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 waters of southern Australia is considered a major threat to benthic marine assemblages and commercial bivalve species. We compare the impact of the seastar in experiments on three soft-sediment assemblages. The experiments were carried out immediately beyond the current range of the seastar in southeast Tasmania. This allows us to assess the repeatability, and hence predictability, of the type and magnitude of the impact of A. amurensis on soft-sediment assemblages. Responses to manipulations at the species level were dissimilar across the three experiments, reflecting marked initial 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 the different assemblages. When potential prey taxa were separated into functional groups reflecting ecological availability (e.g. surface dwelling vs. deep burrowing bivalves), seastar effects were largely restricted to the surface bivalves. While the effect of seastar predation on surface bivalves was common in all three assemblages, the magnitude of the impact varied both among and within assemblages. Variability in the impact of A. amurensis at both levels appeared to be largely due to differences in relative availability of prey species. Overall, the results of these experiments indicate that 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 in soft-sediment assemblages and the seastar's responses to them.