Abstract
Coral reef ecosystems are under increasing pressure from local and regional stressors and a changing climate. Current management focuses on reducing stressors to allow for natural recovery, but in many areas where coral reefs are damaged, natural recovery can be restricted, delayed or interrupted because of unstable, unconsolidated coral fragments, or rubble. Rubble fields are a natural component of coral reefs, but repeated or high-magnitude disturbances can prevent natural cementation and consolidation processes, so that coral recruits fail to survive. A suite of interventions have been used to target this issue globally, such as using mesh to stabilise rubble, removing the rubble to reveal hard substrate and deploying rocks or other hard substrates over the rubble to facilitate recruit survival. Small, modular structures can be used at multiple scales, with or without attached coral fragments, to create structural complexity and settlement surfaces. However, these can introduce foreign materials to the reef, and a limited understanding of natural recovery processes exists for the potential of this type of active intervention to successfully restore local coral reef structure. This review synthesises available knowledge about the ecological role of coral rubble, natural coral recolonisation and recovery rates and the potential benefits and risks associated with active interventions in this rapidly evolving field. Fundamental knowledge gaps include baseline levels of rubble, the structural complexity of reef habitats in space and time, natural rubble consolidation processes and the risks associated with each intervention method. Any restoration intervention needs to be underpinned by risk assessment, and the decision to repair rubble fields must arise from an understanding of when and where unconsolidated substrate and lack of structure impair natural reef recovery and ecological function. Monitoring is necessary to ascertain the success or failure of the intervention and impacts of potential risks, but there is a strong need to specify desired outcomes, the spatial and temporal context and indicators to be measured. With a focus on the Great Barrier Reef, we synthesise the techniques, successes and failures associated with rubble stabilisation and the use of small structures, review monitoring methods and indicators, and provide recommendations to ensure that we learn from past projects.
Highlights
The degradation of coral reefs worldwide is affecting the socio-ecological and economic value of reef ecosystems and the livelihoods of millions of people [1,2,3]
Coral reef restoration has been dominated by small-scale (10s to 100s of m2) projects using a few fast-growing species, with primarily aesthetic results, and their success or failure has been difficult to ascertain due to [13] a common lack of adequate monitoring [14]
In environments without direct anthropogenic pressures, natural processes may allow the stabilisation and cementation of coral rubble over time, through a succession of processes that culminates in the return of rubble fields to consolidated reef and living coral assemblages
Summary
The degradation of coral reefs worldwide is affecting the socio-ecological and economic value of reef ecosystems and the livelihoods of millions of people [1,2,3]. Synthesise the techniques, successes and failures associated with active intervention to stabilise substrate and add small structures as coral restoration methods
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