Do urban structures influence local abundance and diversity of subtidal epibiota? A case study from Sydney Harbour, Australia

In an age when human modification of natural substrata is increasingly cited as an agent of population decline and extinction, understanding the role of artificial surfaces as surrogate habitats for natural surfaces is critical. It has been predicted that the addition of new habitats to an area can lead to increases in species abundance and diversity. We tested this hypothesis by contrasting assemblages of subtidal epibiota on natural reef and six common urban surfaces in Sydney Harbour, Australia. All surfaces were in shallow water and consisted of rocky reef, sandstone (brick) retaining walls, fibreglass and concrete pontoons, concrete pilings and wooden pilings with bark and stripped of bark. Assemblages of epibiota on sandstone surfaces (natural rocky reefs and sandstone retaining walls) differed from non-sandstone surfaces. The major distinguishing features of sandstone surfaces were the large cover of coralline algae and small number of taxa. Assemblages on pilings and pontoons were most different from those on sandstone surfaces and relatively similar to each other. There were, however, some differences which seemed to be consistent with features such as type of surface (concrete vs wood) and arrangement of surface (floating pontoons vs fixed pilings). We suggest that artificial structures may increase the abundance and diversity of subtidal epibiota in the shallow areas of an estuary, but are not surrogate surfaces for epibiotic assemblages that occur on nearby natural rock. It would appear that urbanisation of estuarine habitats has consequences for the identity, diversity and abundance of subtidal epibiota.