Coral restoration - A systematic review of current methods, successes, failures and future directions.

Coral reef ecosystems have suffered an unprecedented loss of habitat-forming hard corals in recent decades. While marine conservation has historically focused on passive habitat protection, demand for and interest in active restoration has been growing in recent decades. However, a disconnect between coral restoration practitioners, coral reef managers and scientists has resulted in a disjointed field where it is difficult to gain an overview of existing knowledge. To address this, we aimed to synthesise the available knowledge in a comprehensive global review of coral restoration methods, incorporating data from the peer-reviewed scientific literature, complemented with grey literature and through a survey of coral restoration practitioners. We found that coral restoration case studies are dominated by short-term projects, with 60% of all projects reporting less than 18 months of monitoring of the restored sites. Similarly, most projects are relatively small in spatial scale, with a median size of restored area of 100 m2. A diverse range of species are represented in the dataset, with 229 different species from 72 coral genera. Overall, coral restoration projects focused primarily on fast-growing branching corals (59% of studies), and report survival between 60 and 70%. To date, the relatively young field of coral restoration has been plagued by similar 'growing pains' as ecological restoration in other ecosystems. These include 1) a lack of clear and achievable objectives, 2) a lack of appropriate and standardised monitoring and reporting and, 3) poorly designed projects in relation to stated objectives. Mitigating these will be crucial to successfully scale up projects, and to retain public trust in restoration as a tool for resilience based management. Finally, while it is clear that practitioners have developed effective methods to successfully grow corals at small scales, it is critical not to view restoration as a replacement for meaningful action on climate change

Substrate stabilisation and small structures in coral restoration: State of knowledge, and considerations for management and implementation.

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

A stratified transect approach captures reef complexity with canopy-forming organisms

On the Great Barrier Reef (GBR), persistent changes to reef communities have begun to be documented, and on inshore reefs these shifts may favour the proliferation of macroalgae. Critical to understanding changes to reef community structure in response to anthropogenic impacts is developing effective methods to accurately document the abundance of different reef organisms. Effective monitoring must be time and cost efficient, replicable, and able to sufficiently and accurately detect disturbances to allow development of strategies to mitigate their impacts. Traditional techniques to document coral reef communities (i.e. photo-quadrats, benthic intercept transects) rely on planar views, which tend to either over- or under-represent canopy-forming organisms. As canopy-forming organisms are likely to be affected by anthropogenic influences (corals negatively, algae positively), it is essential for monitoring programs to implement methods sufficient to document changes to the vertical dimension of coral reefs. Here we build on previous work to document the canopy effect in coral-dominated ecosystems and propose a new survey approach suitable for implementation in algal-dominated systems. A vertically stratified transect, modified from a traditional point intercept transect, captures benthic and canopy-forming members of reef communities and provides information on three-dimensional complexity. To test the capability of the new method to detect changes in vertical reef structure, seaweed was removed from experimental quadrats and monitoring techniques were applied before and after four months of regrowth. A stratified method more accurately captured the three-dimensional change resulting from algal canopy growth, while resolving the over- and under-representation of algal biomass in two traditional techniques. We propose that a stratified transect method improves abundance estimates of canopy-forming organisms whilst maintaining data compatibility with traditional methods

