L'écologie a vu se développer, depuis une vingtaine d'années, une vision systémique se concentrant sur les interactions entre composantes de l'écosystème. L'association de ces composantes fait émerger des propriétés globales et agit en retour sur les propriétés des composantes. Ces propriétés sont dites émergentes. Leur étude a été appliquée aux réseaux trophiques de différents écosystèmes, pour la plupart littoraux, par des approches numériques visant à construire et analyser des modèles statiques. Une telle approche fait le choix de négliger certains aspects : structures spatiales, dynamiques temporelles, forçage des processus, dans l'objectif de favoriser la prise en compte d'un maximum de compartiments dans une vision synthétique. Ce travail repose entièrement sur les résultats acquis sur les différents compartiments et processus, avec comme objectif d'en faire une synthèse, de les confronter, et de dégager des propositions d'étude de nouveaux processus ou compartiments peu renseignés. De ma thèse à aujourd'hui, j'ai travaillé sur des questions relatives aux réseaux trophiques planctoniques avec, dans chaque situation, un contexte appliqué et un objectif fondamental. Celui-ci consiste à comprendre le fonctionnement de ce réseau planctonique. L'axe directeur utilisé pour discuter de la nature de ces différents réseaux trophiques planctoniques est essentiellement celui de Legendre et Rassoulzadegan (1996) reposant sur un gradient entre deux états : le réseau microbien et le réseau herbivore classique. On a ainsi pu dégager de nouveaux modes de fonctionnement des réseaux trophiques planctoniques et ces propriétés dégagées viennent enrichir la réflexion menée sur le problème appliqué soulevé dans chaque milieu. Suite à mon recrutement à l'Université de La Rochelle, la demande du LBEM était que je travaille sur les vasières intertidales tempérées, en couplant benthos et colonne d'eau. Le choix de conserver une approche en modèles statiques afin de prendre en compte au maximum la diversité fonctionnelle imposait de modifier les outils numériques pour les adapter à cet écosystème macrotidal. Les innovations apportées ont principalement concerné la modification de l'algorithme de l'analyse inverse par une approche de Monte Carlo, le couplage spatial par les flux de transport et des saisons, le développement d'indices basés sur les chaînes de Markov (liens lents ou rapides entre compartiments). L'application de ces méthodes à l'ensemble des données acquises sur le site de Brouage dans les Pertuis Charentais a permis de dégager les caractéristiques suivantes : i) rôle primordial du réseau microbien benthique : microphytobenthos, bactéries hétérotrophes, foraminifères, ii) rôle de cul-de-sac de la matière organique par les foraminifères (la place exacte des foraminifères dans le réseau reste à déterminer), iii) la nématofaune est la plaque tournante du réseau, iv) le réseau est importateur de détritus et exportateur de matière de « qualité », au sens de l'exergie. A partir de ce modèle, des scénarios ont pu être simulés sur des scenarii de culture et la conséquence d'une chute des productions primaires sur la nourriture disponible. Le modèle couplé en boites spatiales a permis de dégager 3 modes de fonctionnement distincts en fonction de la hauteur sur la zone intertidale. Outre ce travail sur la vasière de Brouage un travail de comparaison inter – sites a été initié par l'établissement d'un modèle à deux saisons sur la vasière de l'Aiguillon, au nord des Pertuis Charentais. Le fonctionnement trophique de cette vasière est très proche de celui de Brouage. Il s'en démarque surtout par un rôle moindre des bivalves cultivés et une forte exportation de matière par les poissons brouteurs (mulets). Outre une indication sur l'évolution naturelle des écosystèmes, les propriétés émergentes des réseaux trophiques peuvent être indicatrices d'un état d'évolution sous la pression de l'Homme. Il est alors nécessaire de développer des indicateurs d'écosystèmes capables d'évaluer les changements des environnements côtiers. L'utilisation des indices d'analyse des réseaux devrait permettre de passer à une vision systémique, s'intéressant aux interactions plutôt qu'aux stocks. Seule une comparaison étendue de nombreux réseaux en utilisant toujours les mêmes techniques permettra d'aller plus loin, en cherchant à observer vers quelles propriétés évoluent les systèmes, naturellement et sous pression anthropique.

