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Benthic composition changes on coral reefs at global scales

Abstract

Globally, ecosystems are being reconfigured by a range of intensifying human-induced stressors. Coral reefs are at the forefront of this environmental transformation, and if we are to secure their key ecosystem functions and services, it is important to understand the likely configuration of future reefs. However, the composition and trajectory of global coral reef benthic communities is currently unclear. Here our global dataset of 24,468 observations spanning 22 years (1997–2018) revealed that particularly marked declines in coral cover occurred in the Western Atlantic and Central Pacific. The data also suggest that high macroalgal cover, widely regarded as the major degraded state on coral reefs, is a phenomenon largely restricted to the Western Atlantic. At a global scale, the raw data suggest decreased average (± standard error of the mean) hard coral cover from 36 ± 1.4% to 19 ± 0.4% (during a period delineated by the first global coral bleaching event (1998) until the end of the most recent event (2017)) was largely associated with increased low-lying algal cover such as algal turfs and crustose coralline algae. Enhanced understanding of reef change, typified by decreased hard coral cover and increased cover of low-lying algal communities, will be key to managing Anthropocene coral reefs.

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Fig. 1: Benthic composition of the world’s coral reefs.
Fig. 2: Coral reef benthic composition among major realms.
Fig. 3: Hard coral, macroalgal and low-lying algal community dynamics in major marine realms.
Fig. 4: Hard coral, macroalgal and low-lying algal community dynamics in key habitats across marine realms.

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Data availability

All data used in this study were attained from publicly available databases and previous literature. The sources of all data and links to databases are provided at the appropriate section in the manuscript, in Supplementary Text 3, and are publicly available on Figshare (https://doi.org/10.6084/m9.figshare.21267924.v1). The derived data from published studies are also publicly available on Figshare (https://doi.org/10.6084/m9.figshare.21267924.v1).

Code availability

Code supporting the findings of this study is publicly available on Figshare (https://doi.org/10.6084/m9.figshare.21267924.v1).

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Acknowledgements

We thank Reef Life Survey, Reef Check Foundation, Reef Check Australia and their volunteers as well as the authors of all other data sources listed in Supplementary Text 3 for the collection and provision/publication of benthic cover data, and L. Lutzenkirchen and A. Siqueira for logistical support and feedback. Funding was provided by the Australian Research Council (CE140100020 and FL190100062, D.R.B.) and an Australian Government Research Training Program Scholarship (S.B.T.).

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S.B.T., S.R.C. and D.R.B. conceived the study; S.B.T. compiled the data; S.B.T. and S.R.C. conducted the analyses; S.B.T., S.R.C. and D.R.B. interpreted the analyses; S.B.T. drafted the initial version of the manuscript; S.B.T., S.R.C. and D.R.B. contributed to editing and revising the manuscript, approved the submitted version and agree to be personally accountable for their contributions.

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Correspondence to Sterling B. Tebbett.

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Nature Ecology & Evolution thanks Lorenzo Alvarez-Filip, Florian Roth and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Extended data

Extended Data Fig. 1 Coral reef benthic composition among major realms.

a) World map showing the delineation of the major marine realms based on the data from 1997, 2007 and 2017 used in the ordination plots (b–d). Multivariate ordination plots based on the Morisita–Horn index and constrained by realm, habitat, year and depth, showing coral reef benthic composition in the four realms in b) 1997, c) 2007, and d) 2017. The coloured polygons (matching the realm colours in [a]) in the ordinations are based on 50% kernel density distributions, denoting where the data points are concentrated in multivariate space. Note the vectors in b) show the relationship between the benthic categories and how they influence the benthic composition data points in the ordination plots. The vectors in c) show how the constraining factors correlate with multivariate space (note the strongest correlations are driven by differences across realms). The lines in d) denote the areas of multivariate space typified by the three major benthic categories.

Extended Data Fig. 2 Hard coral, macroalgal and low-lying algal community dynamics in major marine realms with varying y-axis ranges.

The benthic cover of a) hard corals, b) macroalgae and c) low-lying algal communities on coral reefs in the Western Atlantic (n = 5071 cover observations for each benthic category), Indo-West Pacific (n = 8382 cover observations for each benthic category), Central Pacific (n = cover 8786 observations for each benthic category) and Indian Ocean (n = 1713 cover observations for each benthic category). Solid lines denote the mean fit from generalised additive mixed effects models, while the shaded areas denote the 95% confidence intervals.

Extended Data Fig. 3 Relative frequency distribution of the benthic composition data among habitats.

Frequency distribution of benthic composition data (1997–2018) across habitats in the a) Central Pacific, b) Indian Ocean, c) Indo-West Pacific and d) Western Atlantic.

Extended Data Fig. 4 Frequency histogram of the benthic composition data among realms through time.

The number of benthic composition observations in the dataset in each year in the a) Central Pacific, b) Eastern Atlantic, c) Indian Ocean, d) Indo-West Pacific, e) Tropical Eastern Pacific, and f) Western Atlantic.

Extended Data Fig. 5 The cover of hard corals, macroalgae and low-lying algal communities on the world’s coral reefs.

Ternery plots of hard coral, macroalgae and low-lying algal community cover on the world’s coral reefs in 2017/2018 in the a) Indo-West Pacific, b) Western Atlantic, c) Indian Ocean, and d) Central Pacific. Colouring of the hexagons corresponds to the number of data points that fall within each hexagon.

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Tebbett, S.B., Connolly, S.R. & Bellwood, D.R. Benthic composition changes on coral reefs at global scales. Nat Ecol Evol 7, 71–81 (2023). https://doi.org/10.1038/s41559-022-01937-2

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