Create small adjoining cavities or ‘swimthrough’ habitats (≤100 mm) on intertidal artificial structures

Overall effectiveness category Awaiting assessment
Number of studies: 2
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Background information and definitions

Definition: ‘Small adjoining cavities or ‘swimthrough’ habitats’ are adjoining internal cavities sheltered from, but with access to/from, outside the structure. Dimensions can vary but are ≤100 mm in any direction.

Small adjoining cavities or ‘swimthrough’ habitats are not well-studied in intertidal rocky habitats. They may form through weathering of softer rocks, amongst loosely-consolidated cobbles/boulders, or amongst three-dimensional structures created by living organisms. They likely provide organisms refuge from desiccation, temperature fluctuations and predation, in the same way crevice, hole and rock pool habitats do (Menge & Lubchenco 1981; Williams & Morritt 1995). They could also serve as corridors, connecting adjacent refuge habitats. The size and density of cavities or swimthroughs 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, limit their growth and get rapidly filled-up (Firth et al. 2020). Large habitats can be used by larger-bodied organisms but may not provide sufficient refuge from predators for smaller organisms. By default, cavities and swimthroughs contain shaded surfaces, which can be associated with the presence of non-native species (Dafforn 2017).

Cavities/swimthroughs are sometimes present on marine artificial structures made of consolidated boulders or blocks (Sherrard et al. 2016) or gabion baskets (Firth et al. 2014), but are absent from many other structures. Small adjoining cavities or ‘swimthrough’ habitats can be created on intertidal artificial structures by adding or removing 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.}

^{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.}

^{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., 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.}

^{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.}

^{Sherrard T.R.W., Hawkins S.J., Barfield P., Kitou M., Bray S. & Osborne P.E. (2016) Hidden biodiversity in cryptic habitats provided by porous coastal defence structures. Coastal Engineering, 118, 12–20.}

^{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

Two studies examined the effects of creating small adjoining cavities or ‘swimthrough’ habitats on intertidal artificial structures on the biodiversity of those structures. One study was on an open coastline in the UK and in an estuary in the Netherlands and one was on an open coastline in South Africa.

COMMUNITY RESPONSE (2 STUDIES)

Invertebrate community composition (1 study): One replicated, controlled study in South Africa found that creating small swimthrough habitats on intertidal artificial structures did not alter the mobile invertebrate community composition on structure surfaces.
Overall richness/diversity (1 study): One replicated study in the UK and the Netherlands found that varying the size and arrangement of small swimthrough habitats created on intertidal artificial structures did not increase the combined macroalgae and invertebrate species richness in and on the structures.
Invertebrate richness/diversity (1 study): One replicated, controlled study in South Africa found that creating small swimthrough habitats on intertidal artificial structures did not increase the mobile invertebrate species richness or diversity on structure surfaces.

POPULATION RESPONSE (2 STUDIES)

Invertebrate abundance (2 studies): One replicated, controlled study in South Africa found that creating small swimthrough habitats on intertidal artificial structures increased the mobile invertebrate abundance on structure surfaces. One replicated study in the UK and the Netherlands found that varying the size and arrangement of small swimthrough habitats altered the invertebrate abundance in and on structures.

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 study in 2011–2014 on 30 rock gabions on open coastline in the Irish Sea, UK and in the Eastern Scheldt estuary, Netherlands (Firth et al. 2014) found that gabions with small swimthrough habitats created amongst rocks of mixed sizes (small and large) supported similar macroalgae and invertebrate species richness and abundance to those amongst rocks of regular sizes (small or large), but that abundance was higher on gabions with regular small rocks than regular large ones. In the UK, after 12 months, 12 mobile and non-mobile invertebrate species were recorded in and on gabions with swimthroughs. Species richness and abundance was similar in and on gabions with swimthroughs amongst mixed-sized rocks (9 species/gabion, 252 individuals/gabion) and regularly-sized ones (8 species/gabion, 191–366 individuals/gabion). Abundance was higher in and on gabions with small regular rocks (366 individuals/gabion) than large regular ones (191/gabion). In the Netherlands, after 16 months, 14 macroalgae, mobile and non-mobile invertebrate species were recorded and overall species richness was similar on all gabion designs (data not reported). Small swimthrough habitats were created amongst rocks in gabion baskets (500 × 500 × 300 mm; 76 mm mesh size). Swimthroughs were either amongst rocks of mixed sizes (small: 60–100 mm and large: 180 mm), or amongst regularly-sized small or large rocks. Five of each design were placed at midshore on a boulder beach in the UK in April 2011 and at lower-midshore on a sandy beach in the Netherlands in September 2012. UK gabions were dismantled and invertebrates counted after 12 months. In the Netherlands, macroalgae and invertebrates on external horizontal gabion surfaces were counted after 16 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 (year not reported) in two intertidal boulder fields on open coastline in the Indian Ocean, South Africa (Liversage et al. 2017) found that small swimthrough habitats created under concrete blocks supported similar mobile invertebrate species richness, diversity and community composition to blocks without swimthroughs, but higher mobile invertebrate abundance. Swimthrough habitats supported similar mobile invertebrate species richness, diversity and community composition (data reported as statistical model results) but higher mobile invertebrate abundance (3 individuals/dm²) compared with blocks without swimthroughs (1/dm²). Small swimthrough habitats (length: 290 mm; width: 70 mm; height: 20 mm) were created on the undersides of concrete blocks (250 × 150 × 40 mm) using a mould. Twelve blocks with swimthroughs and 12 without were placed on the seabed in each of two natural boulder-fields (material, shore level and month/year not reported). Mobile invertebrates on the horizontal surface (roof) of swimthrough habitats and on the equivalent undersurface of blocks without swimthroughs were counted from photographs after seven weeks. Some blocks were missing and no longer provided habitat (numbers not reported).
Study and other actions tested
Referenced paper
Liversage K., Cole V., Coleman R. & McQuaid C. (2017) Availability of microhabitats explains a widespread pattern and informs theory on ecological engineering of boulder reefs. Journal of Experimental Marine Biology and Ecology, 489, 36-42.

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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