Structural complexity enhancement as a potential coral-reef restoration tool

One of the major challenges for coral reef science is to find sound management tools to reverse the rapid degradation of the world's reefs and their valuable ecosystem services. In this sense, much effort is dedicated to identify reef resilience indicators and traits, the restoration and protection of which can promote coral reef recovery following degradation. Here we examine two widely studied reef resilience traits, i.e. coral recruitment and structural complexity, and potential links between the two on different spatial scales that may be used to promote reef resilience by active restoration. Specifically, we explore the use of structural complexity enhancement (SCE; artificially increasing the complexity of a degraded reef by adding structures to it) as a potential restoration tool for increasing coral recruitment. Our SCE experiments resulted in an increase of 400-600% in coral recruitment, which was correlated with some of the spatial scales of complexity. From a management perspective, the current study supports the idea that, under certain circumstances, SCE may serve as an effective restoration tool to enhance coral recruitment, which could be an alternative or addition to coral transplantation. The results also emphasize the importance of measuring and enhancing structural complexity on different spatial scales (from mm to m) for the promotion of different functional groups (e.g. corals and fish). This approach, if further developed, may serve as a key element in restoration projects of structurally degraded coral reefs.