Create small ridges or ledges (1–50 mm) on subtidal artificial structures
Overall effectiveness category No evidence found (no assessment)
Number of studies: 0
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 subtidal rocky habitats. They can provide organisms refuge from predation or grazing (Wahl & Hoppe 2002) and have positive effects on fish populations (Morris et al. 2018). Some species preferentially recruit to habitats with high vertical or horizontal relief (Andrews & Anderson 2004). 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 artificial structures such as cable mattresses (Lacey & Hayes 2020) or quarried boulders (MacArthur et al. 2020), but are often absent from other types of structures. Small ridges and ledges can be created on subtidal artificial structures by adding material, either during construction or retrospectively.
There is a body of literature investigating the effects of creating these habitats on artificial reefs (e.g. Gratwicke & Speight 2005; Morris et al. 2018). These studies are not included in this synopsis, which focusses on in situ conservation actions to enhance the biodiversity of structures that are engineered to fulfil a primary function other than providing artificial habitats.
See also: Create textured surfaces (≤1 mm) on subtidal artificial structures; Create natural rocky reef topography on subtidal artificial structures; Create small protrusions (1–50 mm) on subtidal artificial structures; Create large protrusions (>50 mm) on subtidal artificial structures; Create large ridges or ledges (>50 mm) on subtidal artificial structures; Create groove habitats and small protrusions, ridges or ledges (1–50 mm) on subtidal artificial structures.
Andrews K.S. & Anderson T.W. (2004) Habitat-dependent recruitment of two temperate reef fishes at multiple spatial scales. Marine Ecology Progress Series, 277, 231–244.
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.
Gratwicke B. & Speight M.R. (2005) Effects of habitat complexity on Caribbean marine fish assemblages. Marine Ecology Progress Series, 292, 301–310.
Lacy N.C. & Hayes P. (2020) Epifauna associated with subsea pipelines in the North Sea. ICES Journal of Marine Science, 77, 1137–1147.
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.
Morris R.L., Porter A.G., Figueira W.F., Coleman R.A., Fobert E.K. & Ferrari R. (2018) Fish-smart seawalls: a decision tool for adaptive management of marine infrastructure. Frontiers in Ecology and the Environment, 16, 278–287.
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.