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Continuité écologique, fragmentation et dynamique spatio-temporelle des communautés en rivières intermittentes

Abstract : Ecology aims at understanding how species and individuals are distributed in space and time. Progress has been recently made in spatial analyses of communities but structuring mechanisms are often considered as static or stable over time. This could affect the understanding of the role of deterministic (such as the influence of local environmental factors) and stochastic processes (such as random fluctuations of mortality rates) in maintaining ecosystems, especially for dynamic ecosystems regularly exposed to drastic changes of environmental conditions. My objective in this thesis was to improve knowledge on spatial and temporal structures of communities in dynamic ecosystems, based on the study of intermittent rivers, experiencing interruption of flow and/ or loss of surface water. The Chapter 3 contributed to improve a statistical tool routinely used in spatial analyses of community. Mantel tests, measuring the correlation between two distance matrices and its significance, are frequently used to study to which extent environmental filtering and dispersal structure metacommunities. We showed that Moran Spectral Randomization allowed us to correct the overestimation of the correlation between distance matrices when they presented spatial autocorrelation. Our results suggest that community analyses based on those tests may have overestimated the structuring role of environmental filtering and that our method will allow future analyses to avoid this bias. In addition, Mantel tests being used in other domains such as genetics, the improvement suggested here will have larger outcomes than community ecology. In Chapter 4, I showed that spatial and temporal distributions of drying event influenced community dynamics in intermittent rivers. The influence of the spatial pattern of disturbances, defined here as an extreme change of environmental conditions, had been previously tested with simulations and mesocosms but not in situ. Upstream-drying river basins harboured more dissimilar communities compared to downstream-drying basins. Upstream drying events may restrain recolonization for aquatic dispersers and lead to a lesser homogenisation of communities. Frequency and duration of drying events also influenced the temporal dynamics of communities and generally led to a higher temporal variability of taxonomic and functional compositions; this relationship was, however, dependent on the spatial drying pattern. By comparing French rivers that have been exposed to drying for centuries and Czech rivers recently exposed to flow intermittence caused by anthropic disturbances, I showed that aquatic communities responded differently to intermittence (Chapter 5). Measuring spatial and temporal beta diversity highlighted the higher variability of communities induced by drying in Czech rivers from both taxonomic and functional perspectives. In addition, a higher sensitivity of resilient taxa to increasing flow intermittence was found in Chapter 5 for non-natural intermittent rivers and this suggests that the increase of drying duration and drying length induced by climate change might jeopardize the recovery of community of these ecosystems after drying in near future
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Julie Crabot. Continuité écologique, fragmentation et dynamique spatio-temporelle des communautés en rivières intermittentes. Ecologie, Environnement. Université de Lyon, 2019. Français. ⟨NNT : 2019LYSE1319⟩. ⟨tel-02482647v2⟩