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Effect of land use, soil characteristics and rainfall conditions on landslide occurrence in the tropical mountains of NW Rwanda
No full textLandslides are a widespread geo-hydrological hazard that cause loss of life, damage to infrastructure, and long-term disruption of livelihoods. Reducing their impacts requires a better understanding of the processes that govern their occurrence in both time and space. Considerable progress has been made in landslide hazard assessment through systematic mapping, susceptibility analysis, and early warning systems, but such advances remain concentrated in the Global North. In contrast, the Global South continues to face severe data scarcity and methodological gaps, which limit the effectiveness of hazard assessment tools. Motivated by the frequent and severe impacts of landslides in Rwanda, this thesis investigates the role of anthropogenic factors in landslide occurrence, with a focus on the northwestern provinces, the country’s hotspot of recurrent events.
The research consists of two complementary studies. The first examines how agricultural terraces influence landslide occurrence during three major rainfall-triggered events in 2016, 2018, and 2020. Terraces are widely promoted in tropical mountains as a soil and water conservation strategy on intensively cultivated hillslopes. However, their impact on slope stability remains poorly understood. Using an inventory of more than 4,600 landslides triggered by these events, I analyzed the relation between landslides, terraces, and other controlling factors. Results indicate that landslides were, on average, about three times more likely on terraced hillslopes than on non-terraced ones. The effect varied across events: it was strongest for the 2020 event, weaker in 2018, and negligible or negative in 2016. Terraces were most influential on moderately to highly susceptible slopes, but had little additional effect where susceptibility was already very high. Landslides on terraces also tended to be smaller in size. These findings demonstrate the complex and context-dependent role of terraces in modulating slope stability, with direct implications for susceptibility mapping and land management.
The second study addresses the scarcity of soil moisture (SM) data in tropical Africa by developing and analyzing one of the first large-scale field-based SM datasets in the region. Soil moisture is a critical variable for rainfall-triggered landslides, yet remains rarely measured in the Global South. Between November 2021 and November 2024, I collected 195,026 daily volumetric SM measurements along six representative hillslope transects that covered two soil textures (clayey and sandy loam) and three land management types (forested, cultivated without terraces, cultivated with terraces). The dataset reveals strong spatial and temporal variability in SM linked to soil depth, texture, rainfall conditions, and land use. Clayey soils were generally wetter at 30–60 cm depth, whereas sandy loam soils under cultivation and terraces often reached the highest values at 100 cm depth. Land use effects were soil-dependent: forests on sandy loam were much drier at depth than cultivated slopes, while in clayey soils forests were slightly wetter than cultivated areas. Correlations between SM and antecedent rainfall varied widely with season, soil, and land use. Notably, during the long wet season—the peak landslide period—the predictive power of simple antecedent rainfall indices was low, especially on cultivated and terraced slopes.
Together, these studies highlight the strong influence of land management on both the spatial distribution of landslides and the dynamics of soil moisture in tropical mountainous environments. The results challenge the assumption that rainfall alone can serve as a reliable proxy for slope instability and call into question the uniform use of fixed antecedent rainfall thresholds in early warning systems. By demonstrating the context-dependent interactions among rainfall, soil, and land use, this thesis underscores the need for soil- and land use–specific calibration of landslide hazard assessments in the Global South. Ultimately, the work advances understanding of landslide processes in Rwanda and similar tropical settings, with important implications for risk reduction strategies and sustainable land management
AI in the era of new international dynamics: global transformations and power rivalries
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Etude des vagues de chaleur marines dans un zone côtière semi-fermée: détection, formation, causes et conséquences, en combinant des données in situ et satellite
Get PDFMarine heatwaves (MHWs) and marine cold spells (MCSs) are discrete temperature anomalies events that can persist for weeks or months, often causing severe ecological and socio-economic impacts. This thesis investigates the development, drivers and consequences of MHWs and MCSs in Central and Southern Chile (29°S-55°S), with a particular focus on Northern Patagonia, a region of oceanographic interest and of an economic importance due to intensive aquaculture activity. The main objective is to characterise the surface and subsurface dynamics of MHWs and MCSs, assess their spatial variability among large basins and narrow fjords, identify their drivers, and evaluate their ecological implications.
