Published: August 29, 2011

r 2011 American Chemical Society 8204 dx.doi.org/10.1021/es201924b | Environ. Sci. Technol. 2011, 45, 8204–8207

POLICY ANALYSIS

pubs.acs.org/est

Ecologically Informed Engineering Reduces Loss of IntertidalBiodiversity on Artificial ShorelinesMark A. Browne*,†,‡ and M. Gee Chapman†

†Centre for Research on Ecological Impacts of Coastal Cities, School of Biological Sciences, University of Sydney, NSW 2006, Australia‡School of Biology & Environmental Science, University College Dublin, Dublin, Ireland

bS Supporting Information

’ INTRODUCTION

Urbanization has transformed the earth’s surface and urbaninfrastructure—housing, utilities, transportation and commerce—have replaced natural habitats over large areas. These trans-formed landscapes typically support fewer species, althoughsome species reach pest proportions. In addition, species foundin cities tend to be homogenized, with similar species found incities worldwide.1 Coastal infrastructure, often called coastal“armouring”,2 is built along natural shorelines or on “reclaimed”land to protect infrastructure fromwaves and erosion. It is gettingtaller and spreading3 and with increasing urbanization, rising sea-levels and more stormy weather predicted, is set to increase. Inparts of Japan,4 U.S.,2 Europe,5 and Australia,6 more than half ofthe coastline has been replaced by artificial structures. The mostextensive type of coastal “armouring” is revetments and seawalls,which may be the only “rocky” habitat for kilometres in majorports and estuaries in and around cities (Figure 1a).

Although there have been many attempts to restore degradedintertidal habitats around cities, for example, marshes,7 mangroves,8

and beaches,9 rocky shores have been considered resilient tosuch change because many visually dominant species appearto thrive on artificial habitats.10 Closer study has revealed,however, that many intertidal species, particularly larger mobileanimals such as starfish, urchins, and large gastropods, do not liveon seawalls,11,12 whereas those that do may be genetically lessdiverse,13 grow or reproduce slowly,14 or change their intra- orinterspecific interactions,15�17 particularly when structures arecolonized by invasive species.18 Thus expanding urbanization has

potentially serious implications for intertidal assemblages fromrocky shores, a problem that is only recently been recognized.18,19

The value of “armoured” shores as ecological habitat in urbanareas has mostly been investigated with respect to plants and fish.Thus, Francis and Hoggart20 showed more than three times asmany plants living on walls on the River Thames in centralLondon than on intertidal foreshores and the habitat in andaround piers and docks support large diversities of fish.21,22

Numerous programmes of restoration involve the addition of“natural” habitat offshore from “armoured” shores,23 both todecrease impacts on the shoreline itself and to add habitat such asoyster reef, vegetation, or saltmarshes to bare sediments. Therehave, however, been few attempts to change the quality of thehabitat provided by the walls themselves for species that live onthem, even though walls may be valuable habitat for specieswhose natural rocky habitat may be lost to infrastructure, ordisturbed by other human activities.19,24 In addition, many suchexperiments have been small-scale, addressing focused problems.

Ecological-engineering25�29 - melding engineering criteriaand ecological knowledge to create better urban environments -has revolutionized modern building. Growing plants on wallsand roofs of buildings can improve aesthetics, produce crops,remove air-borne toxicants or cools cities.28 Natural marshes can

Received: June 6, 2011Accepted: August 29, 2011Revised: August 27, 2011

ABSTRACT:Worldwide responses to urbanization, expanding populations and climatic changemean biodiverse habitats are replaced with expensive, but necessary infrastructure. Coastal citiessupport vast expanses of buildings and roads along the coast or on “reclaimed” land, leading to“armouring” of shorelines with walls, revetments and offshore structures to reduce erosion andflooding. Currently infrastructure is designed to meet engineering and financial criteria, withoutconsidering its value as habitat, despite artificial shorelines causing loss of intertidal species andaltering ecological natural processes that sustain natural biodiversity. Most research onameliorating these impacts focus on soft-sediment habitats and larger flora (e.g., restoringmarshes, encouraging plants to grow on walls). In response to needs for greater collaborationbetween ecologists and engineers to create infrastructure to better support biodiversity, we showhow such collaborations lead to small-scale and inexpensive ecologically informed engineeringwhich reduces loss of species of algae and animals from rocky shores replaced by walls. Addingexperimental novel habitats to walls mimicking rock-pools (e.g., cavities, attaching flowerpots)increased numbers of species by 110% within months, in particular mobile animals most affected by replacing natural shores withwalls. These advances provide new insights about melding engineering and ecological knowledge to sustain biodiversity in cities.

