Do Urban Structures Influence Local Abundance and Diversity of Subtidal Epibiota a Case Study From Sydney Harbour Australia

Do urban structures in¯uence local abundanceand diversity of subtidal epibiota? A case study

from Sydney Harbour, Australia

S.D. Connell*, T.M. GlasbyCentre for Research on Ecological Impacts of Coastal Cities, Marine Ecology Laboratories A11,

University of Sydney, Sydney, NSW 2006, Australia

Received 6 April 1998; received in revised form 14 September 1998; accepted 12 October 1998

Abstract

In an age when human modi®cation of natural substrata is increasingly cited as an agent ofpopulation decline and extinction, understanding the role of arti®cial surfaces as surrogate

habitats for natural surfaces is critical. It has been predicted that the addition of new habitatsto an area can lead to increases in species abundance and diversity. We tested this hypothesisby contrasting assemblages of subtidal epibiota on natural reef and six common urban sur-

faces in Sydney Harbour, Australia. All surfaces were in shallow water and consisted of rockyreef, sandstone (brick) retaining walls, ®breglass and concrete pontoons, concrete pilingsand wooden pilings with bark and stripped of bark. Assemblages of epibiota on sandstonesurfaces (natural rocky reefs and sandstone retaining walls) di�ered from non-sandstone sur-

faces. The major distinguishing features of sandstone surfaces were the large cover of corallinealgae and small number of taxa. Assemblages on pilings and pontoons were most di�erentfrom those on sandstone surfaces and relatively similar to each other. There were, however,

some di�erences which seemed to be consistent with features such as type of surface (concretevs wood) and arrangement of surface (¯oating pontoons vs ®xed pilings). We suggest thatarti®cial structures may increase the abundance and diversity of subtidal epibiota in the shal-

low areas of an estuary, but are not surrogate surfaces for epibiotic assemblages that occur onnearby natural rock. It would appear that urbanisation of estuarine habitats has consequencesfor the identity, diversity and abundance of subtidal epibiota. # 1999 Elsevier Science Ltd.All rights reserved.

Keywords: Fouling organisms; Sessile; Arti®cial habitats; Biodiversity; Disturbance

Marine Environmental Research 47 (1999) 373±387

0141-1136/99/$Ðsee front matter # 1999 Elsevier Science Ltd. All rights reserved.

PII: S0141-1136(98)00126-3

* Corresponding author. Tel.: +61-2-9351-4933; fax: +61-2-9351-6713; e-mail: sconnell@zoology.

adelaide.edu.au

1. Introduction

Habitat structure has long been considered an important determinant of thenumber, identity and abundances of species present in biological communities(Abele, 1974; Heck, 1979; Jones, 1991; MacArthur, MacArthur, & Preer, 1962;Menge & Sutherland, 1976; Williams, 1943). One particular feature of habitatstructure is the material from which the habitat is made, i.e. the type of substratum.In an age when human modi®cation of natural substrata is increasingly cited as anagent of population decline and extinction, understanding the role of arti®cial sur-faces 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 naturalhabitats, however, continue to be destroyed and replaced with man-made structures,particularly vertical surfaces (e.g. sandstone walls and pilings), which are made fromdi�erent substrata (e.g. concrete, ®breglass, wood). Notwithstanding the wide use ofarti®cial surfaces to investigate fouling assemblages (see review in Cairns, 1982), fewstudies have speci®cally compared arti®cial and natural surfaces for the purposes ofunderstanding their relative e�ects on species diversity and abundance (but seeMcGuinness, 1989).Many studies have investigated the population dynamics of epibiota in the marine

environment. Traditionally, a variety of types of arti®cial surfaces have been used assurfaces for settlement because they are manipulable and easy to use. The results ofthese studies suggest that di�erent substrata can be expected to a�ect 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 tosessile assemblages, few studies have speci®cally addressed the e�ect of urbanstructures on subtidal epibiota (Glasby, in press). In this study, we tested the pre-diction that assemblages of epibiota associated with urban structures of particularmaterial di�er and that these are distinct from assemblages on the natural reef.A noteworthy feature that characterises an assemblage is the number of con-

