Structural complexity of hard substrates shapes shallow marine benthic communities

Structurally complex habitats offer niche diversity, with varying biotic and abiotic conditions, generally leading to higher biodiversity compared to simpler habitats. However, our understanding of the response of ecological communities to increased complexity and the underlying drivers remain ambiguous due to discrepancies in how complexity is measured across ecosystems, warranting further experimental testing. Here, we used 3D-printed reef structures to assess how structural complexity affects recruitment in a shallow coastal ecosystem in a highly standardised manner. We deployed fractal-like (self-similar) pyramid-shaped structures of three levels of standardized complexity on sandy intertidal and subtidal sandflats, keeping material, shape, and external dimensions consistent across levels. More complex structures had a higher surface area within the same totalvolume and more empty space. We examined the effects on taxonomic diversity, abundances, densities, and spatial patterning of five reef taxa: barnacles, mussels, tunicates, anemones, and algae. Increased complexity resulted in higher taxonomic richness in the intertidal zone but not in the subtidal. Taxonomic accumulation curves indicated increased within-sample heterogeneity and niche diversity in complex substrates. The effects of complexity on abundances (total n per structure) and densities (n per cm²) varied by taxon and tidal zone, suggesting life-strategies shape these responses. Mussels benefited from increased structural complexity regardless of tidal elevation, which we hypothesise is due to reduced predation. Other taxa benefited from the increased surface area in more complex structures. Only barnacles, mainly found on the middle and outer parts of the structures, showed clear spatial patterning within structures, which could potentially be driven by a tradeoff between feeding efficiency and predation risk. Our findings demonstrate how standardised structural complexity shapes composition and diversity of epibenthic communities in coastal ecosystems. We argue that applying structural complexity in restoration efforts could enhance biodiversity in structurally complex ecosystems such as reefs, mangroves and streams.