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Environmental Science & Technology POLICY ANALYSIS

clean wastewater better than do treatment works.29 To date,despite the cogent argument that walls are underused habitats incities and that relatively cheap and easy engineering can improvetheir value as habitat for many plants and animals,19,30 there hasbeen relatively little application of the concept of ecological-engineering to the design of coastal “armouring” in order toincrease local levels of biodiversity.30 This is particularly so forintertidal marine animals and seaweeds.19

’EXPERIMENTAL MODIFICATIONS OF SEAWALLS INSYDNEY HARBOUR

Here we describe an extensive set of experiments in Australiainvestigating adding novel habitats to intertidal seawalls to createbetter environments for intertidal species. These experiments allinvolved close collaborations between ecologists and engineersbuilding or repairing seawalls, who were very responsive to theneeds of replication and good experimental design. The earlierexperiments have been described in detail elsewhere. Theoriginal experiments added new habitats to walls at a small-scale

because of concern by authorities that adding larger habitats toseawalls would alter their aesthetic value.20 These experimentsincluded adding holes and crevices in the fac-ades of the blocksmaking up the wall19 and indenting the mortar between blocks tocreate narrow crevices.19,31 Although successful in enhancing thenumber of species living on the walls in the short term, longerterm they were less successful as they filled with sessile animals,so the available habitat was lost.

These were followed by attempts to create larger cavities in thevertical fac-ades of walls. These cavities retain water during low-tide and, thus, imitate rock-pools, although unlike natural poolsthey are always shaded by the wall. First, they were created byusing sand-bags as bloks of masonry during repair of existingwalls.19 When removed, they created hollows in the wall thatwere colonized by nudibranchs, octopuses and urchins, speciesthat generally cannot live on vertical intertidal walls. A muchlarger experiment on a newly built wall omitted masonry-blocks,leaving cavities in the walls, to which small-lips were added tocreate enclosed pools.32 This was a large-scale experiment, withreplicate “pools” created at three heights in each of three replicatesites. After a year, these had increased the number of speciesliving on the walls, particularly midshore, with 57% more speciesof algae and 42% more species of sessile animals. Such ap-proaches are cheap and easy, requiring no additional material, butrequire the cooopration of the engineers building walls tosupervise changes in plans. They can, however, only be usedwhen building walls out of blocks of material.

Adding similar microhabitat into concrete or existing walls isnot usually feasible. So, inspired by the concept of “livingwalls”,27,29 concrete flower pots were used to mimic rock-pools.Flower pots have been used for thousands of years to grow plantsin novel locations and inhospitable habitats. Here, they alsoprove to be a tractable method for ecologically engineeringseawalls to increase intertidal biodiversity. Concrete pots, builtto withstand wave-action, were attached at mid- and highshoretidal levels (1.0�1.3 m 1.6�1.9 m above chart datum, re-spectively) to seawalls at Cremorne Point (33� 500 50.3298S;151� 130 49.0578E) and Careening Cove (33� 500 42.7194S,151� 130 6.1464E) in Sydney Harbour in December, 2009, withsix replicate pots at each height in each site. These moreresembled natural pools than did the earlier engineered habitatsin that they created pools of water during low tide that wereexposed to the sun, rather than continually shaded and they wereflushed by the tide each high tide. Two sizes of pots (10 and 6 L)were used; these had the same diameter (360 mm) but differentdepths (380 and 220 mm respectively). All animals and plantsfound in the pots and plots on the walls were counted weekly for6 weeks, then at monthly and 3monthly intervals. After 7 monthsthere were still clear differences in the assemblages living in potsand on walls so further sampling was not required.

Some pots were lost due to wave action, so here we describecolonization of animals and plants into 10 L pots at the midshorelevel at both sites after 7months. Square plots of similar area to theinternal surface area of the 10 L pots (2500 cm2) were randomlyinterspersed among the pots on the fac-ade of the wall for com-parsison with the assemblage living on the wall. It was notpossible to clear the assemblage living on the wall due to restric-tions by the authorities, so the existing assemblage was mature, asdescribed by Chapman and Bulleri6 for other seawalls in theharbor. It was comprised of large amounts of bare space (>70%)withsome barnacles, mussels (Mytilus sp.), oysters (Saccostrea glomerata),and brown (Ralfsia verrucosa, Endarachne binghamiae), and red

Figure 1. (A) Port of Vancouver (Canada) illustrating the extent ofartificial shorelines composed of featureless seawalls. (B) Flower potscreating novel habitat on seawalls in Sydney Harbour (Australia). (C)Total number of taxa unique to wall fac-ades (black), unique to flowerpots (white) and shared (gray) at midshore levels; data summarized overtwo sites and seven months, using published methods.17 (D) Potsprovided habitat for many algae (e.g., turf-forming red algae such asCorallina officinalis - i), sessile animals, such as ascidians, filter-feedingtubeworms and sponges, and mobile animals, such as omnivorous crabs(e.g., Leptograpsus variegates - ii), grazing starfish (e.g., Patiriella exigua - iii),grazing amphipods andmany gastropods (e.g.,Austrocochlea concamerata - iv,Alaba opiniosa - vi).

