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AbstractThe spread of non-indigenous species represents a significant threat to the native diversity globally, and are costly to control, manage and eradicate. Classical studies of bio-invasion focused on environmental matching or community level traits. Recently however, the role of propagule pressure is being recognised as fundamental to invasion processes. Establishment success of indigenous terrestrial plants, ungulates, birds and mammals is found to increase under higher propagule pressure regimes. Despite this, our understanding of propagule pressure in marine systems is relatively limited. Very few studies have experimentally manipulated propagule pressure in the marine sub-tidal, possibly due to the intractable nature of marine larvae. There are calls to elucidate general `dose-response' functions between propagule pressure and colonisation success. Propagule pressure is, however, a composite concept and can include not only the total numbers of arriving larvae, but also the quality, genetic diversity, and frequency at which they arrive. Failing to understand these `intrinsic' parameters of propagule pressure may confound our interpretations of general invasion processes. In this thesis I test several hypothesis regarding the role of total propagule pressure, and the frequency and genetic richness of arriving propagules. Throughout this thesis I repeatedly show strong effects of these intrinsic parameters of propagule pressure to the colonisation and early survival of Australian non-indigenous temperate sessile invertebrate fauna. I then used a factorial manipulation of metamorphosed native bi-valves Saccostrea glomerata and non-native C. gigas to test how recruit size and density interact to determine population survival in this system. Again, proportional survival of early recruits increased with greater initial population density, however this pattern varied between coloniser size classes. Predator exclusion did increase overall survival, independent of oyster size, species, or coloniser density. Lastly, I give a real world example of propagule pressure effects in the marine sub-tidal. Here I test the colonisation rates of non-indigenous species at different distances to large nonindigenous populations on an artificial shipping structure. Predictably we find greater colonisation of several ubiquitous nonindigenous species at distances close to the structure. My thesis therefore adds to the growing body of work that is highlighting the role of non-indigenous propagule pressure in determining invasion success in the marine sub-tidal.