Un paradigme émergent prédit que les perturbations influencent les processus qui déterminent l'organisation des communautés. L'assèchement des rivières (disparition complète d'eau de surface pour une durée déterminée) est une perturbation naturelle affectant les cours d'eau dans de nombreuses régions du monde. De plus en plus de cours d'eau pérenne s'assèchent en réponse aux changements globaux. Toutefois, l'effet de ces assèchements sur les communautés aquatiques et les processus impliqués dans leur résilience restent mal quantifié et mal compris. A travers quatre études allant de suivis de terrain à des expérimentations in-situ et des mésocosmes en laboratoire, une résilience exceptionnelle des communautés d'invertébrés a été mesurée dans huit rivières alluviales. La zone hyporhéique semble être la principale source de recolonisation expliquant la forte résilience de ces communautés. En laboratoire, l'augmentation de la température et de la compétition intra-spécifique entraine une migration de Gammarus pulex dans la zone hyporhéique. L'augmentation de la profondeur de la zone hyporhéique lors d'assèchements pourrait réduire la résilience et avoir des effets sur les fonctions de l'écosystème (décomposition de litière). Ces résultats montrent que les assèchements n'ont pas toujours des effets sévères sur les communautés d'invertébrés des rivières alluviales qui semblent même très résilientes. La zone hyporhéique joue un rôle primordial dans la résilience des communautés des cours d'eau. Un accent devrait être mis sur la protection de la zone hyporhéique dans les rivières alluviales afin de préserver cette capacité de résilience face aux changements globaux / Understanding community response to disturbance is essential to identifying processes that determine their assembly and to predicting the future effects of climate change on biodiversity and ecosystem functions. Drying (complete loss of surface water) is a natural disturbance affecting 50% of rivers worldwide and is occurring more in perennial rivers due to climate change. However, its effects on aquatic invertebrate communities and the underlying processes contributing to their resilience (i.e. return to pre-drying or undisturbed levels) have not been well quantified. Using 4 congruous field and mesocosm experiments to quantify community resilience and identify its primary sources in environmentally harsh alluvial rivers. First, I found communities in 8 alluvial rivers were highly resilient to moderate and severe drying. Second, I showed that the hyporheic zone (saturated interstitial sediments) can be the primary source of colonists, promoting high community resilience. Third, I found high water temperature and intraspecific competition caused Gammarus pulex, a common benthic detritivore, to migrate into the hyporheic zone. Fourth, I found increasing depth to the water table diminished the hyporheic zone’s role as a source of colonists by reducing survival of G. pulex. My results support an emerging concept that harsh ecosystems are highly resilient and indicate that the effects of drying on biodiversity and ecosystem functions could vary across river systems. In alluvial rivers, the hyporheic zone can contribute strongly to community resilience and management should focus on protecting and restoring vertical connectivity to maximize resilience to climate change

<p>Patterns of site occupancy among boreal stream insects were studied in central Sweden with focus on sparsely distributed species and the role of dispersal and niche limitations.</p><p>In the study of dispersal limitation, I found that effects of an extraordinarily harsh winter in small to medium-sized streams were strongest in sites located in small streams and far from lake outlets. Species richness and the total abundance of macroinvertebrates and trout returned to pre-disturbance levels after three years. However, some species showed slow recolonization and the proportion of holoaquatic taxa was still reduced after three years. In a second study, I found a positive correlation between site occupancy in stream caddisflies and morphological traits associated with fast and energy-efficient flight, whereas specialized spring caddisflies showed a negative correlation to these traits compared to stream species. This suggested that streams, but not springs, select for strong dispersal ability in caddisflies. In a survey of springs in central Sweden, hydrogeology was found to be a useful predictor of the occurrence of spring specialists. Two of these, <i>Crunoecia irrorata</i> Curtis and <i>Parachiona picicornis</i> (Pictet), were found exclusively in glaciofluvial springs, characterized by a stable discharge and temperature. Less specialized members of the spring fauna (i.e. species also occurring in streams, ponds or lakes) also occurred in moraine and limestone springs characterized by more unstable conditions. </p><p>Niche limitations were studied by contrasting large-scale distributions of closely related rare and common stoneflies. Differences in temperature requirements in the juvenile stages and life cycles suggested that the rare species, <i>Isogenus nubecula</i> Newman, was restricted by a limited tolerance to low stream temperatures, whereas the two common species, <i>Isoperla grammatica</i> (Poda) and <i>Diura nanseni</i> (Kempny), appeared to have a broader tolerance to climatic conditions in the study area. In a second study of niche limitations, macroinvertebrate assemblages in 88 streams in Central Sweden showed a nested distribution pattern. Most species deviating from expected distributions occurred in small streams, indicating competitive exclusion from species-rich sites, predator avoidance, or specialization to unique habitat features of small streams. In the last paper, the longitudinal distribution of filter-feeding caddisflies in a lake-outlet stream demonstrated patterns concordant to feeding specialization. </p>