<p>Most urban areas of developing countries were hitherto experiencing unprecedented growth in their population, the phenomenon commonly referred to as urbanization, which in this study can be said to be the proportion of urban population relative to the total population of a region. </p><p>This phenomenon has opportunities vis-à-vis challenges, whose impact on natural resources in general and water resources in particular, cannot be over- emphasized. It is within the context of these urban challenges that this study attempts to look into and possibly assess the situation in the urban and peri- urban areas of Kumasi Metropolitan Area (KMA) in Ghana, whose urbanization processes is prejudiced by this study to be increasing rapidly. </p><p>The study specifically attempted to assess thechanges in the urbanization patterns and the possible future urbanization trend of the area within a specific time frame. The study also attempts to look at the causes of this urbanization, and its impact on water resources in the Area, both qualitatively and quantitatively, envisaged to be the results of socio-economic activities taking place in the Area. Finally, the study attempts to look into the measures put in place to curb these challenges. </p><p>Amid paucity of data, however, the study reveals that the most single contributor of urbanization processes in the area has been migration from the countryside, and in particular from the northern part of the country. The study also reveals the fact that even though the general quality standard of some water resources is better, the largely uncontrolled socio-economic activities, coupled with the deplorable sanitary conditions in the KMA, has potential to degrade the water resources in the KMA. In addition, the study reveals that little attention is given, in terms of policy formulation to curb this urbanization processes and to protect water resources in the area.</p>

<p>An understanding of the impacts on freshwater systems from water management and development practices and the recognition that freshwater ecosystems are dependent on various aspects of the natural hydrological variability has given rise to a relatively new field of science commonly referred to as environmental flows. The assessment and application of environmental flows has advanced considerably in the last ten years. To coincide with the emergence and expansion of the environmental flows concept around the world, this survey study was undertaken aiming to identify people’s perceptions and interpretations of the environmental flows concept and its role in water management. It also aims to add to existing knowledge of the extent to which the concept is being applied, how it is being translated into policy and practice and the major challenges and opportunities that exist for continued understanding and implementation. The survey was distributed to a range of people representing different water-related sectors and regions around the world. A total of 272 responses representing 64 countries in the six major regions of the world was received. The responses were compiled and analysis of aspects of the respondents backgrounds as well as the questions were conducted using the computer statistical program SPSS. Representation of specific groups, particularly water user groups, and specific regions in the survey however was low, demonstrating possible limitations of the survey distribution method as well as the lack of concept awareness and application in many parts of the world. The ways in which people define and interpret the concept varied widely. The degree to which the concept was applied shows the growing recognition around the world of the need to consider the environmental water requirements when making decisions on water allocations. Despite the growing recognition many areas do not yet apply the concept. The survey allowed the opportunity for respondents to highlight what they perceived asthe major obstacles and difficulties for the concept within their respective areas. Lack of understanding among stakeholders of the socio-economic costs and benefits associated with concept implementation and a lack of political will were the two most common obstacles for the continued adoption and application of environmental flows around the world. Overall, the survey delivered promising signs for the continued evolution of environmental flows within water management. There was widespread opinion that the concept of environmental flows was an essential element in the efforts to achieve sustainable management of water resources.</p>

