Patchiness and fluctuations on moderately exposed rocky shores

Small-scale changes in a mid-tide level patchy fucoid/barnacle community were investigated for 7 years on a moderately exposed shore in the Isle of Man. A fixed area was repeatedly sampled non-destructively to assess the abundance and spatial pattern of the major species present. Considerable natural change was recorded on this unpolluted shore, especially in seaweed cover and number of limpets. Concurrent manipulative field experiments were run to clarify the role of biological interactions in generating and maintaining the observed fluctuations and patchiness. Local reductions in Patella vulgata grazing induced development of Fucus vesiculosus clumps. Such clumps dislodged up to 80% of settling Semibalanus balanoides cyprids, but encouraged aggregation and recruitment of Patella, Actinia equina and Nucella lapillus. Dense barnacles inhibited Patella grazing and growth. A simple model of the generation and maintenance of patchiness is presented.