Coral reef restoration in Indonesia:A review of policies and projects

Indonesia's coral reefs have been severely damaged by global and local stressors, and a range of active restoration techniques are now being used in attempts to rebuild degraded reefs. However, it is difficult to summarise Indonesia's restoration efforts as a whole due to a lack of consistent reporting. Here, we first discuss Indonesia's legal policy framework concerning reef restoration; this is included in the agenda of two government ministries (Marine Affairs and Fisheries, and Environment and Forestry), and comprises national laws and governmental, presidential and ministerial regulations. We then provide an extensive review of reef restoration projects in Indonesia, documenting 533 records across the country between 1990 and 2020. Most (73%) of these records come from the past ten years, and many (42%) are reported in online news articles rather than scientific reports or papers. This review identified 120,483 units of artificial reef installed across Indonesia, along with 53,640 units of coral transplantation (including both coral nurseries and direct out-planting onto reefs); in total, 965,992 fragments of hard coral have been planted across Indonesia. The most favoured restoration materials are concrete (46%) and steel structures (24%). Projects are organised by a diverse range of governmental, NGO, private and community-led organisations. This review demonstrates that Indonesia's policy has encouraged a diverse range of practitioners to implement reef restoration, but projects are often not coordinated with wider networks of restoration practitioners or scientists, and only 16% of the identified projects included a post-installation monitoring framework. Incorporating clear objectives and long-term monitoring programmes in project planning stages, while prioritising knowledge exchange and engagement with international scientific community, will substantially improve restoration outcomes in Indonesia. This will allow the country to fulfil its considerable potential as a global leader in rebuilding damaged coral reefs

Strong intraspecific competition and habitat selectivity influence abundance of a coral-dwelling damselfish

Coral reef ecosystems are experiencing a global decline in coral cover, with direct effects on reef fishes. A decline in habitat may lead to crowding of live coral specialists into remnant habitat patches, intensifying intraspecific competition. Increased local densities of conspecifics are known to negatively affect key demographic processes, but the magnitude of density effects and the role of crowding in response to habitat loss are poorly understood. In this study we examined habitat use and relationships between habitat availability and population density in a coral-dwelling damselfish — Chrysiptera parasema. First, we conducted habitat use and availability surveys to establish the level of habitat selectivity. We then investigated the evidence for crowding due to habitat loss by comparing densities within juvenile aggregations on natural reefs with high and low cover of the preferred habitat. Finally, we used a manipulative patch-reef experiment to measure the potential effects of crowding on mortality of juvenile C. parasema. Surveys revealed that 97% of juvenile C. parasema were associated with Acropora corals. Furthermore, C. parasema densities were closely related to the cover of bottlebrush Acropora, the preferred growth form. Contrary to predictions, there was no evidence of crowding on natural reefs with low coral cover, but rather, reefs with abundant Acropora cover supported larger aggregations with double the density of juveniles. We hypothesized that low densities of C. parasema on natural reefs with low coral cover could be explained by intense intraspecific competition. Experimental manipulations showed that juvenile mortality was density-dependent, with mortality 20% higher on high-density experimental patch-reefs compared to low-density reefs. Behavioural observations on the patch-reefs revealed that the frequency of agonistic interactions and distance to shelter were both unrelated to conspecific densities, highlighting the need for further research into mechanisms underpinning density dependent mortality. These results suggest that intraspecific competition may play an important role in reducing reef fish abundance as a consequence of habitat loss. Given that coral reef systems are currently under threat, with a global decline in coral cover, this study adds to the growing body of knowledge of how disturbances to habitat may affect reef fish communities

Loss of live coral compromises predator-avoidance behaviour in coral reef damselfish

Tropical reefs have experienced an unprecedented loss of live coral in the past few decades and the biodiversity of coral-dependent species is under threat. Many reef fish species decline in abundance as coral cover is lost, yet the mechanisms responsible for these losses are largely unknown. A commonly hypothesised cause of fish decline is the loss of shelter space between branches as dead corals become overgrown by algae. Here we tested this hypothesis by quantifying changes in predator-avoidance behaviour of a common damselfish, Pomacentrus moluccensis, before and after the death of their coral colony. Groups of P. moluccensis were placed on either healthy or degraded coral colonies, startled using a visual stimulus and their sheltering responses compared over a 7-week period. P. moluccensis stopped sheltering amongst the coral branches immediately following the death of the coral, despite the presence of ample shelter space. Instead, most individuals swam away from the dead coral, potentially increasing their exposure to predators. It appears that the presence of live coral rather than shelter per se is the necessary cue that elicits the appropriate behavioural response to potential predators. The disruption of this link poses an immediate threat to coral-associated fishes on degrading reefs