<p>The minimum ecologically viable population size (MEVP) of a species describes the minimum size at which the species itself or another species in the same ecosystem goes extinct as a result of the loss of inter-specific interactions. The MEVP shows a good potential for use as a tool for exploring the mechanisms behind species extinctions, but presently only a small amount of research has been done that takes advantage of this. In this study the MEVP is used to investigate what properties of species can be used as good indicators of ecological importance. 100 large computer generated food webs were created with an assembly model and the reduction in density that was necessary to induce an extinction event in the web was subsequently determined for each species within the webs. This change in density was then correlated with 28 different properties, measured for each species. The results show that properties that measure how well connected a species is, as well as measures of the species role as a prey item in the web are the ones with the greatest potential to find species with high MEVP. Further, the results put emphasis on the importance of regarding the web as a whole when working with species extinctions, while also highlighting the usefulness of the MEVP concept.</p>

<p>The aim of this work is to analyze if a change in the benthic community can be detected two years after a restoration of a small stream. The samples were taken in a small stream at Tinnerö Eklandskap just south of Linköping. In addition to the restored area, two reference sites upstream and downstream of the restored area were sampled to compare to the restored site. The method used for sampling of benthic fauna in the stream was kick sampling. ASPT, Berger-Parker and Renkonen-indices were used to find out if there was any difference between the reference areas and the restored area. In addition to indices, rank-abundance curves and species lists were made to see if there was any trend difference between the different areas. The only index that showed a difference between the different areas was Berger-Parker diversity index. The reason why there were no greater differences between the areas may be due to the fact that two years is too short to allow time for the benthos to re-colonize the restored area.</p>

<p>The condition of the herring (<em>Clupea harengus</em>) in the Baltic Sea has decreased during the past 30-40 years. This decrease could be explained by different factors; (1) change in diet due to changes in zooplankton community, (2) changes in water temperature and salinity, (3) increasing nutrient inputs and (4) competition for food with other species such as sprat (<em>Sprattus sprattus</em>). In this study the change in condition was analysed using the Fulton’s condition index, and by looking at age and sex of the fish as well as the season and locationthe fish was caught, the differences between these factors were presented. Data from the national Swedish contaminant monitoring programme where used from four locations in the Baltic Sea and two locations at the Swedish West coast. The data was analysed using multiple regressions in R Commander. The result show that the condition, and the temporal trends in condition value, varies at different locations, with higher condition values and increasing temporal trends at the Swedish West coast, compared to the Baltic Sea with lower condition values and where three of four locations show decreasing temporal trends. The condition varied between spring and autumn caught herring as well, while age and sex showed less significant differences.</p>

<p>Female reproductive decisions have been suggested to be highly influenced by mate quality. I have studied whether offspring quality may be adjusted by females to match the attractiveness of males and how strong control females have over their reproductive investment focusing on egg size. This was done in the Banggai cardinalfish (Pterapogon kauderni), a sex-role reversed obligate paternal mouthbrooder where males invest heavily into reproduction. As this species is suitable for both laboratory experiments and field studies it is an ideal candidate for the study of reproductive investment. </p><p>Mating was size-assortative and both males and females benefited from pairing with large partners. However, male size determined the reproductive output of a pair. Females courted large males more intensively and produced larger, but not fewer eggs when mated to large males as compared to small males. Further, this matching of egg size to mate attractiveness may be fast. Female courtship behaviours contained honest information regarding both clutch weight and egg maturity, traits that may be highly important for male mate choice. Surprisingly, males played an important part in territory defence suggesting relatively equal sex-roles in this species. Also, this species showed stable group structures which may be important for the evolution of female plasticity in reproductive investment due to high variance in quality of available mates. </p><p>This thesis suggest that females have a remarkable control over their reproductive investments and that male quality may be highly influential on reproductive decisions regarding offspring quality. Furthermore, it suggest that sexual selection may have strong effects on the evolution of egg size and parental care on a whole.</p>

