Create pit habitats (1–50 mm) on intertidal artificial structures

Overall effectiveness category Awaiting assessment
Number of studies: 22
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Background information and definitions

Definition: ‘Pit habitats’ are depressions with a length to width ratio ≤3:1 and depth 1–50 mm (Strain et al. 2018). Intertidal pits may or may not retain water during low tide.

Pit habitats provide organisms refuge from desiccation and temperature fluctuations during low tide in intertidal rocky habitats (Williams & Morrit 1995). They also provide shelter from predation or grazing (Menge & Lubchenco 1981) and some species preferentially settle into them (Skinner & Coutinho 2005). The size and density of pits is likely to affect the size, abundance and variety of organisms that can use them. Small pits can provide refuge for small-bodied organisms but may exclude larger organisms, limit their growth and get rapidly filled-up (Firth et al. 2020). Large pits can be used by larger-bodied organisms but may not provide sufficient refuge from predators for smaller organisms.

Pits are sometimes present on boulders used in marine artificial structures as a result of quarrying processes (Hall et al. 2018), and can form on other structures through erosion. However, these are often filled or repaired during maintenance works (Moreira et al. 2007) and are absent from many structures (Martins et al. 2010). Pit habitats can be created on intertidal artificial structures by adding or removing material, either during construction or retrospectively.

There is a body of literature investigating the effects of pit habitats on ecological interactions and processes in intertidal rocky habitats (e.g. Beck 2000; Chapman & Underwood 1994). These studies are not included in this synopsis, which focusses on in situ conservation actions to enhance the biodiversity of marine artificial structures.

^{Beck M.W. (2000) Separating the elements of habitat structure: independent effects of habitat complexity and structural components on rocky intertidal gastropods. Journal of Experimental Marine Biology and Ecology, 249, 29–49.}

^{Chapman M.G. & Underwood A.J. (1994) Dispersal of the intertidal snail, Nodolittorina pyramidalis, in response to the topographic complexity of the substratum. Journal of Experimental Marine Biology and Ecology, 179, 145–169.}

^{Firth L.B., Airoldi L., Bulleri F., Challinor S., Chee S.-Y., Evans A.J., Hanley M.E., Knights A.M., O’Shaughnessy K., Thompson R.C. & Hawkins S.J. (2020) Greening of grey infrastructure should not be used as a Trojan horse to facilitate coastal development. Journal of Applied Ecology, 57, 1762–1768.}

^{Hall A.E., Herbert R.J.H., Britton J.R. & Hull S.L. (2018) Ecological enhancement techniques to improve habitat heterogeneity on coastal defence structures. Estuarine, Coastal and Shelf Science, 210, 68–78.}

^{Martins G.M., Thompson R.C., Neto A.I., Hawkins S.J. & Jenkins S.R. (2010) Enhancing stocks of the exploited limpet Patella candei d’Orbigny via modifications in coastal engineering. Biological Conservation, 143, 203–211.}

^{Menge B.A. & Lubchenco J. (1981) Community organization in temperate and tropical rocky intertidal habitats: prey refuges in relation to consumer pressure gradients. Ecological Monographs, 51, 429–450.}

^{Moreira J., Chapman M.G. & Underwood A.J. (2007) Maintenance of chitons on seawalls using crevices on sandstone blocks as habitat in Sydney Harbour, Australia. Journal of Experimental Marine Biology and Ecology, 347, 134–143.}

^{Skinner L.F. & Coutinho R. (2005) Effect of microhabitat distribution and substrate roughness on barnacle Teraclita stalactifera (Lamarck, 1818) settlement. Brazilian Archives of Biology and Technology, 48, 109–113.}

^{Strain E.M.A., Olabarria C., Mayer-Pinto M., Cumbo V., Morris R.L., Bugnot A.B., Dafforn K.A., Heery E., Firth L.B., Brooks P.R. & Bishop M.J. (2018) Eco-engineering urban infrastructure for marine and coastal biodiversity: which interventions have the greatest ecological benefit? Journal of Applied Ecology, 55, 426–441.}

^{Williams G.A. & Morritt D. (1995) Habitat partitioning and thermal tolerance in a tropical limpet, Cellana grata. Marine Ecology Progress Series, 124, 89–103.}

Study locations

Key messages

Twenty-two studies examined the effects of creating pit habitats on intertidal artificial structures on the biodiversity of those structures. Ten studies were on open coastlines in the UK, the Netherlands and the Azores, six were on island coastlines in the Singapore Strait, three were in estuaries in southeast Australia and the UK, one was in a port in the Netherlands, one was in an estuary and on an open coastline in the UK, and one was on island coastlines in the Singapore Strait and in estuaries in the UK.

COMMUNITY RESPONSE (16 STUDIES)

