Many rocky shore species have a planktonic dispersal phase. These species produce propagules or larvae that spend their early life in the open sea and may eventually settle on shore some distance from where they originated. This strategy allows species to rapidly colonise new areas that become available and reduces the risk of extinction as long as there are some enclaves of adults producing planktonic juveniles. The supply of larvae and propagules in any year is dependent not only on the size of the reproducing population but also on numerous physical factors.

The level of larval supply and its fluctuations play a considerable role in structuring rocky shore communities and have been appreciated for a long time (Southward and Crisp, 1956, Lewis, 1964, Kendall et al., 1985). Biological interactions, including intraspecific competition, are more intense in areas of high recruitment and recruitment fluctuations can cause instability (Gaines and Roughgarden, 1985; Menge et al., 1985; Menge and Olson, 1990; Menge, 1992; Gaines and Bertness,). The continued coexistence of the barnacles Semibalanus balanoides and Chthamalus on the mid-shore in southwest England probably owes a great deal to recruitment variation. Sometimes space is undersaturated due to low recruitment (Burrows, 1988; Southward, 1991). Chthamalus produce abundant larvae in warm years while Semibalanus performs better in cool years. The favoured species will have a high settlement rate and increase its abundance. Fluctuations in annual temperatures prevent either species from excluding the other. This contrasts with the situation at Millport, where Semibalanus larvae are usually abundant and the species excludes Chthamalus from the mid-shore (Connell, 1961a,b). Dense settlements of barnacles can lead to severe intraspecific competition. Individuals increase in height but are unable to grow laterally. This can lead to feeble attachment and the dislodgement of whole sheets of fragile individuals (Barnes and Powell, 1950). Dense settlements of prey species can ensure that at least some juveniles escape predation (Sebens and Lewis, 1985).

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