The use of sandstone blocks to test hypotheses about colonization of intertidal boulders

Abstract

This study tested the hypothesis that regularly-shaped sandstone blocks in intertidal boulder-fields are colonized by mobile macrofauna at a similar rate to colonization of natural boulders. In addition, the sessile component on these plates (three levels of sessile assemblages) and their position in the boulder-field (three different positions relative to other boulders) were varied to test hypotheses about effects of these features on colonization. The epibiota varied among the three sets of plates and the natural boulders at the start of the experiment. The boulders were very rapidly colonized by a suite of mobile animals, particularly chitons, gastropods and echinoderms. Colonization was sparse and patchy to start with, causing great variability among replicates. Nevertheless, it did not differ among any of the treatments, indicating that even at this early stage, colonization did not differ between natural boulders and blocks, nor according to the sessile assemblage, or the proximity of other boulders. By six months, there was little variability among replicates and the assemblages had converged. This indicates that sandstone plates/blocks can form a standardized unit of habitat which can be used to test models about spatial variation in this relatively specialized fauna.