A multi-scale approach was applied, moving from offshore to coastal environments and from surface to subsurface layers. At large scale, satellite reanalysis data at low resolution (0.25°) were used to identify patterns, long-term trends and main drivers of MHWs in Central and Southern Chile, regions previously unexplored in this context. At finer scale, a novel methodology was developed, merging over three decades of in situ observations with satellite products to generate high-resolution (900 m) temperature fields for Northern Patagonia. This allowed the detection of extreme events across fjords and basins and the assessment of their spatial and temporal variability. Subsurface dynamics where also explored with a hydrodynamic model to evaluate the vertical extent of MHWs and MCS and the relation between surface and subsurface events. Additionally, the potential role of MHWs in promoting harmful algal blooms (HABs) was investigated.
Results show that Central Chile and Northern and Southern Patagonia experienced more than 70 MHWs over the past four decades. Northern Patagonia displays the strongest warming trends and is the only subregion with a significant increase in MHW frequency, with more than 30 events in the past decade alone, while MCSs show an overall decline. In this region, the spatial heterogeneity is particularly pronounced, depending mostly on the topography and stratification: stratified fjords experienced stronger anomalies (up to 5°C), whereas more homogeneous basins exhibited weaker events (below 2°C). Major MHWs occurred in 1997-98, 2008 and 2016-17, with contrasting subsurface penetration. The 1997-98 event shown rapid propagation to the bottom layers, while the 2016-17 event remained confined to the upper layers. Similarly, a prolonged series of MCSs affected intermediate layers from 2019 to 2024, while the main events were registered in 2007-08. For both MHWs and MCSs, subsurface anomalies were generally more intense below the low-salinity surface layer and considerably longer-lasting than those at the surface, with events extending over several months or years, with the extreme example of a MCS that lasted from 2022 to 2024 in the deep layers of Reloncaví Sound. Atmospheric forcings, particularly wind and solar radiation, were identified as key local drivers, while large-scale climate modes such as El Niño Southern Oscillation (ENSO) and Southern Annular Mode (SAM) amplified event intensity and duration. Finally, a link was established between MHWs and HAB development, with evidence that MHWs act as a catalyst for A. catenella blooms by creating favourable environmental conditions when sufficient nutrients are available.
This work provides the first integrated assessment of MHWs and MCSs in Central and Southern Chile, particularly in Northern Patagonia, demonstrating the importance of high-resolution approaches for capturing their complexity in coastal systems and highlighting implications for ecosystem functioning, fisheries and aquaculture management.Les vagues de chaleur marines (MHWs) et les vagues de froid marines (MCSs) sont des évènements discrets d’anomalies de température qui peuvent persister plusieurs semaines, voire plusieurs mois, ayant de sévère répercussions environnementales et socio-économiques. Cette thèse analyse le développement, les causes et les conséquences des MHWs et MCSs dans le centre et le sud du Chili (29°S-55°S) avec une attention particulière sur le nord de la Patagonie, une région d’importance économique en raison de la présence de nombreux centres aquacoles. L’objectif principal de la thèse est de caractériser les dynamiques de surface et de subsurface de ces évènements extrêmes, de comprendre leur variabilité spatiale et temporelle entre les grands bassins et les fjords étroits, d’identifier leurs causes ainsi que leurs impacts.
Une approche multi-échelle a été adoptée, allant des zones océaniques offshores jusqu’aux écosystèmes côtiers, et de la surface vers les fonds marins. A grande échelle, un produit satellitaire de réanalyse à faible résolution (0,25°) a été utilisé pour identifier les patterns, les tendances à long terme et les facteurs des MHWs dans le centre et le sud du Chili, des régions qui n’avaient jamais été étudiées sous cet angle. A plus fine échelle, une nouvelle méthodologie a été développée, combinant plus de trois décennies de données in situ et des observations satellitaires pour générer des champs de température de l’eau à une résolution de 900 m pour la Patagonie du Nord. Cette approche a permis de détecter les évènements extrêmes dans les fjords et les bassins, ainsi que d’établir leur variabilité spatio-temporelle. La dynamique de subsurface a également été explorée à l’aide d’un modèle hydrodynamique afin d’évaluer la propagation verticale des MHWs et MCSs et leur relation avec les évènements de surface. Enfin, le rôle potentiel des MHWs dans le développement d’efflorescences d’algues toxiques (HABs) a été examiné.
