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.

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.

INTRODUCTION
With dramatic declines in coral cover worldwide, especially in the last 3-5 years (Pandolfi et al., 2003;Hughes et al., 2017Hughes et al., , 2018, it has become clear that bolder actions are necessary at both global and local scale to secure a future for coral reefs. Coral reef restoration, in particular, is increasingly employed as a management strategy to halt declines in coral cover and support reef resilience. Increased interest in coral reef restoration is illustrated by the central role restoration is taking in national and international commitments under various multilateral environmental agreements. For example, the United Nations General Assembly has put "rehabilitating our environment" at the heart of the 2030 Agenda for Sustainable Development and declared 2021-2030 as the UN Decade on Ecosystem Restoration. The 4th United Nations Environment Assembly in 2019 also passed a resolution specific to the sustainable management of coral reefs (Resolution 4/13) recognizing the role of restoration to achieve biodiversity goals (United Nations Environment Assembly (UNEA), 2019). A recent ICRI report (McLeod I. M. et al., 2019) revealed that 88% of ICRI members are interested in the development of new international commitments and policies specifically dedicated to coral reef restoration. At the national level, initiatives such as the Reef Restoration and Adaptation Program in Australia (RRAP, Bay et al., 2019), NOAA's restoration strategy within the coral reef conservation strategy (National Oceanic and Atmospheric Administration (NOAA), 2018), the Coral Reef Restoration Protocol in Costa Rica (AIDA-Americas, 2019), or specific Coral Reef Action Plans in Thailand (Suraswadi and Yeemin, 2013) highlight increased interest in investing in coral reef restoration.
However, some confusion arises from an active debate among coral reef scientists on the value of coral reef restoration in the face of large-scale disturbances such as warming temperatures and increased ocean acidification. Two IPCC reports (IPCC, 2018;Bindoff et al., 2019) summarize the existing projections of future coral bleaching to state that coral reefs as we know them will all but disappear in a scenario of up to 2 • C warming, and up to 90% of coral reefs could be lost even with an increase of 1.5 • C. In this context, many experts argue that coral reef restoration is merely a band-aid solution and a distraction from global actions on threat reduction (Bellwood et al., 2019;Morrison et al., 2020). Other experts argue that even if greenhouse gas emissions were to be drastically reduced immediately, global ocean temperatures could still take decades to stabilize (Hansen et al., 2007), and that bold active management actions at the local level such as coral reef restoration are necessary to sustain and re-build reef ecosystems, alongside climate action and protection measures (Rinkevich, 2019;Duarte et al., 2020). Climate action, albeit critical, is only one part of the big equation we need to solve to ensure a future for coral reefs, and restoration can create a necessary bridge to rescue corals at local scales while global threats are being addressed (Coral Restoration Consortium (CRC), 2020).
Adding to the confusion is the largely experimental nature of the practice coral reef restoration (Bayraktarov et al., 2016Hein et al., 2017;Boström-Einarsson et al., 2020). Apart from a few notable examples of positive long-term outcomes (In Fiji Coral for Conservation, 2020;in Belize Fragments of Hope, 2020), there is limited evidence that it can be an effective management strategy to support reef resilience. A lack of longterm monitoring of existing projects (coral restoration projects have a median monitoring duration of 12 months, Boström-Einarsson et al., 2020), and reporting of success focused on a few technical metrics (e.g., coral growth and survival) rather than metrics related to ecosystem function and health or socio-cultural and economic outcomes (Hein et al., 2017;Boström-Einarsson et al., 2020) make it difficult to assess and share general best practices (Leocadie et al., 2020). In the last few years, there has been an explosion of research and development on cutting-edge solutions to scale-up current coral reef restoration techniques (National Academies of Sciences Engineering and Medicine (NASEM), 2019; Bay et al., 2019, RRAP). These developments are necessary to help corals persist. However, the novelty of this research creates a gulf between existing practices and what is recommended, leaving managers, practitioners, decision-makers, and funding agencies with a lack of guidance for what coral restoration can realistically achieve.
In 2019, the United Nations Environment Assembly adopted Resolution 4/13 on sustainable coral reefs management requesting UNEP and ICRI to better define best practices for coral restoration, as appropriate, for the maintenance of ecosystem services, including for coastal defense and restoration of fish nursery areas. In response, a report was prepared by 20 global coral reef restoration experts to assist practitioners, managers, and decision-makers in deciding whether and how to use of coral reef restoration as a strategy to protect coral reefs locally, regionally, and globally (Hein et al., 2020a, UNEP). Here, we synthesize these experts' perspectives on: (a) goals and methods of coral reef restoration on the eve of the UN Decade on Ecosystem Restoration; (b) arguments for and against restoring coral reefs in the face of climate change; and (c) recommendations on how current methods can be used for particular goals and situations.

