Climate change and a range-extending sea urchin : catastrophic-shifts and resilience in a temperate reef ecosystem

The barrens-forming sea urchin Centrostephanus rodgersii (Diadematidae) has undergone recent poleward range-extension to eastern Tasmania. This thesis examines multiple processes influencing the shift from seaweed beds to sea urchin barrens and integrates the findings within the conceptual framework of catastrophic-shifts between the alternative reef states. This thesis identifies two processes acting to alter this temperate reef system: 1. Climate change - resulting in the poleward migration of a habitat-modifying species; and 2. Fishing - resulting in reduced ecosystem resilience. Examination of C. rodgersii population dynamics across the extension-region reveals that recent warming has led to a coastal regime where sea temperature is now suitable for C. rodgersii larval development. Furthermore, the timing of the sea urchins' arrival, age-structure and spatial distribution across the extension-region is consistent with patterns in sea temperature and dispersal potential driven by the EAC. As in the species historic range, C. rodgersii in eastern Tasmania is now found in association with barrens habitat; and field experimentation reveals that creation of barrens by this sea urchin results in local biodiversity loss in the order of ‚ÄövÑvÆ150 taxa that associate with Tasmanian seaweed beds. Furthermore, seaweed-sea urchin dynamics are observed to be broadly consistent with that observed from within the sea urchins' historical range, suggesting that the ecological importance of C. rodgersii will be similar across the extension-region. Field experiments identify the spiny lobster (Palinuridae) as the chief predator of C. rodgersii within the extension-region and trials inside/ outside Marine Protected Areas (where size and abundance of lobsters has recovered following cessation of fishing) demonstrate that C. rodgersii survival rates are decreased in the presence of large predatory lobsters. Indeed, the predatory interaction between lobsters and C. rodgersii is highly size-specific with only lobsters well above the harvested size-limit physically capable of predating on C. rodgersii. Importantly, such predatory capable lobsters have been functionally extirpated from much of the Tasmanian coastline due to intensive fishing for well over a century. The culmination of this thesis is that fishing, by removing large predators, equates to a reduction in ecosystem resilience and increased risk of catastrophic-shift in the face of changing climate.