Dynamique des populations de coraux scléractiniaires en milieu insulaire fragmenté (Polynésie française) : importance des traits d’histoire de vie, susceptibilité aux perturbations naturelles, et implications pour la trajectoire des populations et la résilience des récifs coralliens

Abstract : Coral reefs are experiencing increased frequency, intensity, and diversity of disturbances that threaten not only the corals themselves, but many associated species as well. The reefs located in the Society archipelago, French Polynesia, recently underwent two major disturbances: an outbreak of the corallivorous sea star Acanthaster planci and a cyclone. This investigation focuses on the dynamics of coral populations and of the major associated communities of the outer reef slopes of the islands Moorea and Raiatea in the context of these two disturbances. Coral population dynamics, as well as reef community dynamics, are governed by a combination of intrinsic factors specific to species (e.g., recruitment, growth, and mortality rates, and susceptibilities to various regulations) and extrinsic factors characteristic of their environment (e.g., quality of habitats, levels of biotic and abiotic stress, frequency and intensity of disturbances). In French Polynesia, these dynamics are strongly influenced by natural disturbances that affect the size, the structure, and the composition of communities. The first section of this PhD estimates the impact of two successive disturbances, the aforementioned Acanthaster outbreak and cyclone, on the dynamics of reef communities, and evaluates their consequences for species diversity and abundance by detailing the progression of these disturbances and quantifying their effects, including their spatial and temporal scales. For the sea star outbreak, on both studied islands Acanthaster densities were first observed in relatively restricted locations situated at the bottom of the outer reef slopes, where these predators greatly depleted coral populations before moving successively to adjacent sites, progressively spreading the outbreak to the whole insular reef system. Aside from this mode of progression, Acanthaster displayed strong feeding preferences among coral taxa, with the genera Acropora, Montipora, other scleractinians, and Pocillopora hierarchically preyed upon before these predators significantly impacted the genus Porites. While the cyclone did not display a significant impact on the coral cover of these reefs, which had already been highly impacted by Acanthaster; however, the cyclone did strongly damage the physical structure of the reef habitats upon which many species rely. Overall, the decline of corals resulted in an increase in the cover of turf algae, enabled by the relaxed competition for space, and it indirectly affected the trophic chain leading to a collapse of corallivorous fish assemblages (family Chaetodontidae) and a relative increase of herbivore fish assemblages (family Scaridae). This response of the ecosystem, with respect to consumer pressures which favor algae regulation and coral recovery, seems to characterize an ecosystem with high resilience potential; however, the exceptional severity of recent disturbances and the progressive drift in the structure of coral communities facing recurrent perturbations are of concern for the maintenance of the ecological services and the diversity of reefs in French Polynesia. The second section of this thesis investigates the spatial and temporal variability of coral population dynamics and their link with species life history traits and environmental conditions. This second part qualifies the size structure and quantifies the recruitment, growth, and mortality rates of the dominant coral species in French Polynesia, and identifies the life strategies of these taxa. Pocillopora displays an opportunistic strategy, with high production and dispersion of larvae, and small, relatively short-lived colonies. In contrast, Porites engages in a strategy based on the persistence of colonies on the reef, with a low rate of habitat colonization counterbalanced by elevated longevity and high resistance to disturbances. Acropora engages in a different strategy from both Pocillopora and Porites, which is based on competition and which displays an intermediate recruitment rate and relatively fast growth, but also high susceptibility to disturbances. Considering their respective life strategies, these coral taxa must rely on different environmental scenarios for their maintenance and for dominating reef habitats. The third section of this doctoral investigation focuses on the elaboration of a mathematical model that allows predicting the trajectory of coral assemblages under various environmental scenarios. This third part simulates the population dynamics of the major scleractinian taxa in French Polynesia both (1) during the colonization of the reef habitat, and (2) in the face of a hypothetical increase in the frequency of Acanthaster outbreaks. In accordance with the life strategies identified in the second section of this thesis, Pocillopora appears as the best colonizer of reef habitats, Acropora dominates the assemblages in a relatively stable environment, and Porites is the less impacted by repeated disturbances. The recurrence of disturbances affects the structure and the perennity of coral assemblages. Thanks to the relatively low and spatially restricted anthropic pressures in French Polynesia, coral reefs in this region are among the most resilient on our planet. As on many reefs, however, long term monitoring programs show an increase in the frequency and intensity of disturbances that threaten the maintenance of corals, as well as their associated communities. Today, coral reefs are in decline globally, and they seem to be heralds of a dynamic shared by many natural ecosystems. In the present era of anthropic disturbances and global climate change, the investigation of community regulation and resilience processes appears crucial for the durable management of these valuable ecosystems.
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Mohsen Kayal. Dynamique des populations de coraux scléractiniaires en milieu insulaire fragmenté (Polynésie française) : importance des traits d’histoire de vie, susceptibilité aux perturbations naturelles, et implications pour la trajectoire des populations et la résilience des récifs coralliens. Biodiversité et Ecologie. École Pratique des Hautes Études (USR 3278 CNRS – CRIOBE – EPHE, Moorea, Polynésie française); Université de Perpignan (Laboratoire d’Excellence ‘‘CORAIL’’, Perpignan, France), 2011. Français. ⟨tel-01251936⟩

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