CORAL REEF RESTORATION ON THE EVE OF THE UN DECADE
Ecological restoration is defined by the Society for Ecological Restoration (SER) as "the process of assisting the recovery of an ecosystem that has been degraded, damaged, or destroyed" (Society for Ecological Restoration International Science and Policy Working Group, 2004). In the past, the goal of restoration has been to restore an ecosystem back to a historical baseline. This view also implied that the threat(s) responsible for the degradation, damage or destruction could be removed. However, this may not be possible for coral reefs because the threat of rising ocean temperatures and ocean acidification will continue for decades even if greenhouse gas emission targets are met. The goal of coral reef restoration has therefore shifted toward recovering or maintaining key ecosystem processes, functions, and services through the next few decades of climate change, rather than restoring to a historical baseline.
Here, we suggest that the term "coral reef restoration" be used to describe an active intervention aimed to assist the recovery of reef structure, function, and key reef species in the face of rising climate and anthropogenic pressures, promoting reef resilience and the sustainable delivery of reef ecosystem services. These interventions include reducing impacts, remediation, and rehabilitating ecosystem function, following standards developed by SER (Gann et al., 2019, Figure 1). Actions aimed at protecting and enabling recovery (e.g., waste and water quality management) can be broadly categorized as "proactive, " and they support "reactive" actions, commonly referred to as "restoration." These terms are meant to replace "passive" and "active" on the basis that "passive" has a negative connotation of implying that no action is necessary. "Reactive" actions are aimed at repairing ecosystem function and assisting the recovery of a degraded reef system, should it not be able to recover on its own (Figure 1). Restoring corals should never be the first point of action in a reef management strategy, but rather part of a strategy in a carefully planned ecosystem management framework (Edwards, 2010). Avoiding and mitigating local impacts to reefs should always be the priority, and restoration should never be used as an excuse to justify degradation in another area.

Goals of Coral Reef Restoration
Defining clear goals is critical to effective planning, implementation, and monitoring of restoration. In conservation, goals are commonly defined as the ultimate impact you hope to achieve by conducting interventions over the medium to long term (e.g., 5-20 years; Open Standards for the Practice of Conservation, Conservation Measures Partnerships (CMP), 2020). The overarching goal of most coral reef restoration projects is to recover a functioning and self-sustaining reef ecosystem, and coral reef restoration efforts should be planned as a long-term intervention. However, there are narrower, but still important goals that motivate managers and practitioners. Below is a list of common goals for coral reef restoration ( Table 1).
These goals are non-exclusive and may often complement one another. However, in planning coral restoration, clearly articulating the project goal(s) should be the first action (Shaver et al., 2020). Then, objectives can be defined to track, and accomplish the goals over short time periods (e.g., 1-3 years). To manage ecosystems effectively, objectives should be Specific, Measurable, Achievable, Relevant, and Time-bound (SMART). Objectives should be informed by reference ecosystems but should consider future-anticipated environmental change (McDonald et al., 2016;Gann et al., 2019;Goergen et al., 2020). Examples of SMART objectives specific to coral reef restoration include: XX genotypes from XX coral species outplanted on XX reefs in the first year resulting in XX% increase in genetic diversity, or XX increase in coral cover at XX site within 3 years resulting in XX% reduced wave action (Shaver et al., 2020).