Les résultats montrent que le centre du Chili et la Patagonie ont subi plus de 70 MHWs au cours des quatre dernières décennies. Le nord de la Patagonie se distingue par des tendances de réchauffement particulièrement marquées et constitue la seule sous-région où la fréquence des MHWs augmente avec plus de 30 évènements sur la seule dernière décennie, tandis que les MCSs y présentent une tendance à la diminution. Dans cette même région, l’hétérogénéité spatiale est particulièrement importante, et résulte surtout de la topographie et de la stratification : les fjords, fortement stratifiés, sont touchés par des intensités beaucoup plus marquées (jusqu’à 5°C) que les bassins plus homogénéisés (moins de 2°C). Les MHWs les plus importantes se sont produites en 1997-98, 2008 et 2016-17, avec schémas de propagation vertocale différents : celle de 1997-98 a très rapidement atteint les couches de subsurface, tandis que celle de 2016-17 est restée confinée à la superficie. De manière similaire, une série de MCSs s’est produite dans les couches intermédiaires entre 2019 et 2024, tandis que les évènements les plus importants ont été observées en 2007-08. Pour les MHWs comme pour les MCSs, les anomalies situées sous la couche de surface peu saline sont plus intenses. Les évènements de subsurface, sont également considérablement plus longs que ceux en surface, avec certains épisodes qui durent plusieurs mois voire plusieurs années. Cela a notamment été le cas lors d’une MCS présente pendant trois ans, de 2022 à 2024, dans le Golfe de Reloncaví. Les forçages atmosphériques, en particulier les vents et le rayonnement solaire, jouent un rôle déterminant, tandis que les modes climatiques à grande échelle, tels que El Niño Southern Oscillation (ENSO) and Southern Annular Mode (SAM), amplifient l’intensité et la durée des évènements. Enfin, un lien a été mis en évidence entre les MHWs et la prolifération de HABs d’Alexandrium catenella, qui trouvent dans ces conditions un environnement favorable à leur développement.
Ce travail propose la première évaluation des MHWs et MCSs dans le centre et le sud du Chili, en particulier dans le nord de la Patagonie, mettant en évidence l’importance d’utiliser des approches à haute résolution pour observer la complexité de ces évènements dans les environnements côtiers et souligne leurs conséquences potentielles sur le fonctionnement des écosystèmes ainsi que sur les secteurs de la pêche et de l’aquaculture.Las Olas de Calor Marinas (MHWs) y Olas de Frio (MCSs) son diferentes eventos anómalos de temperatura que pueden persistir por semanas o meses, a menudo causando severos impactos ecológicos y socio-economicos. Esta tesis investiga el desarrollo, causas y consecuencias de MHWs y MCSs en el centro y sur de Chile (29°S-55°), enfocándose particularmente en la Patagonia Norte de Chile, una región de interés oceanográfico y de importancia económica debido a la acuicultura. El objetivo principal de este trabajo es caracterizar las dinámicas de las MHWs y MCSs en superficie y subsuperficie, evaluar su variabilidad espacial entre cuencas y fiordos, identificando sus causas y evaluando sus implicaciones ecológicas.
