Effects of grazing on the structure of mid-shore, intertidal assemblages on moderately exposed rocky shores of the Swedish west coast

A manipulative experiment was done on the Swedish west coast to test hypotheses about effects of grazing on assemblages of algae and sessile animals on moderately exposed shores.The experiments, which lasted for 15 mo, involved a total of 144 experimental units at 16 sites in 2 areas 3 km apart.Experimental treatments involved fences to exclude grazing snails, open fences to control for effects of fences, and control areas, which were not manipulated in any way.In contrast to results from moderately exposed shores in other parts of the world and in contrast to sheltered areas in the region, grazers did not appear to significantly influence the structure and composition of these assemblages.Fences and manual removal of snails were only partly successful in removing grazers.Additional observational evidence, however, supports the conclusion that grazers have a limited impact on structure and abundance of these intertidal assemblages.