Current Methods of Coral Reef Restoration
Methods of coral reef restoration are evolving rapidly with investment in research and development. A number of emerging interventions are currently being tested experimentally across various scales, from individual corals (e.g., genetics, reproduction, physiology), to coral populations, reef communities, and ecosystems. The US National Academies of Science, Engineering, and Medicine (NASEM) and the Reef Restoration and Adaptation Program (RRAP) have recently provided an extensive review of a number of interventions that could increase the physiological resilience of corals to TABLE 1 | Goals and associated rationales of coral reef restoration.

Socio-economic goals
a. Sustain or recover coastal protection Sustain or re-establish the regulating ecosystem services provided by reefs to protect coastal communities and infrastructure by attenuating wave energy and mitigating disturbances such as erosion and coastal flooding b. Sustain or recover fisheries production Sustain or re-establish the provisioning services delivered by reefs in providing habitat and nursery areas for commercially important fisheries c. Sustain or enhance local tourism opportunities Maintain reef aesthetics to support local reef tourism and/or provide opportunities for eco-tourism experiences Adopt an effective "no net loss" mitigation policy whereby if a disturbance (e.g., coastal development) cannot be avoided, it should be minimized and offset for example by relocating anticipated ecological losses prior to disturbance climate change (Bay et al., 2019; National Academies of Sciences Engineering and Medicine (NASEM), 2019). The 23 intervention types investigated by NASEM include novel approaches such as cryopreservation, managed relocation of corals to promote assisted gene flow (AGF), or microbiome manipulations (National Academies of Sciences Engineering and Medicine (NASEM), 2019). The Reef Restoration and Adaptation Program (RRAP) in Australia is evaluating "moonshot" solutions that can operate across the entire scale of the Great Barrier Reef, including assisting the evolutionary adaptation of reef species to warmer waters, and mass production and release of coral larvae to seed reefs (Bay et al., 2019). Other field experiments are underway in places like Fiji and Kiribati to facilitate natural processes of reef recovery by capitalizing on innate reef resilience (Coral for Conservation, 2020). There, the focus is on using colonies that have survived recent episodes of coral bleaching as well as encouraging ecological synergies by actively removing coral predators and re-introducing fish and sea urchins to control macro-algae overgrowth (Coral for Conservation, 2020). These

Coral gardening
Transplanting coral colonies or fragments with an intermediate nursery phase. Nurseries can be in situ (in the ocean) or ex situ (flow through aquaria).

Substrate addition (artificial reef)
Adding artificial structures for purposes of coral reef restoration as a substrate for coral recruitment, coral planting, and/or for fish aggregation 3. 1 Electro-deposition Adding artificial structures that are connected to an electrical current to accelerate mineral accretion.

Green engineering
Adding artificial structures designed to mimic natural processes and be integrated into reef landscapes (nature-based solutions, eco-designed structures, living shorelines).

Substrate manipulation
Manipulating reef substrates to facilitate recovery processes.

Substrate stabilization
Stabilizing substratum or removing unconsolidated rubble to facilitate coral recruitment or recovery.

Algae removal
Removing macroalgae to facilitate coral recruitment or recovery.

Larval propagation
Releasing coral larvae at a restoration site, after an intermediate collection and holding phase, which can be in the ocean or on land in flow through aquaria.

Deployment of inoculated substrate
Deploying settlement substrates that have been inoculated with coral larvae.

Larval release
Releasing larvae directly at a restoration site proposed interventions represent a substantial body of research and potential for improving reef restoration, yet most are still in the research and development phase, and may take years before becoming feasible for large-scale implementation. In contrast, five coral reef restoration methods have already been widely applied and tested in the field ( Table 2). Some are more widely used than others. For example, a recent review by Boström-Einarsson et al. (2020) found that the majority of documented projects (almost 70%) involved coral planting (e.g., direct transplantation, coral gardening). Other methods are far less popular, for example substrate manipulation methods comprised only 10% of all projects, and larval propagation 1% of all projects .

