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Rupture des interactions mutualistes plantes à fruits charnus-vertébrés frugivores, et conséquences sur la régénération des forêts tropicales dans les Mascareignes

Abstract : Tropical forests are mostly dominated by fleshy-fruited plants, which are dispersed by frugivorous vertebrates. The global collapse of large vertebrates therefore raises questions about the resilience of these ecosystems, particularly on islands, which concentrate the bulk of extinctions. The Mascarenes are a remarkable system to study disruption of frugivory interactions because the pristine fauna, plethoric until human colonization in the 17th century and now largely extinct, is well known as well as its diverse flora which is particularly at risk. La Réunion is still home to indigenous forests along strong environmental gradients and active volcanism that offers the opportunity to explore the long-term consequences of defaunation. In addition, the varying levels of vertebrate extinctions between La Réunion (main extant frugivore, mass=55 g) and Mauritius (450 g) makes itpossible to use these islands as pseudo-replicates to test various hypotheses. This thesis is organised in three parts which aim to (1) describe the spatial distribution patterns of dispersal traits on La Réunion and Mauritius, and understand the implications for vertebrate extinctions which have been particularly rapid on La Réunion; (2) evaluate the consequences of frugivory disruption on the reconstruction of forest ecosystems on the lava flows of the Piton de la Fournaise; (3) assess the consequences of frugivory disruption on the maintenance of diversity in forests established before human colonisation. (1) Proportions of fleshy-fruited plants strongly decrease with elevation and this decrease is all the more pronounced the larger the fruit are. By comparing the main factors of vertebrate extinctions between La Réunion and Mauritius, we show that the early destruction of favorable lowland habitats on La Réunion probably played a central role in the rapidity of frugivore extinctions. (2) After expanding the chronosequence of the Piton de la Fournaise flows, we show thatfrugivore extinctions have profoundly altered the capacity of lowland forests to rebuild since the 18th century and that the refaunation of ecosystems with alien frugivores mainly benefits alien fleshy-fruited plants. Reinstating dispersal, native large-seeded plants are nevertheless capable of settling on historical flows where recruitment is overwhelmingly dominated by invasive plants. (3) By comparing two reference forests of La Réunion and Mauritius, we show that the Mauritian flying fox allows a much better recruitment of many plant species at Brise-Fer than the Réunion bulbul at Mare-Longue, except for large-seeded plants which regenerate poorly in both islands. An experiment finally shows how the persistence of fruit flesh alone can strongly limit tree recruitment, but that the latter can be modulated by introduced fauna. Our overall worrying results show the urgency to protect large native frugivores wherever they still exist and of encouraging their return where they had disappeared. At the same time, large-scale sowing should be envisaged in protected areas where the maintenance or even the return of native forest dynamics are imperative.
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2020LARE0035_S_Albert.pdf
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  • HAL Id : tel-03126708, version 1

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Sébastien Albert. Rupture des interactions mutualistes plantes à fruits charnus-vertébrés frugivores, et conséquences sur la régénération des forêts tropicales dans les Mascareignes. Biodiversité et Ecologie. Université de la Réunion, 2020. Français. ⟨NNT : 2020LARE0035⟩. ⟨tel-03126708⟩

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