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algae (Hildenbrandia rubra,Gelidium pusillum). The experimentthus examined what additional species would be added to amature “wall assemblage” (the wall was many decades old andwas unlikely to have been cleaned in that time) by the additionof pots, rather than comparing colonization of the pots and thebare wall.

Using a similar approach to Chapman & Blockley,32 the taxawere divided into three groups: those found (i) only on the walls,(ii) only in pots, and (iii) in both habitats. This allowed us to testthe prediction that species living in rock-pools on natural shores,but not generally on seawalls, would be found in these pots, butnot in the uncleared surrounding plots. By 7 months, 25 specieshad colonized the pools which were not found in the assemblageon the walls (an increase of 64%). These included algae (50%more species), sessile animals (39% more) and mobile animals(118% more; Figure 1b�d, Supporting Information Table S1).

The taxa unique to the pots were diverse, including red (e.g.,Polysiphonia sp.) and green algae (e.g., Ulva lactuca, Bryposis sp.)and many sessile animals, such as ascidians (e.g., Pyura stolonifera),sponges (e.g., Haliclona sp.) and tubeworms (e.g., Galeolariacaespitosa, Hydroides sp.). Although many sessile animals live onseawalls, soft-bodied sponges, ascidians and branching bryozoansseldom do so at midshore levels unless protected in deep crevices.Similarly, these species are largely confined to rock-pools atmidshore levels of rocky shores. More importantly, this assem-blage included many mobile animals, such as grazing snails (e.g.,Littorina unifasciata, Nodilittorina pyramidalis, Austrocochlea porca-ta, Austrocochlea concamerata, Alaba opiniosa), limpets (e.g.,Patelloida alticostata, P. mufria, Kerguelenela lateralis), amphipods(e.g., Caprellids), and starfish (e.g., Patiriella exigua), and crabs(e.g., Leptograpsus variegatus). Many of these species are not foundon unmodified intertidal seawalls6 and thus are most threated byreplacement of natural shores by walls.

’WHERE TO FROM HERE

As cities grow more habitat will be lost as infrastructure spreads.Although restoration is an important tool for conservation,33�35

it is unlikely to be an option once natural habitats have beenreplaced by expensive infrastructure. It has been estimated that itwill cost £590 billion to protect the world’s coastal cities fromrising sea-levels and increases in storms36 and 20�97% of thiswill be spent building new seawalls or increasing the stability,height and length of existing walls.3,36 Our work shows thatcollaboration between experimental ecologists and engineersduring the process of designing and building this infrastructure,as promulgated by Francis and Hoggart,20 can lead to a range ofcheap and easy engineering techniques that can be applied to newand existing coastal infrastructure. For instance adding cavitiesinto seawalls was actually cheaper than building a seawall withoutthem because fewer expensive sandstone blocks were used,whereas the cost of a flower pot, without a metal bracket andfixings is less than AUS$200 each. Work is now needed todetermine how durable the flower pots and their fixings are overlonger periods, but they do seem to be a useful approach to try tosustain, rather than erode, levels of intertidal biodiversity in andaround cities.

’ASSOCIATED CONTENT

bS Supporting Information. Table S1. This material isavailable free of charge via the Internet at http://pubs.acs.org.

’AUTHOR INFORMATION

Corresponding Author*Phone: +353 (0) 870 916 484; fax: +353 (0) 1 716 1152; e-mail:mark.browne@ucd.ie.

’ACKNOWLEDGMENT

We thank North Sydney Council, Woollahra MunicipalCouncil and the City of Sydney for support and access to studysites. J. Thompson of John Nixon Engineering Pty Ltd andMacLeod Consultants were particularly helpful in many of theseexperiments. A. Luck, B. Panayotakos, G. Deavin, C. Myers,M. Day, J. Sidie, B. Twist, J. Commins, D. Beechey are thankedfor assistance and photographs. The Centre for Research onEcological Impacts of Coastal Cities (University of Sydney), ECSServices, Antique Stone and various local government authoritiesprovided support for this research. Manuscript was improved bycomments from S. J. Simpson, R. Shine, A. J. Underwood andthree anonymous referees.

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