Overall community composition (9 studies): Four of six replicated, controlled studies (including four randomized and two before-and-after studies) in Australia, Singapore and the UK found that creating pit habitats on intertidal artificial structures altered the combined macroalgae and invertebrate community composition on structure surfaces. One study found that creating pits did not alter the community composition. One found that creating pits, along with grooves, small protrusions and ridges, had mixed effects depending on the size and arrangement of pits and other habitats and the site, while one found that varying the pit size and arrangement had no significant effect. Three of these studies, along with three other replicated, controlled studies (including one that was randomized) in the UK and Singapore, reported that pit habitats, along with grooves and ridges in one, supported macroalgae, invertebrate and/or fish species that were absent from structure surfaces without added habitats.
Fish community composition (1 study): One replicated, randomized, controlled study in Singapore found that pit habitats created on an intertidal artificial structure, along with grooves, altered the fish community composition on and around structure surfaces, and supported species that were absent from surfaces without pits and grooves.
Overall richness/diversity (12 studies): Eight of 12 replicated controlled studies (including six randomized and two before-and-after studies) in the UK and Singapore found that creating pit habitats on intertidal artificial structures, along with grooves, or grooves, small protrusions and ridges in two studies, increased the combined macroalgae and invertebrate species richness and/or diversity on structure surfaces. Two studies found that creating pits did not increase the species richness, while two found that creating pits, along with grooves or using environmentally-sensitive material, had mixed effects depending on the site. One of the studies found that varying the pit size and arrangement resulted in higher species richness, while one found that this had mixed effects depending on the shore level. Two of the studies found that varying the pit size did not affect species richness. One of them found that increasing the density and fragmentation of pits, along with grooves, had mixed effects on species richness.
Algal richness/diversity (1 study): One replicated, randomized, controlled study in Singapore reported that creating pits on an intertidal artificial structure, along with grooves and small ridges, increased the macroalgal species richness on structure surfaces.
Invertebrate richness/diversity (2 studies): One of two replicated, randomized, controlled studies in Australia and the Azores reported that creating pits on an intertidal artificial structure increased the limpet and periwinkle species richness on structure surfaces, and that their richness and diversity varied depending on the pit arrangement. One found that creating pits did not affect the limpet species richness, regardless of the pit size.
Fish richness/diversity (1 study): One replicated, randomized, controlled study in Singapore found that creating pit habitats on an intertidal artificial structure, along with grooves, increased the fish species richness on and around structure surfaces.

POPULATION RESPONSE (15 STUDIES)

Overall abundance (5 studies): Two of five replicated, controlled studies (including three randomized and two before-and-after studies) in Singapore and the UK found that creating pit habitats on intertidal artificial structures, along with grooves in one study, increased the combined macroalgae and invertebrate abundance on structure surfaces. One study found that creating pits decreased their abundance and one found no effect. One found that creating pits, along with grooves, small protrusions and ridges, had mixed effects on abundance depending on the pit size and arrangement, shore level and site.
Algal abundance (4 studies): Three of four replicated, controlled studies (including two randomized and two paired sites studies) in the Netherlands, Singapore and the Azores found that creating pit habitats on intertidal artificial structures, along with grooves and small ridges in one study, did not increase the macroalgal abundance on structure surfaces. One study found that creating pits had mixed effects on abundance depending on the pit size and arrangement and the site.
Invertebrate abundance (9 studies): Three of eight replicated, controlled studies (including six randomized and two paired sites studies) in the Azores, the Netherlands, Australia and the UK found that creating pit habitats on intertidal artificial structures did not increase the combined invertebrate or mobile invertebrate abundance on structure surfaces. Three studies found that creating pits, along with grooves in one study, had mixed effects on barnacle and/or mobile invertebrate abundances, depending on the site, the species, the size of animals, and/or the pit size and arrangement. Two studies found that creating pits, along with using environmentally-sensitive material in one, increased barnacle and/or mobile invertebrate abundances. Two of the studies found that the pit size or arrangement did not affect abundances, while two found that the effects of pit size and arrangement varied depending on the site and species. One replicated randomized study in the UK found that increasing pit density increased periwinkle abundance, but pit arrangement did not.
Fish abundance (1 study): One replicated, randomized, controlled study in Singapore found that creating pit habitats on an intertidal artificial structure, along with grooves, increased the fish abundance on and around structure surfaces.

BEHAVIOUR (6 STUDIES)

Use (5 studies): Two replicated, randomized, controlled studies in the Azores reported that occupancy of pit habitats created on intertidal artificial structures by limpets and/or periwinkles varied depending on the pit size and arrangement, the size of animals, the species and/or site. Three replicated studies (including two paired sites, controlled studies) in the Netherlandsand in Singapore and the UK reported that pit habitats were used by periwinkles, macroalgae and invertebrates.
Fish behaviour change (1 study): One replicated, randomized, controlled study in Singapore found that creating pit habitats on an intertidal artificial structure, along with grooves, increased the number of bites fishes took from structure surfaces.

About key messages

Key messages provide a descriptive index to studies we have found that test this intervention.

Studies are not directly comparable or of equal value. When making decisions based on this evidence, you should consider factors such as study size, study design, reported metrics and relevance of the study to your situation, rather than simply counting the number of studies that support a particular interpretation.