<p>Chemical signals are the most widely used form of sexual communication throughout the living world. However, there is in general little knowledge about what these signals actually communicate. The role of chemical signals, i.e. pheromones, in sexual behaviour has traditionally been seen as restricted to mate attraction and species recognition. This thesis reviews the evidence for pheromones as indicators of mate quality, and then investigates three important factors for mate quality signals – individual variation, heritability and cost – by using the male pheromone of the lekking fruit fly <i>Drosophila grimshawi</i> as a model.</p><p>The experiments presented indicate that the pheromone of <i>D. grimshawi</i> has multiple functions, and that these functions vary with social context. Thus, with regard to females, the pheromone seems to act mainly as a species/mate recognition signal, since females show little preference for the amount of pheromone deposited by a male. Moreover, males invest less in pheromone production when subjected to females as compared to when subjected to rival males. However, the pheromone seems to be costly in production since males that invest much in pheromone deposition has a shorter lifespan. This suggests a function for the pheromone in male-male interactions. Males can distinguish their own pheromone depositions from those of a strange male, and also discriminate between pheromone depositions from one and two strange males. This might give them the ability to assess the size of a lek and the competitive capacities of rivals, information that should be useful when optimizing sexual behaviour.</p><p>In conclusion, the pheromone seems to act as an honest mate/competitor quality signal in some social contexts, and as a non-costly species/mate recognition signal in other. In addition, I show for the first time that a chemical signal has differential fitness costs, and that an insect is able to distinguish between individual odour signatures.</p>

The objective of this thesis is to show that by separating the two major anthropogenic changes caused in riverine input to the north shore of the Black Sea, it is possible to describe distinct linkages between ongoing ecological changes in the Black Sea, and 1) flow reduction, and 2) nutrient load increase. An attempt is made to quantify the pertinent changes in the inputs of water, salt, and nutrients by modelling individual fluxes over time. The results of these computations are used to argue how the ecology of the Black Sea may be realistically partially restored to something like its former productivity. The origins of input changes to this system are evaluated in terms of their sources. The economic and social costs that will be involved in altering the quality and quantity of those inputs to allow recovery of the Black Sea are examined along with the benefits that may accrue through the restoration of water quality, fisheries and tourism. Mechanisms that may be relevant in the process of achieving the alteration of the inputs are discussed. / Graduation date: 1996

The work in this thesis deals with effects of changed species richness on process rates among stream-living macroinvertebrates. Global biodiversity is decreasing rapidly and it is poorly known what the consequences of this loss may be for ecosystems and the services they provide. Hence, it is important to investigate the potential effects of losing species. In streams, deforestation, introduction of non-native species, pollution and channelization are examples of events that may affect species richness negatively. In this thesis emphasis is on changes in species richness within functional feeding groups (FFGs) of stream-living macroinvertebrates. The FFGs used were shredding detritivores, grazers, filter feeders and predators - all of which uphold important ecological processes in streams. Along with an observational field study, species richness was manipulated in laboratory and field experiments to investigate the effects of changed species richness on process rates and thus ecosystem functioning. The results show that effects of changed species richness on process rates may be dramatic. Among the shredding detritivores there were negative effects on leaf mass loss, regardless whether fixed, random or predicted sequences of species loss was investigated. These effects could be attributed to either species richness per se or species composition. However, among the other FFGs the relationship between species richness and process rates was less consistent. In filter feeders, there was no or a negative effect of decreasing species richness while both grazers and predators showed positive effects of species loss. The results also show that the most important interactions between species in an experiment, thus potentially in a natural community, are likely to determine what the effect of species loss on process rates will be. Facilitation and niche differentiation lead to reduced process rates if species are lost, while mechanisms, such as interspecific resource or interference competition, produce the opposite effect. Furthermore, in systems with a diminishing resource, the first two mechanisms may become more important over time enhancing the effect of species loss in the long term. In conclusion, effects of species loss may be dramatically negative or positive even if lost species are classified as redundant. The effect in the short term most likely depends on which species are lost, on the original species composition and on the underlying mechanisms. Questions remaining to be answered are how important the observed effects are in more complex systems and if they are persistent over time? Future studies will tell.