Field experiments with “cageless” methods to manipulate grazing gastropods on intertidal rocky shores

Excluding particular species or guilds of grazing gastropods from experimental areas on intertidal rocky shores is a manipulation frequently used by marine ecologists. Manual removals, cages or fences are classical methods, but they usually require large areas or multiple procedural controls to avoid experimental artefacts. In order to minimise some of those constraints, alternative methods have been developed and are increasingly being used. The effects and limitations of these methods are, however, generally assumed and hardly ever tested. The experiment described here was designed to compare the effectiveness of manual removals, copper paint and sticky barriers in excluding intertidal grazing gastropods from experimental areas on a natural sandstone platform in South-eastern Australia. The response of the grazing assemblage was evaluated for individual species and collectively (integrating all species) by direct counts of individuals and from radular marks on wax discs. The observed patterns varied considerably among species. As predicted, barriers of copper paint effectively excluded the limpet Cellana tramoserica, for several weeks, while having no apparent effect on co-occurring species of grazing gastropods. Sticky barriers were also effective in excluding C. tramoserica, but for shorter periods of time. Contrary to expectations, however, sticky barriers were generally incapable of deterring the coiled snails Bembicium nanum and Nerita atramentosa. Barriers of copper paint are, therefore, a cost-effective way to manipulate diversity (densities of individuals, number and composition of species) and function (intensity of grazing) in this type of assemblage. Care must be taken to clarify the underlying mechanisms and to identify adequate procedural controls.