Two closely located islands on the west coast of South Africa support widely different benthic communities. The biota at Malgas Island is dominated by seaweeds and by rock lobsters that consume settling mussels, thereby preventing the establishment of the mussels. They also prey on whelks, although one speces, Burnupena papyracea, is protected from predation by a commensal bryozoan that covers its shell. Marcus Island has extensive mussel beds, but rock lobsters and seaweeds are virtually absent; whelks (mostly Burnupena spp.) occur at high densities. Rock lobsters transferred to Marcus Island were overwhelmed and consumed by the whelks, reversing the normal predatorprey relation between the two species. These two contrasting communities persisted during 4 years and may represent multiple states of the same ecosystem. This effective change of roles between a prey species and its chief predator may provide an intrinsic mechanism to maintain these states following the initial exclusion of the predator.

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