Habitat degradation modifies the strength of interspecific competition in coral dwelling damselfishes

Habitat degradation is predicted to exacerbate competition for critical resources; however, the relationship between habitat quality and competition is poorly understood. In this study, we used a manipulative experiment to test the effects of habitat degradation on competition between two planktivorous, coral-dwelling damselfishes, Chrysiptera parasema and Dascyllus melanurus. Experimental reefs were constructed with either healthy (100% live) or degraded (10% live) Acropora longicyathus coral, stocked with varying densities of these two fish species, and monitored for two months. On healthy habitat, the mortality of C. parasema was density dependent, and increased substantially in the presence of the dominant interspecific competitor D. melanurus. In contrast, on reefs where habitat was degraded, C. parasema mortality was highly variable, density independent, and was no longer influenced by the presence of dominant competitor D. melanurus. Behavioral observations revealed that agonistic interactions for both species increased with density on degraded habitat, but not on healthy habitat. In addition, on degraded reefs, both species displayed a reduced association with reef habitat and ventured further away from shelter with increasing densities of the dominant competitor D. melanurus. These results suggest that reduced habitat quality can have such a profound effect on reef fishes, that it eliminates density-dependent mortality and competitive dominance hierarchies, thereby substantially altering the mechanisms that structure reef fish communities

Rapid resource depletion on coral reefs disrupts competitor recognition processes among butterflyfish species

Avoiding costly fights can help conserve energy needed to survive rapid environmental change. Competitor recognition processes help resolve contests without escalating to attack, yet we have limited understanding of how they are affected by resource depletion and potential effects on species coexistence. Using a mass coral mortality event as a natural experiment and 3770 field observations of butterflyfish encounters, we test how rapid resource depletion could disrupt recognition processes in butterflyfishes. Following resource loss, heterospecifics approached each other more closely before initiating aggression, fewer contests were resolved by signalling, and the energy invested in attacks was greater. By contrast, behaviour towards conspecifics did not change. As predicted by theory, conspecifics approached one another more closely and were more consistent in attack intensity yet, contrary to expectations, resolution of contests via signalling was more common among heterospecifics. Phylogenetic relatedness or body size did not predict these outcomes. Our results suggest that competitor recognition processes for heterospecifics became less accurate after mass coral mortality, which we hypothesize is due to altered resource overlaps following dietary shifts. Our work implies that competitor recognition is common among heterospecifics, and disruption of this system could lead to suboptimal decision-making, exacerbating sublethal impacts of food scarcity

Perspectives on the Use of Coral Reef Restoration as a Strategy to Support and Improve Reef Ecosystem Services

In 2019, the United Nations Environment Assembly requested that the United Nations Environment Programme (UNEP) and the International Coral Reef Initiative (ICRI) define best practices for coral restoration. Guidelines led by the UNEP were prepared by a team of 20 experts in coral reef management, science, and policy to catalog the best-available knowledge in the field and provide realistic recommendations for the use of restoration as a reef management strategy. Here, we provide a synthesis of these guidelines. Specifically, we present (1) a case for the value of coral reef restoration in the face of increasing frequency and intensity of disturbances associated with climate change, (2) a set of recommendations for improving the use of coral reef restoration as a reef management strategy, tailored to goals and current methods. Coral reef restoration can be a useful tool to support resilience, especially at local scales where coral recruitment is limited, and disturbances can be mitigated. While there is limited evidence of long-term, ecologically relevant success of coral reef restoration efforts, ongoing investments in research and development are likely to improve the scale, and cost-efficiency of current methods. We conclude that coral reef restoration should not be seen as a “silver bullet” to address ecological decline and should be applied appropriately, with due diligence, and in concert with other broad reef resilience management strategies

Appendix A. Detailed results and additional figures of logistic regressions of mortality of C. parasema and D. melanurus.

Detailed results and additional figures of logistic regressions of mortality of C. parasema and D. melanurus