Trophic downgrading of ecosystems necessitates a functional understanding of trophic cascades. Identifying the presence of cascades, and the mechanisms through which they occur, is particularly important for seagrass meadows, which are among the most threatened ecosystems on Earth. Shark Bay, Western Australia provides a model system to investigate the potential importance of top-down effects in a relatively pristine seagrass ecosystem. The role of megagrazers in the Shark Bay system has been previously investigated, but the role of macrograzers (i.e., teleosts), and their importance relative to megagrazers, remains unknown. The objective of my dissertation was to elucidate the importance of teleost macrograzers in transmitting top-down effects in seagrass ecosystems. Seagrasses and macroalgae were the main food of the abundant teleost Pelates octolineatus, but stable isotopic values suggested that algae may contribute a larger portion of assimilated food than suggested by gut contents. Pelates octolineatus is at risk from numerous predators, with pied cormorants (Phalacrocorax varius) taking the majority of tethered P. octolineatus. Using a combination of fish trapping and unbaited underwater video surveillance, I found that the relative abundance of P. octolineatus was greater in interior areas of seagrass banks during the cold season, and that the mean length of P. octolineatus was greater in these areas compared to along edges of banks. Finally, I used seagrass transplants and exclosure experiments to determine the relative effect of megagrazers and macrograzers on the establishment and persistence of three species of seagrasses in interior microhabitats. Teleost grazing had the largest impact on seagrass species with the highest nutrient content, and these impacts were primarily observed during the warm season. My findings are consistent with predictions of a behaviorally-mediated trophic cascade initiated by tiger sharks (Galeocerdo cuvier) and transmitted through herbivorous fishes and their predators.

Dans cette thèse, nous étudions deux applications des processus stochastiques au marketing internet. Le premier chapitre s’intéresse au scoring d’internautes pour les enchères en temps réel. Ce problème consiste à trouver la probabilité qu’un internaute donné réalise une action d’intérêt, appelée conversion, dans les quelques jours suivant l’affichage d’une bannière publicitaire. Nous montrons que les processus de Hawkes constituent une modélisation naturelle de ce phénomène mais que les algorithmes de l’état de l’art ne sont pas applicables à la taille des données typiquement à l’œuvre dans des applications industrielles. Nous développons donc deux nouveaux algorithmes d’inférence non-paramétrique qui sont plusieurs ordres de grandeurs plus rapides que les méthodes précédentes. Nous montrons empiriquement que le premier a de meilleures performances que les compétiteurs de l’état de l’art, et que le second permet une application à des jeux de données encore plus importants sans payer un prix trop important en terme de pouvoir de prédiction. Les algorithmes qui en découlent ont été implémentés avec de très bonnes performances depuis plusieurs années à 1000 mercis, l’agence marketing d’avant-garde étant le partenaire industriel de cette thèse CIFRE, où ils sont devenus un actif important pour la production. Le deuxième chapitre s’intéresse aux processus diffusifs sur les graphes qui constituent un outil important pour modéliser la propagation d’une opération de marketing viral sur les réseaux sociaux. Nous établissons les premières bornes théoriques sur le nombre total de nœuds atteint par une contagion dans le cadre de graphes et dynamiques de diffusion quelconques, et montrons l’existence de deux régimes bien distincts : le régime sous-critique où au maximum $O(sqrt{n})$ nœuds seront infectés, où $n$ est la taille du réseau, et le régime sur-critique ou $O(n)$ nœuds peuvent être infectés. Nous étudions également le comportement par rapport au temps d’observation $T$ et mettons en lumière l’existence de temps critiques en-dessous desquels une diffusion, même sur-critique sur le long terme, se comporte de manière sous-critique. Enfin, nous étendons nos travaux à la percolation et l’épidémiologie, où nous améliorons les résultats existants. / In this thesis, we study two applications of stochastic processes in internet marketing. The first chapter focuses on internet user scoring for real-time bidding. This problem consists in finding the probability for a given user to perform an action of interest, called conversion, in the next few days. We show that Hawkes processes are well suited for modelizing this phenomena but that state-of-the-art algorithms are not applicable to the size of datasets involved. We therefore develop two new algorithms able to perform nonparametric multivariate Hawkes process inference orders of magnitude faster than previous methods. We show empirically that the first one outperforms state-of-the-art competitors, and the second one scales to very large datasets while keeping very high prediction power. The resulting algorithms have been implemented with very good performances for several years in 1000mercis, a pioneering marketing agency being the industrial partner of this CIFRE PhD, where they became an important business asset. The second chapter focuses on diffusion processes graphs, an important tool for modelizing the spread of a viral marketing operation over social networks. We derive the first theoretical bounds for the total number of nodes reached by a contagion for general graphs and diffusion dynamics, and show the existence of two well distinct regimes: the sub-critical one where at most $O(sqrt{n})$ nodes are infected, where $n$ is the size of the network, and the super-critical one where $O(n)$ nodes can be infected. We also study the behavior wrt to the observation time $T$ and reveals the existence of critical times under which a long-term super-critical diffusion process behaves sub-critically. Finally, we extend our works to different application fields, and improve state-of-the-art results in percolation and epidemiology.

