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

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Abstract

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.

Introduction

Habitat structure has long been considered an important determinant of the number, identity and abundances of species present in biological communities (Abele, 1974, Heck, 1979, Jones, 1991MacArthur, MacArthur, & Preer, 1962Menge & Sutherland, 1976, Williams, 1943). One particular feature of habitat structure is the material from which the habitat is made, i.e. the type of substratum. 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.

In estuaries around Sydney, rocky reefs provide the predominant natural habitat for marine plants and animals that attach to subtidal hard substrata. These natural habitats, however, continue to be destroyed and replaced with man-made structures, particularly vertical surfaces (e.g. sandstone walls and pilings), which are made from different substrata (e.g. concrete, fibreglass, wood). Notwithstanding the wide use of artificial surfaces to investigate fouling assemblages (see review in Cairns, 1982), few studies have specifically compared artificial and natural surfaces for the purposes of understanding their relative effects on species diversity and abundance (but see McGuinness, 1989).

Many studies have investigated the population dynamics of epibiota in the marine environment. Traditionally, a variety of types of artificial surfaces have been used as surfaces for settlement because they are manipulable and easy to use. The results of these studies suggest that different substrata can be expected to affect settlement (Crisp & Ryland, 1960(Cuomo, 1985)Russ, 1977, Walters & Wethey, 1996) and subsequent development of intertidal and subtidal assemblages (Anderson & Underwood, 1997, Butler, 1991, Keough, 1984, McGuinness, 1989, Walters & Wethey, 1996). Despite evidence of the potential importance of substratum type to sessile assemblages, few studies have specifically addressed the effect of urban structures on subtidal epibiota (Glasby, in press). In this study, we tested the prediction that assemblages of epibiota associated with urban structures of particular material differ and that these are distinct from assemblages on the natural reef.

A noteworthy feature that characterises an assemblage is the number of constituent organisms, i.e. species diversity. Species diversity has become a major issue in conservation biology (Chapin et al., 1998, Tilman et al., 1997), particularly in conjunction with the modification of habitat (Dunning, Danielson, & Pulliam, 1992). It has been suggested that the creation of new habitats through urbanisation not only increases habitat diversity, but this in turn causes an increase in species diversity (Rebele, 1994). There has, however, been no direct test of this in the marine environment. The idea that habitat diversity promotes species diversity was recognised as early as the 1940s (Williams, 1943). Subsequently, it has been demonstrated that increases in the diversity of habitats can increase the number of species in an assemblage (Bohninggaese, 1997, Douglas & Lake, 1994). Yet in some instances, an increase in habitat diversity has actually decreased the number of species in an assemblage and/or the abundance of individuals (Heck, 1979, McGuinness & Underwood, 1986). Hence, we also tested whether species diversity (number of taxa) of subtidal epibiota growing on vertical surfaces was enhanced by the presence of artificial surfaces.

This study was designed to compare the number of taxa, types of organisms and abundance of individuals of subtidal epibiota among rocky reefs and common artificial surfaces in Sydney Harbour, Australia. At least six types of artificial surfaces are common in shallow subtidal areas of Sydney harbour: fibreglass pontoons, concrete pontoons, wooden pilings with bark, wooden pilings stripped of bark, concrete pilings and artificial sandstone retaining walls. The major surfaces of most of these structures are orientated vertically, hence we compared the vertical surfaces of these six structures to the vertical surfaces of the predominant natural hard surface, sandstone rocky reef.

Section snippets

Sites and sampling methods

Assemblages of subtidal epibiota were studied in December 1997 (summer) at two sites in Middle Harbour, the northern part of Sydney Harbour, Australia (34°01′ S, 151°11′ E, Fig. 1). Epibiota were defined as assemblages of marine algae and sessile invertebrates growing on hard substrata. The vertical surfaces of seven types of substrata were sampled: natural rocky reef (sandstone), artificial sandstone retaining walls (bricks of sandstone), wooden pilings with bark surfaces, wooden pilings

Comparisons of assemblage structure among surfaces

Natural rocky reefs and in particular sandstone retaining walls (both sandstone) supported the most distinct assemblages (Fig. 2 and Fig. 3). Correspondingly, measures of dissimilarity indicated that assemblages on sandstones surfaces were most different from the other surfaces (Table 1), and pairwise tests indicated that the two sandstone structures (rocky reefs and sandstone walls) did not differ from each other (ANOSIM: p>0.002), but differed from the other surfaces (ANOSIM: p<0.002).

Of the

Discussion

The results of this work lead to two main conclusions about assemblages of epibiota in shallow depths of estuaries. First, the structure of assemblages on natural surfaces may differ greatly from artificial surfaces at similar depths, and any differences among artificial surfaces are likely to be smaller. Second, 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

Acknowledgements

The Centre for Research on Ecological Impacts of Coastal Cities and this work was supported by an ARC Special Research Centre Grant. We thank G. Chapman, M. Lindegarth and A. Underwood for their advice and J. Cunningham and G. Housefield for assistance with fieldwork and persisting with remarkably average diving.

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