Scaling up eco-engineering: The role of topographic complexity and spatial variability in shaping biodiversity on coastal structures

In response to the depauperate biodiversity often associated with the largely homogenous surfaces of artificial structures, ecological (eco-) engineering has emerged as a tool to introduce topographic complexity to coastal development. Although relatively small-scale studies using topographically complex panels and artificial rockpools are commonplace, determining how the configuration of these interventions works over greater and more realistic spatial scales has received comparatively little attention. Given the importance of spatial variability in habitat complexity for shaping community composition and, therefore, regional diversity, filling this knowledge gap is key to enhancing the 'design catalogue' for future eco-engineering installations. Here, we manipulated topographic complexity using individual concrete panels placed into larger arrays to generate two different spatial configurations, and in doing so explore the potentially interactive roles of small-scale panel complexity and larger-scale variability on biodiversity. More topographically complex panels supported greater taxon richness and abundance than low complexity panels, whilst the complexity of the panels in interaction with their spatial arrangement within larger arrays influenced community composition between treatment groups. Our results corroborate studies showing how small-scale variation in surface topography benefits biodiversity, but we also demonstrate that spatial variability in how this complexity is deployed over larger areas impacts community composition. These effects were especially evident for the invasive non-native species that frequently colonise and dominate newly engineered coastal structures, often at the expense of natives. Given the ongoing expansion of coastal infrastructure, studies such as this that explore means of 'scaling up' eco-engineering to better represent the inherent spatial variability of natural habitats are essential to achieving biodiversity comparable to, and potentially greater than, these habitats.