Limited effects of the sea urchin Echinometra mathaei (de Blainville) on the recruitment of benthic algae and macroinvertebrates into intertidal rock platforms at Rottnest Island, Western Australia

Abstract

The sea urchin Echinometra mathaei occurs at densities of up to 100 per m 2 on intertidal rock platforms at Rottnest Island, Western Australia. Contrary to expectation, the experimental removal of these urchins from urchin-dominated areas at two sites on the Island produced little evidence to suggest that the presence of E. mathaei was either necessary for or specifically preventing the recruitment of any particular species of macroalga or macroinvertebrate. At Cape Vlamingh where urchins grazed extensively, removal of urchins was followed by a large increase in the cover of non-encrusting algae, which were the same species that occurred in lesser numbers in uncleared plots. At Radar Reef, where grazing was confined to the urchins' individual burrows, removal of urchins had little effect on the cover of the algae. Furthermore, there was no “keystone” effect of urchins on the algal composition at either site, as domination of the understorey by canopy species was prevented by the physical conditions at Radar Reef and the failure of the canopy species to recruit at Cape Vlamingh. The number of recruits of all species of macroinvertebrates was low (<10 per m 2), and there was no difference in the identity of the species that recruited into the cleared and uncleared plots at either site, although the presence of urchins did have an effect on the abundance and spatial distribution of recruits of conspecifics and the limpet Siphonaria zelandica, and on the distribution of adults of the mobile gastropod Turbo intercostalis. With such a limited effect on recruitment, E. mathaei is much less of a controlling factor in this intertidal community than has been reported for other species of sea urchins in subtidal communities. Processes affecting the supply of propagules to these platforms appear to be the more important determinants of community membership and structure.