Patterns of site occupancy among boreal stream insects were studied in central Sweden with focus on sparsely distributed species and the role of dispersal and niche limitations. In the study of dispersal limitation, I found that effects of an extraordinarily harsh winter in small to medium-sized streams were strongest in sites located in small streams and far from lake outlets. Species richness and the total abundance of macroinvertebrates and trout returned to pre-disturbance levels after three years. However, some species showed slow recolonization and the proportion of holoaquatic taxa was still reduced after three years. In a second study, I found a positive correlation between site occupancy in stream caddisflies and morphological traits associated with fast and energy-efficient flight, whereas specialized spring caddisflies showed a negative correlation to these traits compared to stream species. This suggested that streams, but not springs, select for strong dispersal ability in caddisflies. In a survey of springs in central Sweden, hydrogeology was found to be a useful predictor of the occurrence of spring specialists. Two of these, Crunoecia irrorata Curtis and Parachiona picicornis (Pictet), were found exclusively in glaciofluvial springs, characterized by a stable discharge and temperature. Less specialized members of the spring fauna (i.e. species also occurring in streams, ponds or lakes) also occurred in moraine and limestone springs characterized by more unstable conditions. Niche limitations were studied by contrasting large-scale distributions of closely related rare and common stoneflies. Differences in temperature requirements in the juvenile stages and life cycles suggested that the rare species, Isogenus nubecula Newman, was restricted by a limited tolerance to low stream temperatures, whereas the two common species, Isoperla grammatica (Poda) and Diura nanseni (Kempny), appeared to have a broader tolerance to climatic conditions in the study area. In a second study of niche limitations, macroinvertebrate assemblages in 88 streams in Central Sweden showed a nested distribution pattern. Most species deviating from expected distributions occurred in small streams, indicating competitive exclusion from species-rich sites, predator avoidance, or specialization to unique habitat features of small streams. In the last paper, the longitudinal distribution of filter-feeding caddisflies in a lake-outlet stream demonstrated patterns concordant to feeding specialization.

The global effort to rehabilitate and restore destroyed mangrove forests is unable to keep up with the high mangrove deforestation rates which exceed the average pace of global deforestation by three to five times. Our knowledge of the underlying processes of mangrove forest regeneration is too limited in order to find suitable techniques for the restoration of degraded mangrove areas. The general objective of my dissertation was to improve mangrove restoration by understanding regeneration processes and local plant-plant interaction in a regenerating Avicennia germinans forest. The study was conducted in a high-shore mangrove forest area on the Ajuruteua peninsula, State of Para, Northern Brazil. The dwarf forest consisting of shrub-like trees is recovering from a stand-replacing event caused by a road construction in 1974 which interrupted the tidal inundation of the study area. Consequently, infrequent inundation and high porewater salinity limit tree growth and canopy closure. All trees and seedlings were stem-mapped in six 20 m x 20 m plots which were located along a tree density gradient. Moreover, height, crown extent, basal stem diameter of trees were measured. The area of herbaceous ground vegetation and wood debris were mapped as well. The mapped spatial distribution of trees, seedlings and covariates was studied using point pattern analysis and point process models, such as Gibbs and Thomas point process, in order to infer underlying ecological processes, such as seed dispersal, seedling establishment, tree recruitment and tree interaction. In the first study (chapter 2), I analyzed the influence of abiotic and biotic factors on the seedling establishment and tree recruitment of A. germinans during the recolonization of severely degraded mangrove sites using point process modeling. Most seedlings established adjacent to adult trees especially under their crown cover. Moreover, seedling density was higher within patches of the herbaceous salt-marsh plants Blutaparon portulacoides and Sesuvium portulacastrum than in uncovered areas. The higher density of recruited A. germinans trees in herb patches indicated that ground vegetation did not negatively influence tree development of A. germinans. In addition, tree recruitment occurred in clusters. Coarse wood debris had no apparent effect on either life stage. These results confirm that salt-marsh vegetation acts as the starting point for mangrove recolonization and indicate that the positive interaction among trees accelerates forest regeneration. In the second study (chapter 3), I analyzed how intraspecific interaction among A. germinans trees determines their growth and size under harsh environmental conditions. Interaction among a higher number of neighboring trees was positively related to the development of a focal tree. However, tree height, internode length and basal stem diameter were only positively associated in low-density forest stands (1.2 trees m-2) and not in forest stands of higher tree density (2.7 trees m-2). These results indicated a shift from facilitation, i.e. a positive effect of tree interaction, towards a balance between facilitation and competition. In the third study (chapter 4), I used point process modeling and the individual-based model mesoFON to disentangle the impact of regeneration and interaction processes on the spatial distribution of seedlings and trees. In this infrequently inundated area, propagules of A. germinans are only dispersed at a maximum distance of 3 m from their parent tree. Furthermore, there is no evidence that the following seedling establishment is influenced by trees. I was able to differentiate positive and negative tree interactions simulated by the mangrove model mesoFON regardless of dispersal processes based on static tree size information using the mark-correlation function. The results of this dissertation suggest that mangrove forest regeneration in degraded areas is a result of facilitative and not competitive interactions among mangrove trees, seedling and herbaceous vegetation. This has important implications for the restoration of degraded mangrove forest. Degraded mangrove areas are usually restored by planting a high number of evenly spaced seedlings. However, high costs constrain this approach to small areas. Assisting natural regeneration could be a less costly alternative. Herbaceous vegetation plays a crucial role in forest recolonization by entrapping propagules and possibly ameliorating harsh environmental conditions. So far only competition among mangrove trees has been considered during restoration. However, facilitative tree interactions could be utilized by planting seedling clusters in order to assist natural regeneration instead of planting seedlings evenly-spaced over large areas. This dissertation also showed that point pattern analysis and point process modeling can enable forest ecologists to describe the spatial distribution of trees as well as to infer underlying ecological processes.