Una aproximación multiescalar fue aplicada, moviéndose desde mar abierto hacia ambientes costeros y desde la superficie hacia capaz subsuperficiales. A gran escala, reanálisis de datos satelitales en baja resolución (0.25°) fueron usados para identificar patrones, tendencias a largo plazo y principales causas de MHWs en el centro y sur de Chile, región no estudiada en esta materia. A fina escala, una novedosa metodología fue desarrollada, uniendo tres décadas de observaciones in situ con productos satelitales para generar una alta resolución (900 m) en campos de temperatura para la Patagonia Norte de Chile. Esto permitió la detección de eventos extremos a través de los fiordos y cuencas, y la evaluación de su variabilidad espacial y temporal. Las dinámicas subsuperficiales también fueron investigadas con un modelo hidrodinámico para evaluar la extensión vertical de MHWs y MCSs y la relación entre los eventos superficiales y subsuperficiales, además del rol potencial de MHWs en promover floraciones algales nocivas (HABs).
Los resultados muestran que la zona central, en conjunto con el norte y sur de la Patagonia de Chile experimentaron alrededor de 70 MHWs en las últimas cuatro décadas. La Patagonia Norte de Chile demuestra fuertes tendencias al calentamiento, siendo la única subregión con un incremento significante en la frecuencia de MHWs, con más de 30 eventos solamente en la última década, mientras que MCSs muestran una disminución en general. En esta región, la espacialidad heterogénea es particularmente pronunciada, despendiendo mayoritariamente de la topografía y estratificación: los fiordos estratificados experimentaron fuertes anomalías (sobre 5°C), mientras que cuencas más homogéneas presentaron eventos débiles (bajo los 2°C). Las MHWs más intensas ocurrieron en 1997-98, 2008 y 2016-17, con diferentes niveles de penetración subsuperficial. El evento de 1997-98 indico una rápida propagación hacia capas profundas, mientras que el evento de 2016-17 se mantuvo confinado en capas superiores. Similarmente, una serie prolongada de MCSs afectó las capas intermediarias desde 2019 a 2024, pero los principales eventos fueron registrados en 2007-08. Para ambos (MHWs y MCSs), las anomalías subsuperficiales fueron generalmente mas intensas debajo de la capa superficial de baja salinidad y considerablemente mas duraderas que las superficiales, con eventos extendidos por varios meses o años, con el ejemplo extremo de una MCSs desde 2022 a 2024 en capas profundad del Seno Reloncaví. Forzantes atmosféricos, particularmente vientos y radiación solar, fueron identificados como causas locales claves, mientras que eventos climáticos a gran escala como El Niño (ENSO) y el Modo Anular del Sur (SAM) amplifican la intensidad y duración de los eventos. Finalmente, se encontró una conexión entre MHWs y desarrollo de HABs, con evidencia que las MHWs actúan como precursor de la proliferación de Alexamdrium catenella, creando condiciones ambientales favorables cuando la suficiente cantidad de nutrientes son disponibles.
Este trabajo promueve la primera evaluación integrada de MHWs y MCSs en el centro y sur de Chile, particularmente en la Patagonia Norte, demostrando la importancia de la alta resolución aproximada para capturar la complejidad de su sistema de costas y resalta las implicaciones para los ecosistemas, pesquería y acuicultura
Mechanisms of Community Assembly through the lens of Phylogenetic Diversity: a Critical Reappraisal.
