Persistence of sea urchin (Heliocidaris erythrogramma) barrens on the east coast of Tasmania: inhibition of macroalgal recovery in the absence of high densities of sea urchins

Sea urchin barrens occur commonly in temperate regions throughout the world and have significant implications for ecological processes on subtidal rocky reefs because they constitute areas of low productivity and diversity compared with habitats dominated by macroalgae. On the east coast of Tasmania, the occurrence of sea urchin (Heliocidaris erythrogramma) barrens in sheltered bays has additional implications in that they represent an important habitat of the introduced kelp Undaria pinnatifida. Identifying the factors responsible for ongoing maintenance of the barren habitat is essential in defining management options to promote recovery of native canopy-forming species. We used transplant experiments to investigate whether inhibition of recovery of native canopy-forming algae can occur in the absence of intense sea urchin grazing. High densities of native canopy-forming species successfully colonised paving blocks deployed in a dense algal bed adjacent to a sea urchin barren. Transplanting these paving blocks to plots on the barren from which sea urchins were removed resulted in >80% mortality of recruits after three months, and 100% mortality after seven months. The decline in macroalgal recruits on paving blocks transplanted to the urchin barren was associated with an increase in the cover anddepth of sediment. A persistent cover of sediment also developed on paving blocks deployed on the urchin barren, where no native canopy-forming algal recruits were observed. While sea urchins are undoubtedly important in creating urchin barrens, our results suggest that other mechanisms can influence recovery of native canopy species. In sheltered and semi-exposed bays on the east coast of Tasmania, sedimentation appears to play a critical role in inhibiting early developmental stages of native macroalgae, thereby contributing to a positive feedback that acts to maintain the barren habitat