Ecological consequences of density-dependence in the sea urchin Centrostephanus rodgersii

Herbivory by sea urchins can dictate macroalgal cover on subtidal reefs. The ecological impacts of removing sea urchins on kelp, invertebrates and fishes are well-established, but we have little understanding of the density-dependent processes that influence the persistence of barrens habitat. To understand the importance of changes in density on habitat state, we manipulated sea urchin densities on reefs on the temperate east coast of Australia. Centrostephanus rodgersii is the major herbivore on these reefs and determines the presence of barrens habitat. In the first of 2 separate experiments, we manipulated density in barrens to create areas of reef with either full density (controls), 0.66 or 0.33 of full density or zero sea urchins. After 2 yr, sea urchins at reduced densities had high recruitment and per capita fecundity and grew faster than those in unmanipulated controls. Macroalgal cover was significantly greater in all treatments with reduced density and in those with no sea urchins. A second experiment done at a larger scale focused on sea urchin recruitment in response to the removal of all sea urchins. In that experiment, as in the smaller-scale experiment, there was evidence of significant compensatory recruitment of sea urchins to reefs with no sea urchins, although not at all locations. Our results provide evidence of strong density-dependent processes in the demography of C. rodgersii and its effect on reef ecology. We discuss the implications of these results for populations and the fishery for C. rodgersii, the seascape and for interventions to restore kelp forests.