The assemblage of ca. 6–4 Ma volcanic rocks exposed at the Sagehen Research station in the Truckee-Tahoe region of the northern Sierra Nevada, United States, is interpreted to be, within the Ancestral Cascades volcanic arc, a Lassen-type stratovolcano complex. Sagehen is of particular importance because it is one of the few Tertiary arc volcanic centres in California which has not been heavily glaciated during the Pleistocene. The volcanic rocks are variably porphyritic or aphanitic, including abundant plagioclase with clinopyroxene and amphibole. The rocks range from basalt to basaltic-andesite to andesite in composition. Basalts are olivineand clinopyroxene-bearing with minor phenocrysts of plagioclase. The basaltic-andesites are primarily pyroxene bearing while the andesites contain pyroxene-, plagioclase- and hornblende porphyritic phases. Sagehen arc lavas are calc-alkaline and enriched in the large ion lithophile elements and depleted in High Field Strength Elements. The basalts are depleted in Zr and Hf while the andesites are enriched with Zr and Hf relative to the middle rare earth elements. Compared to previously studied Ancestral Cascade arc samples, Sagehen region basalts have lower 143Nd/144Nd isotopic values that do not correspond to proposed mantle-lithosphere mixing lines, while the andesite samples appear to represent the interplay of these two components on a 87Sr/86Sr vs. 143Nd/144Nd. The trace element data and isotopic plots suggest that the melts that produced the basalts are from subduction modified mantle wedge peridotites that ponded near the base of the lithosphere similar to the generation of other subduction related calc-alkaline lavas along convergent continental margins. The andesitic samples appear to be the result of further modification through crustal assimilation as seen in the higher isotopic Sr contents in the andesites and Ce/Smpmn vs. Tb/Ybpmn plots. Finally, the proposed map units from Sylvester & Raines (2007) were found to contain various geochemical facies based on the samples collected indicating that some map units may have to be redefined or sub-divided.

Cette thèse cible l'étude de la structure thermique de la croûte supérieure (<10km) dans les arcs magmatiques continentaux, et son influence sur l'enregistrement thermochronologique de leur exhumation et de leur évolution topographique. Nous portons notre regard sur deux chaînes de montagne appartenant aux Cordillères Américaines : Les Cascades Nord (USA) et la zone de faille Motagua (Guatemala). L'approche utilisée est axée sur l'utilisation de la thermochronologie (U-Th-Sm)/He sur apatite et zircon, couplée avec la modélisation numérique de la structure thermique de la croûte. Nous mettons en évidence la variabilité à la fois spatiale et temporelle du gradient géothermique, et attirons l'attention du lecteur sur l'importance de prendre en compte la multitude des processus géologiques perturbant la structure thermique dans les chaînes de type cordillère, c'est à dire formées lors de la subduction océanique sous un continent.