stituent organisms, i.e. species diversity. Species diversity has become a major issuein conservation biology (Chapin et al., 1998; Tilman et al., 1997), particularly inconjunction with the modi®cation of habitat (Dunning, Danielson, & Pulliam,1992). It has been suggested that the creation of new habitats through urbanisationnot only increases habitat diversity, but this in turn causes an increase in speciesdiversity (Rebele, 1994). There has, however, been no direct test of this in the marineenvironment. The idea that habitat diversity promotes species diversity was recog-nised as early as the 1940s (Williams, 1943). Subsequently, it has been demonstratedthat increases in the diversity of habitats can increase the number of species in anassemblage (Bohninggaese, 1997; Douglas & Lake, 1994). Yet in some instances,an increase in habitat diversity has actually decreased the number of species in anassemblage and/or the abundance of individuals (Heck, 1979; McGuinness &Underwood, 1986). Hence, we also tested whether species diversity (number of taxa)

374 S.D. Connell, T.M. Glasby/Marine Environmental Research 47 (1999) 373±387

of subtidal epibiota growing on vertical surfaces was enhanced by the presence ofarti®cial 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 arti-®cial surfaces in Sydney Harbour, Australia. At least six types of arti®cial surfacesare common in shallow subtidal areas of Sydney harbour: ®breglass pontoons, con-crete pontoons, wooden pilings with bark, wooden pilings stripped of bark, concretepilings and arti®cial sandstone retaining walls. The major surfaces of most of thesestructures are orientated vertically, hence we compared the vertical surfaces ofthese six structures to the vertical surfaces of the predominant natural hard surface,sandstone rocky reef.

2. Methods

2.1. Sites and sampling methods

Assemblages of subtidal epibiota were studied in December 1997 (summer) at twosites in Middle Harbour, the northern part of Sydney Harbour, Australia (34�010 S,151�110 E, Fig. 1). Epibiota were de®ned as assemblages of marine algae and sessileinvertebrates growing on hard substrata. The vertical surfaces of seven types ofsubstrata were sampled: natural rocky reef (sandstone), arti®cial sandstone retainingwalls (bricks of sandstone), wooden pilings with bark surfaces, wooden pilingsstripped of bark, concrete pilings, concrete pontoons and ®breglass pontoons. Pon-toons (�2.5�3.5�0.4 m deep) were moored to concrete pilings which together act asplatforms and anchorage for boating activities.Sampling involved photographing surfaces (15�23 cm) at a depth of 0.2 m below

Mean Low Water Springs. Five replicate photographs were taken of each type ofsurface; these represented di�erent and haphazardly chosen structures. This wasrepeated at each of two sites (Fig. 1). Percentage covers of taxa were estimated fromprojected photographs in which the identities of taxa were recorded under 40 evenlyspaced points across a 5�20 cm transect placed haphazardly on the image. Any taxain quadrats not sampled under the points were noted as occurring in the sample. Apilot study indicated that the size and replication of transects, and number of pointsused to estimate percentage covers of taxa generally gave relatively precise estimatesof abundance (unpublished data).

2.2. Analytical methods

A visual assessment of the dissimilarities in the structure of assemblages amongsurfaces was provided by a dendrogram; dissimilarities among surfaces and betweensites were provided by a non-metric multi-dimensional scaling ordination (nMDS).The centroids used in the nMDS plot used the average of all replicates per surfacefor each site (n=5). The Bray-Curtis similarity measure was used to calculate similar-ities among replicate observations (Bray & Curtis, 1957) which were fourth-root

S.D. Connell, T.M. Glasby/Marine Environmental Research 47 (1999) 373±387 375

transformed. Two one-way analyses of similarities (ANOSIM) were done (Clarke,1993) on all replicates to test for di�erences among surfaces and between sites. Thesigni®cance level was adjusted for multiple comparisons associated with pairwisetests (Bonferroni procedure; Rice, 1989); a=0.002 for comparisons of surfaces and0.007 for `Site'.Similarity percentages (SIMPER) were used to determine which taxa primarily

