Impact of the northern Pacific seastar Asterias amurensis on soft sediment assemblages, including commercial species, in southeast Tasmania

Introduced species are having major impacts in terrestrial, freshwater and marine ecosystems worldwide. In Australia, the introduced northern Pacific seastar (Asterias amurensis) was first recorded in southeast Tasmania in 1986, where it has become the dominant invertebrate predator in the Derwent River Estuary. Despite indirect indications based on seastar foraging behavior, stomach contents, and estimates of feeding electivity that suggest the potential for considerable impact on native benthic marine assemblages, the impact of the seastar has not previously been examined directly or quantitatively in either its native or introduced range. Because of the absence of baseline data prior to the arrival of the seastar and the presence of other anthropogenic stressors in the estuary, estimating the impact of the seastar is difficult. To overcome these difficulties and the limitations of any one method of impact assessment, I used multiple methods at different scales to provide independent tests of impact: (a) experiments in which seastar density is manipulated at several sites immediately beyond the current range of the seastar; (b) experiments in which seastar density is manipulated following recruitment of prey; ( c) experiments in which the density of both seastars and another introduced benthic predator (Carcinus maenas) are manipulated to examine their interaction; (d) comparative analysis of prey taxa in the sediments and in seastar stomachs; and ( e) spatially hierarchical surveys to examine the relationship between soft sediment assemblages and seastar abundance at several sites in southeast Tasmania. The combination of these methods provide, for the first time, a robust estimate of the impact of the seastar. In the Derwent River Estuary where the seastar occurs at high densities, live adult bivalves are rare despite the presence in sediments of numerous recent remains (intact shells) of adults. Experiments conducted immediately beyond the current range clearly demonstrated a large impact of Asterias amurensis on adult bivalve populations and on the commercial cockles Fu/via tenuicostata and Katelysia rhytiphora in particular. Manipulative experiments also demonstrated that Asterias amurensis has a large impact on the survivorship of bivalve recruits in the estuary, effectively arresting significant recruitment events. Observations of diet and prey switching show that while the seastar has clear food preferences, it is a generalist predator able to switch to other prey when preferred prey become relatively rare. This finding and results of experiments conducted at several sites demonstrated that the exact nature of seastar effects is site and time specific given the inherent natural variability in soft sediment assemblages and the seastar's responses to them. In the event of spatial overlap with the introduced predatory European green crab (Care in us maenas), experiments suggest that both predators may coexist because of resource partitioning on the basis of prey size and/or habitat requirements, and that the impact on bivalves may be greater in the presence of both species. I use the results from experimental manipulations, feeding observations and large scale surveys to provide a broad synthesis of the immediate and predicted impacts on native assemblages and commercial species. There is strong evidence that predation by the seastar is responsible for the decline and subsequent rarity of bivalve species that live just below or on the sediment surface in the Derwent River Estuary. Recent modelling of dispersal of seastar larvae indicates that the large majority of larvae produced in the estuary are likely to be advected from it (Morris & Johnson in prep). It seems clear that should seastar densities in other areas on the Tasmanian coast attain the levels that occur in the Derwent River Estuary, there are likely to be large direct effects on native assemblages, particularly on populations of large bivalves (including commercial species) that live just under or on the sediment surface. Given the seastar's ability to exploit a range of other food resources and the importance of bivalves as a functional component of native systems, I also predict broader direct and indirect effects on native assemblages. Overall, these important consequences of the establishment and potential spread of this introduced predator warrant management efforts to control its spread and impact.