Interaction between a fucoid canopy and limpet grazing in structuring a low shore intertidal community

The relative roles of a macroalgal canopy, Fucus serratus L., and a grazer, Patella vulgata L., in structuring a low shore community of a sheltered, rocky intertidal site were examined using a factorial experimental design. Canopy removal and grazer exclusion treatments were used to determine the effects of both factors on the understorey community. Re-establishment of the canopy was studied in relation to grazing pressure and substrate type in order to determine how this macroalga maintains its spatial and temporal dominance on the low shore. In addition, the size of canopy clearances were varied to determine the effect of gap size on community structure. Removal of the F. serratus canopy alone resulted in little change to the understorey community. Few understorey species were adversely affected by exposure to increased light levels and the grazing pressure of limpets was sufficient to prevent large scale recruitment of algae. Removal of limpets alone resulted in little change except for a greatly enhanced recruitment of F. serratus juveniles, indicating that grazing pressure rather than the canopy was inhibiting recruitment of the canopy species. Large changes in community structure only occurred following removal of both the canopy and grazers together since either the canopy or limpet grazing alone was sufficient to prevent large scale recruitment of algae. Limpet grazing did not prevent the re-establishment of a F. serratus canopy, although recovery was delayed in comparison to plots where limpets were removed. The size of gaps in the canopy had no discernible effect on the species composition or rate of succession in experimental treatments.  1999 Elsevier Science B.V. All rights reserved.