(i.e. accounted for 80% of the similarity) contributed to average similarity within asurface and to provide measures of the relative dissimilarity among surfaces (Clarke,1993). Multivariate analyses were done using a suite of multivariate techniquesincluded in the PRIMER program (Plymouth Routines In Multivariate EcologicalResearch; Clarke, 1993).Taxa that were present in more than 15% of the 70 photographs were analysed

with analysis of variance (ANOVA) according to Winer, Brown, and Michels (1991)and Underwood (1981). Two-factor ANOVA treated `Surface' as ®xed and `Site'as random and orthogonal to `Surface'. Post-hoc pooling was used to provide amore powerful test of `Surface' when the interaction `Surface�Site' was non-signi®cant (p>0.25; Winer, Brown, & Michels, 1991). Prior to analysis, data weretested for homogeneity of variances using Cochran's C-Test (Underwood, 1981).

Fig. 1. Map showing the two study sites within Middle Harbour, Sydney Harbour.


Heterogeneous data were Arc-sine transformed and if this transformation did notremove heteroscedasticity, the raw data were used for the analysis with the moreconservative probability of 0.01.

3. Results

3.1. Comparisons of assemblage structure among surfaces

Natural rocky reefs and in particular sandstone retaining walls (both sandstone)supported the most distinct assemblages (Figs. 2 and 3). Correspondingly, measuresof dissimilarity indicated that assemblages on sandstones surfaces were most di�er-ent from the other surfaces (Table 1), and pairwise tests indicated that the twosandstone structures (rocky reefs and sandstone walls) did not di�er from each other(ANOSIM: p>0.002), but di�ered from the other surfaces (ANOSIM: p<0.002).Of the remaining surfaces, assemblages on ®breglass pontoons were the most

distinctive (Figs. 2 and 3, Table 1). Although assemblages on concrete pontoonswere most similar to ®breglass pontoons (Table 1), pairwise tests revealed that

Fig. 2. Dendrogram showing the relative dissimilarities of assemblage structure among the seven surfaces.


assemblages di�ered signi®cantly between these surfaces (ANOSIM: p<0.002).Other than sandstone substrata, surfaces with the most similar assemblage structurewere concrete pilings and concrete pontoons (Table 1). Assemblages on these con-crete substrata did not di�er from those on wooden pilings (barked and unbarked),

Fig. 3. Multi-dimensional scaling ordination showing the relationship between surfaces and sites.

~=sandstone retaining walls, &=rocky reef, !=wooden striped-pilings, +=concrete pilings,

�=concrete pontoons, ^=wooden barked-pilings, *=®breglass pontoons. The symbols in lighter ink

refer to Site 1, those in heavier ink to Site 2.

Table 1

Bray-Curtis measures of dissimilarities between surfaces (the larger the value the more dissimilar the

assemblage)

Surface Measure of dissimilarity

Fibreglass pontoon

Concrete pontoon 46.7

Concrete piling 58.7 42.3NS

Bark piling 55.9 45.3NS 43.5NS

Stripped piling 56.7 43.5NS 46.7NS 50.8

Natural reef 69.7 65.5 65.3 70.6 66.1

Sandstone wall 79.9 81.4 80.4 84.3 82.1 50.9NS

Surface F. pontoon C. pontoon C. piling B. piling S. piling Natural reef

All pairwise comparisons are signi®cantly di�erent (p<0.002) unless indicated NS.


but assemblages on the two types of wooden pilings di�ered from each other(ANOSIM: p<0.002).In general, spatial variability (between sites) did not appear to have a greater e�ect

than surface-type on assemblage structure (Fig. 3). No signi®cant di�erences weredetected between sites for any surface, except possibly sandstone retaining walls.There was, however, evidence to suggest that the composition of assemblages oneach surface was somewhat dependent on site; points representing Site 1 (Fig. 3) arefurther up the plot than points from the equivalent surfaces at Site 2. For six of theseven surfaces there were no signi®cant di�erences in the composition of assem-blages between the two sites (p>0.02). The Bonferroni correction for multiplecomparisons reduced a from 0.05 to 0.007, but the smallest possible level of prob-ability for pairwise comparisons between sites (n=5) was p=0.008. This signi®cancelevel occurred for the comparison between sites for assemblages on sandstoneretaining walls. Thus, it is possible that the sandstone wall sites di�ered and this wassupported by the large R value (0.656) for that comparison.