Supporting evidence from individual studies

A replicated, controlled study in 2001–2002 on two intertidal breakwaters on open coastline in the English Channel, UK (Moschella et al. 2005) reported that creating pit habitats on the breakwaters increased the macroalgae and invertebrate species richness on breakwater surfaces. After 12 months, settlement plates with pits supported five species in total, while plates without pits supported two species (data not statistically tested). Concrete settlement plates (300 × 300 mm) were made with and without pit habitats. Plates had six large (diameter: 30 mm) or 13 small (15 mm) round pits (depth: 20 mm), or a mixture of four large and four small pits (spacing/arrangement not reported). Four plates of each and four without pits were attached to horizontal midshore surfaces on each of two granite boulder breakwaters in 2001 (year: M. Hanley pers. comms.; month not reported). Macroalgae and invertebrates were counted on plates with and without pits during low tide after 12 months.
Study and other actions tested
Referenced paper
Moschella P.S., Abbiati M., Åberg P., Airoldi L., Anderson J.M., Bacchiocchi F., Bulleri F., Dinesen G.E., Frost M., Gacia E., Granhag L., Jonsson P.R., Satta M.P., Sundelöf A., Thompson R.C. & Hawkins S.J. (2005) Low-crested coastal defence structures as artificial habitats for marine life: using ecological criteria in design. Coastal Engineering, 52, 1053-1071.
A replicated, randomized, controlled study in 2006–2007 on an intertidal seawall on open coastline in the Atlantic Ocean, Azores (Martins et al. 2010; same experimental set-up as Martins et al. 2016) found that creating pit habitats on the seawall increased abundances of recently-recruited and juvenile limpets Patella candei at one of two sites, but not adults. After four months, at one of two sites, recruits and juveniles were more abundant on surfaces with pits (3–6 limpets/surface) than without (0–1/surface). At the second site, no recruits were recorded and juvenile abundance was similar on surfaces with and without pits (both 0/surface). At both sites, adult abundance was statistically similar on surfaces with pits (2–8/surface) and without (0–3/surface). At the first site, recruits occupying pits were more abundant in high-density pits (4–6/surface) than low (1–2/surface), while adults occupying pits were more abundant in large pits (9–11/surface) than small (1–2/surface). Pit habitats were created by drilling into a basalt boulder seawall in November 2006. Arrays of large (diameter: 24 mm) and small (12 mm) round pits (depth: 10 mm) were evenly-spaced on 250 × 250 mm seawall surfaces with high (16 pits/array) or low (8/array) densities. There were five surfaces with each size-density combination and five without pits, randomly arranged at midshore in each of two sites along the seawall. Limpets were removed from surfaces when pits were created, then were counted on surfaces with and without pits during low tide after four months.
Study and other actions tested
Referenced paper
Martins G.M., Thompson R.C., Neto A.I., Hawkins S.J. & Jenkins S.R. (2010) Enhancing stocks of the exploited limpet Patella candei d’Orbigny via modifications in coastal engineering. Biological Conservation, 143, 203-211.
A replicated, randomized, controlled study in 2000–2003 on an intertidal seawall in Sydney Harbour estuary, Australia (Chapman & Underwood 2011) found that creating pit habitats on the seawall did not alter the macroalgae and invertebrate community composition or increase limpet (Patellidae and/or Siphonariidae, Fissurellidae) species richness or abundance, or chiton (Polyplacophora) abundance on seawall surfaces. After three months, seawall surfaces with pits supported similar macroalgae and invertebrate community composition to surfaces without (data reported as statistical model results). After 27 months, limpet species richness and abundance were similar in large pits (0 species and individuals/array), small pits (1 species/array, 2 individuals/array) and on surfaces without pits (1 species/surface, 3 individuals/surface). The same was true for chiton abundance (large pits: 0 individuals/array; small pits: 2/array; no pits: 0/surface). Pit habitats were created in 2000 (month not reported) by drilling into a vertical sandstone seawall during reconstruction. Large (diameter: 50 mm) and small (25 mm) round pits (depth: 5 mm) were drilled in arrays of 16 (spacing/arrangement not reported) on 1 × 0.4 m seawall surfaces. There were five surfaces with each of large, small and no pits, randomly arranged (shore level not reported). Macroalgae and invertebrates were counted on surfaces with and without pits during low tide after three months. Mobile invertebrates were counted in pits and on surfaces without after 27 months.
Study and other actions tested
Referenced paper
Chapman M.G. & Underwood A.J. (2011) Evaluation of ecological engineering of “armoured” shorelines to improve their value as habitat. Journal of Experimental Marine Biology and Ecology, 400, 302-313.
A replicated, randomized study in 2006–2008 on an intertidal seawall on open coastline in the English Channel, UK (Skov et al. 2011) found that pit habitats created on the seawall supported similar periwinkle Melarhaphe neritoides abundance regardless of the pit patchiness, but that increasing the pit density increased their abundance. Over 24 months, seawall surfaces with patchy pits supported similar periwinkle abundance (132–176 individuals/surface) to surfaces with evenly-spaced pits (170–208/surface). Abundance increased with increasing pit density (4 pits: 52 individuals/surface; 16 pits: 178/surface; 36 pits: 285/surface; 64 pits: 343/surface) but it was not clear which densities differed significantly from which. Pit habitats were created by drilling into a vertical concrete seawall in June 2006. Arrays of round pits (diameter: 10 mm; depth: 7 mm) were patchy (four patches/surface) or evenly-spaced on 500 × 500 mm seawall surfaces, with different densities (4, 16, 36 or 64 pits/surface). There were three surfaces with each arrangement-density combination randomly arranged at highshore. Existing cracks and holes were filled with cement and organisms were removed from surfaces when pits were created, then small periwinkles were counted on surfaces during low tide over 24 months.
Study and other actions tested
Referenced paper
Skov M.W., Hawkins S.J., Volkelt-Igoe M., Pike J., Thompson R.C. & Doncaster C.P. (2011) Patchiness in resource distribution mitigates habitat loss: insights from high‐shore grazers. Ecosphere, 2, 1-17.
