Invasive species have been identified as one of the major threats to biodiversity and ecosystem functioning, but the nature and magnitude of their effects depends on the environmental context and on the abundance of the invader. The Pacific oyster, Crassostrea gigas, is a globally invasive ecosystem engineer which can monopolise shores and alter native biodiversity. Less is known, however, about its effects on ecosystem functioning or whether its effects differ in different habitats or at different abundances. This research used an interdisciplinary approach to characterise the impact of invasive oysters on biodiversity and ecosystem functioning and to determine how these impacts would vary in different habitats and at increasing abundances.

First, the effects of increasing cover of C. gigas on biodiversity in intertidal boulder-fields was assessed. Furthermore, the effects of the physical structure and biological activity were separated using dead and living oysters. C. gigas increased diversity on boulders, but effects were non-linear with regards to the cover of C. gigas. When present at low levels of cover, C. gigas increased biodiversity on boulders, but at higher levels there was no further increase in biodiversity and boulders became heavily dominated by macroalgae, Fucus Vesiculosus, and a key grazer, Littorina Littorea, which possibly indirectly affected the establishment of other species. Either directly or indirectly, the establishment of a protected biogenic habitat built by the honeycomb worm, Sabellaria alveolata, on the undersides of boulders was reduced with increasing cover of C. gigas on their upper surfaces. The effects of C. gigas on the establishment of other species were found to be mostly attributable to the physical structure rather than their biological activities.

Additionally, plots with increasing cover of C. gigas were set-up in mussel-beds and mud-flats within two estuaries and were sampled after 4 and 15 months. The effects on biodiversity were mostly context-dependent: biodiversity increased with increasing cover of C. gigas in mud-flats, but was unaffected or reached a threshold and decreased with the highest level of cover in mussel-beds, depending on the estuary. Some species, such as L. littorea and an invasive barnacle, Elminius modestus, were facilitated by C. gigas regardless of location or habitat.

Ecosystem functioning in mussel-beds and mud-flats in one of the estuaries was also affected by C. gigas. Several biogeochemical properties and processes were altered, but responses were non-linear with regards to cover and some differed between habitats. Sediment-water fluxes and benthic turnover rates of NH4+ were greatest at medium cover of C. gigas in both habitats, but for Si(OH)4 they increased with increasing cover of oysters in mud-flats but decreased at the greatest cover of oysters in mussel-beds. Community respiration was only affected at the highest cover of C. gigas.

The increase in community respiration was further investigated in this study where the effects of increasing cover of C. gigas in mud-flats on ecosystem processes and associated microbial assemblages were tested. The increase in community respiration was at least party attributable to an increase in microbial activity with high covers of C. gigas. Ecosystem processes and microbial assemblage structure responded non-linearly with regards to the cover of C. gigas. The assemblage composition of methanogens and ammonia-oxidising microbes in anoxic sediments were only altered by low covers of C. gigas while ammonia-oxidisers in oxic sediments were only altered by high covers of C. gigas. At any level of cover, C. gigas increased gaseous carbon emission from sediments. NH4+ flux reached a threshold at medium cover of C. gigas as it did in Chapter IV and indirect mediation from algae facilitated by high covers of C. gigas is suggested as a mechanism.

This research has shown that C. gigas can significantly alter biodiversity and several ecosystem processes. The nature and magnitude of many of these effects differed depending on the type of habitat, the location and on the level of cover of C. gigas. At a larger scale, high covers of C. gigas may impact the conservation status and alter the capacity of estuaries to provide ecosystem services, such as commercial shellfish production.

The data uploaded from this project has contributed to the following papers published in peer reviewed international journals:

Green DS, Boots B, & Crowe TP (2012) Effects of Non-Indigenous Oysters on Microbial Diversity and Ecosystem Functioning. PLoS ONE 7(10): e48410. doi:10.1371/journal.pone.0048410

Green, D.S. & Crowe, T.P. (2013). Physical and biological effects of introduced oysters on biodiversity in an intertidal boulder-field. Marine Ecology Progress Series, 482, 119-132.

Green DS, Rocha C, Crowe TP (2013). Effects of Non-indigenous Oysters on Ecosystem Processes Vary with Abundance and Context. Ecosystems, 16: 881-893.

Green, D.S. and Crowe, T.P. Context- and density-dependent effects of introduced oysters on biodiversity. Accepted in Biological Invasions.

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SAFER-Data Display URL	https://eparesearch.epa.ie/safer/iso19115/display?isoID=3031
Resource Keywords	ecosystem functioning, invasive species, biodiversity, Crassostrea gigas, protected habitats
EPA/ERTDI/STRIVE Project Code	2007-B-CD-1-S1
EPA/ERTDI/STRIVE Project Theme	Biodiversity
Resource Availability:	Semi-Private
Limitations on the use of this Resource	Time restrictions based on publishing peer reviewed articles from this research are requested. Please contact Dr Dannielle Green for more details danniellesgreen@gmail.com
Number of Attached Files (Publicly and Openly Available for Download):	0
Project Start Date	Tuesday 1st April 2008 (01-04-2008)
Earliest Recorded Date within any attached datasets or digital objects	Monday 1st September 2008 (01-09-2008)
Most Recent Recorded Date within any attached datasets or digital objects	Sunday 30th June 2013 (30-06-2013)
Published on SAFER	Thursday 24th October 2013 (24-10-2013)
Date of Last Edit	Wednesday 4th December 2013 at 17:15:51 (04-12-2013)
Datasets or Files Updated On	Thursday 24th October 2013 at 00:25:25 (24-10-2013)

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Description of Geographical Characteristics of This Project or Dataset
This study was done at two locations in Ireland, Lough Foyle (Quigley?s Point, County Donegal: 55° 7' 14.87", -7° 11' 53.59) and Lough Swilly (Ballylin Point, County Donegal, Ireland: 55° 2' 36.12", -7° 33' 36.09").

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