May 2004
Alternative states in the oceanEcologists expect natural communities to vary. Occasionally, however, such variation can be severely abrupt and lead to formation of alternative and potentially persistent states.
In the July issue of Ecology Letters, Paine and Trimble describe a dramatic assemblage shift on a rocky intertidal shore in Washington State (USA). The change was initiated during the extreme 1997/98 El Ni�o and has involved replacement of the original algal dominated assemblage by a mussel bed.
Starfish are capable of controlling the distribution and abundance of these mussels. The authors argue for two possible outcomes: gradual recovery of the algal assemblage as the mussels are consumed, or formation of a persistent alternative state if the mussels grow to a sufficient size to escape further predation.
The study emphasizes the ecological importance and generality of size escapes, a mechanism also characterizing potentially permanent transformations of terrestrial grasslands to forests.
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