No full textpeer reviewedDarwin was one of the first to hypothesize a connection between niche differentiation and competition and species relatedness, offering an appealing framework to disentangle community assembly processes based on phylogenetic diversity patterns. Community assembly is, however, the result of several processes including potentially confounding factors associated with dispersal limitations and spatial effects, casting doubt about the application of phylogenetic diversity metrics to infer community assembly processes. We implemented a spatially-explicit model involving limited dispersal, drift, trait-based selection and competition to simulate community composition under competing assembly processes in a landscape with contrasted habitat connectivity. The phylogenetic structure of communities globally varied depending on assembly processes and the combination thereof, validating the assumption, made by a large number of studies but seldom tested in a spatially-explicit context, that different assembly processes indeed lead to significantly different patterns of community phylogenetic structure. All the investigated alpha metrics exhibited a poor ability to detect overdispersion under stabilizing processes, and some even unduly recovered a signal of clustering. Some of the most widely used metrics, such as UniFrac, carry a redundant signal with non-phylogenetic metrics, and hence, poorly capture the phylogenetic signal in the data. We identified three metrics, namely Bst or Pst for abundance data and PIst for occurrence data, which best retrieved the correct signal of phylogenetic structure under different assembly processes. Spatial effects may blur the phylogenetic structure of communities and decrease our ability to infer underlying processes. However, meaningful results may be obtained when the appropriate comparisons are made. In particular, phylogenetic clustering under equalizing processes must be tested on inter-habitat comparisons because it is the differential filtering of species between habitats that reveals the impact of equalizing processes. Our simulations further suggest that a significant phylogenetic structure of communities can be retrieved even in species-poor communities, except when the communities being compared are dominated by a single, most abundant species. We therefore conclude with best practices to adequately infer assembly processes with useful phylogenetic diversity metrics
An integrated high-resolution bathymetric model for the Danube Delta system
Get PDFpeer reviewedActing as a buffer between the Danube and the Black Sea, the Danube Delta plays an important role in regulating the hydro-biochemical flows of this land-sea continuum. Despite its importance, very few studies have focused on the impact of the Danube Delta on the different fluxes between the Danube and the Black Sea. One of the first step to characterize this land-sea continuum is to describe the bathymetry of the Delta. However, there is no complete, easily accessible bathymetric data on all three branches of the Delta to support hydrodynamic, biogeochemical or ecological studies. In this study, we aim to fill this gap by combining 4 different datasets, three in the river and one for the riverbanks, each varying in density and spatial distribution, to create a high-resolution bathymetry dataset. The bathymetric data was interpolated on a hybrid curvilinear-unstructured mesh with an anisotropic Inverse Distance Weighting (IDW) interpolation method. The resulting product offers resolutions ranging from 2 m in a connection zone to 100 m in one of the straight unidirectional channel. Cross validation of the dataset underlined the importance of the data source spatial pattern, with average Root Mean Square Error (RRMSE) of 0.55 %, 6.3 % and 27.6 %, for river segments covered by the densest to the coarsest dataset. These error rates are comparable to those observed in bathymetry interpolation in rivers with similar source datasets. The bathymetry presented in this study is the first unique, high-resolution, comprehensive and easily accessible bathymetric model covering all three branches of the Danube Delta. It will serve as an input in a hydrodynamic model of the Danube Delta, with the aim of better understanding the role of the Delta in the land-sea continuum between the Danube and the Black Sea. The dataset is available at https://doi.org/10.5281/zenodo.14055741 (Alaerts et al., 2024)
Living together but apart: spatial and trophic niche segregation of two termite species sharing the same nest
No full textpeer reviewedResource differentiation and segregation is widely recognized as a key factor enabling species coexistence. However, patterns of niche segregation remain poorly understood in faunal assemblages confined by physical barriers, such as species cohabiting in the same nest. In termite–termite symbiosis where a species (inquiline) is hosted in the nest built by another species (host), resource partitioning within the nest appears critical for species coexistence. Here, we aim at disentangling the habitat and trophic niche segregation between Constrictotermes cavifrons and its inquiline, Inquilinitermes inquilinus. We assess how spatial segregation contributes to reducing competition by analyzing where the inquiline constructs its galleries within the host nest. Using an isotopic niche approach, we also examine whether I. inquilinus imposes costs on its host by depleting shared food resources or mitigates conflict through niche differentiation, by exploiting distinct dietary resources. Our findings suggest that the inquiline’s persistence within the nest is linked to spatial segregation, with the inquiline occupying zones rich in dark organic material, while the host inhabits clay-rich, friable galleries constructed by itself. Isotopic analyses further revealed dietary segregation between the two species, likely reducing competition and facilitating coexistence. The actual food used by the inquiline is most probably the dark mineral organic material found in the bottom of the host nest. These observations are in favor of a commensal symbiosis, wherein the inquiline would impose no alimentary or spatial cost upon the host.15. Life on lan
Morphological variation and craniofacial allometry in feliform carnivorans
No full textpeer reviewedWith over 260 recognized extant species, the order Carnivora is one of the most diverse mammalian groups today, with a history tracing back approximately 60 million years. Crown carnivorans are divided into two suborders: Feliformia (cats, genets, hyenas, mongooses, etc.) and Caniformia (dogs, bears, raccoons, weasels, skunks, seals, etc.). Among the evolutionary trends shaping phenotypic diversity over macroevolutionary scales, Craniofacial Evolutionary Allometry (CREA) describes a pattern in which smaller species tend to have proportionally shorter faces and larger braincases. CREA has been documented in two caniform families (Canidae and Mustelidae) and one feliform family (Herpestidae), but not in Felidae. While feliforms generally exhibit shorter faces than caniforms, the reasons behind the felids’ deviation from the CREA pattern remain unclear. A large-scale study quantifying allometry in both fossil and living feliforms might provide key insights into this question.