<p>I have, in this thesis, studied the interactions between gray-sided voles (Clethrionomys rufocanus) and tundra vegetation, on islands in, and mainland sites close to the lake Iešjávri, in northern Norway. As isolated islands are virtually free of predation, I have been able to compare plant-herbivore interactions in the presence and absence of predators. I transplanted vegetation from an island with predators and voles, to predator-free islands with and with out voles. The results reveal the existence of a terrestrial trophic cascade as voles had a severe impact on the transplanted vegetation on the predator-free islands, but only minor effects on the mainland where predators are present. Moreover, this study shows that plant defence was only a successful strategy when predators were present. Voles reduced the abundance of all available plants during winter on the predator-free islands. The results imply that cascading effects of predation are most important for well-defended plants with grazing-sensitive morphology as these plants escape herbivore impacts in the presence of predators but are vulnerable in their absence</p><p>I studied the recovery of intensively grazed vegetation by building exclosures on islands that have been heavily grazed by voles for almost a decade.This study shows that the collective biomass of vascular plants recovered completely on three years, when voles were excluded. Although most species that are dominating the ungrazed vegetation recovered rapidly in the exclosures, the vegetation did not simply return to its ungrazed state. Herbaceous plants increased and there were pronounced differences in response among evergreen dwarf-shrub species. The semi-prostrate and tannin rich crowberry (Empetrum nigrum), showed the strongest recovery of all species, while the erect lingonberry (Vaccinium vitis-idaea) only showed weak signs of recovery. Thus, growth form determined the potential of plants to recover, whereas the trade-off between defensive investments and capacity to recover was weak or absent.</p><p>I studied the interaction between gray-sided voles and their main winter food plant, bilberry (Vaccinium myrtillus) on islands in and mainland sites close to the lake Iešjávri. I compared the abundance, population structure and palatability of bilberry ramets between vole-free islands, islands with voles but no predators and mainland sites with both voles and predators. Voles decreased the abundance of bilberry on the mainland, but the effect was much stronger on predator-free islands. Bilberry was fairly tolerant to grazing as it partially compensated for the lost tissue by producing more new ramets. Moreover, a cafeteria experiment showed that voles preferred the ramets from predator-free islands, which is inconsistent with conjectures emphasizing inducible plant defenses. The vole-bilberry interaction lacks features of delayed density dependence that could explain the vole cycles. I conducted a clipping and fertilization experiment to further investigate the effects of herbivory on palatability of bilberry shoots. Fertilization decreased the concentration of condensed tannins in shoots of bilberry and voles preferred fertilized and clipped shoots. I found no indication of induced defense that could reduce the palatability of bilberry twigs in response to herbivory.</p><p>The relationships between gray-sided vole densities, levels of invertebrate herbivory and chemical quality of leaves of Northern willow (Salix glauca) were studied on islands and mainland sites with contrasting vole densities. I found a positive correlation between level of invertebrate herbivory and vole density. The number of leaves per shoot, leaf size and leaf nitrogen content were also positively correlated with vole densities, while leaf C/N ratios were negatively correlated with vole densities. The positive correlation between vole densities and level of invertebrate herbivory is probably due to a facilitative effect of voles on invertebrate herbivores, mediated through changes in plant chemistry.</p>

