Abiotic and biotic factors influencing the invasion of Carcinus maenas in southern New South Wales, Australia

Non-indigenous species are among the greatest threats to global biodiversity and collectively cause significant economic damage. The European shore crab Carcinus maenas is a global marine pest that has been present in Australia for over 100 years, but has been little studied in this part of the world. This thesis aimed to enhance our knowledge of the distribution and abundance of C. maenas in New South Wales (NSW) and the role that the unique abiotic and biotic features of the Intermittently Open and Closed Lakes and Lagoons (ICOLLs) that dominate the NSW coastline play in the invasion. ICOLLs can spend periods of days to years disconnected from the ocean by a sand bar, and display high spatio-temporal variability in their abiotic and biotic conditions. Trapping surveys across 14 estuaries (including ICOLLs) and spanning 8 dates found that in NSW the abundance of C. maenas is low and displays considerable spatial variation at the scales of estuaries and habitat. The detection of crabs in estuaries was positively correlated with the proportion of time estuarine entrances were open. Within estuaries, abundances were greater in mangroves than in saltmarsh, seagrass, oyster or bare sediments and were negatively correlated with the abundance of several native predators. Hence the opening and closing of estuarine entrances influenced C. maenas invasion by determining opportunities for recruitment, and possibly by determining the predator and habitat assemblages within these. Recruitment and cohort analyses indicated that C. maenas larvae primarily settle in the late Austral winter to spring, retaining the reproductive phenology of the northern hemisphere. Overall, this study suggests that although C. maenas has been present in Australia for over a century its abundance, and hence impact, remains low along the NSW coast due to a combination of abiotic and biotic factors.