We collected 3D landmark data from 112 specimens representing 49 species of extant and extinct feliforms and stem groups. Using 3D geometric morphometrics, we quantified cranial morphological variation and tested for allometric patterns within and across feliform families. Our preliminary results show weak allometric signals overall, with the strongest allometric relationship (highest adjusted R²) found in Felidae. Regressions of facial versus braincase size against centroid size were also weak. The lowest allometric signals are observed in Nimravidae and Viverridae, but are also very weak in Hyaenidae. hese preliminary results suggest that feliform carnivorans tend to deviate from the CREA pattern, highlighting the complexity of morphological disparity in Feliformia. As previously suggested in the literature, CREA is not universal, and the evolution of skull shape likely reflects a balance between functional adaptation (e.g., feeding ecology, ecological specialization) and developmental constraints, modularity, or evolutionary rates, which may obscure craniofacial allometry
Subsistence Strategies of Upper Palaeolithic Human Groups in the Dniester Area (Ukraine)
No full texteditorial reviewedThe archaeological research conducted in the last decades allowed to highlight important data about the human occupations and paleoenvironmental conditions during the Upper Paleolithic in the East Carpathian area. More recently new sites were discovered and excavated (such as Doroshivtsi III and Korman IX), and new analyses were conducted on collections to better understand the Gravettian and Epigravettian occupations and subsistence practices in this area. We focus on the Carpathian and part of Middle-Podolsk regions of the Dniester basin, bordered by the outer Eastern Carpathians, in Ukraine. We present here an updated synthesis including the last discoveries and studies on known sites, considering chronological and paleoenvironmental data as well as behavioural data from lithic and, particularly from faunal remains. Our objective is to better understand the evolution of subsistence strategies as well as the modalities of territory occupation during the Upper Palaeolithic by the nomadic societies in the western segment of the East European Plain
CRIS'EAU SEC, a local drought crisis simulator
Get PDFLe jeu sérieux Cris’Eau Sec simule une tension sur la ressource en eau à l’échelon local en plongeant les joueurs au cœur d’une cellule de crise communale. Divisé en trois temps scénaristiques distincts, le jeu sérieux modélise la montée en tension lente typique d’une sécheresse.
Les participants qui endosseront les rôles de diverses acteurs décisionnels d’une commune fictive wallonne – Sec-en-Ville – , devront dès lors se concerter, communiquer et prendre des décisions afin de gérer au mieux un territoire et sa population face à diverses menaces en lien avec les stress hydriques.
Cette expérience offre une potentialité importante d’entraînement et de sensibilisation face au risque de sécheresse émergent et voué à s’accroître sous nos latitudes en raison du réchauffement climatique.Cris'Eau Sec is a serious game that simulates the stress on local water resources by putting players at the heart of a local crisis unit. Divided into three distinct scenarios, the serious game models the slow build-up of tension typical of a drought.
Participants will take on the roles of various decision-makers in a fictitious Walloon municipality - Sec-en-Ville - and will have to work together, communicate and take decisions in order to best manage the area and its population in the face of various threats linked to water stress.
This experiment offers significant potential for training and raising awareness in the face of the emerging risk of drought, which is set to increase in our latitudes as a result of global warming.11. Sustainable cities and communities13. Climate action16. Peace, justice and strong institution