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Critical thresholds of adult patch density and spacing during coral fertilization

Nature Ecology & Evolution (2025)Cite this article

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Abstract

Climate change and other anthropogenic stressors have severe impacts on the ecological functioning of marine ecosystems by causing widespread declines in population sizes and, for surviving individuals, limiting the capacity for population recovery through sexual reproduction. Ecological theory suggests that affected populations can suffer local extinction because of Allee effects, where reduced population densities prevent gamete encounters, resulting in reproductive failure. Without understanding the relationship between the density or spacing of spawning individuals and fertilization success, coral reefs may unknowingly pass a critical population threshold, further complicating conservation efforts. In this study we conducted a series of independent manipulative field experiments using three common simultaneous hermaphroditic spawning Acropora species in two locations (One Tree Island, Great Barrier Reef, and Ngermid Bay, Palau) to assess evidence of Allee effects in small populations. Experimental ‘patches’ of corals were structured with mean intercolonial distances ranging from 1 m to 2 m, resulting in low but measurable fertilization success (1.2–8.7%). We developed a mechanistic coral fertilization model and validated its predictions against this empirical data, finding close alignment. Depending on the species and their colony size, the model predicts that adult coral densities need to exceed 13–50 colonies per 100 m2 for reefs to ensure 10% fertilization success.

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Fig. 1: Field sites and fertilization success of three Acropora species.
Fig. 2: Fertilization model overview, predictions and sensitivity analysis.
Fig. 3: Relationships between simulated patch (10 × 10 m2) characteristics and fertilization success.

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Data availability

The datasets generated and analysed during this study are deposited in the CSIRO Data Access Portal at https://doi.org/10.25919/v4ce-de91 (ref. 62). Source data are provided with this paper.

Code availability

The code used to implement the model described in this study is available via GitHub at https://github.com/gerard-ricardo/fert-model.

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Acknowledgements

We thank the Traditional Owners of the Great Barrier Reef, particularly the Byelle, Gooreng Gooreng, Gurang and Taribelang Bunda First Nations people of the Port Curtis Coral Coast, and the Manbarra First Nations people of the Palm Islands, for permission to work in their Sea Country with free prior and informed consent. We thank their Elders, past, present and emerging, and acknowledge their continuing spiritual connection to their Sea Country. Work on the Great Barrier Reef was conducted under Great Barrier Reef Marine Park Authority permit nos. G21/44774.1 and G22/46963.1, and work in Palau under Marine Research Permit RE-22-11. We thank S. Blanchfield, J. Goldman, M. Tonks and staff from One Tree Island Research Station, Heron Island Research Station, Orpheus Island Research Station, the National Sea Simulator at AIMS, and the Palau International Coral Research Center for assistance during the field work. J. Crosswell, T. Malthus and S. Noonan kindly provided equipment. We thank A. Wuppukondur and D. Callaghan for their advice on the hydrodynamic modelling options and A. Teo and P. Todd for providing the source code for their model. We acknowledge the facilities and technical assistance of the Centre for Microscopy and Microanalysis, University of Queensland. This work was supported by the EcoRRAP subprogram (https://gbrrestoration.org/program/ecorrap/), which is part of the Reef Restoration and Adaptation Program (RRAP) (https://gbrrestoration.org/). RRAP is funded by the partnership between the Australian Government’s Reef Trust and the Great Barrier Reef Foundation. The funders had no role in study design, data collection and analysis, decision to publish or preparation of the manuscript.

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Authors and Affiliations

  1. Marine Spatial Ecology Lab, School of the Environment, The University of Queensland, St. Lucia, Queensland, Australia

    Gerard Ricardo, Elizabeth Buccheri, Andrew Khalil & Peter J. Mumby

  2. CSIRO Environment, St. Lucia, Queensland, Australia

    Gerard Ricardo, Christopher Doropoulos, Russell C. Babcock & Elizabeth Buccheri

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  1. Gerard Ricardo
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  2. Christopher Doropoulos
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  3. Russell C. Babcock
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Contributions

Conceptualization: C.D., P.J.M. and G.R. Formal analysis: G.R. and P.J.M. Funding acquisition: C.D. and P.J.M. Investigation: G.R., P.J.M., R.C.B. and A.K. Methodology: G.R., P.J.M., C.D., R.C.B. and E.B. Visualization: G.R. Supervision: P.J.M. and C.D. Writing—original draft preparation: G.R. Writing—review and editing: all authors.

