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

Beds of dead coral rubble are widely reported to impede coral recovery, and causation has been linked to rubble instability. However, additional environmental factors, including flow and sedimentation, may also impact coral recruitment in this habitat. Indeed, the drivers of coral recruitment likely differ with the type of rubble. Here, we experimentally remove the influence of rubble mobility and evaluate how different rubble environments influence the process of coral settlement and recruitment (a loose vs. interlocked rubble habitat). Stabilised rubble recruitment tiles were deployed both flush with the substrate and elevated 20 cm above the substrate to separate substrate/environmental effects from differential larval supply among treatments. Coral settlement was greatest on stabilised tiles deployed in the more structurally complex habitat (reef carbonate without rubble). Settlement declined by 2 to 4-fold in rubble habitats with high interlocking structure and declined even further (8-fold) in structurally simple, loose rubble. These trends reflect environmental drivers rather than any differences in rubble stability. Post-settlement mortality was high across stabilised tiles regardless of habitat. While recruitment tracked the improvement in settlement from loose to interlocked rubble, recruitment from interlocked rubble to reef habitat became comparable, 12 months post-deployment. Even if stabilised, loose rubble beds are likely to have low recovery prospects, whereas interlocked rubble beds can provide suitable coral substrates under the right environmental conditions. Rubble stabilisation alone will not increase coral recovery within certain types of rubble beds, and this should be considered when determining how or if restoration intervention is required.