A replicated, controlled study in 2011–2013 on an intertidal breakwater on open coastline in the English Channel, UK (Firth et al. 2014a) found that creating pit habitats on the breakwater increased the macroalgae and invertebrate species richness on breakwater surfaces. After 24 months, macroalgae and invertebrate species richness was similar on surfaces with large (10 species/surface) and small (9/surface) pits, and higher on both than on surfaces without pits (3/surface). Six invertebrate species groups recorded on surfaces with pits were absent from those without. Pit habitats were created in August 2011 by drilling into the vertical sides of concrete breakwater blocks. Arrays of 100 large (diameter: 22 mm) and small (14 mm) round pits (depth: 25 mm) were evenly-spaced on 1 × 1 m breakwater surfaces. There was one surface with each of large, small and no pits on each of eight blocks at mid-lowshore. Pits were angled to retain water. Macroalgae and invertebrates were counted on surfaces with and without pits during low tide after 24 months.
Study and other actions tested
Referenced paper
Firth L.B., Thompson R.C., Bohn K., Abbiati M., Airoldi L., Bouma T.J., Bozzeda F., Ceccherelli V.U., Colangelo M.A., Evans A.J., Ferrario F., Hanley M.E., Hinz H., Hoggart S.P.G., Jackson J.E., Moore P., Morgan E.H., Perkol-Finkel S., Skov M.W., Strain E.M., van Belzen J. & Hawkins S.J. (2014) Between a rock and a hard place: environmental and engineering considerations when designing coastal defence structures. Coastal Engineering, 87, 122-135.
A replicated, controlled study in 2010–2011 on an intertidal seawall in the Teign estuary, UK (Firth et al. 2014b) found that pit habitats created on the seawall supported similar macroalgae and invertebrate species richness to seawall surfaces without pits. After 19 months, macroalgae and invertebrate species richness was similar in pits (2 species/array) and on surfaces without pits (1/surface). Pit habitats were created in May 2010 by pushing a stick into wet mortar between blocks during construction of a vertical sandstone seawall. Arrays of four round pits (diameter: 25 mm; depth: 25 mm) were evenly-spaced on 150 × 150 mm seawall surfaces. There were 15 surfaces with pits and 15 without at highshore. Pits were angled to retain water. Macroalgae and invertebrates were counted in pits and on surfaces without pits during low tide after 19 months. One array of pits and seven surfaces without had been buried by sediment and no longer provided habitat.
Study and other actions tested
Referenced paper
Firth L.B., Thompson R.C., Bohn K., Abbiati M., Airoldi L., Bouma T.J., Bozzeda F., Ceccherelli V.U., Colangelo M.A., Evans A.J., Ferrario F., Hanley M.E., Hinz H., Hoggart S.P.G., Jackson J.E., Moore P., Morgan E.H., Perkol-Finkel S., Skov M.W., Strain E.M., van Belzen J. & Hawkins S.J. (2014) Between a rock and a hard place: environmental and engineering considerations when designing coastal defence structures. Coastal Engineering, 87, 122-135.
A replicated, controlled study in 2012–2013 on an intertidal groyne on open coastline in the Irish Sea, UK (Firth et al. 2014c) reported that pit habitats created on a concrete block placed in the groyne supported similar macroalgae and invertebrate species richness to groyne surfaces without pits. Data were not statistically tested. After 13 months, a total of three species were recorded in deep pits, two in shallow pits, and four on groyne surfaces without pits. Pit habitats were created on two vertical sides of a concrete block (1.5 × 1.5 × 1 m) using a mould. Two arrays of each of 16 deep (50 mm) and 16 shallow (20 mm) round pits (diameter: 20 mm) were evenly-spaced in 250 × 250 mm areas on each side. The block was placed at midshore in a boulder groyne during construction in February 2012. Surfaces with pits were compared with vertical surfaces of adjacent groyne boulders (dimensions/material not reported). Macroalgae and invertebrates were counted in pits and on groyne surfaces without during low tide after 13 months.
Study and other actions tested
Referenced paper
Firth L.B., Thompson R.C., Bohn K., Abbiati M., Airoldi L., Bouma T.J., Bozzeda F., Ceccherelli V.U., Colangelo M.A., Evans A.J., Ferrario F., Hanley M.E., Hinz H., Hoggart S.P.G., Jackson J.E., Moore P., Morgan E.H., Perkol-Finkel S., Skov M.W., Strain E.M., van Belzen J. & Hawkins S.J. (2014) Between a rock and a hard place: environmental and engineering considerations when designing coastal defence structures. Coastal Engineering, 87, 122-135.
A replicated, paired sites, controlled study in 2008–2010 on an intertidal breakwater on open coastline in the North Sea, Netherlands (Paalvast 2015a) reported that settlement plates with pit habitats supported similar abundances of macroalgae and invertebrates to plates without pits. Data were not statistically tested. After 28 months, there were no clear differences in macroalgal or invertebrate abundances on plates with and without pits (data not reported). Periwinkles Littorina saxatilis and Littorina neritoides were seen using pits. Concrete settlement plates (250 × 250 mm) were made with and without pit habitats using a mould. Plates with pits had 25 variable pits/plate (diameter: 12–35 mm; depth: 25–50 mm). One plate with pits and one without were placed on each of 10 horizontal and 10 vertical surfaces on each side of a concrete-block breakwater (wave-exposed, wave-sheltered) in May 2008. On the wave-exposed side, plates were at mid-highshore, while on the wave-sheltered side, plates were at low-midshore. Macroalgae and invertebrates on plates were counted during low tide over 28 months.
Study and other actions tested
Referenced paper
Paalvast P. (2015) The role of geometric structure and texture on concrete for algal and macrofaunal colonization in the marine and estuarine intertidal zone. RECIF Conference on artificial reefs: From materials to ecosystems, Caen, France, 77-84.
A replicated, paired sites, controlled study in 2009 on 14 jetty pilings in Rotterdam Port in the Rhine-Meuse estuary, Netherlands (Paalvast 2015b) reported that settlement plates with pit habitats supported similar abundances of macroalgae and invertebrates to plates without pits. Data were not statistically tested. After nine months, there were no clear differences in macroalgal or invertebrate abundances on plates with and without pits (data not reported). Periwinkles Littorina saxatilis were seen using pits. Concrete settlement plates (250 × 250 mm) were made with and without pit habitats using a mould. Plates with pits had 25 variable pits/plate (diameter: 12–35 mm; depth: 25–50 mm). One plate with pits and one without were attached to vertical surfaces on each of 14 wooden pilings at lowshore in March 2009. Macroalgae and invertebrates on plates were counted during low tide over nine months.
