Impact of the introduced New Zealand screwshell Maoricolpus Roseus on soft-sediment assemblages in southeast Tasmania

Non-indigenous species (NIS) continue to have major impacts on the integrity, diversity and health of native terrestrial, freshwater and marine ecosystems globally. The New Zealand Screwshell (Maoricolpus roseus) is a turritellid gastropod inadvertently introduced to Tasmania during the 1920s. It has since become one of Australia's most widespread marine invasive species, occupying vast areas of benthic habitat along Australia's SE coast. In Australia, M roseus occupies a range of habitats from muddy silts to rocky reef although it is most common on shelly substrates and course sands over a depth range of 0 - 200 m, and regularly attains densities of `600 - 2500 m^-2`. Given that M roseus has occupied vast areas of sea floor at high densities for extensive periods of time, the potential exists for this species to invoke large ecological changes to its recipient communities. Despite this, the impact of the screwshells is yet to be either qualitatively or quantitatively examined. Due to the absence of baseline data on the abundance and distribution of native species prior to the arrival M roseus ea. 90 years ago, assessing impacts of the species using a before-after comparison was not possible. In this study I employed a combination of qualitative surveys and in situ manipulative experiments at a variety of spatial, temporal and conceptual scales in order to develop a robust quantitative assessment of the impact of M roseus. This approach involved employing several independent assessments of impact: (a) experiments in which impacts of M roseus to community structure are assessed and separated; (b) experiments in which impacts of M roseus to benthic community function are quantified using in situ metabolic chambers; ( c) quantitative surveys which examine the relationship between screwshells and commercially important scallop species; and ( d) experiments in which the impacts of M roseus on growth and development of commercially important scallops are assessed. Importantly, the experimental and survey designs used were developed to address two unique facets. Firstly, the design of the experiments and surveys allowed identification of the spatial and temporal variability of M roseus' impacts. Secondly, the designs allowed for quantification of the separate impacts of M roseus on the structure and function of native soft-sediment communities. In the D'Entrecasteaux Channel (SE Tasmania) where M roseus is known to be patchily distributed at a range of densities from 0 - 2000 m-2, in situ experiments clearly demonstrated a marked impact on the structure and metabolism of the benthic community due to the presence of the screwshells. Typically, communities associated with high densities of screwshells exhibited elevated species richness and abundances of total macroinvertebrates when compared to patches of benthos with low densities of screwshells, and areas devoid of the species. Further, such communities exhibited significantly higher metabolic rates (both respiration and production), although the specific metabolic rates fundamentally depended upon whether screwshells were alive, dead and empty, or supporting hermit crabs. Observations from surveys combined with in situ experiments also highlighted impacts of M roseus on .commercial species. The distribution of three native scallop species in the D 'Entrecasteaux Channel depended upon the distribution and density of M roseus, in addition to the coarseness of the sediments. Further, Pecten fumatus, the most common of the scallop species, actively avoided areas of benthos supporting screwshells at densities `> 200 m^-2`. Caging experiments also revealed that the condition, growth and weight of P. fumatus were significantly impacted by the presence of M roseus. I collate and interpret the results from in situ experimental manipulations and large and small scale surveys to provide a robust estimate of the immediate and potential impacts of M roseus on native soft-sediment ecosystems and commercially important species. The results have highlighted the importance of using multiple methodologies when assessing impacts of NIS, as undoubtedly the most robust information is attained through the use of multiple independent assessments of impact. There is clear evidence that M roseus has drastically altered the structural complexity and integrity of recipient soft-sediment benthic habitats, and the structure and function of the community it supports. Given the magnitude of impact of M roseus on native communities and commercially important species, there is a clear need for a management strategy to be developed for this species. Not only is M roseus a potential threat to the integrity and natural function of native softsediment communities, but it is also has the potential to put important recreational and commercial scallop fisheries in Tasmania at risk.