Regional scale differences in the determinism of grazing effects in the rocky intertidal

Patellid limpets are dominant grazers on intertidal rocky shores of NW Europe with a key role in structuring the eulittoral community. Localised loss of limpets and the subsequent reduction in grazing pressure is known to result in important changes in community structure, through the development of canopy-forming macroalgae, and an associated increase in species diversity and community complexity. The level of determinism in the community level response to localised loss of patellid limpets was assessed at spatial scales from 100s of kilometres to 10s of metres and temporal scales from weeks to months at mid-tide level of exposed rocky shores. Limpets were removed and excluded from experimental plots to simulate localised limpet loss and appropriate controls established. Experimental plots were established in replicate patches at 2 shores at each of 2 regional locations, separated by approximately 500 km: the Isle of Man and SW England. Removals were conducted on 2 dates within each of 2 seasons (summer and winter) and the community level response monitored for a period of 12 mo. There was a clear effect of limpet loss at all spatial and temporal scales, with rapid development of green ephemeral algae followed by a fucoid canopy. However, the degree of determinism in the development of canopy-forming algae differed markedly between the 2 locations. At the northerly location, the Isle of Man, fucoid algae developed quickly and dominated all areas of limpet exclusion; there was little variability between plots. In contrast, in SW England, the abundance of fucoid algae was significantly lower and much more variable. Such geographic changes in the development of macroalgae in the absence of the dominant grazer are discussed in relation to rocky shore community dynamics and the latitudinal change in balance between grazers and algae over the wave exposure gradient.