Survival of kelp adjacent to areas grazed by sea urchins in New South Wales, Australia

The sea urchin Centrostephanus rodgersii grazes areas of reef free of large brown algae (the barrens habitat). Survival of the kelp Ecklonia radiata adjacent to patches of barrens habitat was compared to that in the centre of kelp forests (centre) and edges of forests not adjacent to patches of barrens habitat (ungrazed). Estimates of rates of instantaneous mortality for tagged plants, as described by the slope of a negative exponential model (± 95% CI) were: barrens, 0.078 ± 0.004; centre, 0.051 ± 0.004 and ungrazed edge, 0.065 ± 0.007. Survival of plants was greatest in the middle of forests and least on the margins of patches of barrens habitat. A significant proportion of mortality in the barrens and centre positions was caused by herbivorous fish. When these plants were excluded from analysis there were no significant differences in survival between the barrens and ungrazed positions. It is concluded that C. rodgersii has little impact on the abundance of Ecklonia outside sharply defined boundaries.