The Tangled Web We Weave
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THE TANGLED WEB WE WEAVE HUMAN-MEDIATED SPREAD OF INVASIVE SPECIES VIA TRADE NETWORKS USING NEMATODES AS MODEL ORGANISMS Natalie Clare Banks B.A., B.Sc. Australian National University This thesis is presented for the degree of Doctor of Philosophy of Murdoch University October 2016 i DECLARATION I declare that this thesis is my own account of my research (except where other sources are acknowledged) and contains as its main content work which has not previously been submitted for a degree at any tertiary education institution. This research was conducted under the following Human Ethics Permit numbers: 2013/005 and 2013/158. The original research for this thesis was conducted and authored by myself with the assistance of co-authors in regard to fieldwork (Dr Tangchitsomkid, Dr Hodda, Mr Chanmalee, Ms Sangsawang, Ms Songvilay, Ms Phannamvong, and Mr Thamakhot) and intellectual contributions to chapter drafts (Drs Bayliss, Paini and Hodda). Natalie Banks A NOTE ON CONTENT This PhD thesis comprises a published research paper (Chapter 1 published in Ecology Letters, February 2015) and chapters prepared as papers for publication (Chapter 4 submitted to Biological Invasions, October 2016). These are presented with chapter summaries in the introduction and a general discussion that links the chapters into a coherent and integrated body of research. References and in-text citations have been formatted for consistency. ii ABSTRACT Human trade networks play a major role in the unintended introduction of invasive species to new environments. Network Science has shown that the structural properties of networks influence the movement of goods as well as their associated organisms. This thesis examines how the properties of one type of network, the plant produce trade network, aid the movement of one group of potentially invasive organisms, nematodes. The presence, diversity, abundance and dispersal of nematodes via these networks and points critical to the flow of goods as well as nematodes, were also examined. A survey of markets and farms was conducted in three countries (Australia, Thailand and Lao PDR) and nematodes were extracted from the roots of vegetable produce sampled at each location. Plant-parasitic nematodes were identified to genus and numbers of free-living nematodes recorded. Network analysis software was then used to generate and analyse maps of the trade, nematode and plant-parasitic nematode movement networks in each country. A large range and number of free- living and plant-parasitic nematodes were detected moving locally, nationally and internationally via plant produce trade networks. All networks were broadly similar, containing hubs and shortcuts and were directed, poorly clustered and disassortative networks. The movement of nematodes followed the same structural pattern as the larger trade network, meaning that goods and nematodes moved through these networks in a particular and predictable way. Certain critical points in each network were at greater risk of an incursion or of spreading nematodes and may, therefore, represent effective places to target intervention strategies. This thesis synthesises and applies insights and tools from Network Science to Invasion Science theory and practice. By untangling the role of networks in the invasion process, scientists and managers are in a better position to prepare, predict and prevent the spread of invasive species. iii TABLE OF CONTENTS DECLARATION.…………………………………………………………….……… i A NOTE ON CONTENT.………………….….…………….……………….…….… i ABSTRACT………………………………………………………………………… ii TABLE OF CONTENTS...……………………………………….………………… iii ACKNOWLEDGEMENTS……………………………………………………….… vii LIST OF ABBREVIATIONS………………………………………………….…… viii GENERAL INTRODUCTION……………………………………...…………………1 THE PROBLEM................................................................................................... 2 THESIS STATEMENT……………………………………………………………… 3 THESIS OUTLINE…………………………………………………………………. 3 Chapter 1: The role of global trade and transport network topology in the human- mediated dispersal of alien species …………............................................................. 3 Chapter 2: The Network Topology of Plant Produce Trade Networks and Implications for Pest Movement………………............................................................ 3 Chapter 3: Nematodes Network Too: diversity, abundance and dispersal via Plant Produce Trade networks ………………....................................................................... 4 Chapter 4: Network Analysis and Plant Pest Infestation Risk in Plant Produce Trade Networks ………………............................................................................................... 4 Chapter 5: General Discussion.................................................................................. 4 References................................................................................................................... 5 CHAPTER 1: THE ROLE OF GLOBAL TRADE AND TRANSPORT NETWORK TOPOLOGY IN THE HUMAN-MEDIATED DISPERSAL OF ALIEN SPECIES………………………………………………………………………….... 8 ABSTRACT…........................................................................................................... 8 INTRODUCTION………………………………………….……………………………. 9 NETWORK TOPOLOGY………………………………………….…….……….……. 10 Scale-free network properties.................................................................................... 11 Small-world network properties................................................................................ 11 Directed & undirected networks............................................................................... 15 Mixing patterns......................................................................................................... 17 iv Connectance……………………….............................................................................. 18 A SYSTEMATIC APPROACH TO PREVENTION & MANAGEMENT…….………...…. 20 ISSUES……………………………………………………………...…….…….…… 25 Heterogeneous, interacting components.................................................................. 26 Temporal dynamics……………………….................................................................. 26 Spatial issues………………………………................................................................. 28 Integration………………….…………....................................................................... 29 CONCLUSION….………………………………………….…………….……….…... 30 ACKNOWLEDGEMENTS….………...…………………….…………….……………. 31 REFERENCES.…………………………………………...…………….………….…. 32 CHAPTER 2: THE NETWORK TOPOLOGY OF PLANT PRODUCE TRADE NETWORKS AND IMPLICATIONS FOR PEST MOVEMENT................................... 42 ABSTRACT............................................................................................................. 42 INTRODUCTION...................................................................................................... 43 METHODS.............................................................................................................. 46 Data Collection…...................................................................................................... 46 General Structure……………….......................................................................... 46 Movement of Nematodes..................................................................................... 48 Data Analysis…......................................................................................................... 48 RESULTS................................................................................................................ 49 Degree Distributions................................................................................................. 49 Other Network Parameters........................................................................................ 49 DISCUSSION........................................................................................................... 52 Degree Distributions................................................................................................ 52 Other Network Parameters........................................................................................ 52 Average Path Length........................................................................................... 53 Clustering Coefficient...…................................................................................... 53 Density.…............................................................................................................ 54 Assortativity Coefficient...................................................................................... 55 CONCLUSION......................................................................................................... 56 REFERENCES......................................................................................................... 58 v CHAPTER 3: NEMATODES NETWORK TOO: DIVERSITY, ABUNDANCE AND DISPERSAL VIA PLANT PRODUCE TRADE NETWORKS......................................... 62 ABSTRACT............................................................................................................. 62 INTRODUCTION...................................................................................................... 62 METHODS.............................................................................................................. 64 Data Collection…..................................................................................................... 64 Nematode Extraction................................................................................................