Large-scale interventions may delay decline of the Great Barrier Reef

On the iconic Great Barrier Reef (GBR), the cumulative impacts of tropical cyclones, marine heatwaves and regular outbreaks of coral-eating crown-of-thorns starfish (CoTS) have severely depleted coral cover. Climate change will further exacerbate this situation over the coming decades unless effective interventions are implemented. Evaluating the efficacy of alternative interventions in a complex system experiencing major cumulative impacts can only be achieved through a systems modelling approach. We have evaluated combinations of interventions using a coral reef meta-community model. The model consisted of a dynamic network of 3753 reefs supporting communities of corals and CoTS connected through ocean larval dispersal, and exposed to changing regimes of tropical cyclones, flood plumes, marine heatwaves and ocean acidification. Interventions included reducing flood plume impacts, expanding control of CoTS populations, stabilizing coral rubble, managing solar radiation and introducing heat-tolerant coral strains. Without intervention, all climate scenarios resulted in precipitous declines in GBR coral cover over the next 50 years. The most effective strategies in delaying decline were combinations that protected coral from both predation (CoTS control) and thermal stress (solar radiation management) deployed at large scale. Successful implementation could expand opportunities for climate action, natural adaptation and socioeconomic adjustment by at least one to two decades.


Recommendation? Accept with minor revision (please list in comments)
Comments to the Author(s) Condie et al. present an extensive modeling effort to explore the potential benefits of large-scale intervention efforts to prevent coral decline on the Great Barrier Reef. I commend the authors on a thorough and accessible description of the many parameters and assumptions incorporated in their model. Overall, I found the conclusions to be well supported by the data and the findings of high value to the coral reef management community. In particular, the authors did an excellent job of distilling the core conclusions from their work as key recommendations for intervention planning.
The paper is well written and effectively organized. My only major comment is that the paper would benefit from a more thoughtful discussion of the limitations of their model assumptions and how their interpretation of the data might change if their assumptions varied slightly. I see at least two points worth elaborating on: First, coral adaptation is incorporated in the model but it does not seem like changes in coral community composition towards more resilient (or less tasty to COTs) species have been considered. There are well documented changes in species composition on the GBR, which are likely to influence future coral responses to thermal stress, cyclones, and COTs. How might the projections of coral cover change if a shift in coral composition reduced the amount of food available to COTs, decreased bleaching mortality, and reduced the extent of cyclone damage? This seems like a plausible transformation with widespread impacts on model outputs and warrants some consideration. Second, the concept of shading is put forth as an important intervention strategy with potentially important benefits to corals. The authors briefly acknowledge that the feasibility of this is unknown but it seems reasonable to assume that it is quite literally impossible. Given that, it seems pertinent the authors consider how the strong effects of shading may be influenced by model assumptions. A reduction of 4 DHW seems generous as this is ¼ to 1/3 of the heat experienced by the GBR in 2015/16. If the benefits of shading weren't quite so large, would this intervention still be considered one of the most critical? It seems like some examination of the sensitivity of model output to the shading assumption would help contextualize how seriously this intervention should be considered.
I have provided a few minor comments below to help improve clarity. Minor comments: Line 226 -heat waves occasionally occurs as 2 words. Make sure to be consistent with this term as it is predominantly used, "heatwave" Lines 497-503 -this is slightly confusing with table 1 that shows shading and cooling (?) as equivalent to reductions of 12 dhw. It seems like IS5 from table one is not included in the model while IS 6 is? Relative to 12, a reduction by 4 dhw seems small which doesn't follow well from the sentence in lines 498-501 describing "much higher dhw reductions" I suggest revising this section for clarity, specifying exactly which mitigation measures are being estimated to produce the 4 dhw reduction, and reiterating that while identified in table 1, cooling is not incorporate in the model as an intervention.
This section also poorly characterizes the distinction between regional shading and priority shading/cooling illustrated in figure 6.
Line 666 -there is a comma missing after i.e.
Line 718 -with such a concrete example of local management improvements it seems like this would be worth including in the supplement rather than just stating it is "not shown" Lines 719-726 -this discussion seems to be linked more directly to heating rather than shading. A brief description about why shading is useful seems appropriate. Is it the reduction in irradiance stress during a heatwave or does shading quantifiably reduce temperatures? Given water flow I suspect the former but it is written as though the latter is most likely.
Line 723 -another instance of heat wave being 2 words.