Study and other actions tested
Referenced paper
Paalvast P. (2015) The role of geometric structure and texture on concrete for algal and macrofaunal colonization in the marine and estuarine intertidal zone. RECIF Conference on artificial reefs: From materials to ecosystems, Caen, France, 77-84.
A replicated, randomized, controlled study in 2011–2012 on two intertidal seawalls on island coastlines in the Singapore Strait, Singapore (Loke et al. 2016; same experimental set-up as Loke et al. 2017) reported that concrete settlement plates with pit habitats, along with grooves and small ridges, supported higher macroalgal species richness but similar abundances compared with granite plates without added habitats. After 12 months, settlement plates with pits, grooves and ridges supported a total of five macroalgal species groups, while plates without supported three (data not statistically tested). Abundances of three species groups were statistically similar on plates with pits, grooves and ridges (18–41% cover) and without (5–61%) in five of six comparisons, while one group was more abundant on plates with pits, grooves and ridges (22–27 vs 5%) at one site. Abundances were similar on plates with variable (1–34%) and regular (3–41%) habitats. It is not clear whether these effects were the direct result of creating pits, grooves or ridges. Settlement plates (400 × 400 mm) were moulded with pit habitats, with grooves and small ridges, and with neither. Plates with pits, grooves and ridges were concrete with 36 square pits/plate or four-to-five grooves and ridges/plate. Pits, grooves and ridges were either regular (32 mm width, depth/height and spacing) or variable (8–56 mm). Plates without pits, grooves or ridges were granite fragments set in cement. Granite may be considered an environmentally-sensitive material compared with concrete (see “Use environmentally-sensitive material on intertidal artificial structures”). Five of each design were randomly arranged at lowshore on each of two granite boulder seawalls in July 2011. Macroalgae on plates were counted from photographs after 12 months.
Study and other actions tested
Referenced paper
Loke L.H.L., Liao L.M., Bouma T.J. & Todd P.A. (2016) Succession of seawall algal communities on artificial substrates. Raffles Bulletin of Zoology, 32, 1-10.
A replicated, randomized, controlled study in 2009–2010 on two intertidal seawalls on island coastlines in the Singapore Strait, Singapore (Loke & Todd 2016a; same experimental set-up as Loke & Todd 2016b) found that concrete settlement plates with pit habitats supported different macroalgae and invertebrate community composition with higher species richness but similar abundances compared with granite plates without pits. After 13 months, macroalgae and invertebrate species richness was higher on settlement plates with pits (11 species/plate) than without (3/plate), while abundances were statistically similar (231 vs 178 individuals/plate). Community composition differed on plates with and without pits (data reported as statistical model results). Settlement plates (200 × 200 mm) were moulded with and without pit habitats. Plates with pits were concrete with 36 square pits/plate with either regular (16 mm width, depth and spacing) or variable (4–28 mm) arrangement. Plates without pits were granite fragments set in cement. Granite may be considered an environmentally-sensitive material compared with concrete (see “Use environmentally-sensitive material on intertidal artificial structures”). Eight of each design were randomly arranged at both lowshore and highshore on each of two granite boulder seawalls in November–December 2009. Macroalgae on plates were counted from photographs and invertebrates in the laboratory after 13 months.
Study and other actions tested
Referenced paper
Loke L.H.L. & Todd P.A. (2016) Structural complexity and component type increase intertidal biodiversity independently of area. Ecology, 97, 383-393.
A replicated, randomized, controlled study in 2009–2010 on two intertidal seawalls on island coastlines in the Singapore Strait, Singapore (Loke & Todd 2016b; same experimental set-up as Loke & Todd 2016a) found that concrete settlement plates with pit habitats, along with grooves, small protrusions and small ridges, supported higher macroalgae and invertebrate species richness than granite plates without added habitats, but that abundances and community composition varied depending on the habitat arrangement, shore level and site. After 13 months, macroalgae and invertebrate species richness was higher on settlement plates with pits, grooves, protrusions and ridges than on plates without at lowshore (13–23 vs 6–10 species/plate) and highshore (5–9 vs 2–3/plate). Richness was higher on plates with variable habitats than regular ones at lowshore (22–23 vs 13–16/plate), but not highshore (6–9 vs 5–6/plate). Abundances were higher on plates with added habitats than without in four of eight comparisons (9–833 vs 3–208 individuals/plates), while community composition differed in three of four comparisons (data reported as statistical model results). In all other comparisons, results were similar (abundances: 104–1,957 vs 49–1,162/plate). It is not clear whether these effects were the direct result of creating pits, grooves, protrusions or ridges. However, richness was higher on plate quarters with pits (11 species/quarter) than ridges (6/quarter), but similar to quarters with protrusions and with grooves and ridges (both 8/quarter). Abundances were similar for all four habitats types (88–231 individuals/quarter). Settlement plates (400 × 400 mm) were moulded with and without pit habitats, along with grooves, small protrusions and small ridges. Plates with added habitats were concrete. Each 200 × 200 mm quarter contained either 36 square pits, four-to-five grooves and ridges, 36 protrusions or 12 ridges. All habitats had either regular (16 mm width, depth/height and spacing) or variable (4–28 mm) arrangement. Plates without added habitats were granite fragments set in cement. Granite may be considered an environmentally-sensitive material compared with concrete (see “Use environmentally-sensitive material on intertidal artificial structures”). Eight of each design were randomly arranged at both lowshore and highshore on each of two granite boulder seawalls in November–December 2009. Macroalgae on plates were counted from photographs and invertebrates in the laboratory after 13 months.
