Effect of grazing by limpets on mid-shore species assemblages in northern Spain

Limpets were excluded from barnacle-dominated areas in 2 semi-exposed localities on the shore of northern Spain. We tested the hypotheses that a macroalgal canopy would develop and barnacle cover would decrease in exclusion quadrats. Spatial replication considered localities and sites within localities, while temporal replication considered 2 seasons of removal and 2 start dates within each locality and season. The experimental methods included exclusion quadrats (using fences), procedural controls (half fences) and untouched controls. Results supported both hypotheses. Limpet removal caused exclusion quadrats to change over time more than controls. An algal canopy dominated by Fucus vesiculosus developed and more barnacles were lost in exclusion quadrats than in controls. In addition, more trochids (largely Gibbula spp.) and whelks (Thais lapillus) were found in exclusion quadrats. Site effects were significant for virtually all variables analysed but no effect of locality, season or date was found. An indirect effect mediated by the development of an algal canopy appeared to be responsible for the loss of barnacles in exclusion quadrats. This indirect effect might have been reinforced by an additional indirect effect of algae mediated by increased densities of whelks under the algal canopy. Altogether, our results agree with those in semi-exposed shores in other parts of the world but not with results obtained in the west coast of Sweden, which has contrasting environmental conditions and a different dominant grazer species.