To what extent do mesophotic coral ecosystems and shallow reefs share species of conservation interest? A systematic review

Background Globally, shallow-water coral reef biodiversity is at risk from a variety of threats, some of which may attenuate with depth. Mesophotic coral ecosystems (MCEs), occurring from 30 to 40 m and deeper in tropical locations, have been subject to a surge of research this century. Though a number of valuable narrative reviews exist, a systematic quantitative synthesis of published MCE studies is lacking. We conducted a systematic review to collate mesophotic research, including studies from the twentieth century to the present. We highlight current biases in research effort, regarding locations and subject matter, and suggest where more attention may be particularly valuable. Following a notable number of studies considering the potential for mesophotic reefs to act as refuges, it is important to know how comprehensive these sources of recruits and organisms capable of moving to shallow water reefs may be. Methods We search seven sources of bibliographic data with two search strings, as well as personal libraries. Articles were included if they contained species presence data from both shallower and deeper than 30 m depth on tropical coral reefs. Studies were critically appraised based on the number of species identified and balanced sampling effort with depth. Maximum and minimum depths per species were extracted from each study, along with study region and taxon. We quantified the degree of community overlap between shallow tropical reefs (&lt; 30 m) and reefs surveyed at the same locations below 30 m. Proportions of shallow species, across all studied taxa, observed deeper than 30 m were used to generate log odds ratios and passed to a mixed-effects model. Study location and taxon were included as effect modifiers. Funnel plots, regression tests, fail safe numbers, and analysis of a high validity subgroup contributed to sensitivity analyses and tests of bias. Results Across all studies synthesised we found two-thirds of shallow species were present on mesophotic reefs. Further analysis by taxon and broad locations show that this pattern is influenced geographically and taxonomically. Community overlap was estimated as low as 26% and as high as 97% for some cases. Conclusion There is clear support for the hypothesis that protecting mesophotic reefs will also help to conserve shallow water species. At the same time, it is important to note that this study does not address mesophotic-specialist communities, or the ecological forces which would permit refuge dynamics. As we limit our analysis to species only present above 30 m it is also possible diversity found exclusively deeper than 30 m warrants protection in its own right. Further research into relatively ignored taxa and geographic regions will help improve the design of protected areas in future.</p

To what extent do mesophotic coral ecosystems and shallow reefs share species of conservation interest? A systematic review

Background Globally, shallow-water coral reef biodiversity is at risk from a variety of threats, some of which may attenuate with depth. Mesophotic coral ecosystems (MCEs), occurring from 30 to 40 m and deeper in tropical locations, have been subject to a surge of research this century. Though a number of valuable narrative reviews exist, a systematic quantitative synthesis of published MCE studies is lacking. We conducted a systematic review to collate mesophotic research, including studies from the twentieth century to the present. We highlight current biases in research effort, regarding locations and subject matter, and suggest where more attention may be particularly valuable. Following a notable number of studies considering the potential for mesophotic reefs to act as refuges, it is important to know how comprehensive these sources of recruits and organisms capable of moving to shallow water reefs may be. Methods We search seven sources of bibliographic data with two search strings, as well as personal libraries. Articles were included if they contained species presence data from both shallower and deeper than 30 m depth on tropical coral reefs. Studies were critically appraised based on the number of species identified and balanced sampling effort with depth. Maximum and minimum depths per species were extracted from each study, along with study region and taxon. We quantified the degree of community overlap between shallow tropical reefs ( Results Across all studies synthesised we found two-thirds of shallow species were present on mesophotic reefs. Further analysis by taxon and broad locations show that this pattern is influenced geographically and taxonomically. Community overlap was estimated as low as 26% and as high as 97% for some cases. Conclusion There is clear support for the hypothesis that protecting mesophotic reefs will also help to conserve shallow water species. At the same time, it is important to note that this study does not address mesophotic-specialist communities, or the ecological forces which would permit refuge dynamics. As we limit our analysis to species only present above 30 m it is also possible diversity found exclusively deeper than 30 m warrants protection in its own right. Further research into relatively ignored taxa and geographic regions will help improve the design of protected areas in future.</p

Additional file 4 of To what extent do mesophotic coral ecosystems and shallow reefs share species of conservation interest? A systematic review

Additional file 4. Full text report. All studies with full texts retrieved and identified during the screening process as ecological are listed. Code details the screening decision for the entry based on full text screening

Additional file 8 of To what extent do mesophotic coral ecosystems and shallow reefs share species of conservation interest? A systematic review

Additional file 8. Factor level effects. Predictions of community overlap between shallow and mesophotic reefs by taxon and location, as visualised in Fig. 4. Values are reported as per computer output and in descending order of estimate. The number of studies for each factor level contributing to the model is provided. Reference level indicates which factor level was held constant as the factor of interest was changed. The use of different reference levels should not affect the analysis in the absence of an interaction term in the model

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Additional file 6. Distribution of studies across factor levels. The number of studies included in the full meta-analysis of 52 studies is reported for each combination of taxon and location. The total number of studies for a given factor level is in brackets after the level name. Many combinations are not represented, prompting the decision not to include an interaction term in the statistical analysis

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Additional file 2. Data extraction report. All articles included in the final meta-analysis are reported. We also report all articles identified during full text screening as containing data of interest. The comments column provides explanations for articles which did not contribute data for the final meta-analysis, studies which were assigned to the low validity group, as well as whether the authors were contacted

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Additional file 10. Proportion of mesophotic-specialists reported with maximum study depth. The proportion of all species reported, found exclusively deeper than 30 m, increases with the maximum depth of a study. An absence of deep sampling efforts does not allow us to claim there are low levels of MCE specific biodiversity. As such we limit the discussion in this paper to the shallow taxa occurring on MCEs, not of the total change in community between the two depth zones. In order for a fair comparison to be made between shallow and mesophotic biodiversity we must more strictly define the lower limit of MCEs. Data was limited to studies reporting a maximum depth < 500 m for legibility

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Additional file 5. Unavailable full texts report. List of articles screened at abstract level and assigned an eco tag, which we could not retrieve a full text for

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Additional file 9. Funnel plot. Funnel plot for the full meta-analysis. Dotted lines indicate the 95% confidence limit for expected deviation of study estimates from the model. Studies falling outside the triangle are potential outliers. With 52 studies we would expect fewer than 3. A lack of asymmetry to the plot provides no evidence of publication bias within the collected dataset