I have, in this thesis, studied the interactions between gray-sided voles (Clethrionomys rufocanus) and tundra vegetation, on islands in, and mainland sites close to the lake Iešjávri, in northern Norway. As isolated islands are virtually free of predation, I have been able to compare plant-herbivore interactions in the presence and absence of predators. I transplanted vegetation from an island with predators and voles, to predator-free islands with and with out voles. The results reveal the existence of a terrestrial trophic cascade as voles had a severe impact on the transplanted vegetation on the predator-free islands, but only minor effects on the mainland where predators are present. Moreover, this study shows that plant defence was only a successful strategy when predators were present. Voles reduced the abundance of all available plants during winter on the predator-free islands. The results imply that cascading effects of predation are most important for well-defended plants with grazing-sensitive morphology as these plants escape herbivore impacts in the presence of predators but are vulnerable in their absence I studied the recovery of intensively grazed vegetation by building exclosures on islands that have been heavily grazed by voles for almost a decade.This study shows that the collective biomass of vascular plants recovered completely on three years, when voles were excluded. Although most species that are dominating the ungrazed vegetation recovered rapidly in the exclosures, the vegetation did not simply return to its ungrazed state. Herbaceous plants increased and there were pronounced differences in response among evergreen dwarf-shrub species. The semi-prostrate and tannin rich crowberry (Empetrum nigrum), showed the strongest recovery of all species, while the erect lingonberry (Vaccinium vitis-idaea) only showed weak signs of recovery. Thus, growth form determined the potential of plants to recover, whereas the trade-off between defensive investments and capacity to recover was weak or absent. I studied the interaction between gray-sided voles and their main winter food plant, bilberry (Vaccinium myrtillus) on islands in and mainland sites close to the lake Iešjávri. I compared the abundance, population structure and palatability of bilberry ramets between vole-free islands, islands with voles but no predators and mainland sites with both voles and predators. Voles decreased the abundance of bilberry on the mainland, but the effect was much stronger on predator-free islands. Bilberry was fairly tolerant to grazing as it partially compensated for the lost tissue by producing more new ramets. Moreover, a cafeteria experiment showed that voles preferred the ramets from predator-free islands, which is inconsistent with conjectures emphasizing inducible plant defenses. The vole-bilberry interaction lacks features of delayed density dependence that could explain the vole cycles. I conducted a clipping and fertilization experiment to further investigate the effects of herbivory on palatability of bilberry shoots. Fertilization decreased the concentration of condensed tannins in shoots of bilberry and voles preferred fertilized and clipped shoots. I found no indication of induced defense that could reduce the palatability of bilberry twigs in response to herbivory. The relationships between gray-sided vole densities, levels of invertebrate herbivory and chemical quality of leaves of Northern willow (Salix glauca) were studied on islands and mainland sites with contrasting vole densities. I found a positive correlation between level of invertebrate herbivory and vole density. The number of leaves per shoot, leaf size and leaf nitrogen content were also positively correlated with vole densities, while leaf C/N ratios were negatively correlated with vole densities. The positive correlation between vole densities and level of invertebrate herbivory is probably due to a facilitative effect of voles on invertebrate herbivores, mediated through changes in plant chemistry.

The assemblage of ca. 6–4 Ma volcanic rocks exposed at the Sagehen Research station in the Truckee-Tahoe region of the northern Sierra Nevada, United States, is interpreted to be, within the Ancestral Cascades volcanic arc, a Lassen-type stratovolcano complex. Sagehen is of particular importance because it is one of the few Tertiary arc volcanic centres in California which has not been heavily glaciated during the Pleistocene. The volcanic rocks are variably porphyritic or aphanitic, including abundant plagioclase with clinopyroxene and amphibole. The rocks range from basalt to basaltic-andesite to andesite in composition. Basalts are olivineand clinopyroxene-bearing with minor phenocrysts of plagioclase. The basaltic-andesites are primarily pyroxene bearing while the andesites contain pyroxene-, plagioclase- and hornblende porphyritic phases. Sagehen arc lavas are calc-alkaline and enriched in the large ion lithophile elements and depleted in High Field Strength Elements. The basalts are depleted in Zr and Hf while the andesites are enriched with Zr and Hf relative to the middle rare earth elements. Compared to previously studied Ancestral Cascade arc samples, Sagehen region basalts have lower 143Nd/144Nd isotopic values that do not correspond to proposed mantle-lithosphere mixing lines, while the andesite samples appear to represent the interplay of these two components on a 87Sr/86Sr vs. 143Nd/144Nd. The trace element data and isotopic plots suggest that the melts that produced the basalts are from subduction modified mantle wedge peridotites that ponded near the base of the lithosphere similar to the generation of other subduction related calc-alkaline lavas along convergent continental margins. The andesitic samples appear to be the result of further modification through crustal assimilation as seen in the higher isotopic Sr contents in the andesites and Ce/Smpmn vs. Tb/Ybpmn plots. Finally, the proposed map units from Sylvester & Raines (2007) were found to contain various geochemical facies based on the samples collected indicating that some map units may have to be redefined or sub-divided.