3.2. Comparisons of individual taxa among surfaces

Of 31 taxa sampled across all surfaces and sites (Appendix A), 21 are known tooccur on natural reef in Middle Harbour; 14 were detected in this study and 7 othersin Glasby (1997). Hence, arti®cial surfaces supported 10 species that have not beensampled on rocky reefs. Importantly, no one arti®cial surface supported the samesuite of species that was typical of rocky reefs. Univariate analyses indicated thatthere were signi®cantly more taxa on ®breglass pontoons than sandstone retainingwalls and that there were no di�erences in the number of taxa among other surfaces(Fig. 4a, Table 3a, SNK test).In general, the cover of no taxon varied greatly among the arti®cial surfaces

(®breglass, concrete, wood), but some taxa varied greatly between these and sand-stone surfaces (natural reef and sandstone walls). A major distinguishing featureof sandstone surfaces was the domination of space by coralline algae, particularlyon sandstone retaining walls (Fig. 4b, Tables 2 and 3b). These algae almost com-pletely covered sandstone retaining walls and were very abundant on natural rock,but were absent or uncommon on the other ®ve surfaces. This extensive cover ofcoralline algae was associated with reduced abundance of other taxa, particularly®lamentous algae and serpulid polychaetes (Hydroides spp.; Fig. 4c,d, Table 3b,d).Encrusting bryozoans (primarily Watersipora subtorquata) were abundant on

®breglass and concrete pontoons. Fibreglass pontoons supported the greatestabundance of bryozoans at one site (Site 2, Table 3e). The SNK test was unable toidentify di�erences among surfaces for Site 1, but there was a trend for a greatercover of bryozoans on the two types of pontoons and natural reef than any othersurface (Fig. 4e). Most notably, no bryozoans occurred on sandstone retaining wallsat either site (Fig. 4e).Despite the similarities in the coverage of taxa among the three types of piling,

pilings without bark where notable for the percentage cover of unoccupied (bare)space (Fig. 4f, Table 3f). Spirorbid polychaetes, which are comparatively small


organisms, contributed to a great deal of the total cover on pilings and naturalrock, but very little to the cover on sandstone walls and ®breglass pontoons(Fig. 4g, Table 3g). Although signi®cant di�erences among surfaces were detected

Fig. 4. Graphs of taxa showing the mean percentage cover among surfaces and between sites. Shaded

bars represent Site 1 and unshaded Site 2. FPo, ®breglass pontoons; CPo, concrete pontoons; CPi, con-

crete pilings; BPi, bark pilings; SPi, stripped pilings; NR, natural rock; SW, sandstone retaining walls.

Table 2

Results of SIMPER analysis showing the taxa that primarily (80%) contributed to average similarity

within a surface, i.e. `Characteristic taxa'

Surface Characteristic taxa

Natural rock Coralline algae

Sandstone wall Coralline algae

Fibreglass pontoon Green algae, Bryozoan (Waterspoira subtorquata)

Concrete pontoon Hydroides, spirorbids, barnacles

Concrete piling Hydroides, spirorbids

Bark piling Hydroides, bare spacea

Stripped piling Hydroides, red ®lamentous, barnacles (Site 1), sponge sp. (Site 2)

Green/brown ®lamentous algae was a taxon characteristic of all surfaces, except sandstone retaining walls

(not listed).a Bare space was included in SIMPER analyses, but was not counted as total or novel taxa.


for porifera and red ®lamentous ceramalean algae (Fig. 4h,i, Table 3h,i), SNK testscould not detect which means di�ered.

