr Tracey Boumer is also much appreciated. To all my friends, (and I hope you'll pardon me if I have forgotten to mention anyone) without whom the smaller things of life weigh heavy, I extend my gratitude. Simon Adkins, Kim McBreen, Isabel Castro, Howard Smith, Harani Himona, Cathy Lake, Dale Towers, Vanessa Munro, Phil Battely, Peng Tay, Julie Alley, Jonathan Miles, Gary Bramley, Halema Flannagan, Tamsin Ward-Smith, Jackie Townsend, Edwin Smith, Patricia Murray, Kathryn Whaley, Penny Aspin, Jay McCartney, and Chris Devine, Thank you all. Of course my deepest gratitude goes to my Mother and Brother, who are, and have always been, right behind me. CONTENTS Thesis abstract 1 Chapter 1 Invasions: a continual process challenging 'communities' 4 Introduction 4 Communities 4 The invasion syndrome 6 Ingression and invasion 6 What makes a species invasive ? 8 The invaders 8 System susceptibility to invasion, and the role of disturbance 12 Summary of the invasion process 16 The impact of invaders in New Zealand 16 Thesis outline 18 Chapter 2 Impact of Calluna on vegetation structure and as resource for invertebrates 20 Abstract 20 Introduction 21 Plants as resources for invertebrates 21 History of the invasion of Gal/una vulgaris in New Zealand 22 Aim 26 Methods 27 Site selection 27 Resource modification 35 Biomass 35 Mineral content 35 Nitrogen 36 Plant architecture 36 Results 39 Community diversity assessment 39 Resource assessment 39 Biomass 39 Mineral content 44 Nitrogen 44 Plant architecture 49 Discussion 54 Alteration of resources 55 Plant architecture 57 Conclusion 58 Chapter 3 Impact on invertebrate assemblages after invasion by Calluna 60 Abstract 60 Introduction 61 Methods 63 Pit-fall traps 63 Sweep-netting 64 Bush beating 64 Utter scraps 64 Sampling regime 64 Analysis 65 Results 68 Assemblage compositions 68 Variation between habitats 69 Invertebrate abundance distribution patterns 82 Seasonal variation 82 Feeding guild structure 90 Overseas fauna comparisons 96 Correlations with plant features 96 Discussion 99 Conservation of habitat diversity 99 Invertebrate assemblage responses to change 99 Seasonal variation 100 Diversity 101 Common invertebrates respond first to invasions 102 Abundance distributions in assemblages 102 Conclusions 103 Chapter 4 Impact of native herbivore feeding on Cal/una vulgaris 106 Abstract 106 Introduction 107 Lab feeding trial 108 Herbivory impacts 109 Methods 109 Feeding trials 109 Experimental system 109 Field exclusion trials: Herbivory impacts 116 Treatments 117 Measuring responses 118 Analysis 118 Results 119 Feeding trials 119 Insect survival and profit 119 Diet preferences 119 Location during choice tests 119 Quantity of plant material consumed 122 Herbivore exclusion trials 125 Above ground herbivore load 127 Root herbivore survey 127 Exclusion trials 128 Discussion 131 Insect feeding 131 Insect consumption 132 Gal/una growth and herbivory 134 Threshold damage 134 Native phytophagous diet plasticity 134 Conclusion 135 Chapter 5 Predicting community stability through measures of resilience, persistence and consistency 136 Abstract 136 Introduction 138 Stability and complexity 138 System stability 140 Choice of descriptors 141 Resistance 141 Persistence or consistency 142 Resilience 143 Aim 143 Predictions 144 Methods 145 Sites 145 Experimental design 145 Controls 146 Spraying treatments 148 Sampling 148 Analysis 148 Section-1 : Initial assemblage complexity Species richness and abundance 148 Guilds 148 Randomisation tests 149 Evenness and diversity 149 Connectence 150 Complexity of the assemblages 150 Section-2 : Community stability Resilience 151 Finding a model of the rate of return 152 Consistency 152 Results 153 Section -1 : Complexity of initial assemblages Species richness 153 Abundance 153 Monte Carlo randomisation tests 155 Diversity - Evenness and Dominance 155 Connectence 159 Section -2 : Rates of reinvasion Resilience 162 Mode! 165 Consistency 165 Discussion 169 Complexity 169 Evidence of systems complexity and stability 169 Stability' 171 Implications 172 Chapter 6 The invasion process and its impacts in Tongariro National Park 174 Introduction 174 The invasion process 175 The resource base 178 Disturbance and successional impacts on the invasion process 180 Impacts on invertebrate assemblage structure 180 The invertebrate assemblage of established Gal/una habitat 181 Resource availability for invasion of a biological control agent 182 The biology of the new invader 183 Influence on Gal/una dynamics 185 Management and conservation 186 References 188 1 ABSTRACT Invasion ecology, disturbance and successional ecology, and conservation biology are four areas of increasingly realised importance in the maintenance and understanding of the world's remaining 'natural' ecosystems. The impacts of biological invasions and the role of physical disturbance on whole communities is difficult to comprehensively study, but can often be best viewed via invertebrate assemblages and their plant habitats. In this thesis I have taken the opportunity to study these concepts in an area where conservation is of national importance, Tongariro National Park (New Zealand). The Park, for the last 80 years, has and is, suffering from invasion by Gal/una vulgaris (European heather), which radically changes the plant composition of many of the landscapes in the Park and thus the resource base of the invertebrate communities. This is a community-based study focusing on the structures of invertebrate assemblages, their ability to adapt and be resilient in relation to their changing habitat and resources. It primarily compares features between invaded (disturbed) and 'normal' communities (i.e: their structures, adaptations/impacts caused by the invader, and community cohesiveness before and after invasion). The vegetation composition was investigated in a range of selected communities of lowland tussock grassland and heath lands in T ongariro National Park, Central North Island, New Zealand. These varied in their developmental history and conspicuous native species. Sites were partnered with adjacent communities of comparable composition, but invaded by Gal/una vulgaris. Ecosystem resources were measured through attributes of the invader such as biomass, mineral content, and architectural complexity, and were compared with conspicuous indigenous shrubs. Pair-wise comparisons of uninvaded and invaded communities showed that species richness changed very little with invasion, although the percent cover of the conspicuous indigenous component declined from 90% to 40%. Gal/una was found to add architectural complexity to the indigenous plant communities, many features of which may provide novel invertebrate living space. I conclude that Gal/una has modified the indigenous communities, lowering the obviousness of indigenous plants, and providing a structural form that occupies a greater volume of space with stem and foliage than the indigenous shrubs. To ascertainif invasion and dominance by Gal/una caused local extinctions, reductions, or other modifications to indigenous invertebrate fauna, the impact on the invertebrate assemblages in the invaded indigenous