Multi‐metric complexity drives ecological succession on blue‐green infrastructure across scales

Seawall construction to protect coastlines from the impacts of climate change has resulted in the loss of marine biodiversity and ecosystem services globally. These negative impacts have been associated with reductions in habitat complexity, both topographic and biogenic. In response, habitat rehabilitation through eco‐engineering has been promoted as a means to restore complexity and counteract ecosystem loss in urban seascapes.

However, greater understanding of community establishment on eco‐engineered structures hinges on a stronger form‐independent concept of complexity‐biodiversity relationships, linking reproducible complexity metrics to multiple biodiversity responses, and thus advancing the generalisability and applicability of ecological findings beyond design‐specific contexts. Here, we show that qualitative and quantitative facets of complexity drive patterns of ecological succession on a large‐scale, intertidal eco‐engineering installation across 2 years.

At three habitat scales, complexity facilitated increased alpha and gamma diversity, enhanced habitat‐forming seaweed and grazer abundance, and supported spatial niche partitioning. Multiple quantitative metrics of topographic (e.g. rugosity across scales of 10–100 mm), spatial (e.g. number of neighbouring panels) and biogenic complexity (canopy cover) emerged as significant drivers of key biodiversity metrics.

Synthesis and applications . Our findings demonstrate that restoring coastal habitat complexity can substantially improve ecosystem diversity and functioning, offering a scalable marine conservation approach within increasingly engineered seascapes. We highlight the need for multi‐metric, multi‐scale assessments of spatio‐temporal community formation on eco‐engineered infrastructure and show that both topographic and biogenic complexity support biodiversity gains, driven by biomimetic surface structures and seaweed‐based facilitation cascades, respectively. Topography‐based effects on intertidal foundation species (e.g. seaweeds) deserve particular focus for the long‐term environmental management of coastal habitats, due to the capacity of these species to provide ecosystem services and mitigate climate change impacts. Thus, while we recommend that measurements of habitat complexity should progress towards quantitative, generalisable metrics, practical efforts to restore biodiversity through eco‐engineering should be tailored to key habitat‐forming species of the local environment.