Contrasting shifts in coral assemblages with increasing disturbances

Abstract

Increasing incidence of major disturbances is contributing to extensive and widespread coral loss, thereby undermining the biodiversity, structure and function of reef ecosystems. The composition of coral assemblages is already changing due to selective effects of recurrent disturbances, combined with marked differences in the underlying life-history dynamics of corals, which affects their recovery. This study quantifies the effects of varying disturbance regimes on two groups of corals with divergent life histories: short-lived species with rapid growth (bushy and tabular Acropora) and long-lived species with slow growth (massive and columnar Porites). Inter-decadal shifts in the coral assemblages across four locations suggest that a high frequency of moderate disturbances favours Porites, whereas infrequent, but severe disturbances favour rapidly replenishing Acropora. Using empirical modelling, we expand these observations to show that Acropora continues to dominate so long as the interval between major disturbances is > 2 years. The only disturbance regime we considered that favoured Porites was high frequency (2-year recurrence) of moderate disturbance, whereas high frequency of severe disturbances led to local extirpation of both Acropora and Porites. Our results show that increasing incidence of major disturbances will not necessarily lead to selective loss of species that are most susceptible to disturbance, as long as these species can continue to colonise vacant space and grow quickly in the aftermath of such disturbances. This study highlights the need to consider the sensitivity of taxa to changes in both disturbance frequency and severity when forecasting changes in the composition of coral assemblages under new disturbance regimes.

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Acknowledgements

This manuscript was developed with support from an American Australian Association Sir Keith Murdoch Fellowship (awarded to MSP) and further advanced during extensive discussions at the 2019 Coral Demography Working Group meeting at Catalina Island, funded by the ARC Centre of Excellence for Coral Reef Studies. In particular, this manuscript was greatly improved by discussions with P Edmunds. The authors are grateful to all colleagues and collaborators that have documented changes in coral assemblages in southern Persian Gulf, Chagos and French Polynesia, especially S. Purkis, C. Sheppard and M. Berumen.

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Appendix

Appendix

  • (a) Details of the coral model

    To simulate likely changes in relative abundance of Acropora versus non-branching Porites subject to increasing frequency versus severity of disturbances, we expanded on models used previously to explore Acropora/Porites dynamics (Riegl and Purkis 2009; Riegl et al. 2013). Coral dynamics were simulated using a two-species model, represented by fast-growing, branching corals of the genus Acropora in three life stages (juvenile small colony C1, medium adult C2 and big adult C3) and slow-growing corals, non-branching Porites, in the same three life stages (C4, C5 and C6). “Juvenile” refers here to small colonies from recruit size (few mm) to below puberty (~ 5 cm radius, Trapon et al. 2013) which depend on the other size classes for their existence (since they do not contribute to reproduction themselves) and suffer high mortality. “Small adult” refers to colonies that have achieved puberty and are in the midsize range of the species (roughly 5–50 cm radius), and “large adults” refer to the very large colonies that can be encountered in these genera (> 50 cm radius).

    Simulations generated cover values for each life stage, which was then summed across life stages in Fig. 2 and normalised to range 0–1 by their carrying capacity (e.g. a population at K occupies a proportion of 1 of the maximum cover it can achieve), to provide an easier to read proportional contribution of each species to the community.

  • (a) Acropora population

    $$\frac{\mathrm{d}{C}_{1}}{\mathrm{d}t}=({R}_{1}{C}_{2}+{R}_{2}{C}_{3}-{C}_{1}\left({G}_{1}+p+{d}_{1}\right))(1-\frac{({C}_{1}+{C}_{2}+{C}_{3}+{C}_{5}+{C}_{6)}}{K})$$
    (1)
    $$\frac{\mathrm{d}{C}_{2}}{\mathrm{d}t}={G}_{1}{C}_{1}-{C}_{2}\left(p+{d}_{1}+{G}_{2}\right)-\frac{c{C}_{2}{C}_{3}}{K}$$
    (2)
    $$\frac{\mathrm{d}{C}_{3}}{\mathrm{d}t}={G}_{2}{C}_{2}-{C}_{3}(p+{d}_{1})$$
    (3)
  • (b) Massive Porites population

    $$\frac{\mathrm{d}{C}_{4}}{\mathrm{d}t}=({R}_{3}{C}_{5}+{R}_{4}{C}_{6}-{C}_{4}\left({G}_{3}+p+{d}_{2}\right))(1-\frac{({C}_{4}+{C}_{5}+{C}_{6}+{C}_{3}+{C}_{2)}}{K})$$
    (4)
    $$\frac{\mathrm{d}{C}_{5}}{\mathrm{d}t}={G}_{3}{C}_{4}-{C}_{3}\left(p+{d}_{2}+{G}_{2}\right)-\frac{c{C}_{3}{C}_{5}}{K}$$
    (5)
    $$\frac{\mathrm{d}{C}_{6}}{\mathrm{d}t}={G}_{4}{C}_{5}-{C}_{6}(p+{d}_{2})$$
    (6)

Corals were modelled in three discrete size categories to assign functional differences in fertility, competitiveness and mortality. Growth within each stage and graduation among stages is a continuous flow determined by Gi. Coral-type specificity of parameters R, d, G, is indicated by subscripts. Ci is cover in stage i, K is carrying capacity, Gi is growth from one size class to the next, di and p are mortality parameters (disease or non-outbreak predation), and c is a competition parameter. The smallest size classes (C1, C4) are considered to be non-reproductive. Gamete survivability, post-settlement fate, as well as shrinkage from larger stages are implicit in R and C1,4 which therefore include larvae, recruits and small juveniles. Connectivity between reefs may occur at this stage, but we assumed that populations were effectively closed, whereby there was no meaningful input of propagules from external sources. This assumption is justified where large-scale disturbances affect an entire region simultaneously, thus obviating any advantages of connectivity. This also simplified the model and emphasised changes due to modifications of frequency versus severity of disturbances. Cover \( \left( {\frac{{{\text{d}}C_{{\text{1}}_{\text{i}}} }}{{{\text{d}}t}}} \right) \) changes as a function of fertility in bigger size classes (RiCj, i = 1, 2, 3, 4, j = 2, 3, 5, 6). All settlement is into free space (considering the effect of medium and large corals on K). Losses occur by growth into the next larger size class (Gi,i = 1, 3), predation (p) and diseases (di,i = 1, 2).

The medium-sized stage is fertile (terms RiCj in Eqs. 1, 4), and medium Porites and Acropora lose space in competition (scaled by c) with large Acropora (c1C3C2,5/K is subtracted from C2,5). The competition term differs from that for small corals because medium and large corals increase by growth, not reproduction (Gi not RiCj). In the large size class, even Porites has reached a size refuge where they no longer lose in competition (no term multiplied by c is subtracted). Medium and large corals suffer losses by predators and diseases (p and di, where i = 2, 3 for Acropora and 5, 6 for Porites). Large corals are more fecund than the medium-sized stages (Hall and Hughes 1996; Madin et al. 2014). As a result of K only appearing in Eqs. 1 and 4, large individuals tend to stack up with time, if disease and predation mortality (d and p) are low, as is sometimes observed in real coral populations (Riegl et al. 2012).

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Pratchett, M.S., McWilliam, M.J. & Riegl, B. Contrasting shifts in coral assemblages with increasing disturbances. Coral Reefs 39, 783–793 (2020). https://doi.org/10.1007/s00338-020-01936-4

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Keywords

  • Climate change
  • Coral reefs
  • Community shifts
  • Modelling