There are many factors that have been proposed to contribute to plant invasiveness in nonnative ecosystems. Traits of invading species are one of them. It has been proposed that successful species at a certain invasion stage share particular traits, which could be used to predict the behaviour of potentially invasive plants at the respective stage. Three main stages of invasion are distinguished: introduction, naturalization, and invasion. I conducted a stageand trait-based analysis of available data for the invasion of New Zealand by the flora of southern Africa. Using 3076 southern African native vascular plant species introduced into New Zealand, generalised linear mixed model analysis was conducted to assess association of several species traits with the three invasion stages. The results showed that plant traits were significantly associated with introduction but fewer traits were associated with naturalization or invasion, suggesting that introduction can be predicted better using plant traits. It has been also hypothesized that species may become invasive in non-native ecosystems because they are removed from the regulatory effects of coevolved natural enemies (Enemy Release hypothesis). A detailed field study of the succulent plant Cotyledon orbiculata var. orbiculata L. (Crassulaceae) was conducted in the non-native New Zealand and native Namibian habitats to compare the extent of damage by herbivores and pathogens. C. orbiculata is a southern African species that is currently thriving in New Zealand in areas seemingly beyond the climatic conditions in its native range (occurring in higher rainfall areas in New Zealand than are represented in its native range). As hypothesised, C. orbiculata was less damaged by herbivores in New Zealand but, contrary to expectation, more infected by pathogens. Consequently, the plant was overall not any less damaged by natural enemies in the non-native habitat than in its native habitat, although the fitness impacts of the enemy damage in the native and invaded ranges were not assessed. The results suggest that climatic conditions may counteract enemy release, especially in situations where pathogens are more prevalent in areas of higher rainfall and humidity. To forecast plant invasions, it is concluded that species traits offer some potential, particularly at the early stage of invasion. Predicting which introduced plants will become weeds is more difficult. Enemy release may explain some invasions, but climatic factors may offset the predictability of release from natural enemies.

The spread of exotic species is considered to be one of the most significant threats to ecosystems and emphasises the need for appropriate management interventions. The majority of marine non-indigenous species (NIS) are believed to have been introduced via ship biofouling and their domestic spread continues to take place via this mechanism. In some countries, biosecurity systems have been developed to prevent the introduction of NIS through biofouling. However, implementing biosecurity strategies is difficult due to the challenges around identifying high-risk vectors. Reliable predictors of risk have remained elusive, in part due to a lack of scientific knowledge. Nonetheless, invasion ecology is an active scientific field that aims to build this knowledge. Propagule pressure is of particular interest in invasion ecology as it describes the quantity and quality of the propagules introduced into a recipient region and is considered to be an important determinant in the successful establishment of NIS. Environmental history affects health and reproductive output of an organism and, therefore, it is beneficial to examine this experimentally in the context of biofouling and propagule pressure. The aim of this thesis was to examine how voyage characteristics influence biofouling recruitment, survivorship, growth, reproduction and offspring performance through the ship invasion pathway. This was to provide fundamental knowledge to assist managers with identifying high-risk vessels that are likely to facilitate the introduction or domestic spread of NIS, and to understand the processes affecting biofouling organisms during long-distance dispersal events. Chapter One provides an introduction to the issues addressed in this thesis. Each data chapter (Chapters Two – Five) then focused on a stage of the invasion process and included field experiments using a model organism, Bugula neritina. Finally, Chapter Six provides a summary of key findings, discussion and the implications to biosecurity management. Throughout this thesis, the effect of donor port residency period on the success of recruits was highlighted. Chapter Two focused on recruitment in the donor region. As expected, recruitment increased with residency period. Importantly, recruitment occurred every day on vulnerable surfaces, therefore, periods as short as only a few days are able to entrain recruits to a vessel hull. The study presented in Chapter Three showed that there was high survivorship of B. neritina recruits during 12 translocation scenarios tested. In particular, the juvenile short-residency recruits (1-8 days) survived voyages of 8 days at a speed of 18 knots; the longest and fastest voyage simulated. Interestingly, variation in voyage speed and voyage duration had no effect on the survivorship of recruits, but did have legacy effects on post-voyage growth. Again, B. neritina which recruited over very short residency periods of 1 day continued to perform well after translocation and had the highest level of reproductive output after the voyage scenarios (Chapter Four). Recruits that were older (32-days) and reproductively mature at the commencement of the scenarios failed to release any propagules. Even though the number of ‘at sea’ and ‘port residency’ days were equal, reproductive output was higher after short and frequent voyages than after long and infrequent voyages. Finally, the study presented in Chapter Five examined transgenerational effects of B. nertina. Results showed that although the environmental history of the parent colony had a carry-over effect on offspring performance, it was the offspring environment that was a stronger determinant of success (measured by reproductive output and growth). Although cross-vector spread is possible (i.e. parent and offspring both fouling an active vessel), offspring released from a hull fouling parent into a recipient environment will perform better. In combination, these studies have provided new insights into NIS transport via vessel biofouling. Although shipping pathways are dynamic and complex, these results suggest that juvenile stages that recruit over short residency periods and are then translocated on short voyages, may pose a higher risk for NIS introduction than originally assumed. This has implications for marine biosecurity management as short residency periods are common and short, frequent voyages are typical of domestic vessel movements which are largely unmanaged.