Variability in the impact of an introduced predator (Asterias amurensis: Asteroidea) on soft-sediment assemblages

The introduction and establishment of the predatory asteroid Asterias amurensis in coastal watersof southern Australia is considered a major threat to benthic marine assemblages and commercialbivalve species. We compare the impact of the seastar in experiments on three soft-sedimentassemblages. The experiments were carried out immediately beyond the current range of the seastarin southeast Tasmania. This allows us to assess the repeatability, and hence predictability, of the typeand magnitude of the impact of A. amurensis on soft-sediment assemblages. Responses tomanipulations at the species level were dissimilar across the three experiments, reflecting markedinitial differences in the three assemblages at both the species and functional group levels. However,at the functional group level, there were notable similarities in the impact of the seastar on thedifferent assemblages. When potential prey taxa were separated into functional groups reflectingecological availability (e.g. surface dwelling vs. deep burrowing bivalves), seastar effects werelargely restricted to the surface bivalves. While the effect of seastar predation on surface bivalveswas common in all three assemblages, the magnitude of the impact varied both among and withinassemblages. Variability in the impact of A. amurensis at both levels appeared to be largely due todifferences in relative availability of prey species. Overall, the results of these experiments indicatethat while the impact of the seastar is broadly predictable at the functional group level, the exact nature of seastar effects is likely to be site- and time-specific given the inherent natural variability insoft-sediment assemblages and the seastar's responses to them.