THE VALUE OF CORAL REEF RESTORATION IN THE FACE OF RISING ENVIRONMENTAL CHALLENGES
The Global Climate Change Challenge Clearly the biggest obstacles to natural recovery of coral populations are global climate change and associated mass coral bleaching. Even if global targets set by the Paris Agreement are met in the future, current greenhouse gas emissions are still increasing, and the increase in frequency of massbleaching events in the last 5 years suggest that coral reefs globally are very close to their temperature limits (Hughes et al., 2018). In this context, some scientists argue that active interventions, such as reef restoration, do not address the underlying causes of reef declines (Bruno and Valdivia, 2016;Hughes et al., 2017;Bellwood et al., 2019). Coral reef restoration has been criticized as an expensive, temporary fix that is not deployable at scales that match the scale of disturbances, and a distraction from other conservation strategies that are more focused on addressing the root causes of disturbances (Bellwood et al., 2019;Morrison et al., 2020). However, it is important to differentiate among the portfolio of actions available to tackle climate change and to ensure coral reefs ecosystems and their associated services can persist in the future. Coral reef restoration is not designed to reduce climate impacts, but rather is intended as a complementary tool to support natural recovery following disturbance in key areas. Given the many uncertainties associated with different climate scenarios (Bindoff et al., 2019), the key challenge is to design coral restoration efforts such that the realities of climate change are embedded in the choice of goals, objectives, and methods (Shaver et al., 2020). It is not an "either or" situation, as climate change mitigation does not preclude investment in local management strategies designed to build the resilience and adaptation of the socio-ecological coral reef systems.
Further exacerbating the situation are local causes of reef degradation. Identifying, reducing, and/or removing these local pressures are all critical steps in effective coral reef restoration (Edwards, 2010). There is no point replanting a coral reef where corals have died due to poor water quality if water quality has not been addressed and improved prior to planting. It is also not worth the valuable and limited resources of most local reef managers to undertake restoration if the reef can recover without restoration efforts, which can happen on reefs where coral recruitment is not limited and if there is enough time between predicted disturbance events. If, on the other hand, there is a barrier to recovery that cannot be overcome naturally, then restoration is necessary to kick start system recovery.

Barriers to Natural Recovery
The most common, non-climate related, barriers to natural recovery are substrate limitations and/or recruitment limitations. Substrate limitation refers to instability and suitability, which both affect the capacity of coral larvae to recruit, settle, and grow. For example, unconsolidated coral rubble impedes coral attachment and may create further physical damage , while substrate covered in macroalgae impedes coral settlement (Dixon et al., 2014). Recruitment limitation occurs when the supply of coral larvae (or fragments) from reproductive adult populations is exceedingly low or when a reef is disconnected from larval supply. Finally, physiological barriers to natural recovery have emerged in places where coral growth and survival have become limited by new thermal extremes (Schoepf et al., 2015;Thomas et al., 2018).

Restoration as a Call to Action
There is a growing argument that the risk of doing nothing far outweighs the risks or uncertainties of active interventions (Anthony et al., 2017(Anthony et al., , 2020. The rapid increase in implementation of coral reef restoration strategies is driven by a sense of urgency following catastrophic loss in global coral cover in the last decade. This sense of urgency creates unique scientific uncertainties as there is not enough time to wait for climate action to be enacted, for pressures to stop, or for repeated experimental methods to be published in scientific journals before action is taken. Even in the context of continued coral declines attributed to climate change, goals outlined in Table 1 highlight the varied motives for coral reef restoration across socio-ecological scales. At local scales, and in the short-term, coral reef restoration can provide benefits such as: (1) increasing genetic diversity and thus the potential for adaptation, (2) helping to prevent the extinction of some species, (3) assisting species migration to new locations, (4) continuing to provide critical ecosystem services, and (5) providing tangible mechanisms for people to combat ecological grief. Importantly, coral reef restoration should not be considered as a solution on its own but rather as part of an integrated resilience-based management framework (e.g., McLeod E. et al., 2019) that includes a hierarchical portfolio of actions from threat reduction (i.e., climate change mitigation, water quality controls, fishing regulations), to actions that support the recovery and resistance of ecosystem processes such as marine protected areas or coral predator removal (e.g., crown-of-thorns starfish) as illustrated in Figure 1. As such, coral reef restoration may span beyond planting scleractinian corals to include interventions such as algae removal and fish introduction that support the recovery of reef function. Also, within that framework, the different strategies integrate both social and ecological adaptive capacity to manage for uncertainty and change (McLeod E. et al., 2019). Coral reef restoration can be a useful tool to support resilience, and if well integrated into a resilience-based management framework, can play a key role in meeting Sustainable Development Goals associated with the UN Decade on Ecosystem Restoration (Claudet et al., 2019). Nonetheless, implementation of coral reef restoration actions should not be haphazard and should not divert resources away from other reef management strategies that actively control stressors. Integrating investments for coral reef restoration within funding for resilience-based management may help maximize the positive impacts of current and future strategies.

