Create pit habitats (1–50 mm) on subtidal artificial structures
Overall effectiveness category Awaiting assessment
Number of studies: 1
Background information and definitions
Definition: ‘Pit habitats’ are depressions with a length to width ratio ≤3:1 and depth 1–50 mm (Strain et al. 2018).
Pit habitats provide organisms refuge from predation in subtidal rocky habitats (Nelson & Vance 1979). Some species preferentially settle into them (Nozawa et al. 2011). The size and density of pits is likely to affect the size, abundance and variety of organisms that can use them. Small pits 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 pits can be used by larger-bodied organisms but may not provide sufficient refuge from predators for smaller organisms.
Pits are sometimes present on boulders used in marine artificial structures as a result of quarrying processes (Hall et al. 2018), and can form on other structures through erosion. However, these are often filled or repaired during maintenance works (Moreira et al. 2007) and are absent from many structures. Pit habitats can be created on subtidal artificial structures by adding or removing material, either during construction or retrospectively.
There is a body of literature investigating the effects of creating pit habitats on artificial substrates for coral rearing and gardening (e.g. Nozawa et al. 2011; Okamoto et al. 2010). 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 hole habitats (>50 mm) on subtidal artificial structures; Create groove habitats (1–50 mm) on subtidal artificial structures; Create crevice habitats (>50 mm) on subtidal artificial structures; Create groove habitats and small protrusions, ridges or ledges (1–50 mm) on subtidal artificial structures.
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.
Hall A.E., Herbert R.J.H., Britton J.R. & Hull S.L. (2018) Ecological enhancement techniques to improve habitat heterogeneity on coastal defence structures. Estuarine, Coastal and Shelf Science, 210, 68–78.
Moreira J., Chapman M.G. & Underwood A.J. (2007) Maintenance of chitons on seawalls using crevices on sandstone blocks as habitat in Sydney Harbour, Australia. Journal of Experimental Marine Biology and Ecology, 347, 134–143.
Nelson B.V. & Vance R.R. (1979) Diel foraging patterns of the sea urchin Centrostephanus coronatus as a predator avoidance strategy. Marine Biology, 51, 251–258.
Nozawa Y., Tanaka K. & Reimer J.D. (2011) Reconsideration of the surface structure of settlement plates used in coral recruitment studies. Zoological Studies, 50, 53–60.
Okamoto M., Yap M., Roeroe A.K., Nojima S., Oyamada K., Fujiwara S. & Iwata I. (2010) In situ growth and mortality of juvenile Acropora over 2 years following mass spawning in Sekisei Lagoon, Okinawa (24°N). Fisheries Science, 76, 343–353.
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.
Supporting evidence from individual studies
A replicated, controlled study in 2012–2014 on two subtidal breakwaters on open coastline in the Mediterranean Sea, Israel (Sella & Perkol-Finkel 2015) found that pit habitats created on breakwater blocks, along with holes, grooves and environmentally-sensitive material, supported different macroalgae and invertebrate community composition with higher species diversity than standard-concrete blocks without added habitats, while macroalgae, invertebrate and fish abundances varied depending on the species group. After 24 months, the macroalgae and invertebrate species diversity was higher on blocks with added habitats than without (data reported as Shannon index) and the community composition differed (data reported as statistical model results). Thirty species (7 mobile invertebrates, 14 non-mobile invertebrates, 9 fishes) recorded on and around blocks with added habitats were absent from blocks without. Species abundances varied on blocks with and without added habitats depending on the species group (see paper for results). It is not clear whether these effects were the direct result of creating pits, holes, grooves, or using environmentally-sensitive material. Pit habitats were created on breakwater blocks (1 × 1 × 1 m) using a formliner. Each block had multiple round pits (diameter: 10 mm; depth: 5 mm; T. Hadary pers. comms.) amongst multiple holes and grooves (number/spacing not reported). Five blocks of each of three patented ECOncreteTM materials (lower pH and different cement/additives to standard-concrete) were placed at 5–7 m depth on a concrete-block breakwater during construction in July 2012. Five standard-concrete blocks (1.7 × 1.7 × 1.7 m) without added habitats were placed on a similar breakwater 80 m away. Macroalgae and invertebrates on blocks, and fishes on and around blocks, were counted over 24 months.Study and other actions tested