Abrupt community change on a rocky shore – biological mechanisms contributing to the potential formation of an alternative state

Robert T. Paine* and Alan C. Trimble Department of Biology, Box 351800, University of Washington, Seattle, WA 98195-1800, USA *Correspondence: E-mail: [email protected] Abstract The 1997/1998 El Niño initiated a major shift in the intertidal assemblage on the Washington State outer coast. A 25 year time series (1978–2003) shows stands of dominant canopy algae replaced by mussel beds. A prior experiment had indicated that mussels can become too large to be eaten by starfish; newly initiated starfish removals predict mussel attainment of a size refuge. Such escapes inhibit recovery towards prior community composition and enhance development of alternative community states which may persist long after the originating forcing has lessened or disappeared.