Effects of experimental manipulations of the density of a key grazer Cymbula granatina on rocky-shore community composition in the Benguela ecosystem, South Africa

Abstract

Densities of a commonly-harvested intertidal limpet Cymbula granatina were manipulated at two sites within a no-take marine protected area on the west coast of the Cape Peninsula in South Africa, using four treatment levels ranging from zero to maximum natural densities, together with control plots, to evaluate the effects of different harvesting intensities on rocky-shore community composition. Following removal or thinning of C. granatina, community composition changed: cover of corticated and ephemeral algae increased and that of encrusting corallines decreased. As limpet density fell, abundance of the brown encrusting alga Pseudoralfsia verrucosa increased on rock but decreased on shells. These outcomes were, however, dependent on the time frames considered, as macroalgae underwent annual cycles, and the effects of limpet removal were evident predominantly during the summer upwelling season when macroalgae proliferated. There are important management implications arising from these finding, notably that any reductions of this limpet to levels below 50 % of natural densities will profoundly alter community composition, particularly increasing macroalgae while decreasing encrusting corallines, with likely secondary effects on succession and recruitment of other species.