4. Discussion

The results of this work lead to two main conclusions about assemblages of epi-biota in shallow depths of estuaries. First, the structure of assemblages on naturalsurfaces may di�er greatly from arti®cial surfaces at similar depths, and any di�er-ences among arti®cial surfaces are likely to be smaller. Second, we suggest thatarti®cial structures may increase the abundance and diversity of subtidal epibiota inthe shallow areas of an estuary, but are not surrogate surfaces for epibiotic assem-blages that occur on nearby natural rock.

4.1. E�ects of substratum on assemblage structure

The assemblages of epibiota on natural rock (sandstone) and sandstone retainingwalls di�ered from those on the wooden, concrete and ®breglass surfaces of pilingsand pontoons. The major distinguishing features of sandstone surfaces were thedi�erence in identity of species in the assemblage and the abundance of individual

Table 3

ANOVA comparing the percentage cover of selected taxa among seven surfaces at two sites

Source df MS F p MS F p MS F p

(a) Number of taxa (b) Coralline algae (c) Filamentous algae

Surface 6 25.88 7.42a * 11895.42 43.41 *** 4793.63 7.92 *

Site 1 0.23 0.07 ± 165.09 1.20 ± 2035.80 6.65 ±

Su�Si 6 4.33 1.25 >0.25 274.05 1.99 NS 605.60 1.98 NS

Residual 56 3.56 137.63 306.21

(d) Hydroides (e) Bryozoans (f) Bare space

Surface 6 741.42 6.62 * 742.49 4.00 NS 157.44 8.24 *

Site 1 5.04 0.08 ± 0.75 0.01 ± 2.23 0.06 ±

Su�Si 6 111.91 1.84 NS 185.47 3.22 ** 19.11 0.53 >0.25

Residual 56 61.00 57.67 36.38

(g) Spirorbids (h) Porifera (i) Ceramiales

Surface 6 158.67 3.90a * 39.57 2.38 * 292.09 1.09 NS

Site 1 78.80 1.98 ± 0.36 0.02 ± 272.10 2.44 ±

Su�Si 6 48.84 1.23 >0.25 12.44 0.73 >0.25 266.77 2.39 *

Residual 56 39.84 17.14 111.53

Post-hoc pooling of the interaction term with the residual enabled a more powerful test of the main factor

`Surface' (p>0.25; Winer, Brown, & Michels, 1991). NS, p>0.05; *p<0.05; **p<0.01; ***p<0.001. The

critical value of `a' was adjusted to allow for signi®cant heterogeneity of variances (Cochran's C-test

p<0.05). The comparison of sites was not relevant for testing the hypothesis. Transformations: untrans-

formed=a, b, c, f, h, i; Arc-sine=d, e, g.a F-ratios a�ected by pooling and resultant values are given.


taxa. This appeared to be driven primarily by the domination of space by corallinealgae which correspondingly reduced the abundance of other taxa. Note that it ispossible that other taxa occurred under the thick mat of coralline algae.The assemblages among pilings and pontoons (not sandstone) showed some

striking similarities. The two structures with the most similar assemblages wereconcrete pilings and concrete pontoons. A possible explanation for this is that con-crete has certain physical or chemical properties which a�ect the settlement, growthor survival of organisms. For example, the high alkalinity at the surface of concrete,due to the leaching of calcium hydroxide from cement, enhances the settlement ofoysters under laboratory and ®eld conditions (Anderson, 1996). While stripped-pilings and barked pilings were both made of wood, their physical and chemicalproperties are also likely to vary, and this may explain why the assemblages on thesetwo surfaces di�ered. Pilings with bark are notable by the large amount of barespace (up to 35%) and this may be due to features of the bark that reduce settlementand subsequent recruitment of some taxa. This seems likely given that the bark isleft on pilings to help prevent deterioration of the wood by fouling organisms andboring molluscs.Di�erent types of structures are also likely to be correlated with topographic

