Create short flexible habitats (1–50 mm) on subtidal artificial structures

Overall effectiveness category Awaiting assessment
Number of studies: 3
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Background information and definitions

Definition: ‘Short flexible habitats’ are flexible protruding materials such as rope, ribbon or twine 1–50 mm in length (modified from “Soft structures” in Strain et al. 2018).

Short flexible habitats, such as understory macroalgal blades, turfs and soft-bodied invertebrates, provide other organisms three-dimensional habitat space and refuge from predation in subtidal rocky habitats (Levin & Hay 1996). They can support high biodiversity (Smale et al. 2020) but can also dominate space and have negative effects on other species (O’Brien & Scheibling 2018). The size, density and material properties of flexible habitats are likely to affect the size, abundance and variety of organisms that can use them and the spaces they create.

Some organisms that form flexible habitats tend to be absent or sparse on artificial subtidal structures (Wilhelmsson & Malm 2008), although some readily colonize in suitable conditions. Artificial flexible habitats such as ropes or nets can be present on some structures, but are likely to be temporary and regularly disturbed (e.g. removed and replaced) when present. Short flexible habitats can be created on subtidal artificial structures by adding material, either during construction or retrospectively. Material choice is important for creating flexible habitats, since some flexible materials are unlikely to persist in the marine environment, while those that do may become entanglement hazards or contribute to pollution if dislodged.

There is a body of literature describing the use of artificial turfs as collectors to measure larval supply and settlement in subtidal rocky habitats and to investigate the effects of structural complexity on ecological interactions (e.g. Atilla & Fleeger 2000; Perrett et al. 2006; Underwood & Chapman 2006). 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. Studies that investigate the effects of transplanting live soft-bodied organisms onto structures are not included here, but are considered under the action “Transplant or seed organisms onto subtidal artificial structures”.

^{Atilla N. & Fleeger J.W. (2000) Meiofaunal colonization of artificial substrates in an estuarine embayment. Marine Ecology, 21, 69–83.}

^{Levin P.S. & Hay M.E. (1996) Responses of temperate reef fishes to alterations in algal structure and species composition. Marine Ecology Progress Series, 134, 37–47.}

^{O’Brien J.M. & Scheibling R.E. (2018) Turf wars: competition between foundation and turf-forming species on temperate and tropical reefs and its role in regime shifts. Marine Ecology Progress Series, 590, 1–17.}

^{Perrett L.A., Johnston E.L. & Poore A.G.B. (2006) Impact by association: direct and indirect effects of copper exposure on mobile invertebrate fauna. Marine Ecology Progress Series, 326, 195–205.}

^{Smale D.A., Epstein G., Hughes E., Mogg A.O.M. & Moore P.J. (2020) Patterns and drivers of understory macroalgael assemblage structure within subtidal kelp forests. Biodiversity and Conservation, 29, 4173–4192.}

^{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.}

^{Underwood A.J. & Chapman M.G. (2006) Early development of subtidal macrofaunal assemblages: relationships to period and timing of colonization. Journal of Experimental Marine Biology and Ecology, 330, 221–233.}

^{Wilhelmsson D. & Malm T. (2008) Fouling assemblages on offshore wind power plants and adjacent substrata. Estuarine, Coastal and Shelf Science, 79, 459–466.}

Study locations

Key messages

Three studies examined the effects of creating short flexible habitats on subtidal artificial structures on the biodiversity of those structures. Two studies were in an estuary in southeast Australia and one was in marinas in northwest France.

COMMUNITY RESPONSE (2 STUDIES)

Invertebrate community composition (2 studies): Two replicated, randomized, controlled studies (including one paired sites study) in Australia and France found that creating short flexible habitats on subtidal artificial structures had mixed effects on the mobile and/or non-mobile invertebrate community composition, depending on the density or length of flexible habitats and/or the site. One of the studies found it altered the non-mobile invertebrate community composition.
Invertebrate richness/diversity (1 study): One replicated, randomized, paired sites, controlled study in France found that creating short flexible habitats on subtidal artificial structures did not increase the mobile or non-mobile invertebrate species richness on structure surfaces.

POPULATION RESPONSE (3 STUDIES)

Invertebrate abundance (3 studies): Three randomized, controlled studies (including two replicated and one paired sites study) in Australia and France found that creating short flexible habitats on subtidal artificial structures had mixed effects on the mobile and/or non-mobile invertebrate abundance on and around structure surfaces, depending on the survey week, species group, flexible habitat length, or site. One of the studies found no effect on mobile invertebrate abundance.
Fish abundance (1 study): One randomized, controlled study in Australia found that creating short flexible habitats on subtidal artificial structures had mixed effects on the seahorse abundance on and around structures, depending on the survey week.

