Exposure to disturbances

Coral communities in French Polynesia undergo particularly high frequencies of intense disturbances that decimate populations, yet recover within a decade even from major mass mortality events, constituting a singular vibrant coral system (15; this study). The reef trajectories illustrate well the theoretical disturbance-recovery pattern characterizing non-equilibrium ecosystems (Fig. 1). Within the temporal scope of this study (1994–2008) conducted at a regional scale, a least one major episode of each of the three most impactful disturbances to coral reefs (bleaching, COTS, and cyclone) was recorded. These disturbances affected 42% of surveyed reefs and differed in term of spatial impacts. As a result, coral dynamics were regionally asynchronous, and many reefs were located at differing positions along the theoretical disturbance-recovery pattern. The COTS outbreak was most widespread and affected 24% of reefs, against 18% affected by the 1998 coral-bleaching event, and 6% by the consecutive cyclones in 1997. These statistics have further increased over recent years, with ongoing propagation of COTS throughout the Society and Australes archipelagos until 2010 and the additional passage of a cyclone near Society Islands in 2010, which resulted in major upheaval of reef communities11. Some recent bleaching events were also reported in 2016 and 2017, mainly located in the Tuamotu and Austral archipelagos.

Long-term observations of reef communities around the island of Moorea indicate both COTS outbreaks and cyclones impact reefs in this region with a periodicity of ~20 years, whereas coral-bleaching events occur every ~4 years11,40,41,46. However, in contrast with COTS outbreaks and cyclones which consistently result in abrupt coral decline, most bleaching events do not constitute major disturbances to coral communities in French Polynesia, given the selective decimation of few susceptible taxa46,47. Similar observations are reported from the Great Barrier Reef, another vast coral system of the South Pacific, where most coral decline has historically been also associated with COTS outbreaks and storms while bleaching events were often less impactful over broad spatial scales28,35,39 although this pattern is increasingly challenged after the back-to-back extreme marine heatwaves in 2016 and 201730. However, in contrast with the dynamics observed in French Polynesia, abrupt major disturbance events inducing as much as 33% decline in coral cover have seldom been observed on the Great Barrier Reef where a longer history of observation may be required to capture multiple occurrences of such intense events14,23,34. At a broader scale, the 1998 and 2016 coral-bleaching events and COTS outbreaks observed in 2006–2010 in French Polynesia were part of global phenomena26,48. However, a cloudy weather above the Society archipelago presumably mitigated the effects of the coral bleaching event in 199844 and possibly 2016. Overall, the French Polynesian reef system appears as a dynamic mosaic of coral communities that follow different trajectories in response to more or less localized environmental disturbances.

Vulnerability to disturbances

Despite fundamental differences in their nature and pace of action11,17,40, the episodes of coral bleaching, COTS outbreak, and cyclone intercepted in our dataset induced equivalent declines of coral communities. The average 76% decline in coral cover attributable to these events was however unequally shared among the dominant coral taxa. With respectively 90% and 82% of decline on average, Acropora and Pocillipora populations were most sensitive to disturbances and underwent quasi-extirpation at each event. In contrast, Porites was more resistant with a decline of 34%. These differences in coral vulnerability to disturbances concord with the contrasting life history characteristics distinguishing the three taxa in terms of colony morphology and porosity of the skeleton, thickness in tissue layer, and palatability for predators, which are major determinants of coral susceptibility to cyclone, bleaching, and COTS11,49,50.

