Physical disturbances and their direct effect on an indirect effect: responses of an intertidal assemblage to a severe storm

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Indirect interactions are increasingly viewed as important ecological processes and have been catalogued as two types (interactive chain and modifying interaction). Such interactions on a rocky shore in New South Wales were analysed experimentally to test hypotheses derived from models to explain changes in assemblages following a severe storm. The storm removed canopies of algae ( Hormosira banksii); this was followed by recruitment of barnacles ( Chamaesipho tasmanica) into areas under former canopies, from which they had previously been absent. Controlled, replicated manipulations were done to test hypotheses that barnacles had been influenced by the algae themselves, by predatory whelks ( Morula marginalba) sheltering under the H. banksii, or by encrusting algae found in shade under the canopy. The primary cause of patterns in the cover of barnacles and changes caused by the storms was predation by whelks. Encrusting algae had a minor negative influence on barnacles. A canopy of H. banksii had no direct effect on the barnacles, but canopies indirectly influenced barnacles by providing shelter, therefore increasing the effectiveness (but not the numbers) of whelks. The conclusion is that indirect interactions are important, but cannot always be categorized as one or other of the two types. H. banksii is part of a chain, by providing the habitat necessary for whelks to act. They are also a modifier of the interaction because whelks forage differently under canopies of different sizes. Finally, predictions about interactions are difficult if they are dependent on recovery from unpredictable influences, such as large storms.