Create large ridges or ledges (>50 mm) on subtidal artificial structures

Overall effectiveness category No evidence found (no assessment)
Number of studies: 0
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Effectiveness

not assessed
Certainty

not assessed
Harms

not assessed

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Background information and definitions

Definition: ‘Large ridges and ledges’ are elevations with a length to width ratio >3:1 that protrude >50 mm from the substratum (modified from “Large 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.

Large ridges and ledges create vertical or horizontal (i.e. overhangs) relief in subtidal rocky habitats. They can provide organisms refuge from predation (Meese & Lowe 2020) 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 quarried boulders used in marine artificial structures (MacArthur et al. 2020), but are often absent from other types of structures. Large 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.

^{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.}

^{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.}

^{Meese E.N. & Lowe C.G. (2020) Environmental effects on daytime sheltering behaviours of California horn sharks (Heterodontus francisci). Environmental Biology of Fishes, 103, 703–717.}

^{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.}

Key messages

We found no studies that evaluated the effects of creating large ridges or ledges on subtidal artificial structures on the biodiversity of those structures.

This means we did not find any studies that directly evaluated this intervention during our literature searches. Therefore, we have no evidence to indicate whether or not the intervention has any desirable or harmful effects.

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.

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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Manage or restrict harvesting of species on subtidal artificial structures Action Link	Awaiting assessment	3	Synopsis Link
Create pit habitats (1–50 mm) on subtidal artificial structures Action Link	Awaiting assessment	1	Synopsis Link
Create large protrusions (>50 mm) on subtidal artificial structures Action Link	No evidence found (no assessment)	0	Synopsis Link