A global analysis of complexity–biodiversity relationships on marine artificial structures

Abstract

Aim Topographic complexity is widely accepted as a key driver of biodiversity, but at the patch‐scale, complexity–biodiversity relationships may vary spatially and temporally according to the environmental stressors complexity mitigates, and the species richness and identity of potential colonists. Using a manipulative experiment, we assessed spatial variation in patch‐scale effects of complexity on intertidal biodiversity. Location 27 sites within 14 estuaries/bays distributed globally. Time period 2015–2017. Major taxa studied Functional groups of algae, sessile and mobile invertebrates. Methods Concrete tiles of differing complexity (flat; 2.5‐cm or 5‐cm complex) were affixed at low–high intertidal elevation on coastal defence structures, and the richness and abundance of the colonizing taxa were quantified after 12 months. Results The patch‐scale effects of complexity varied spatially and among functional groups. Complexity had neutral to positive effects on total, invertebrate and algal taxa richness, and invertebrate abundances. However, effects on the abundance of algae ranged from positive to negative, depending on location and functional group. The tidal elevation at which tiles were placed accounted for some variation. The total and invertebrate richness were greater at low or mid than at high intertidal elevations. Latitude was also an important source of spatial variation, with the effects of complexity on total richness and mobile mollusc abundance greatest at lower latitudes, whilst the cover of sessile invertebrates and sessile molluscs responded most strongly to complexity at higher latitudes. Conclusions After 12 months, patch‐scale relationships between biodiversity and habitat complexity were not universally positive. Instead, the relationship varied among functional groups and according to local abiotic and biotic conditions. This result challenges the assumption that effects of complexity on biodiversity are universally positive. The variable effect of complexity has ramifications for community and applied ecology, including eco‐engineering and restoration that seek to bolster biodiversity through the addition of complexity.

Habitat Complexity Affects the Structure but Not the Diversity of Sessile Communities on Tropical Coastal Infrastructure

“…In the global study carried out by Strain et al (2021), the addition of physical complexity consistently enhanced sessile invertebrate species richness in most (11 out of 14 locations) of the study sites regardless of their regional climates. However, our study in Penang showed no differences in the species richness of sessile organisms between complex and flat tiles.…”

“…These results contradicted with the outcomes of Vozzo et al (2021) and Strain et al (2020) where the addition of complexity, and surface area through habitat structure and seeding increased species richness and diversity of sessile taxa in Sydney. Conversely, Strain et al (2021) found negative or no effects of adding physical complexity and surface area on diversity of sessile taxa in 3 locations, across the globe (see also O'Shaughnessy et al, 2021 which was conducted in the subtidal). This suggests that the effects of complexity on the diversity of colonising organisms is context dependent and varies between locations.…”

“…Proof-of-concept trials from tropical regions, where the effects of temperature and desiccation are more pronounced and of greater threat to intertidal organisms, are lacking (but see Loke et al, 2019;Bradford et al, 2020;Chee et al, 2020). The World Harbour Project (WHP) was designed to address this knowledge gap, by deploying complex tiles manufactured using 3D printing technology to mimic natural habitat features on rocky shores on seawalls, globally (Strain et al, 2021).…”

Do positive interactions between marine invaders increase likelihood of invasion into natural and artificial habitats?

“…Worldwide, efforts to enhance biodiversity of artificial structures are on‐going (Morris et al 2019 and O'Shaughnessy et al 2020b for reviews), including the use of transplantation of ecosystem‐engineer species to enhance biodiversity of ordinarily depauperate surfaces (Ng et al 2015, Ferrario et al 2016). To date, these efforts have received relatively little attention, but trials have indicated promising (Perkol‐Finkel et al 2012), but variable results (Strain et al 2021). Whilst the use of habitat‐forming species for restoration efforts has been advocated and may well lead to positive biodiversity outcomes (Byers et al 2006), care must also be taken advocating the use of invasive and non‐native species, without full consideration of the wider environmental implications (Sotka and Byers' (2019) criticism of Ramus et al (2017)) which may yield unexpected results.…”

Spatially Variable Effects of Artificially-Created Physical Complexity on Subtidal Benthos

“…Research and development of habitat creation in coastal environments, however, is decades behind artificial reef research, but has flourished in recent years (for a review see Strain et al, 2017a). IGGI interventions in coastal environments have ranged from the addition of microtexture (Coombes et al, 2015), crevices and grooves (Martins et al, 2010;Borsje et al, 2011) and artificial rock pools (Evans et al, 2016;Firth et al, 2016b) to existing coastal structures, to the use of precast habitat enhancement units and panels (Browne and Chapman, 2014;Perkol-Finkel et al, 2018;Evans et al, 2021;Strain et al, 2021), all yielding promising biodiversity outcomes. Furthermore, habitatforming species such as mussels and oysters (Vozzo et al, 2021), corals (Ng et al, 2015) and canopy-forming algae (Falace et al, 2006;Perkol-Finkel et al, 2012), can be seeded onto artificial structures to encourage settlement of conspecifics and increase biodiversity.…”