complexity and area of surface. Previous work has shown that complex surfacescan provide sites for settlement and improved opportunities for attachment,growth and survivorship of organisms (Walters & Wethey, 1996). Moreover, it iswell known that the number of species within a habitat often increases with anincrease in the size of habitat (Gleason, 1922; McGuinness, 1984; Simberlo� &Abele, 1982) and this can happen at relatively small scales for subtidal epibiota(Butler, 1991; Keough, 1984). Thus, di�erences in size (and shape) of a structurecould be partially responsible for the observed di�erences in assemblages on thesesurfaces.Although the characteristics of the surface appear to be an important determinant

in structure of assemblages, the arrangement of the surface may also be in¯uential.While ®breglass pontoons supported the most distinct assemblages of non-sandstonesurfaces, concrete pontoons supported the least dissimilar assemblage to ®breglasspontoons. This indicates that although ®breglass and concrete surfaces have verydi�erent e�ects on assemblage structure, pontoons themselves have very distinctivee�ects on assemblage structure. A unique aspect of pontoons is that they ¯oat andattached organisms are not subjected to changes in depth associated with the riseand fall of the tide. No studies have compared the development of assemblageswith and without the in¯uence of tides, but it is likely that constant depth andcharacteristics associated with depth (e.g. light) have considerable e�ects on thedevelopment of assemblages.The feeding activities of ®sh and invertebrates have been shown to have profound

e�ects on assemblages of subtidal epibiota (Breitburg, 1985; Russ, 1980). Moreover,it is widely speculated that predatory ®sh are a major structuring force on patternsof abundance of subtidal epibiota (Harris & Irons, 1982; Sebens, 1986; Walters &Wethey, 1996; Young & Chia, 1984) and also contribute to di�erences in assem-blages of intertidal epibiota growing on plates constructed of various substrata


(McGuinness, 1989). In Sydney Harbour, there is substantial variability in theabundance and composition of ®sh around pontoons, pilings and natural reef sepa-rated by only tens of metres (Connell, personal observation) and ®sh are majorpredators of oysters on pilings (Connell, unpublished data). Hence, grazing andpredation particularly by ®sh cannot be discounted as a factor contributing to dif-ferences in assemblages of epibiota among such structures.Finally, variation in the age of the various surfaces may account for a large pro-

portion of the di�erences observed between natural reef and arti®cial surfaces.Urban structures are considerably younger than rocky reef and are likely to havebeen introduced at di�erent times. Variation in age is an intrinsic property of urbanstructures. Consequently, many structures would have a unique history of variousevents of recruitment, mortality and succession. It is well known that assemblagescan be a�ected by temporal heterogeneity (Menge & Sutherland, 1976) associatedwith the timing of biological or physical disturbance (Breitburg, 1985; Dean &Hurd, 1980), and stage of succession (Connell & Slatyer, 1977) or time since dis-turbance (Anderson & Underwood, 1994). Hence, temporal heterogeneity will be animportant source of variation in the structure of assemblages between natural reefand arti®cial surfaces, especially since the addition of urban structures to marinehabitats has and will continue to occur haphazardly through time.

4.2. Habitat diversity and species diversity

The idea that the addition of new habitats can increase species diversity is not newto ecology (Connor & McCoy, 1979; Williams, 1943), and has been adapted as animportant theme in the subdisciplines of conservation biology (Barbault, 1995;Kaplan, 1993; Walting, 1997) and urban ecology (Rebele, 1994). It has been recog-nised that urban structures are habitats for a variety of plants and animals in ter-restrial systems (Rebele, 1994), but until now this has not been studied in marinesystems. We were unable to refute the model that the addition of urban structureshas resulted in a greater number of species of attached plants and animals at shallowdepths.Although it is clear that urban structures modify the identities and numbers of

species within an area, the idea that this has a positive e�ect on species diversityneeds to be treated with extreme caution (Glasby & Connell, in press). The additionof structures to the natural habitats often results in the fragmentation of naturalhabitats and many studies have demonstrated a decline of diversity in habitats afterfragmentation (Andren, 1994; Devries, Denboer, & Vandijk, 1996; Spellerberg,1991). Furthermore, the addition of exotic species, which enhances species diversity,may have catastrophic e�ects on the abundance of plants and animals and on thepreservation of endangered species (Lodge, 1993).Arti®cial surfaces may be surrogate habitats for some species, but our results

