Create small ridges or ledges (1–50 mm) on intertidal artificial structures

Overall effectiveness category Awaiting assessment
Number of studies: 4
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Background information and definitions

Definition: ‘Small ridges and ledges’ are elevations with a length to width ratio >3:1 that protrude 1–50 mm from the substratum (modified from “Small elevations” in Strain et al. 2018). On vertical surfaces, vertically-orientated elevations that fit these criteria are referred to as ‘ridges’, while horizontal ones are referred to as ‘ledges’. On horizontal surfaces, these features are referred to as ‘ridges’ regardless of their orientation.

Small ridges and ledges create vertical or horizontal (i.e. overhangs) relief in intertidal rocky habitats. They can provide organisms refuge from desiccation and temperature fluctuations during low tide (Williams & Morrit 1995) and also shelter from predation or grazing (Wahl & Hoppe 2002). Some species preferentially recruit to habitats with high vertical or horizontal relief (Harmelin-Vivien et al. 1995). The size and density of ridges and ledges is likely to affect the size, abundance and variety of organisms that can use them. Small habitats can provide refuge for small-bodied organisms but may exclude larger organisms and limit their growth. Large habitats can be used by larger-bodied organisms but may not provide sufficient refuge from predators for smaller organisms. By default, horizontal ledges (overhangs) create shaded and downward-facing surfaces, which can be associated with the presence of non-native species (Dafforn 2017).

Ridges and ledges are sometimes present on quarried boulders used in marine artificial structures (MacArthur et al. 2020) but are often absent from other types of structures. Small ridges and ledges can be created on intertidal artificial structures by adding material, either during construction or retrospectively.

^{Dafforn K.A. (2017) Eco-engineering and management strategies for marine infrastructures to reduce establishment and dispersal of non-indigenous species. Management of Biological Invasions, 8, 153–161.}

^{Harmelin-Vivien M.L., Harmelin J.G. & Leboulleux V. (1995) Microhabitat requirements for settlement of juvenile sparid fishes on Mediterranean rocky shores. Hydrobiologia, 300, 309–320.}

^{MacArthur M., Naylor L.A., Hansom J.D. & Burrows M.T. (2020) Ecological enhancement of coastal engineering structures: passive enhancement techniques. Science of the Total Environment, 740, 139981.}

^{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.}

^{Wahl M. & Hoppe K. (2002) Interactions between substratum rugosity, colonization density and periwinkle grazing efficiency. Marine Ecology Progress Series, 225, 239–249.}

^{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

Four studies examined the effects of creating small ridges or ledges on intertidal artificial structures on the biodiversity of those structures. Two studies were on island coastlines in the Singapore Strait and two were in estuaries in Hong Kong and southeast Australia.

COMMUNITY RESPONSE (4 STUDIES)

Overall community composition (2 studies): One of two replicated, randomized, controlled studies in Singapore found that creating small ridges on intertidal artificial structures did not alter the combined macroalgae and invertebrate community composition on structure surfaces. One study found that creating small ridges, along with grooves, small protrusions and pits, had mixed effects on the community composition, depending on the site, and the size and arrangement of ridges and other habitats.
Overall richness/diversity (4 studies): One of two replicated, randomized, controlled studies in Singapore found that creating small ridges on intertidal artificial structures did not increase the combined macroalgae and invertebrate species richness on structure surfaces. One study found that creating small ridges, along with grooves, small protrusions and pits, did increase the species richness, and that varying the habitat size and arrangement had mixed effects, depending on the shore level. Two replicated studies (including one randomized, paired sites study) in Hong Kong and Australia found that small ridges or ledges supported lower species richness than grooves created in between them, but one of them found that species diversity on ridges compared with grooves varied depending on the ridge height.
Invertebrate richness/diversity (1 study): One replicated study in Australia found that small ledges created on intertidal artificial structures supported lower mobile invertebrate species richness than grooves created in between them.
Fish richness/diversity (1 study): One replicated study in Australia found that small ledges created on intertidal artificial structures supported similar fish species richness to grooves created in between them.

POPULATION RESPONSE (3 STUDIES)

Overall abundance (3 studies): One of two replicated, randomized, controlled studies in Singapore found that creating small ridges on intertidal artificial structures did not increase the combined macroalgae and invertebrate abundance on structure surfaces. One study found that creating small ridges, along with grooves, small protrusions and pits, had mixed effects on abundance, depending on the shore level, site, and the size and arrangement of ridges and other habitats. One replicated study in Australia found that small ledges supported similar abundance to grooves created in between them.

Invertebrate abundance (1 study): One replicated study in Australia found that small ledges created on intertidal artificial structures supported lower mobile invertebrate and oyster abundances than grooves created in between them.

Fish abundance (1 study): One replicated study in Australia found that small ledges created on intertidal artificial structures supported similar fish abundance to grooves created in between them.