RECOMMENDATIONS
Restoration is only one in a suite of intervention options available to reef managers. Reef restoration should always be undertaken in concert with complementary strategies and integrated in a resilience-based management framework (Hein et al., 2020a, UNEP). Also, restoration might not always be appropriate. The following considerations, should be made prior to planning and designing: (1) assess the cause(s) of coral decline (e.g., pollution, human activities, bleaching); (2) review factors affecting the potential for natural recovery of corals (e.g., spawning capacity, barriers to coral recruitment, limits to coral growth); and (3) determine which intervention is best suited under the circumstances to achieve the stated goals of the restoration project (Edwards, 2010;Hein et al., 2020a, UNEP). These steps will help identify (a) whether coral reef restoration is necessary, and (b) what might need to done beforehand (e.g., improving water quality, improving the physical integrity of reef substrate, or recovering key ecological processes (Edwards, 2010;Hein et al., 2020a, UNEP).

Planning and Design
Restoration is not a "one size fits all" approach, and each aspect of a restoration program, from goals to methods used, should be tailored to the specific needs and abilities of each location. Key elements of effective and efficient designs include: (1) defining SMART goals and objectives, (2) developing a climate-smart, adaptive strategy, and (3) engaging stakeholders early (Shaver et al., 2020). Pilot studies should be included to refine the choices of sites and methods and the overall action plan prior to full implementation (Shaver et al., 2020). In addition, current information and projections on the specific vulnerability of a reef site to climate change should be incorporated in initial planning to ensure the chosen intervention(s) have a chance to withstand future conditions (West et al., , 2018Shaver et al., 2020). Engaging with stakeholders, local communities, indigenous communities, and traditional owners in all stages of restoration planning and implementation is critical to reduce potential conflicts associated with the use of reef resources and to maximize collaborations and investment opportunities (Gann et al., 2019;DeAngelis et al., 2020). Incorporating traditional or local knowledge of the specific reef system of concern will improve the chances of restoration success. Appropriate engagement and communication are critical to maximize the flow of socio-cultural and economic benefits beyond the people directly involved in the restoration effort, therefore securing longer-term support. Coral reef restoration can be a useful educational tool that encourages tangible behavioral changes and improves the social resilience of local communities, the economic resilience of local reef-reliant industries, as well as the ecological resilience of the reef .

Monitoring and Communication
Appropriate monitoring of coral reef restoration efforts should assess outcomes against initial goals and objectives at appropriate time scales. Monitoring is crucial to inform and facilitate adaptive management, and to increase transparency and accountability. Ideally, restoration efforts should be set up in a way that allows for an assessment of effectiveness with control sites and/or following a before/after/control/impact (BACI) design (see Falk et al., 2006;Gann et al., 2019;Goergen et al., 2020), and monitored and evaluated consistently (Pioch et al., 2017), so improvements can be made as the project evolves and environmental conditions change. Comparing outcomes across projects will necessitate a standardization of monitoring protocols across socio-ecological dimensions (Hein et al., 2017;Goergen et al., 2020). Systematically monitoring a few metrics (e.g., dimension of restored area, genotypic diversity, coral population abundance) as outlined in Goergen et al. (2020) is also important to further the understanding of the effectiveness of coral reef restoration to assist the recovery of degraded reefs. Monitoring outcomes also need to be better communicated to improve collaboration and outreach (DeAngelis et al., 2020). Within a project community, it is important to communicate often to keep the public engaged and to use non-scientific language that is easily understandable and relevant to target audiences. Communication among managers and practitioners is also important to share successes, failures, and foster collaborations to advance the field.

