Instability is not the only driver of rubble impacts on coral settlement and recruitment

Abstract Beds of coral rubble are widely considered to inhibit coral recovery, largely due to rubble mobility. However, environmental factors, such as flow and sedimentation, and differences among rubble types, may also shape recruitment patterns. To isolate these effects, we experimentally removed the influence of rubble mobility by deploying coral recruitment tiles made from rubble fragments fixed to mesh across three habitats: loose rubble, interlocked rubble, and hard reef (control) at Heron and Wistari reefs, southern Great Barrier Reef. We then evaluated coral settlement (at 4 months) and recruitment (at 12 months). Settlement was highest on tiles in the structurally complex reef, two- to fourfold lower in the interlocked rubble, and up to eightfold lower in the loose rubble. Nonetheless, settlement occurred across all habitats, indicating that differences were driven primarily by environmental and ecological factors intrinsic to each substrate rather than by substrate stability alone. High post-settlement mortality led to similar recruitment across habitats after 12 months. These findings suggest that settlement declines with decreasing habitat stability and complexity, even on tiles providing a consistently stable settlement surface, but that recruitment can converge across habitats. Overall, the findings highlight the importance of rubble stabilisation, alongside addressing environmental constraints on settlement in rubble-dominated systems.