suggest that as habitat for assemblages of epibiota, natural reef may not be ade-quately replaced by urban structures. Importantly, arti®cial surfaces create newhabitat. Assemblages of epibiota on them are di�erent from those on natural reefat the same depth, thereby altering the distribution and abundances of epibiotic


organisms. The current ®xation with species diversity, therefore, may be inap-propriate if it diverts attention from understanding the e�ects on the abundance ofthe component species. Alteration of habitat may have profound e�ects on theabundance of particular species and in turn these changes may cause negative orpositive impacts on the broader environment.The notion that urban structures may have positive and negative impacts on the

environment parallels developments in ®sheries science. Although arti®cial struc-tures are seen as e�ective tools to enhance the diversity and productivity of com-mercially important species (Pickering & Whitmarsh, 1997), it is recognised thatthey can degrade the environment (Chou, 1997). Consequently, ®sheries science hasidenti®ed the need for and begun research on how di�erent types of structures a�ecta ®shery (Bassett, 1994; Collins, Jensen, Lockwood, & Lockwood, 1994). In a simi-lar context, assessments of how alternative urban structures a�ect the broader ma-rine environment are required.In conclusion, our results clearly indicate the potential for both the type of surface

(e.g. wooden pilings with vs without bark) and the size/shape of the same type ofsurface (e.g. concrete pontoons and concrete pilings) to a�ect the composition andabundance of the assemblage. These results taken with others (Anderson & Under-wood, 1994; McGuinness, 1989; Michener & Kenny, 1991) highlight that it is notonly the identity of species that vary among surfaces, but also the abundance of thecomponent species. Future research may be well directed in establishing the types ofspecies and how their abundances are a�ected by the addition of urban structures.An increase in the number of urban habitats per se is not necessarily bene®cial to theenvironment if the associated changes in species composition and abundance lead tothe degradation of other components of estuarine areas.

Acknowledgements

The Centre for Research on Ecological Impacts of Coastal Cities and this work wassupported by an ARC Special Research Centre Grant. We thank G. Chapman, M.Lindegarth and A. Underwood for their advice and J. Cunningham and G. House-®eld for assistance with ®eldwork and persisting with remarkably average diving.


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Appendix A

Taxa identi®ed on rocky reef and urban structures. `Surface' indicates the surface from which the

taxon was sampled: A (arti®cial surfaces), N (natural reef) and B (both arti®cial surfaces and natural

reef)

Phylum/Class Taxa Surface

Chlorophyta Enteromorpha spp. Aa

Ulva lactuca Aa

Cladophorales A

Brown/green ®lamentous complex B

Rhodophyta Ceramiales complex B

Encrusting unid. N

Rhodymenia australis Aa

Corallina o�cinals B

Peyssonnelia spp. N

Phaeophyta Colpomenia sinuosa A

Sargassum spp. B

Zonaria sp. A

Dictyota dichotoma A

Anthozoa sp. 1 Aa

Porifera sp. 1 B

sp. 2 A

sp. 3 Aa

sp. 4 Aa

sp. 5 A

Polychaeta Spirorbidae B

Hydroides spp. B

Cirripedia Balanus trigonus B

Bivalvia Saccostrea commercialis N

Mytilus edulis Aa

Anomia sp. Aa

Bryozoa Watersipora subtorquata B

Schizoporella errata A

Fenestrulina mutabilis N

Bugula neritina Aa

Ascidiacea Styela plicata B

Pyura sp. Aa

a Taxa also not detected in a previous study of vertical rocky-reefs in Middle Harbour at 0.5 m below

Mean Low Water Springs (Glasby, 1997). Four 15�15 cm photographic quadrats were sampled in each of

six sites during January 1996.


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Do Urban Structures Influence Local Abundance and Diversity of Subtidal Epibiota a Case Study From Sydney Harbour Australia