Restoration Goals
Defining specific goals and objectives will help managers and practitioners develop targeted monitoring plans and enhance the clarity of reporting on the outcomes of their project(s). In many instances, project(s) will tackle more than one goal at a time and accrue multiple benefits as a result. However, each goal comes with specific challenges. The tables and figures below are provided to help cross reference goals, methods, and other relevant factors. In Table 3 we provide key considerations for various restoration goals. For example, goals associated with sustaining tourism may be accomplished in relatively short time frames (<3 years) if tourism operators are involved in the project early-on, with clear communication plan and sustainable funding schemes ( Table 3). Projects attending to acute disturbances require effective emergency management plans to succeed in a short time frame. On the longer end of the spectrum, reestablishing a self-sustaining, functioning reef ecosystem is a more complex, longer-term goal that depends upon other ecological variables (e.g., water quality, genetic diversity of corals). Choosing goals should be done thoughtfully and with respect not only to the environmental challenges but with respect to the capacity of management (e.g., sustainable funding, interest, personnel).

Method(s) Selection
There are a growing number of methods for coral reef restoration and selecting a method should be done with careful consideration of the projects' goals.
Method(s) selection should be driven by specific goals the coral restoration efforts are designed to achieve. An index matrix prepared by experts in the field informs the suitability of each currently established methods for a particular goal (Figure 2). There, methods were ranked from least to most appropriate in fulfilling specific goals, based on the best-available current knowledge. For example, larval release and the deployment of inoculated substrates were ranked as most appropriate for the goal of mitigating population decline and preserving biodiversity (Figure 2), on the basis that these two methods will maximize genetic diversity at the restored site(s). Note that for most projects, multiple methods may be used to satisfy specific goals and associated objectives. For example, for the goal of responding to acute disturbances to accelerate recovery, both methods of direct transplantation and substrate stabilization were identified as most appropriate (Figure 2). Location and project specific characteristics should guide the choice of methods further (Shaver et al., 2020). Interestingly, for many of the goals (e.g., recover and sustain coastal protection, recover and sustain fisheries production), none of the current methods were ranked as "most appropriate, " further highlighting some critical gaps between the goals and current methods for coral reef restoration. However, given the fast pace at which the field of coral reef restoration is expanding and the increasing level of investment, new methods that may be more appropriate are in development.
Providing guidance on how and when to use various methods of restoration was part of the driving force behind the UNEP Report (Hein et al., 2020a, UNEP). Each of the established coral reef restoration methods comes with its own set of benefits and challenges. The rationale behind selecting one method over another is generally not reported in the literature. The lack of guidance is likely due, again, to a lack of monitoring and reporting of long-term outcomes of coral reef restoration efforts (Hein et al., 2017;Boström-Einarsson et al., 2020), but also to a FIGURE 2 | Method suitability matrix for each coral reef restoration goals. The darker the color, the more appropriate a method is to each specific goal. Note that for many goals multiple methods may be suitable. lack of studies that compare outcomes from different coral reef restoration methods (Hein et al., 2020b). Many different criteria may be considered when selecting one type of intervention over another, many of which will be location-and projectspecific (Shaver et al., 2020). For example, one might consider the flexibility of a method in terms of the ease of implementing and adjusting the effort to adapt to unforeseen disturbances; others might be driven by externalities associated with permit requirements such as ensuring local communities can actively be involved in the restoration process. Three criteria: cost, efficiency, and scalability are particularly important driving forces of that decision-making process.

