Implementing biosecurity in live plant trade networks : A thesis submitted in partial fulfilment of the requirements for the Degree of Doctor of Philosophy at Lincoln University

New Zealand faces unique challenges with regards to invasive species due to a high number of endemic species and correspondingly high risk for the decline of indigenous species as a result of invasive introductions. Many of these introductions have been facilitated by human mediated transport, which promotes long range movement of plants and associated pests and pathogens. The transportation of these invasive species to new environments can negatively affect native plants, crops, and industries reliant on plant production. In particular, live plant trade networks represent effective mediums to disseminate invasive species such as pathogens, pests, and weeds. The movement of live plants through professional nurseries and private retailers in New Zealand is yet to be fully investigated. Performing an assessment of contact structures derived from live plant trades can be used to prevent the introduction and spread of invasive species, thereby safeguarding both businesses and the environment. Network analysis provides suitable tools to study live plant trade systems, such as the bilateral plant nursery network, as well as the itinerary online sales network. Fundamental characteristics related to geographic distribution of plant sales (such as location of origin and destination) and key metrics related to plant product movement within live plant trade networks need to be examined in order to better understand the nature of this structure. This thesis addresses gaps in knowledge related to live plant trade in New Zealand by generating networks that characterise human-mediated movement of horticultural products. Subsequently, developing network models of such systems was used to determine their vulnerability to invasive species acquisition and spread. This was accomplished via the assessment of three live plant trade systems of particular relevance to New Zealand biosecurity. These studies thusly characterise the forestry nursery transport network, the initial incursion of the invasive plant pathogen myrtle rust (Austropuccinia psidii) into New Zealand, and plant sales facilitated by the New Zealand-based online retailer Trade Me. Firstly, assessment of the forestry nursery transport network described the movement of product from seed orchards to nurseries to plantation forests and the biosecurity protocols in place at forestry nurseries. This study identifies seed orchards as the point of production most likely to facilitate widespread dispersal of a pathogen through the industry. The second study system, assessment of the initial incursion of myrtle rust into New Zealand, characterises the sale of nursery plants from myrtle rust positive nurseries during the first three months of the incursion. These networks identified a lack of comprehensive record keeping at ornamental nurseries as a risk for future invasions that are facilitated by contaminated nursery stock. Additionally, assessment of all myrtle rust positive locations during the first two years of the incursion identified delimiting surveys as a highly successful surveillance tool that would benefit from more research into optimisation of the survey radius depending on the nature of the invasive. The third and final study system, which characterised plant sales facilitated by Trade Me, employed the networking of plant sales based on their subcategorisation on the Trade Me site (into one of nine subcategories chosen by Trade Me) as well as the suburb of residence for both the seller and buyer of a product. Additionally, an assessment of the most common taxa sold on the site was conducted. The information derived from this study can be adopted for immediate use in biosecurity responses in the event of an introduction of an unwanted pest or pathogen vectored on ornamental plants, as well as identify the plant product classes at highest risk for long-distance dispersal.
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