Spatial Heterogeneity, Sea Urchin Grazing, and Habitat Structure on Reefs in Temperate Australia

Abstract

The sea urchin Centrostephanus rodgersii is important in determining biogenic habitat structure on rocky reefs in New South Wales. Individual C. rodgersii, in common with other diadematids, are found in shelters during the day, from which they emerge to forage at night and maintain patches of barrens habitat. The effect of availability of shelter on the foraging behavior of C. rodgersii, and on the abundance of benthic invertebrates and algae, was investigated by a field experiment. Physical heterogeneity in the form of large boulders were translocated between the barrens and Ecklonia forest habitats and arranged so as to either maximize or minimize shelter. The fauna and flora on translocated boulders became progressively similar to that found in the surrounding habitat. Most obvious of these changes were reductions in the density of foliose algae on boulders moved from Ecklonia forest to barrens and reductions in the density of limpets in the reverse translocation. C. rodgersii recruited to all treatments irrespective of where the boulders came from, where they were moved to, and the amount of shelter provided. The availability of shelter was clearly a sufficient condition for the creation of areas of barrens habitat. The reliance on shelter by these sea urchins makes their local distribution and, therefore, that of the barrens habitat, more predictable than in other temperate regions.