Past extensive changes in land use have resulted in fragmentation of species habitats. Changes in landscape structure have provoked discussion about the prospects of species of fragmented habitats in the future agricultural landscape. Landscape dynamics of plant species is basically a result of extinction and colonization rates. Many studies have confirmed the former expectation that a number of habitats suitable for a plant species in a landscape stay unoccupied. The differences in species distribution and proportion of suitable habitats occupied by a given species are hypothesized to be due to the area and isolation of suitable habitats and colonization and survival ability of a given species. To understand species dynamics in a changing landscape, first we have to reveal the relationships between the distribution of species and their proportion of occupied habitats, habitat age, habitat configuration and species colonization and survival ability. After the complete understanding of the determinants of species dynamics we will be able to predict reliably species prospect in the future. The very basic aim of this thesis was to reveal the determinants of landscape dynamics of dry grassland species in terms of their distribution and frequency in the landscape. The importance of correct identification...

L’établissement correspond à la formation d’une population pérenne dans l’aire d’introduction. Les populations introduites ayant des effectifs faibles, elles sont sujettes à plusieurs mécanismes augmentant leurs risques d’extinction. La structure spatiale de l’aire d’introduction, une mosaïque hétérogène de patchs d’habitat appelée « paysage », peut affecter la persistance de la population introduite. L’objectif de cette thèse est d’étudier l’interaction entre cette structure spatiale et ces mécanismes, ainsi que leur impact sur l’établissement. Les recherches entreprises ont été conduites en utilisant des modèles stochastiques afin de simuler des invasions et faire émerger des prédictions, et en testant expérimentalement ces prédictions grâce à des introductions artificielles de Trichogramma chilonis en microcosmes. Ces travaux ont permis d’identifier un effet fort de la connectivité du site d’introduction, qui peut diminuer les chances d’établissement au niveau local en favorisant l’émigration depuis le site d’introduction, et augmenter les chances d’établissement à un niveau plus large en permettant la colonisation d’autres patchs dans l’aire d’introduction. Au niveau du paysage, nous avons identifié l’impact des hubs, des patchs concentrant les flux de dispersion, qui accroissent fortement la vitesse de colonisation mais diminuent le taux d’établissement. L’établissement était également favorisé par l’agrégation de la ressource et la colonisation par sa dissémination à travers le paysage. La nature stochastique des dynamiques de colonisation est telle qu’il est nécessaire de les prendre en compte pour étudier l’établissement. / Establishment is an important stage of biological invasions, which corresponds to the formation of a persistent population in the introduction area. It is not trivial, as introduced populations are often small, and subject to various specific mechanisms, which increase extinction risks. The spatial structure of the introduction area, which is usually a heterogeneous mosaic of habitat patches called a “landscape”, can interact with those mechanisms and impact the introduced population persistence. This thesis objective is to study the interaction between this spatial structure and those mechanisms, as well as their impact on establishment. On the one hand, we used stochastic models to simulate invasions and formulate predictions. On the other hand, we tested these predictions by performing artificial introductions of Trichogramma chilonis in laboratory microcosms. We were able to identify the impact of the introduction site connectivity, which could decrease establishment probabilities at a local level by increasing the emigration rate from the introduction site, and increase establishment at the landscape level by increasing the colonisation rate of other patches in the introduction area. At the landscape level, we identified the impact of hubs, i.e. patches concentrating dispersal fluxes. They strongly increased colonisation speed, but also decreased establishment. The clustering of resources increased establishment, while its scattering increased colonization. Our results show that introduced population dynamics are highly sensitive to their size. The stochastic nature of colonization dynamics is also necessary to study establishment.

La sylviculture intensive modifie la composition, la structure et la dynamique des peuplements, réduisant notamment la densité et la diversité des attributs forestiers caractéristiques des stades de sénescence (bois mort et micro-habitats portés par les arbres). Ces éléments représentent des habitats essentiels pour les espèces lignicoles. A l’échelle du massif, la planification des opérations sylvicoles va créer une discontinuité spatiale de ces micro-habitats, pouvant être perçue comme une source de fragmentation pour de nombreuses espèces. Cependant, les effets ont été peu explorés au sein des forêts tempérées exploitées. L’objectif de cette thèse est d’évaluer l’effet de la disponibilité des micro-habitats (quantité et configuration) à de multiples échelles spatiales sur les communautés et les populations d’espèces lignicoles, afin de fournir in fine des connaissances pour la spatialisation des stratégies de conservation. Nous avons proposé une méthodologie statistique permettant de prédire la distribution des micro-habitats au sein du massif de Compiègne (France). En s’appuyant sur cette cartographie prédictive, nous avons montré que différentes guildes de coléoptères saproxyliques, définies en fonction de traits d’histoire de vie liés aux capacités de dispersion, répondaient à la structure du paysage à différentes échelles spatiales. La modélisation de la dynamique des métapopulations d’une bryophyte épixylique (Dicranum viride) nous a permis de mettre en évidence l’importance de la dispersion sur les processus de colonisation/extinction qui sous-tendent les patrons d’occurrence des espèces observés à l’échelle du massif. Nos résultats soulignent le rôle central du bois mort et des micro-habitats, qui peuvent être considérés comme un levier de gestion pertinent. Augmenter leur quantité et optimiser leur arrangement spatial intra et inter-peuplement apparaît essentiel pour assurer la persistance des espèces lignicoles au sein des massifs forestiers gérés. / Intensive forestry induces a simplification of forest stand structure, composition, and dynamics, resulting in a decrease of density and diversity of old-growth forest attributes (deadwood and tree-related microhabitats). These elements represent critical habitats for lignicolous species. At the landscape scale, forest management planning create a spatial discontinuity of these microhabitats, which can be perceived as a fragmentation process for many species. Although habitat fragmentation is currently recognized as one of the major threats to biodiversity, its impact on species in managed temperate forest are still poorly understood. The objective of this work was to evaluate the effect of microhabitat availability (quantity and configuration) on species communities and populations, at multiple spatial scales, in order to provide evidence-based recommendations regarding the spatial arrangement of conservation strategies. We proposed a statistical methodology to predict the microhabitats distribution within the complex of the Compiègne forest (France). Based on this predictive mapping, we showed that different guilds of saproxylic beetles, defined according to their life history traits related to dispersal abilities, respond to the landscape structure at different spatial scales. Modelling metapopulation dynamics of an epixylic bryophyte (Dicranum viride) has highlighted the importance of dispersal on colonization/extinction processes that underlie the species’ occurrence patterns observed at the forest scale. Our results emphasize the key role of deadwood and tree-related microhabitats, which can be considered as relevant management leverage. Increase their quantity and optimize their spatial arrangement within and between forest stands, appears to be essential for persistence of the lignicolous species within managed forests.

