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Abstract

Introduced species are having major impacts in terrestrial, freshwater, and marine ecosystems worldwide. Given
that resources for management are limited and that only a small percentage of invaders are likely to cause large
ecological change, management priorities should be based on the severity of immediate and anticipated impacts on
native assemblages and commercial species. This paper synthesizes work on the current and predicted impacts of an
introduced predatory seastar (Asterias amurensis) on soft sediment assemblages, including native species subject to
commercial fishing, in the Derwent Estuary and other areas of southeast Tasmania. Due to 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 help overcome the weaknesses of any single method, our assessment of
impact rests on 'weight of evidence' from multiple approaches. Results from experimental manipulations at small
scales, detailed observations of feeding, and field surveys over a range of spatial scales in areas with and without
the seastar provide strong evidence that predation by the seastar is likely to be responsible for the decline and
subsequent rarity of bivalve species that live just below or on the sediment surface in the Derwent Estuary. The data
suggest that should seastar densities in other areas on the Tasmanian coast attain the current levels in the Derwent
Estuary, there are likely to be large direct effects on native assemblages, particularly on populations of large surface
dwelling bivalves, including several commercial species. Given the seastar's ability to exploit a broad range of food
resources other than bivalves, and the functional importance of bivalves in native systems, we predict broader direct
and indirect effects on native assemblages. We would be unable to reach these same conclusions from a single
approach to assessing impacts. The overall picture from the combination of methods at different scales provides
more information than the sum of the results of the separate lines of investigation.

Item Type:	Article
Authors/Creators:	Ross, DJ and Johnson, CR and Hewitt, CL
Keywords:	Asterias amurensis, ecological variability, impacts, introduced species, multiple methods, predation, seastar, soft sediments, Tasmania
Journal or Publication Title:	Biological Invasions
DOI / ID Number:	https://doi.org/10.1023/A:1024019428616
Additional Information:	The original publication is available at www.springerlink.com
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