Study and other actions tested
Referenced paper
Loke L.H.L. & Todd P.A. (2016) Structural complexity and component type increase intertidal biodiversity independently of area. Ecology, 97, 383-393.
A replicated, randomized, controlled study in 2006–2014 on an intertidal seawall on open coastline in the Atlantic Ocean, Azores (Martins et al. 2016; same experimental set-up as Martins et al. 2010) found that creating pit habitats on the seawall had mixed effects on macroalgae and invertebrate abundances depending on the species, site and pit size and density. After seven years, abundance was higher on seawall surfaces with pits than those without for limpets Patella candei in three of four comparisons (1–20 vs 2 individuals/surface), for barnacles Chthamalus stellatus in two of four comparisons (8–27 vs 11% cover), and for periwinkles Tectarius striatus in one of four comparisons (2–11 vs 1 individuals/surface). Limpets and barnacles were more abundant on surfaces with large pits (limpets: 8–20/surface; barnacles: 25–27%) than small (limpets: 1–8/surface; barnacles: 8–12%). The opposite was true for periwinkles (large pits: 2/surface; small: 7–11/surface). Limpets were more abundant on surfaces with high-density pits (8–20/surface) than low-density (1–8/surface), whereas abundance did not significantly differ for barnacles (high-density: 12–27%; low: 8–25%) or periwinkles (high: 2–7/surface; low: 2–11/surface). Results were variable for small periwinkles Melarhaphe neritoides and macroalgae (see paper for results). Pit habitats were created by drilling into a basalt boulder seawall. Arrays of large (diameter: 24 mm) and small (12 mm) round pits (depth: 10 mm) were evenly-spaced on 250 × 250 mm seawall surfaces with high (16 pits/array) or low (8/array) densities. There were five surfaces with each size-density combination and five without pits, randomly arranged at midshore in each of two sites along the seawall. Limpets were removed from surfaces when pits were created in November 2006, then macroalgae and invertebrates were counted on surfaces with and without pits during low tide after 87 months.
Study and other actions tested
Referenced paper
Martins G.M., Jenkins S.R., Neto A.I., Hawkins S.J. & Thompson R.C. (2016) Long-term modifications of coastal defences enhance marine biodiversity. Environmental Conservation, 43, 109-116.
A replicated, randomized, controlled study in 2011–2012 on two intertidal seawalls on island coastlines in the Singapore Strait, Singapore (Loke et al. 2017; same experimental set-up as Loke et al. 2016) found that concrete settlement plates with pit habitats supported higher macroalgae and invertebrate species richness and different community composition compared with granite plates without pits. After 12 months, settlement plates with variable pits supported a total of 49 macroalgae and invertebrate species, while plates with regular pits supported 35 species and plates without pits supported 22 (data not statistically tested). Average richness was similar on plates with variable (17 species/plate) and regular (14/plate) pits, and higher on both than on plates without pits (7/plate). Community composition was similar on plates with variable and regular pits, but both differed to plates without pits (data reported as statistical model results). Settlement plates (400 × 400 mm) were moulded with and without pit habitats. Plates with pits were concrete with 36 square pits/plate with either regular (32 mm width, depth and spacing) or variable (8–56 mm) arrangement. Plates without pits were granite fragments set in cement. Granite may be considered an environmentally-sensitive material compared with concrete (see “Use environmentally-sensitive material on intertidal artificial structures”). Five of each design were randomly arranged at lowshore on each of two granite boulder seawalls in July 2011. Macroalgae and invertebrates on plates were counted in the laboratory after 12 months.
Study and other actions tested
Referenced paper
Loke L.H.L., Bouma T.J. & Todd P.A. (2017) The effects of manipulating microhabitat size and variability on tropical seawall biodiversity: field and flume experiments. Journal of Experimental Marine Biology and Ecology, 492, 113-120.
A replicated, controlled, before-and-after study in 2014–2015 on an intertidal seawall on open coastline in the North Sea, UK (Hall et al. 2018a) found that creating pit habitats on the seawall altered the macroalgae and invertebrate community composition and increased their species diversity, richness and abundance on seawall surfaces. After 12 months, the macroalgae and invertebrate species diversity (data reported as Shannon index), richness and abundance were higher on surfaces with pits (3 species/surface, 99 individuals/surface) than without (1 species/surface, 26 individuals/surface), and also compared with surfaces before pits were created (0 species and individuals/surface). Community composition differed on surfaces with and without pits (data reported as statistical model results). One macroalgal species recorded on surfaces with pits was absent from those without. Pit habitats were created by drilling into vertical surfaces of a granite boulder seawall. Round pits were in arrays of four (diameter: 16 mm; depth: 20 mm; 70 mm apart) on 200 × 200 mm seawall surfaces. There were 16 surfaces with pits and 16 without at mid-lowshore. Pits were angled to retain water. Organisms were removed from surfaces when pits were created in October 2014, then macroalgae and invertebrates were counted on surfaces with and without pits during low tide over 12 months.
Study and other actions tested
Referenced paper
Hall A.E., Herbert R.J.H., Britton J.R. & Hull S.L. (2018) Ecological enhancement techniques to improve habitat heterogeneity on coastal defence structures. Estuarine, Coastal and Shelf Science, 210, 68-78.
