Community-level impacts of the invasive isopod Cirolana harfordi

The invasive isopod Cirolana harfordi occurs in great densities in oyster-beds in Sydney Harbour and is the second most abundant taxon in terms of biomass. A previous observational study did not find any relationships between densities of C. harfordi and variability in local assemblage structure; this result could, however, be confounded by the enormous variation in the composition of the assemblage between locations. Using manipulative experiments in mesocosms to standardise for such local contingencies, this study evaluated the impacts of this invasive species on assemblages in experimental oyster-beds. We deployed experimental oyster-beds among natural oyster-beds in locations within Hawkesbury River, Australia, where C. harfordi was absent, to be colonised by the assemblage of organisms for 2 months. C. harfordi were then introduced to these experimental oyster-beds in mesocosms. Assemblages in invaded experimental oyster-beds were compared to those in non-invaded experimental oyster-beds in mesocosms and in natural oyster-beds in Sydney Harbour. The results showed no change in the structure of assemblages where C. harfordi were introduced, and those assemblages were similar to those in natural oyster-beds, thus supporting the initial patterns observed in the field. The evidence did not support the general assumption that non-indigenous species with high biomass and abundance cause impacts on local assemblages. This quantitative assessment of the potential impact of C. harfordi provides valuable information for managers to decide on control strategies.