Depuis le début du 20ième siècle, les changements d'usage des terres, la disparition des prédateurs et les régulations de la chasse ont provoqué une augmentation des populations de cervidés dans les forêts tempérés et boréales. Ce phénomène, qui est un grand succès de la conservation de ces espèces, a toutefois conduit à des surabondances qui ont entrainé des effets négatifs en cascades sur la végétation et les communautés animales qui en dépendent. J'ai utilisé l'expérience naturelle qu'est l'introduction du cerf à queue noire sur l'archipel d'Haïda Gwaii pour étudier les conséquences de sa surabondance sur un écosystème tempéré peu perturbé par les activités anthropiques. J'ai ainsi pu mettre en évidence les contrôles descendants directs et indirects qu'exerce l'herbivore sur les Bryophytes (positifs), les plantes vasculaires (négatifs) et sur l'avifaune (négatifs) quand il n'est pas limité par les prédateurs ou la chasse. J'ai complété ces résultats par une analyse régionale sur 20 ans pour montrer que la perte de biodiversité enclenchée par la surabondance de ces cerfs était un phénomène d'érosion continu se prolongeant bien au-delà de l'impact initial. A l'échelle de l'Amérique du Nord, j'ai ensuite pu montrer, conformément aux prédictions faite à partir des études locales, qu'il existait une relation entre surabondance des populations d'ongulés et déclin de l'avifaune du sous-bois du continent. Enfin, j'ai analysé les suivis d'une expérience de réduction des densités de cerfs entamée sur deux îles il y a 13 ans. La végétation et l'avifaune se sont partiellement reconstituées démontrant qu'il est possible (1) de restaurer les réseaux trophiques fortement modifiés et (2) qu'une telle restauration si elle est possible prendra du temps et ne convergera pas (rapidement) vers un état initial. Afin de limiter les conséquences dommageables provoquées par la surabondance des cervidés, la conservation des prédateurs et l'augmentation de la chasse doivent être favorisés.

Reversible axial flow fans are used as emergency ventilation fans to discharge the smoke generated on the probable fires occurring in the underground transportation systems and mines as quickly as possible, without causing any harm to people exposed to it. The fans which are placed in different configurations according to the location of fire must be able to work bi-directionally, namely reversible. Due to this fact, the blade profiles of the fan must possess the same aerodynamic performance while working on either discharge or suction condition of the fan, dictated by direction of the rotation. This manuscript consists of the computation of the aerodynamic performances of symmetrical blade profiles of fully reversible axial fans by computational fluid mechanics (CFD) methods, developing a methodology for the design of reversible axial fans and analysis of the designed fan with CFD methods. The aerodynamic performances of the blade cascades are evaluated using FLUENT 6.0 software for different Reynolds numbers, solidities and angle of attacks of the cascade. The results of these computations are embedded into the developed methodology. Performance analysis of the reversible axial flow fan, which is designed with the developed methodology, is done with CFD techniques.

Social media is a vast, fluctuant domain that is difficult to grasp, overlook and explain. It is important in our daily lives as well as for organisations, such as traditional media. Traditional media is moving from analogue news propagation to propagating news online, where social media plays a significant role. This study contributes to the understanding of traditional media propagation in social media, through quantitative and network analysis of articles’ spread in social media. The study also contributes to refining social media network analysis methodology, from the perspective of traditional media propagation in social media. The study is conducted as a survey where web documents of social media posts were collected and analysed. The scope of the study were Swedish traditional media. Two analysis methods were used: a quantitative statistical analysis of the propagation of articles and a network analysis comparing the usefulness of two common network analysis metrics: indegree centrality and PageRank. The results show that an overall of 22,34% of traditional media articles in this study, were propagated in social media. The findings include what categories of articles are most propagated on different social media platforms. Different kinds of newspapers were also compared, and variances were found. Local press articles were more propagated on Facebook than on Twitter, in opposite to national press that were more propagated on Twitter than on Facebook. Indegree centrality was found to be the most useful metric for examining traditional media propagation amongst Swedish newspapers, when compared to PageRank. Lack of cross-platform research in social media is pointed at, since this study identifies a prominent need for evolving cross-platform social media research.