Mass gatherings stress local and global health care systems as they bring together individuals from all over the world that have very different health conditions. We firstly provide an overview of the concepts and results of mathematical epidemiology and public health. Secondly, we present an introduction to the mathematical modelling of measles using deterministic and stochastic approaches for both single and multiple populations. Lastly, we develop a model for mass gatherings and present an application to measles during the Hajj by studying an SIR deterministic metapopulation model with residency and its stochastic analogue. The models incorporate real world country data and time dependent movement and transmission rates, accounting for realistic volume of international travel and seasonality of measles activity. Numerical results for the deterministic system are presented. We conclude with a discussion on further work.

Mass gatherings stress local and global health care systems as they bring together individuals from all over the world that have very different health conditions. We firstly provide an overview of the concepts and results of mathematical epidemiology and public health. Secondly, we present an introduction to the mathematical modelling of measles using deterministic and stochastic approaches for both single and multiple populations. Lastly, we develop a model for mass gatherings and present an application to measles during the Hajj by studying an SIR deterministic metapopulation model with residency and its stochastic analogue. The models incorporate real world country data and time dependent movement and transmission rates, accounting for realistic volume of international travel and seasonality of measles activity. Numerical results for the deterministic system are presented. We conclude with a discussion on further work.

A metapopulation is defined as a set of potential local populations among which dispersal may occur. Metapopulation theory has grown rapidly in recent years, but much has focused on the mathematical properties of metapopulations rather than their relevance to real systems. Indeed, barring some notable exceptions, metapopulation theory remains largely untested in the field. This thesis investigates the importance of metapopulation structure in the ‘real world’, firstly by building additional realism into metapopulation models, and secondly through a 3-year field study of a real metapopulation system. The modelling analyses include discrete-and continuous-time models, and cover single species, host-parasitoid, and disease-host systems, with and without stochasticity. In all cases, metapopulation structure enhanced species persistence in time, and often allowed long-term continuance of otherwise non-persistent interactions. Spatial heterogeneity and patterning was evident whenever local populations were stochastic or deterministically unstable in isolation. In metapopulations, the latter case often gave rise to self-organising spatial patterns. These were composed of spiral wave fronts (or ‘arcs of infection’ in disease models) of different sizes, and were related to the stability characteristics of local populations as well as the dispersal rates. There was no evidence for self-organising spatial patterns in the host-parasitoid system studied in the field (the weevil Sitona discoideus and its braconid parasitoid Microctonus aethiopoides), and a new model for the interaction suggested that this is probably due to the strong host density-dependence and stabilising parasitism acting on local populations. Dispersal may be important because of very high mortality in dispersing weevils, which may be related to the scarcity of their host plant in the landscape. If this is the case, the model suggested that local weevil density may be sensitive to the area of crop grown. Stochastic models showed that species in suitably large metapopulations may persist for very long times at relatively low overall density and with very low incidence of density-dependence. This suggests that metapopulation processes may explain a general inability to detect density-dependence in many real populations, and may also play an important part in the persistence of rare species. For host-parasitoid metapopulation models, persistence often depended on the way in which they were initialised. Initial conditions corresponding to a biological control release were the least likely to persist, and the maximum host suppression observed in this case was 84%, as compared with 60% for the corresponding non-spatial models and >90% often observed in the field. Metapopulation structure also allowed persistence of ‘boom-bust’ disease models, although the dynamics of these were particularly dependent on assumptions about what happens to disease classes at very low densities. Models assuming infinitely divisible units of density, models incorporating a non-zero extinction threshold, and individual-based models all gave differing results in terms of disease persistence and rate of spatial spread. Fitting models to overall metapopulation dynamics often gave misleading results in terms of underlying local dynamics, emphasising the need to sample real populations at an appropriate scale when seeking to understand their behaviour.