A replicated, controlled, before-and-after study in 2015–2016 on two intertidal groynes on open coastline in the English Channel, UK (Hall et al. 2018b) found that creating pit habitats on the groynes altered the macroalgae, invertebrate and fish community composition and increased their species diversity and richness but not abundance on groyne surfaces. After 12 months, macroalgae, invertebrate and fish species diversity (data reported as Shannon index) was higher on surfaces with pits than without, and also compared with surfaces before pits were created. Species richness on surfaces with pits (2 species/surface) was statistically similar to surfaces without (1/surface), but higher than before pits were created (1/surface). Abundances were lower on surfaces with pits (7 individuals/surface) than without (65/surface), and statistically similar to before pits were created (33/surface). Community composition differed on surfaces with and without pits (data reported as statistical model results). Five species (2 mobile invertebrates, 2 non-mobile invertebrates, 1 fish) recorded on surfaces with pits were absent from those without. Pit habitats were created by drilling into vertical surfaces of two limestone boulder groynes. Round pits were in arrays of four (diameter: 16 mm; depth: 20 mm; 70 mm apart) on 200 × 200 mm groyne surfaces. There were 48 surfaces with pits and 48 without at lowshore. Pits were angled to retain water. Organisms were removed from surfaces when pits were created in March 2015, then macroalgae, invertebrates and fishes were counted on surfaces with and without pits during low tide over 12 months.
Study and other actions tested
Referenced paper
Hall A.E., Herbert R.J.H., Britton J.R. & Hull S.L. (2018) Ecological enhancement techniques to improve habitat heterogeneity on coastal defence structures. Estuarine, Coastal and Shelf Science, 210, 68-78.
A replicated, randomized, controlled study in 2013–2016 on an intertidal seawall on open coastline in the Atlantic Ocean, Azores (Cacabelos et al. 2019) reported that pit habitats created on the seawall supported more limpets Patella candei, periwinkles Tectarius striatus and small periwinkles Melarhaphe neritoides than seawall surfaces without pits, and found that their species richness and diversity (but not abundance) varied depending on the pit patchiness. After 30 months, average limpet and periwinkle species richness was 2 species/surface with pits and 1/surface without, while average abundances were 2–22 individuals/surface with pits and 0/surface without (data not statistically tested). Species diversity and richness varied depending on the pit patchiness, but average abundances did not. Pit occupancy and the effects of patchiness on total abundances varied by species (see paper for details). Pit habitats were created in December 2013 by drilling into a basalt boulder seawall. Arrays of 16 round pits (diameter: 12 mm; depth: 10 mm) on 250 × 250 mm surfaces had three levels of patchiness: high (4 patches of 4); moderate (2 patches of 8); and low (evenly-spaced). There were five surfaces of each and five without pits, randomly arranged at midshore in each of two sites along the seawall. Seasnails were counted on surfaces with and without pits during low tide after 30 months.
Study and other actions tested
Referenced paper
Cacabelos E., Thompson R.C., Prestes A.C.L., Azevedo J.M.N., Neto A.I. & Martins G.M. (2019) Patchiness in habitat distribution can enhance biological diversity of coastal engineering structures. Aquatic Conservation: Marine and Freshwater Ecosystems, 29, 127-135.
A replicated, randomized, controlled study in 2014–2015 on an intertidal seawall on an island coastline in the Singapore Strait, Singapore (Loke et al. 2019) found that creating pit habitats on the seawalls, along with grooves, increased the macroalgae and invertebrate species richness on seawall surfaces, and that increasing the density and fragmentation of pits and grooves had mixed effects on species richness. After 12 months, macroalgae and invertebrate species richness was higher on seawall surfaces with pits and grooves (13–29 species/surface) than on surfaces without (3/surface). Species richness varied on surfaces with high-density (19–29/surface), medium-density (14–27/surface) and low-density (13–16/surface) pits and grooves, depending on their arrangement, and vice versa (unfragmented arrangement: 14–20/surface; moderately-fragmented: 13–29/surface; highly-fragmented: 15–20/surface). It is not clear whether these effects were the direct result of creating pits or grooves. Concrete settlement plates (200 × 200 mm) were moulded with 37 round pit habitats amongst seven grooves, both with variable length, width and depth (2–56 mm). Plates with pits and grooves were attached to 2.4 × 2.4 m seawall surfaces in varying densities (high: 30 plates/surface; medium: 20/surface; low: 10/surface) and arrangement (unfragmented, moderately-fragmented, highly-fragmented). Four surfaces with each density-fragmentation combination and four with no plates were randomly arranged, spanning low-highshore, on a granite boulder seawall in February 2014. Macroalgae on seawall surfaces with and without plates were counted from photographs and invertebrates in the laboratory after 12 months.
Study and other actions tested
Referenced paper
Loke L.H.L., Chisholm R.A. & Todd P.A. (2019) Effects of habitat area and spatial configuration on biodiversity in an experimental intertidal community. Ecology, 100, e02757.
A replicated, randomized, controlled study in 2016–2017 on two intertidal seawalls in the Clyde estuary and on open coastline in the English Channel, UK (MacArthur et al. 2019a) found that creating pit habitats on seawall surfaces, along with using environmentally-sensitive material, increased the macroalgae and invertebrate species richness on surfaces at one of two sites, and increased invertebrate abundances at both sites. After 18 months, at one of two sites, macroalgae and mobile invertebrate species richness was higher on settlement plates with pits (2 species/plate) than without (1/plate), but was statistically similar on plates with and without pits at the second site (2 vs 1/plate). At both sites, plates with pits had higher mobile invertebrate abundance (4–11 individuals/plate) and barnacle (Cirripedia) cover (49–74%) than plates without (mobiles: 1/plate; barnacles: 22–34%). It is not clear whether these effects were the direct result of creating pits or using environmentally-sensitive material. Settlement plates (150 × 150 mm) were moulded with and without pit habitats. Plates with pits had multiple irregular pits (maximum depth: 30 mm). Eight limestone-cement (environmentally-sensitive material) plates with pits and eight concrete plates without were randomly arranged at upper-midshore on each of two vertical concrete seawalls in April–May 2016. Macroalgae and invertebrates on plates were counted from photographs over 18 months.
Study and other actions tested
Referenced paper