BEHAVIOUR (0 STUDIES)

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, 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 small ridges supported similar macroalgae and invertebrate community composition, species richness and abundance to granite plates without ridges. After 13 months, macroalgae and invertebrate community composition (data reported as statistical model results), species richness and abundance were statistically similar on settlement plates with small ridges (6 species/plate; 97 individuals/plate) and without (3 species/plate, 178 individuals/plate). Settlement plates (200 × 200 mm) were moulded with and without small ridges. Plates with ridges were concrete with 12 serrated ridges/plate, with either regular (16 mm width, height and spacing) or variable (4–28 mm) arrangement. Plates without 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”). 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 small ridges, along with grooves, small protrusions and pits, 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 ridges, grooves, protrusions and pits than 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/plate), 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 ridges, grooves, protrusions or pits. However, richness was lower on plate quarters with ridges (6 species/plate) than pits (11/plate), but similar to quarters with protrusions and with grooves and ridges (both 8/plate). Abundances were similar for all habitat types (88–231 individuals/quarter). Settlement plates (400 × 400 mm) were moulded with and without small ridges, along with grooves, small protrusions and pits. Plates with added habitats were concrete. Each 200 × 200 mm quarter contained either 12 serrated ridges, four-to-five grooves and ridges, 36 protrusions or 36 pits. 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, paired sites study in 2016–2017 on two intertidal seawalls in the Pearl River estuary, Hong Kong (Bradford et al. 2020) found that small ridges created on the seawalls supported lower macroalgae and invertebrate species richness than groove habitats created in between them, while species diversity varied depending on the ridge height. After 12 months, macroalgae and invertebrate species richness on settlement plates was similar on tall and short ridges (both 3–4 species/plate), and lower on both than in the grooves in between (8–9/plate). The same was true for species diversity, except that tall ridges supported similar diversity to grooves (data reported as Shannon index). Concrete settlement plates (250 × 250 mm) were moulded with five tall (height: 50 mm) or short (25 mm) vertical small ridges (length: 250 mm; width: 17–65 mm) between four grooves (length: 250 mm; width: 15–50 mm; depth: 50 or 25 mm). Five of each were randomly arranged at midshore on each of two vertical concrete seawalls in November 2016 (month/year: M. Perkins pers. comms.). Plates had textured surfaces. Macroalgae and invertebrates on ridges and in grooves were counted in the laboratory after 12 months. One plate with tall ridges was missing and no longer provided habitat.
Study and other actions tested
Referenced paper
Bradford T.E., Astudillo J.C., Lau E.T.C., Perkins M.J., Lo C.C., Li T.C.H., Lam C.S., Ng T.P.T., Strain E.M.A., Steinberg P.D. & Leung K.M.Y. (2020) Provision of refugia and seeding with native bivalves can enhance biodiversity on vertical seawalls. Marine Pollution Bulletin, 160, 111578.
A replicated study in 2015–2016 of two intertidal seawalls in Sydney Harbour estuary, Australia (Strain et al. 2020) found that small ledges created on the seawalls supported lower macroalgae and invertebrate species richness and lower mobile invertebrate and oyster Saccostrea glomerata abundances than groove habitats created in between them, but that macroalgae and other non-mobile invertebrate abundance, fish species richness and fish abundance were similar on ledges and in grooves. After 12 months, small ledges supported lower macroalgae and non-mobile invertebrate species richness (4 species/ledge) than the grooves in between (6/groove). The same was true for mobile invertebrates (4/ledge vs 6/groove), but not fishes (both 2/sample). Abundances were lower on ledges than in grooves for mobile invertebrates (1 individual/ledge vs 18/groove) and oysters (14 vs 56% cover), but were similar for macroalgae and other non-mobile invertebrates (36 vs 38% cover) and fishes (both 1 individual/sample). Concrete settlement plates (250 × 250 mm) were moulded with five horizontal small ledges (length: 250 mm; width: 17–65 mm; height: 50 mm) between four grooves (length: 250 mm; width: 15–50 mm; depth: 50 mm). Five plates were attached at midshore on each of two vertical sandstone seawalls in November 2015. Plates had textured surfaces. Macroalgae and invertebrates were counted on ledges and in grooves during low tide, from photographs and in the laboratory after 12 months. Fishes were counted from time-lapsed photographs during two high tides.
Study and other actions tested
Referenced paper
Strain E.M.A., Cumbo V.R., Morris R.L., Steinberg P.D. & Bishop M.J. (2020) Interacting effects of habitat structure and seeding with oysters on the intertidal biodiversity of seawalls. PLoS ONE, 15, e0230807.

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.

Where has this evidence come from?

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This Action forms part of the Action Synopsis:

Biodiversity of Marine Artificial Structures

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Create short flexible habitats (1–50 mm) on intertidal artificial structures Action Link	Awaiting assessment	1	Synopsis Link
Use environmentally-sensitive material on intertidal artificial structures Action Link	Awaiting assessment	8	Synopsis Link
Control or remove non-native or nuisance species on intertidal artificial structures Action Link	No evidence found (no assessment)	0	Synopsis Link