The non-indigenous isopod Cirolana harfordi in Australia: Does a non-native in great abundance impact local assemblages?

Background/Question/Methods

It is generally reported that biological invasions usually have adverse consequences for native communities and, in some cases, may have beneficial effects. Also, highly abundant species are usually expected to have strong biological interactions with sympatric organisms. Cirolana harfordi is a non-indigenous isopod in Australia and also a species found in great densities in oyster-beds in Sydney Harbour (up to 900 individuals/m²). Little is known, however, about its potential impact on local communities. In addition, introductions of marine isopods are poorly studied worldwide. The aims of this study were to evaluate the impact of C. harfordi on the assemblage of invertebrates in oyster-beds. Natural oyster-beds in Sydney Harbour were sampled and correlations between the densities of C. harfordi and its sympatric organisms were determined. Also, artificial oyster-beds created using live oysters were deployed in the field to be colonised by invertebrate assemblages. These were taken to the laboratory and placed in aquaria. C. harfordi was introduced to half of them and, after 7 days, diversity was assessed.

Results/Conclusions

In the observational experiment, we found no relationship between the density of C. harfordi and that of other groups of organisms. In the manipulative experiment, there were no significance differences in the assemblages between the treatments, either considering abundances or presence/absence of the organisms. These results indicate that C. harfordi does not appear to have any impact on the assemblage of invertebrates living in oyster-beds. These results contradict the general model that invasive species exert negative effects on the assemblages in invaded areas and that highly abundant species have strong biological interactions.