Interactions Among Subtidal Australian Sea Urchins, Gastropods, and Algae: Effects of Experimental Removals

Abstract

The subtidal region at Cape Banks, New South Wales, Australia, is characterized by large areas covered solely by encrusting coralline algae in association with large densities of invertebrate grazers. Orthogonal removals of these grazers: the sea urchin Centrostephanus rodgersii; the limpets Patelloida alticostata, P. mufria, and Cellana tramoserica; and the turbinids Australium tentiforme and Turbo torquata, were done at two sites and two times to investigate the interactions among these species and their effects on the algal community. These manipulations had large effects on both the algal and animal communities, although the type and intensity of these interactions altered at different temporal and spatial scales. Initially, the removal of urchins and/or all species of limpets resulted in a rapid increase in the amount of noncrustose algae in these areas. Removal of turbinids, however, had no significant effect. Where both limpets and urchins had been removed, the cover of foliose algae continued to increase quickly and was ≈80—100% after 12 mo. In areas where only urchins were removed, the increase in the cover of algae was slower, and only approached 100% after 18—24 mo. Where only limpets had been removed, 3 mo after an initial increase in the cover of filamentous algae the cover declined to be only slightly more than in control areas. These results were similar at all sites and times. In one series of removals, the effects of some of the species of limpets were separated. In this case, the increase in the cover of algae was found to be fastest when only P. mufria was left (the urchins and other limpets removed), than when all limpets and urchins had been removed. Initially, the removal of the urchins generally resulted in a large increase in the recruitment and density of the limpets. Approximately 1 yr following the removal of the urchins,however, the density of adult and juvenile limpets of all species had declined to near zero. Removal of the larger species of limpets, P. alticostata and Cellana, also increased the rate of recruitment of the smaller species, P. mufria. Thus, these invertebrate grazers, especially the sea urchins, appeared to be necessary for the maintenance of the areas of crustose algae. Similarly, the sea urchins were found to be necessary for the continued presence of the limpets within these areas. The strong effects of the urchins on this community, and the application of the situation to theories concerning keystone species and facilitation, are discussed in relation to the important but more subtle effects of the limpets.