Vercelloni J et al. (2019), Exposure, vulnerability, and resiliency of Fren...

ECB-ART-46927

Sci Rep 2019 Jan 31;91:1027. doi: 10.1038/s41598-018-38228-5.

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Exposure, vulnerability, and resiliency of French Polynesian coral reefs to environmental disturbances.

Vercelloni J , Kayal M , Chancerelle Y , Planes S .

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Preserving coral reef resilience is a major challenge in the Anthropocene, yet recent studies demonstrate failures of reef recovery from disturbance, globally. The wide and vigorous outer-reef system of French Polynesia presents a rare opportunity to assess ecosystem resilience to disturbances at a large-scale equivalent to the size of Europe. In this purpose, we analysed long-term data on coral community dynamics and combine the mixed-effects regression framework with a set of functional response models to evaluate coral recovery trajectories. Analyses of 14 years data across 17 reefs allowed estimating impacts of a cyclone, bleaching event and crown-of-thorns starfish outbreak, which generated divergence and asynchrony in coral community trajectory. We evaluated reef resilience by quantifying levels of exposure, degrees of vulnerability, and descriptors of recovery of coral communities in the face of disturbances. Our results show an outstanding rate of coral recovery, with a systematic return to the pre-disturbance state within only 5 to 10 years. Differences in the impacts of disturbances among reefs and in the levels of vulnerability of coral taxa to these events resulted in diverse recovery patterns. The consistent recovery of coral communities, and convergence toward pre-disturbance community structures, reveals that the processes that regulate ecosystem recovery still prevail in French Polynesia.

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	Figure 1. A general theoretical disturbance-recovery model describing the resilience of complex natural ecosystems following disturbances in a limiting environment. The sigmoid recovery response encompasses (I) a linear latency phase of slow recolonization of habitats, (II) an exponential acceleration phase of increasing growth through the utilization of resources in a non-restrictive environment, and (III) a logarithmic deceleration phase of decreasing growth under escalating environmental limitations until reaching an asymptotic saturation at the carrying capacity threshold. This theoretical recovery model can be partially or fully represented mathematically by five functional models: from 1–5 representing, respectively, Linear, Exponential, Logarithmic, Logistic and Gompertz models (see equations in Methods).
	Figure 2. Geographical locations of the monitored reefs. Modified from Adjeroud et al. 200542.
	Figure 3. Coral dynamics (mean cover ± 95% confidence intervals) as measured on the 17 reef locations surveyed throughout French Polynesia. Letters on graphs indicate statistically different groups of cover values. Shaded areas on graphs indicate occurrences of major disturbances inducing ≥33% coral mortality: coral bleaching (▽), predator crown-of-thorns starfish outbreak (✳), cyclone (○). The general patterns of coral dynamics on each reef is synthesized using arrows: increase (↗), stagnation (↔), decrease (↘). Island names in bold are those where a disturbance-recovery cycle was observed.
	Figure 4. Impacts of the three major environmental disturbances on coral communities and populations of dominant coral genera. For each coral category, box plots from left to right correspond respectively to all disturbances confounded (coloured bars with a different colour code for each group, nreef = 6), coral bleaching (▽, nreef = 3), predator crown-of-thorns starfish outbreak (✳, nreef = 2 and 4 reef locations), and cyclone (○, nreef = 1). Boxplots show the distributions of data with medians represented by the tick lines and 95% of the data delimited by the boxes and the minimum and maximum values represented by the thin lines. Letters on graph indicate statistical differences in the susceptibility of coral genera to disturbances. The average percentage of coral loss values (±SE) are also displayed as text.
	Figure 5. Recovery trajectory of coral communities and populations of dominant coral genera on the three islands where a disturbance-recovery cycle was observed. Panels (A–C) illustrate recovery in population and community size as expressed by absolute cover. Panels (D–F) illustrate recovery in community structures as expressed by relative-contribution of populations to communities. Dots indicate observations (filled dots for post-bleaching recovery and hollow dots for cyclone), lines are estimated functional responses and shaded areas show the 95% confidence intervals of the regressions. Note that the panels (D–F) do not show regression intervals because they were sometimes too wide to be displayed. Asterisks indicate the pre-disturbance sizes (A–C) and relative-contributions (D–F) of the different populations and communities. The equation describing each regression is indicated on the graphs (equations 1–5, refer to the core of the manuscript for the mathematical formulae). See Supplementary Materials 2 for the estimated parameters of each equation and the associated p-values.
	Figure 6. Relationships between the pre-disturbance sizes (A) and compositions (B) of communities and the estimated values of these variables when reaching saturation in coral community at the end of the recovery process. The panel C illustrates the relationship between disturbance intensity (%loss) and duration of recovery (time required to reach the pre-disturbance cover value, see Electronic Supplementary Material). Dots indicate observations (filled dots for post-bleaching recovery and hollow dots for cyclone) and lines are functional responses. The equation describing each regression is indicated on the graphs (equations 1–5, refer to the core of the manuscript for the mathematical formulae). See Electronic Supplementary Material for the estimated parameters of each equation and the associated p-values.

References [+] :

Adjeroud, Recovery of coral assemblages despite acute and recurrent disturbances on a South Central Pacific reef. 2018, Pubmed