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


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Ross, Donald Jeffrey 2001 , 'Impact of the northern Pacific seastar Asterias amurensis on soft sediment assemblages, including commercial species, in southeast Tasmania', PhD thesis, University of Tasmania.

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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.

Item Type: Thesis - PhD
Authors/Creators:Ross, Donald Jeffrey
Keywords: Asterias amurensis, Exotic marine organisms
Copyright Holders: The Author
Copyright Information:

Copyright 2001 the Author - The University is continuing to endeavour to trace the copyright
owner(s) and in the meantime this item has been reproduced here in good faith. We
would be pleased to hear from the copyright owner(s).

Additional Information:

Thesis (Ph.D.)--University of Tasmania, 2001. Includes bibliographical references

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