Cost, Efficiency, and Scalability
Eleven coral reef restoration experts assessed each of the most established coral reef restoration methods. Experts were selected from the ICRI ad hoc committee on coral reef restoration as well as from the CRC leadership team and ranged from academics, to managers, and practitioners from various reef regions around the globe. Scores were provided for three criteria: cost, efficiency and scalability, providing a qualitative comparison among methods (Figure 3). Results, presented as violin plots, help identify consensus and variability among the experts' opinions and display variability in the responses. For example, there was consensus on the high cost of substrate addition methods, but high variability on the efficiency and scalability of this method. Electro-deposition ranked as the least efficient and scalable, and among the costliest methods (Figure 3). There was high variability in the scores overall-most plots spanning almost the whole range from 0 to 10 (Figure 3), which is likely due to the lack of rigorous monitoring and the limited implementation of some of the methods (e.g., larval restoration, Boström-Einarsson et al., 2020). With appropriate monitoring (as suggested by Goergen et al., 2020), estimates of cost-effectiveness and scalability could improve given increasing investment in coral reef restoration. However, for most methods, the overall trend of high costs but medium to low efficiencies (Figure 3. The discrepancies of opinions among experts for most metrics also reflect the relative youth of coral reef restoration science and highlight the future opportunities for innovations and solutions that are more scalable, affordable and effective building upon the body of work and experiences gained in the field to date. Challenges and recommendations for each of method are highlighted in Table 4. While not prescriptive, Table 4 is intended to provide guidance, beyond the suitability of methods to goals outlined in Figure 2, and the relative cost, efficiency, and scalability illustrated in Figure 3. For example a group interested in restoring a reef for the goals of "preserving biodiversity" as well as "sustaining local tourism opportunities, " may choose to combine at least two methods-larval propagation methods would help ensure long-term coral genetic variation and potential for  adaption, while coral gardening could engage local tourists and create a sustainable funding mechanism. Another group may want to increase fisheries productions while protecting their coastline. This group may use artificial substrate to protect their coastline, and plant branching coral from a nearby nursery (or coral garden) on the substrate to provide fish with complex habitat. If these methods are too costly, substrate stabilization and direct transplantation of corals of opportunity could be substituted. We hope the series of tables and figures provided here are a helpful guide to thinking through the various goals and methods of restoration, which vary widely depending on local environmental condition, available capacity, and funding.

CONCLUSION AND RECOMMENDATIONS
The need for restoration is accelerating as coral reefs around the world continue to experience catastrophic declines in coral health and cover. One of the roles of the UNEP is to provide expert guidance on how coral reef restoration interventions may be used to protect and enhance the delivery of reef ecosystem services in the future. In this synthesis, several key recommendations emerge. First, it is important to recognize that coral reef restoration is not a "silver bullet" designed to address the rising threats of climate change and anthropogenic disturbances. It should never be used as an excuse to justify reef degradation. Second, 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. Third, coral reef restoration interventions should be integrated within a resilience management framework, as a continuum of reactive and proactive actions, focusing not just on restoring hard corals but the overall function of the reef community. Fourth, monitoring of appropriate metrics over time is essential so that management decisions can be more scientifically robust. Finally, applying coral reef restoration methods effectively and efficiently requires "climate-smart" designs that account for future uncertainties and changes West et al., 2017West et al., , 2018. Current information and projections on the specific vulnerability of a reef site to climate change should be incorporated in initial planning to ensure the chosen intervention(s) have a chance to withstand future conditions (Van Hooidonk et al., 2016;Shaver et al., 2020). Following recommendations from the Society for Ecological Restoration, we suggest coral reef restoration strategies follow four critical directions: (1) planning and assessing around specific goals and objectives, (2) identifying adaptive strategies to balance risks and trade-offs, (3) engaging communities in all stages of the restoration efforts, (4) developing longterm monitoring plans to allow for adaptive management and improving the understanding of methods' effectiveness for specific goals. With ongoing and further investment in research and development, the cost-effectiveness of established and new methods should improve the scalability and effectiveness of coral reef restoration interventions. Supporting such investment is critical to improving the capacity to intervene locally and globally and improve the chances for coral reefs to thrive into the future.

AUTHOR CONTRIBUTIONS
MH, IM, ES, TV, SP, LB-E, MA, and GG conceived the manuscript and reviewed and edited the manuscript. MH, IM, TV, and GG wrote the manuscript. All authors contributed to the article and approved the submitted version.

FUNDING
This work was funded by the Swedish International Development Cooperation Agency, the Principality of Monaco, and the Australian Government's National Environmental Science Programme Tropical Water Quality Hub (NESP TWQ) funding to IM, MH, and LB-E.