The tangled web we weave: Human-mediated spread of invasive species via trade networks using nematodes as model organisms

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 freeliving 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