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 randomized, controlled study in 2008 on two subtidal swimming-enclosure nets in Sydney Harbour estuary, Australia (Hellyer et al. 2011) found that creating short flexible habitats (frayed-netting) on enclosure-net panels had mixed effects on seahorse Hippocampus whitei and mobile invertebrate abundances, depending on the survey week and invertebrate species group. Over two months, net panels with frayed-netting had higher seahorse abundance (1–3 individuals/panel) than panels without flexible habitats (0–1/panel) during six of seven surveys, but similar abundance during the other survey (frayed-netting: 1/panel; without: 0/panel). Mobile invertebrate abundances on panels with and without flexible habitats varied depending on the species group and survey week (see paper for results). Short flexible habitats were created on polyethylene rope swimming-enclosure nets (100 mm mesh size) in March 2008 by attaching clumps of frayed nylon netting (50 mm length) at knot intersections (‘frayed-netting’). Three net panels (length: 0.3 m, height: 1 m) with frayed-netting and three panels without were randomly arranged along each of two enclosure nets (depth not reported). In May 2008, sixty-three seahorses were released onto the nets. Seahorses were counted on panels with and without flexible habitats over two months and mobile invertebrates (seahorse prey) were surveyed using a suction-pump over three months.
Study and other actions tested
Referenced paper
Hellyer C.B., Harasti D. & Poore A.G.B. (2011) Manipulating artificial habitats to benefit seahorses in Sydney Harbour, Australia. Aquatic Conservation: Marine and Freshwater Ecosystems, 21, 582-589.
A replicated, randomized, controlled study in 2012–2013 on a subtidal dock in Sydney Harbour estuary, Australia (Lavender et al. 2017) found that creating short flexible habitats (polyethylene turf) on settlement plates altered the non-mobile invertebrate community composition on plates and had mixed effects on the mobile invertebrate community composition and invertebrate abundances, depending on the turf length and species group. After three months, non-mobile invertebrate community composition differed on settlement plates with longer and shorter turf, and both differed to plates without turf (data reported as statistical model results). Plates with longer turf also supported different mobile invertebrate composition to plates with shorter turf and without turf, which were similar. Non-mobile invertebrates were less abundant on plates with turf (0–7% cover) than without (4–28%) in nine of 14 comparisons, but similar in the other five comparisons (with turf: 5–25%; without: 4–28%). Mobile invertebrates were more abundant on plates with turf (2–324 individuals/plate) than without (0–50/plate) in 22 of 28 comparisons, but similar in six comparisons (with turf: 2–58/plate; without: 1–50/plate). Plastic settlement plates (100 × 100 mm) were made with and without short flexible habitats (polyethylene turf). Plates with turf had either longer (18 mm) or shorter (2–3 mm) blades (1.5 mm width). Twelve of each were randomly arranged at 3 m depth beneath a dock with turf facing downwards in October 2012. Invertebrates on plates were counted in the laboratory after three months.
Study and other actions tested
Referenced paper
Lavender J.T., Dafforn K.A., Bishop M.J. & Johnston E.L. (2017) Small-scale habitat complexity of artificial turf influences the development of associated invertebrate assemblages. Journal of Experimental Marine Biology and Ecology, 492, 105-112.
A replicated, randomized, paired sites, controlled study in 2014 on eight subtidal pontoons in two marinas in the English Channel and the Élorn estuary, France (Leclerc & Viard 2018) found that creating short flexible habitats (polypropylene turf) on settlement plates did not increase the invertebrate species richness or the mobile invertebrate abundance on plates, but had mixed effects on the non-mobile invertebrate abundance and the community composition, depending on the turf density and site. Mobile invertebrate species richness and abundance was similar on plates with high-density turf (22–33 species/plate, 189–1,093 individuals/plate), low-density turf (23–34 species/plate, 194–1,132 individuals/plate) and plates without turf (19–27 species/plate, 132–1,019 individuals/plate). The same was true for non-mobile invertebrate species richness (high-density: 6–10 species/plate; low-density: 8–11/plate; no turf: 7–12/plate), and their abundance at one of two sites (high-density: 95–143% cover; low-density: 90–114%; no turf: 101–119%). At the second site, abundance was lower on plates with turf (high-density: 108–156%; low-density: 117–151%) than without (120–192%). Invertebrate community composition differed on plates with and without turf in four of eight comparisons, but was similar in the other four (data reported as statistical model results). Plastic settlement plates (180 × 180 mm) were made with and without short flexible habitats (polypropylene turf). Plates with turf (blade length: 30 mm; width: 2 mm) had either high (100% cover) or low (50%) turf density. One of each was randomly arranged vertically at 1 m depth beneath each of four pontoons in each of two marinas in May 2014. Invertebrates on plates were counted in the laboratory after three months.
Study and other actions tested
Referenced paper
Leclerc J.-C. & Viard F. (2018) Habitat formation prevails over predation in influencing fouling communities. Ecology & Evolution, 8, 477-492.

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

Related Actions

Actions	Effectiveness	Studies	Category
Create long flexible habitats (>50 mm) on subtidal artificial structures Action Link	Awaiting assessment	5	Synopsis Link
Create small ridges or ledges (1–50 mm) on subtidal artificial structures Action Link	No evidence found (no assessment)	0	Synopsis Link
Cease or alter maintenance activities on subtidal artificial structures Action Link	Awaiting assessment	2	Synopsis Link