Recovery from disturbances

Despite undergoing a sustained regime of intense disturbances of multiple types, the French Polynesian outer-reef system shows a particularly high resilience capacity, with full recovery in coral cover repeatedly observed within 5–10 years following mass mortality events (15, this study). This fast replenishment of communities seems to be at the highest level of recovery achievable for slow growing, habitat-forming organisms such as reef-building corals. A similar pattern is reported on tropical forests where systems that have evolved in more frequently disturbed natural environments also show the highest aptitude for recovery22. Comparably, on Australia’s Great Barrier Reef where many coral communities are exposed to different types of disturbances, the shortest coral community recovery periods observed extend 7–10 years, and are reported along with examples of longer duration and failure in recovery23,28,34. Reports of full coral recovery from mass mortality events are even scarcer in other regions, and extend beyond the decadal timescale51. Importantly, in our broad-scale survey, no major decline without subsequent recovery was observed on any reef, confirming that the coral communities systematically bounce back following mortality events and thus the reefs possess a high resiliency. This high recovery capacity of reefs is shown to be driven by the elevated ability of coral larvae to repopulate reefs shortly after the impacts of disturbances21,52,53, which implies a sustained connectivity among populations on spatial scales that extend beyond the range of disturbances. As such, the fragmented insular reef system in French Polynesia can be seen as a network of inter-connected coral communities with asynchronous dynamics, which guarantees an ever preserved stock of adult populations whose reproductive output can provide larvae for replenishment of disturbed reefs40 and maintain the extrinsic resistance of these communities to major ecological shifts36. Nevertheless, coral decline is of increasing concern on some localized lagoonal reef habitats of French Polynesia that are exposed to higher influence from human populations and follow different dynamics from that of outer-reef coral communities54.

Coral communities on the three recovering reefs showed a comparable symmetrical-sigmoid trajectory shape, suggesting that coral replenishment was governed by similar regulatory processes determining habitat colonization and saturation at a large spatial scale. The three reefs, however, recovered at differing rates, each tending toward a distinct saturation threshold that probably varies across reefs21,55. Taxonomic differences in recovery trajectories of coral populations concord with the contrasting life history characteristics of these species in French Polynesia. Indeed, coral community recovery was consistently dominated by Pocillopora which has the highest reproduction rate and a life strategy promoting a fast recolonization of habitats at a large spatial scale56. The sigmoid recovery trajectory, depicting an accelerating expansion of its populations after a relatively short period of latency and a slowing rate of expansion as approaching a saturation threshold concords with findings that suggest a density dependent regulation of recruitment in this taxon52,57. Acropora and Porites showed slower demography with recovery trajectories often restricted to partial representations of the theoretical sigmoid recovery pattern as illustrated in Fig. 2. Interestingly, coral recovery dynamics led to the rise of Porites on reefs that were impacted by bleaching, and of Acropora following cyclone. Hence, among the dominant coral taxa in French Polynesia, Porites is the most resistant to bleaching40,46,50, and the high capacity of Acropora to propagate through fragmentation confers a strong potential for positive responses to cyclonic events52.

Despite differing levels of vulnerability of taxa to disturbances and different history of disturbance on reefs, the recovering coral populations and communities converged toward their pre-disturbed states thus preserving their community abundances and structures. These findings attest of the resiliency of the French Polynesian coral system and contrast with the globally increasing examples of altering coral communities and ecological shifts to alternative stable reef states26. The estimated rates of coral recovery on the relatively unaltered French Polynesian outer-reefs can thus constitute a valuable baseline for evaluating reef resilience in other regions and in the future. Our results particularly show that not only French Polynesian outer-reefs still possess strong ecological attractors that keep these ecosystems in a conventional state of coral dominance across disturbance-recovery cycles, but also that coral trajectories on recovering reefs converge to a predictable and preserved community structure regardless of disturbance history and species life history.

Systematic convergence toward pre-disturbance community structures

Dramatic decline in the quantity and quality of natural ecosystems has drawn much research and conservation efforts toward assessment of ecosystem trajectory and resiliency1,2,23,24,58. In this endeavour, both theoretical and empirical approaches have been developed with differing degrees of complexity and over different scales of biological organization6,15,19,20,22,37. Yet, a standardized framework that bridges between these different approaches, and provides a common ground that facilitates quantitative understanding of community dynamics and inter-system comparisons, has been lacking. By confronting observed species dynamics with ecological theory of resilient systems, our approach allows estimating persistency in community sizes and composition beyond system fluctuations. In particular, the functional models we provide constitute a flexible and accurate set of tools for modelling specific portions of species trajectories that are of ecological interest, as illustrated here for recovery processes, and should benefit understanding and predicting future community dynamics. Applied to long-term data on reef-building coral dynamics from the frequently disturbed yet resilient French Polynesia reef system, this approach revealed systematic convergence in community recovery trajectories over a broad spatial scale which, we argue, can be used as a measure of ecosystem resistance to ecological shifts. As the frequency and intensity of disturbances are expected to keep increasing with ongoing human pressures and climate change, our approach opens new paths to detecting early signs of failure in community recovery and predicting forthcoming trajectories in coral reefs and other ecosystems.