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Correspondence to Gerard Ricardo.

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Extended data

Extended Data Fig. 1 Biological imagery and model parameterisation experiments.

(a) Scanning electron microscopy of coral egg-sperm bundles, eggs, and sperm. Parameterisation of each gamete stage and process influences the predictions from the fertilisation model. The relative sizes of gametes have been adjusted for clarity. (b) The effects of egg concentration on coral fertilisation success at two sperm concentrations, analysed with a binomial GLMM. The test was two-sided, and p-values were adjusted for multiple comparisons using the Holm-Bonferroni method. Darker points and line represent 106 sperm mL−1 (n = 18 biological replicates), and lighter points and line represent 104 sperm mL−1 (n = 19 biological replicates). Lines show the estimated mean, and shaded bands represent the 95% confidence intervals. (c) Egg ascent rates of Acropora cf. tenuis (n = 11 biological replicates) compared to bundle ascent rates of Acropora cf. tenuis (n = 13 biological replicates) previously reported in Ricardo, et al.60. (d) Fertilisation success of Acropora cf. tenuis following time from spawning (min) with a 5-min sperm contact exposure time, analysed with a binomial GLMM. The test was two-sided, and p-values were adjusted for multiple comparisons using the Holm-Bonferroni method. Darker points and line represent 7×107 sperm mL−1 (n = 24 biological replicates), and lighter points and line represent 2×106 sperm mL−1 (n = 21 biological replicates). Lines show the estimated mean, and shaded bands represent the 95% confidence intervals. Note the log scale on the x-axis.

Source Data

Extended Data Fig. 2 Validation and releases of tracer dyes to simulate coral sperm dispersion.

(a) Vertical decay of coral sperm and tracer dyes over time within a single water column. Curves represent concentration profiles for sperm (n = 20 time measurements), rhodamine WT (n = 9), and fluorescein (n = 9). Inset: Absorbance spectral profiles of coral sperm at four sperm concentrations (sperm mL−1), rhodamine WT dye at multiple concentrations, and fluorescein dye at multiple concentrations. (b) (Left) A representative example of a fluorescein dye release from a research vessel imaged with a UAV. (Right) The same image after the colour band indexing. (c) Current velocity (n = 11762) measured 1 m above the benthos along a north-facing reef slope at Wistari Reef during a ~ 4 mo. deployment. Measurements represent a time series recorded by a single tilt meter. Dotted lines represent the median and 2.5th and 97.5th percentiles. Inset: Density plot of the current velocities. Thicker bar denotes ~1 SD from the median, thinner bar denotes ~2 SDs from the median.

Source Data

Extended Data Fig. 3 Intercolonial distances of A. hyacinthus adult colonies on the reef slope and crest at Uchelbeluu Reef.

Intercolonial distances of A. hyacinthus adult colonies on the reef slope (n = 1 transect) and crest (n = 3 transects) at Uchelbeluu Reef in Palau. Dashed lines indicate the median distances for each habitat.

Source Data

Extended Data Fig. 4 Simulated relationships between mean intercolonial distances of adult colonies and species-specific coral cover or colony density.

Simulated relationships between mean intercolonial distances of adult colonies and (a) species-specific coral cover or (b) colony density. Dashed lines denote mean intercolonial distances of 1 m.

Supplementary information

Supplementary Information

Supplementary Methods 1, Discussion 1, Tables 1–4 and References.

Reporting Summary

Source data

Source Data Fig. 1 and Extended Data Figs. 1–3

Spatial distribution of Acropora sperm concentration and field fertilization success data.

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Ricardo, G., Doropoulos, C., Babcock, R.C. et al. Critical thresholds of adult patch density and spacing during coral fertilization. Nat Ecol Evol (2025). https://doi.org/10.1038/s41559-025-02844-y

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