Recovery of canopy-forming macroalgae following removal of the enigmatic grazing sea urchin Heliocidaris erythrogramma

The sea urchin Heliocidaris erythrogramma (Valenciennes, 1846) is considered an ecologically important member of shallow sub-tidal reef assemblages across temperate Australia. However, defining its ecological role has remained elusive due to a paucity of evidence demonstrating the ability of this species to graze and maintain sea urchin barrens. Here we present critical evidence for a grazing effect of H. erythrogramma at typically observable densities (~ 4-6 individuals m− 2) by demonstrating recovery of canopy-forming brown algae following two controlled removals of the sea urchin from ostensibly barrens reef at sheltered locations in eastern Tasmania. In experimental plots where H. erythrogramma was removed, canopy-forming algae gradually recovered to demonstrate an average 5 times (ranging nil to 10 times) increase in percentage cover at 24 months post manipulation (chiefly driven by the habitat-formers Cystophora spp., Macrocystis pyrifera, Acrocarpia paniculata, and Sargassum spp.). While divergence in the overall algal community was indicated by both experiments, a statistically significant shift, based on percentage cover community data, was observed for only one location at 24 months post sea urchin removal, suggesting that a complete canopy-driven shift in community structure will be gradual and will be contingent on urchin density remaining below approximately 1.5 m− 2 in the longer term. The relatively slow and highly variable rates of algal recovery across plots suggests that seasonal coexistence of annual (chiefly filamentous algae) and some perennial macroalgae on H. erythrogramma barrens is likely due to a relatively low and variable intensity of selective grazing. We conclude that H. erythrogramma, at sufficient densities, is capable of grazing and maintaining a sea urchin barrens state however this phenomenon is observed to manifest in sheltered waters only. In contrast, H. erythrogramma occurring on exposed Tasmanian reefs achieves similar densities but here barrens are not observed and the sea urchin occurs cryptically within crevices and, consistent with experiments elsewhere in Australia, the sea urchin under these conditions appears to feed exclusively on an abundance of detached 'drift' macroalgae. Top-down and bottom-up factors leading to increased abundance of H. erythrogramma will be important in determining the potential influence of this enigmatic grazer on rocky reef communities, however critical tests of mechanisms controlling the switch between foraging modes are needed. Taken together, correlative patterns and results of experimental removals across the range of H. erythrogramma demonstrate that models of temperate Australian reef dynamics must account for the highly context-dependent effects of sea urchin foraging.