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Successions écologiques et potentiel de récupération des communautés coralliennes : structure, démographie et recrutement dans le sud-ouest de l'océan Indien

Abstract : Coral reefs support millions of people’s livelihood around the world. However, the effects of climate change and the increase in frequency and intensity of disturbances are leading to their accelerated degradation and to the decline of scleractinian coral communities. Current concerns relate to the resilience of these vulnerable ecosystems. In this context, it is essential to improve our understanding of the mechanisms underlying maintenance of coral communities, which may also improve conservation and management efforts that are urgently needed for these ecosystems. This PhD work aims to analyze demographic processes, including recruitment, and the structure of coral assemblages at several scales in different island environments of the southwestern Indian Ocean region. These descriptors were addressed through ecological succession on reefs and underwater lava flows of Reunion Island, and through the assessment of the reef recovery potential of five island systems in the area. Results show that coral recruitment in the Mascarene Islands (Reunion and Rodrigues) is low and highly variable at all spatial scales, from a few centimeters to several hundreds of kilometers. This spatial variability is also observed between sites in terms of benthic cover, density, size structure, mortality and recovery potential of coral communities. However, this spatial variability is not clearly linked to the protection levels of the sites, nor to the theoretical patterns of ecological succession (studied in Reunion Island). A succession pattern is yet highlighted through the increase in coral size and species richness over time until interspecific interactions (e.g. competition for space) lead to their decline. In addition, a strong dominance of the Pocillopora genus is recorded at all lava flow sites, confirming its pioneering and competitive nature. The recovery index (RI) that we developed suggests that the recovery potential of the reefs of the Mozambique Channel is higher (especially for Europa) than that of the reefs of the Mascarene Islands, which are more subject to direct anthropogenic pressures. These results are consistent with past observations of recovery trajectories of the studied reefs following various disturbances. The addition of recruitment rates to the calculation of RI for Reunion and Rodrigues islands clearly modifies the recovery potential of these islands sites: the sites with the highest recruitment rates are also those with the highest RI. This project thus provides essential information on Indian Ocean reef communities that may improve management strategies for coral reef conservation.
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Florian Jouval. Successions écologiques et potentiel de récupération des communautés coralliennes : structure, démographie et recrutement dans le sud-ouest de l'océan Indien. Ecologie, Environnement. Université de la Réunion, 2019. Français. ⟨NNT : 2019LARE0019⟩. ⟨tel-02872269⟩

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