Pushing boundaries of range and resilience: a review of range-extension by a barrens-forming sea urchin

The barrens-forming sea urchin Centrostephanus rodgersii (Diadematidae) has undergone recent poleward range-extension to the Tasmanian coastline (SE Australia). By compiling field observations (including SST spanning > 60yrs), broad-scale surveys and manipulative experiments conducted during the past decade, this review details knowledge on the response of this key sea urchin species to climate change and dually explores multiple processes influencing the ultimate ecological consequence of catastrophic-shift from productive kelp beds to urchin barrens, as now observed within the range-extension region. As a result of changing regional climate, eastern Tasmania has become increasingly suitable for Centrostephanus larval development with the timing of the sea urchins’ arrival, age-structure and spatial distribution across the extension-region consistent with patterns in warming sea temperatures and current-driven dispersal potential. Furthermore, consistency in temperature dependency of larval development plus lack of genetic differentiation of the species across its entire range, confirm the critical role of changing climate in driving the range-extension.

Continued warming predicted for this region will favour increased larval survival, promoting ongoing population expansion and ultimately increased likelihood of populations reaching sufficient density to affect widespread overgrazing. As such, knowledge of patterns and mechanisms promoting overgrazing are also reviewed for the purpose of identifying kelp beds at greatest risk given climate trends, reef substratum types and predator abundance (chiefly spiny lobsters) as influenced by intense fishing pressure. Finally, this review shows how management of non-climatic local scale stressors can be used to increase resilience of kelp beds against overgrazing given large-scale climatedriven increases in key sea urchin populations.