Decision letter (RSOS-201296.R0)
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Dear Dr Condie
The Editors assigned to your paper RSOS-201296 "Large-scale interventions may delay decline of the Great Barrier Reef" have now received comments from reviewers and would like you to revise the paper in accordance with the reviewer comments and any comments from the Editors. Please note this decision does not guarantee eventual acceptance.
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Reviewer comments to Author: Reviewer: 1 Comments to the Author(s) Condie et al. present an extensive modeling effort to explore the potential benefits of large-scale intervention efforts to prevent coral decline on the Great Barrier Reef. I commend the authors on a thorough and accessible description of the many parameters and assumptions incorporated in their model. Overall, I found the conclusions to be well supported by the data and the findings of high value to the coral reef management community. In particular, the authors did an excellent job of distilling the core conclusions from their work as key recommendations for intervention planning.
The paper is well written and effectively organized. My only major comment is that the paper would benefit from a more thoughtful discussion of the limitations of their model assumptions and how their interpretation of the data might change if their assumptions varied slightly. I see at least two points worth elaborating on: First, coral adaptation is incorporated in the model but it does not seem like changes in coral community composition towards more resilient (or less tasty to COTs) species have been considered. There are well documented changes in species composition on the GBR, which are likely to influence future coral responses to thermal stress, cyclones, and COTs. How might the projections of coral cover change if a shift in coral composition reduced the amount of food available to COTs, decreased bleaching mortality, and reduced the extent of cyclone damage? This seems like a plausible transformation with widespread impacts on model outputs and warrants some consideration. Second, the concept of shading is put forth as an important intervention strategy with potentially important benefits to corals. The authors briefly acknowledge that the feasibility of this is unknown but it seems reasonable to assume that it is quite literally impossible. Given that, it seems pertinent the authors consider how the strong effects of shading may be influenced by model assumptions. A reduction of 4 DHW seems generous as this is ¼ to 1/3 of the heat experienced by the GBR in 2015/16. If the benefits of shading weren't quite so large, would this intervention still be considered one of the most critical? It seems like some examination of the sensitivity of model output to the shading assumption would help contextualize how seriously this intervention should be considered.
I have provided a few minor comments below to help improve clarity. Minor comments: Line 226 -heat waves occasionally occurs as 2 words. Make sure to be consistent with this term as it is predominantly used, "heatwave" Lines 497-503 -this is slightly confusing with table 1 that shows shading and cooling (?) as equivalent to reductions of 12 dhw. It seems like IS5 from table one is not included in the model while IS 6 is? Relative to 12, a reduction by 4 dhw seems small which doesn't follow well from the sentence in lines 498-501 describing "much higher dhw reductions" I suggest revising this section for clarity, specifying exactly which mitigation measures are being estimated to produce the 4 dhw reduction, and reiterating that while identified in table 1, cooling is not incorporate in the model as an intervention.
This section also poorly characterizes the distinction between regional shading and priority shading/cooling illustrated in figure 6.
Line 666 -there is a comma missing after i.e.
Line 718 -with such a concrete example of local management improvements it seems like this would be worth including in the supplement rather than just stating it is "not shown" Lines 719-726 -this discussion seems to be linked more directly to heating rather than shading. A brief description about why shading is useful seems appropriate. Is it the reduction in irradiance stress during a heatwave or does shading quantifiably reduce temperatures? Given water flow I suspect the former but it is written as though the latter is most likely.
Line 723 -another instance of heat wave being 2 words.

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Author's Response to Decision Letter for (RSOS-201296.R0) See Appendix A.

Do you have any ethical concerns with this paper? No
Have you any concerns about statistical analyses in this paper? No

Comments to the Author(s)
The authors can be commended on an extensive and well-excuted study. I look forward to seeing this in print.

Decision letter (RSOS-201296.R1)
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Dear Dr Condie, It is a pleasure to accept your manuscript entitled "Large-scale interventions may delay decline of the Great Barrier Reef" in its current form for publication in Royal Society Open Science. The comments of the reviewer(s) who reviewed your manuscript are included at the foot of this letter.
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Associate Editor Comments to Author (Dr Melita Samoilys):
Please accept our apologies for the delay in issuing a decision. It has been unusually difficult to secure even one report on your paper -indeed, the long delay to get to this point, and the comprehensive report provided by the reviewer motivates the editors to make a recommendation on one report. While the reviewer is broadly positively inclined towards your paper, we are issuing a revision to encourage you to fully address their concerns -it is likely that the paper will be returned to this referee for a further assessment after submission of the revision, unless the editors are persuaded your changes obviate the need for this step. We wish you every success, and all the best for the festive season.
Reviewer comments to Author: Reviewer: 1 Comments to the Author(s) Condie et al. present an extensive modeling effort to explore the potential benefits of large-scale intervention efforts to prevent coral decline on the Great Barrier Reef. I commend the authors on a thorough and accessible description of the many parameters and assumptions incorporated in their model. Overall, I found the conclusions to be well supported by the data and the findings of high value to the coral reef management community. In particular, the authors did an excellent job of distilling the core conclusions from their work as key recommendations for intervention planning.
Our thanks for such positive encouragement.
The paper is well written and effectively organized. My only major comment is that the paper would benefit from a more thoughtful discussion of the limitations of their model assumptions and how their interpretation of the data might change if their assumptions varied slightly. I see at least two points worth elaborating on: First, coral adaptation is incorporated in the model but it does not seem like changes in coral community composition towards more resilient (or less tasty to COTs) species have been considered. There are well documented changes in species composition on the GBR, which are likely to influence future coral responses to thermal stress, cyclones, and COTs. How might the projections of coral cover change if a shift in coral composition reduced the amount of food available to COTs, decreased bleaching mortality, and reduced the extent of cyclone damage? This seems like a plausible transformation with widespread impacts on model outputs and warrants some consideration.
All of the processes mentioned already operate in the model and the suggested scenario of shifting coral composition certainly occurs. Specifically, the model includes 5 coral groups (plus the thermally tolerant group) that all differ in their preference by CoTS and susceptibilities to cyclones and heat stress. The thermal tolerance of each coral group can also independently adapt in response to heatwave events. However, apart from the high-level view provided by the declining trend in coral diversity (evenness index for coral groups) presented in Figure 5, we have not attempted to explore trajectories of individual coral groups in this paper. While