Spatial structure on moderately exposed rocky shores: Patch scales and the interactions between limpets and algae

The spatial pattern of limpet grazing has been previously hypothesised to drive a mosaic of algal patches on moderately exposed shores in the northeast Atlantic. The presence of differently aged algal patches and spatial effects associated with limpets were tested for using spatially referenced data sets. Algal and limpet occurrences in 0.01 m² cells were recorded in a series of 25 m² quadrats on shores on the Isle of Man. All sites showed small-scale spatial autocorrelation for limpets and mature and juvenile algae. Hence the sites surveyed had an uneven distribution of limpets superimposed on a mosaic of differently aged patches of algae. The local density of limpets was related to the probability of a cell containing juvenile algae in 7 of the 10 quadrats surveyed. Where they had an effect, limpets generally reduced the probability of juvenile algae being recorded locally, as would be predicted by the grazer-driven mosaic hypothesis.