MacArthur M., Naylor L., Hansom J.D., Burrows M.T., Loke L.H.L. & Boyd I. (2019) Maximising the ecological value of hard coastal structures using textured formliners. Ecological Engineering X, 1, 100002.
A replicated, randomized, controlled study in 2016–2017 on two intertidal seawalls in the Clyde and Forth estuaries, UK (MacArthur et al. 2019b) found that creating pit habitats on the seawalls, along with grooves, had mixed effects on the macroalgae and invertebrate species richness and invertebrate abundances, depending on the site. After 18 months, at one of two sites, macroalgae and mobile invertebrate species richness and mobile invertebrate abundances were higher on settlement plates with pits and grooves (4 species/plate, 11 individuals/plate) than without (1 species/plate, 1 individual/plate), but barnacle (Cirripedia) cover was similar on plates with and without pits and grooves (15 vs 22%). At the second site, richness and mobile invertebrate abundances were similar on plates with and without pits and grooves (2 vs 1 species/plate, both 3 individuals/plate), while barnacle cover was lower on plates with pits and grooves (73 v 83%). It is not clear whether these effects were the direct result of creating pits or grooves. Concrete settlement plates (150 × 150 mm) were moulded with and without pit habitats and grooves. Plates with pits and grooves had 37 round pits amongst seven grooves, both with variable dimensions (maximum depth: 30 mm). Eight plates with pits and grooves and eight without were randomly arranged at upper-midshore on each of two vertical concrete seawalls in April–May 2016. Macroalgae and invertebrates on plates were counted from photographs over 18 months.
Study and other actions tested
Referenced paper
MacArthur M., Naylor L., Hansom J.D., Burrows M.T., Loke L.H.L. & Boyd I. (2019) Maximising the ecological value of hard coastal structures using textured formliners. Ecological Engineering X, 1, 100002.
A replicated study in 2018–2019 on four intertidal seawalls on island coastlines in the Singapore Strait, Singapore, and in the Plym and Tamar estuaries, UK (Hsiung et al. 2020) reported that settlement plates with pit habitats supported macroalgae and invertebrates. Over 12 months, settlement plates with pits supported 67 invertebrate species in total (Singapore: 54; UK: 13). After 12 months, there were 3–21 species/plate and 6–216 individuals/plate. Plates supported 1–5% cover of limpets (Patellidae, Fissurellidae, Siphonariidae, Lottioidea), 18–25% cover of barnacles (Cirripedia), 4–8% cover of ephemeral green macroalgae, and 29–35% cover of encrusting macroalgae. Concrete settlement plates (200 × 200 mm) were moulded with 15 water-retaining round pit habitats (diameter: 6–28 mm; depth not reported) over half their surfaces. Plates had either reduced pH (environmentally-sensitive material) or standard pH. Twenty-four of each were attached at a 60° angle at midshore on each of two seawalls in both Singapore and the UK during February–March 2018. Macroalgae on plates were counted from photographs and invertebrates in the laboratory over 12 months. Eight plates were missing and no longer provided habitat.
Study and other actions tested
Referenced paper
Hsiung A.R., Tan W.T., Loke L.H.L., Firth L.B., Heery E.C., Ducker J., Clark V., Pek Y.S., Birch W.R., Ang A.C.F., Hartanto R.S., Chai T.M.F. & Todd P.A. (2020) Little evidence that lowering the pH of concrete supports greater biodiversity on tropical and temperate seawalls. Marine Ecology Progress Series, 656, 193-205.
A replicated, randomized, controlled study in 2018–2019 on an intertidal seawall on an island coastline in the Singapore Strait, Singapore (Taira et al. 2020) found that creating pit habitats on the seawall, along with grooves, increased the macroalgae and non-mobile invertebrate abundance, fish species richness and abundance, and altered the fish community composition and behaviour on and around seawall surfaces. After 12 months, macroalgae and non-mobile invertebrate abundance was higher on seawall surfaces with pits and grooves (17% cover) than on surfaces without (4%). Over 12 months, fish community composition differed on and around surfaces with and without pits and grooves (data reported as statistical model results). Fish species richness and maximum abundance were higher on and around surfaces with pits and grooves (9–15 species and 14–29 individuals/60-minute survey) than without (7–14 species/survey, 10–25 individuals/survey), and fishes took more bites from surfaces with pits and grooves (18–456 vs 4–17 bites/survey). Eleven fish species recorded on and around surfaces with pits and grooves were absent from those without. It is not clear whether these effects were the direct result of creating pits or grooves. Concrete settlement plates (200 × 200 mm) were moulded with 37 round pit habitats amongst seven grooves, both with variable length, width and depth (2–56 mm). Twenty plates with pits and grooves were attached to 2.4 × 2.4 m seawall surfaces in seven irregularly-spaced patches. Plates had been naturally-colonized since February 2015. Six surfaces with plates and six without were randomly arranged, spanning low-highshore, on a granite boulder seawall in February 2018. Macroalgae and non-mobile invertebrates on seawall surfaces with and without plates were counted from photographs, while fishes and the number of bites they took were counted from 60-minute videos during each of seven high tides over 12 months.
Study and other actions tested
Referenced paper
Taira D., Heery E.C., Loke L.H.L., Teo A., Bauman A.G. & Todd P.A. (2020) Ecological engineering across organismal scales: trophic-mediated positive effects of microhabitat enhancement on fishes. Marine Ecology Progress Series, 656, 181-192.

Please cite as:

Evans, A.J., Moore, P.J., Firth, L.B., Smith, R.K., and Sutherland, W.J. (2021) Enhancing the Biodiversity of Marine Artificial Structures: Global Evidence for the Effects of Interventions. Conservation Evidence Series Synopses. University of Cambridge, Cambridge, UK.

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This Action forms part of the Action Synopsis:

Biodiversity of Marine Artificial Structures

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Actions	Effectiveness	Studies	Category
Create large ridges or ledges (>50 mm) on intertidal artificial structures Action Link	Awaiting assessment	3	Synopsis Link
Create large protrusions (>50 mm) on intertidal artificial structures Action Link	Awaiting assessment	2	Synopsis Link
Create textured surfaces (≤1 mm) on intertidal artificial structures Action Link	Awaiting assessment	4	Synopsis Link