Experimental Studies on the Ecology of Intertidal Environments at Heron Island. II. The Effect of Substratum

Abstract

Different kinds of rocks were cemented to the beach rock at Heron Island both upon the open surface to permit fish browsing, and within cages to prevent it. During a 12-month period, a negligible number of intertidal animals colonized these rocks, and their sparsity upon the surrounding beach rock is evidently not due to a substratum effect per se. Minor effects of the substratum on the flora were noted. On the harder rocks the brown alga, Ralfsia, was a conspicuous newcomer, while green algae were best developed on the calcareous substrata. The intensity of fish browsing was inversely related to the hardness of the rock surface, but the effect was concluded to be an indirect one, hardness affecting the thickness of the algal cover, and this, in turn, affecting the browsing intensity. Hard concrete placed upon the beach 7 years previously was appreciably higher than the general surface, and (after making certain assumptions) it appears that rock is removed by fish browsing to a thickness of about 0.5 mm per year. In a preliminary experiment acorn barnacles were transplanted from the reef crest to the beach rock, and a higher survival rate was observed on caged specimens.