Summary

Habitat destruction and introductions of exotic species are common events in estuaries and together are believed to be two of the greatest threats to native biodiversity. Estuarine habitats are under increasing threat from invasion, due in large part to large numbers of ships travelling among ports and potentially transporting many types of exotic species, either in ballast water or on their hulls. Simultaneously, estuarine habitats are becoming more urbanized, in many instances creating hard surfaces in regions where none previously existed. These surfaces are typically associated with shipping and aquaculture – activities which are often linked with the transfer of exotic marine species (primarily sessile invertebrates and algae). In this paper, we argue that habitat creation (in the form of artificial structures) may also be a considerable threat to native biodiversity.

Although the occurrence of exotic species on artificial structures such as pilings and pontoons has been recognised in the past, patterns of colonisation of exotic versus native species have not been quantified, nor have there been explicit tests of why particular patterns exist. We addressed these issues by sampling the growth on various artificial structures and rocky reefs and by using panels constructed of different materials in a series of experiments done in Port Jackson, Sydney. The panels were submerged for up to 8 months at different sites in Port Jackson, after which exotic or native species were identified and counted. The most common exotics sampled were species of sea squirts (ascidians), lace corals (bryozoans), marine worms (polychaetes) or seaweeds, whilst many of the natives were seaweeds, barnacles or oysters.

We found that well-established communities of marine species (of various ages) growing on artificial structures had a greater number of exotic species than did communities on rocky reefs, even though the structures were not in close proximity to commercial shipping. Interestingly, however, when we examined the development of new communities (on clean panels), different patterns emerged. That is, a large number of exotic species recruited to reefs, although similarly to our initial study, there were relatively few native species on panels attached to pontoons. When comparing surfaces made of different materials (sandstone, concrete or wood), numbers of exotics were similar on each type of panel, but wooden panels had the fewest native species. Finally, in an experiment designed to determine the effect that surface movement had on species, the number of exotic species was found to increase with increasing movement of panels.

These experiments are the first to demonstrate that artificial structures tend to support a greater proportion of exotic species than do natural surfaces. Furthermore, they have provided evidence that this pattern is due, at least in part, to the materials that the structures are made of, their position in the water column and the extent to which they move. Unlike many native species, exotics colonised moving or fixed surfaces composed of various materials. Thus, exotic species tend to be more versatile than native species and so colonise most surfaces whereas natives tend to be associated primarily with rocky reefs. Consequently, by adding artificial structures to waterways, it is likely that we are facilitating the spread of exotic species, which may use these structures, much like stepping stones, to gain access to areas that were previously uninvaded.

Results of this research will assist greatly with NSW DPI’s plan to monitor waterways for marine pests and exotic species. Early warning sampling devices will be devised to float near the surface of the water and rotate so as to maximise the possibility of detecting exotic species in our estuaries. Extension of this work may help provide information about ways of minimising the colonisation of artificial structures by exotic species.