An experimental analysis of interactions among several intertidal organisms

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Observations were made on organisms on a rocky shore in southeastern Australia. Algae growing over barnacles seemed to attract carnivorous whelks at low tide, yet these whelks were less likely to attack individual barnacles completely covered by algae than those partially covered. A field experiment tested whether these patterns arose directly as the effect of an interaction of algae and whelks on the survival of barnacles. The communities in small plots were manipulated to provide a choice of bare space, barnacles only or barnacles and algae as discrete patches within a matrix of algal-covered barnacles. Whelks were released in each of these patches at half of the sites, and their subsequent location and activity were recorded, together with the mortality of barnacles, percentage of cover of algae, and the numbers of mobile grazing gastropods (as alternative prey for the whelks) in each plot. Whelks were observed eating barnacles and gastropods during the experiment. Although whelks ate many barnacles, the direct effect of algae on mortality of barnacles was only occasionally important, whelks did not seem to affect the numbers of grazers. We conclude that the observed pattern is due to a number of different interactions among these organisms. The experimental design is discussed and patterns of complexity within the community are summarized.