2,690 research outputs found

    Vertical distribution and composition of phytoplankton under the influence of an upper mixed layer

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    The vertical distribution of phytoplankton is of fundamental importance for the dynamics and structure of aquatic communities. Here, using an advection-reaction-diffusion model, we investigate the distribution and competition of phytoplankton species in a water column, in which inverse resource gradients of light and a nutrient can limit growth of the biomass. This problem poses a challenge for ecologists, as the location of a production layer is not fixed, but rather depends on many internal parameters and environmental factors. In particular, we study the influence of an upper mixed layer (UML) in this system and show that it leads to a variety of dynamic effects: (i) Our model predicts alternative density profiles with a maximum of biomass either within or below the UML, thereby the system may be bistable or the relaxation from an unstable state may require a long-lasting transition. (ii) Reduced mixing in the deep layer can induce oscillations of the biomass; we show that a UML can sustain these oscillations even if the diffusivity is less than the critical mixing for a sinking phytoplankton population. (iii) A UML can strongly modify the outcome of competition between different phytoplankton species, yielding bistability both in the spatial distribution and in the species composition. (iv) A light limited species can obtain a competitive advantage if the diffusivity in the deep layers is reduced below a critical value. This yields a subtle competitive exclusion effect, where the oscillatory states in the deep layers are displaced by steady solutions in the UML. Finally, we present a novel graphical approach for deducing the competition outcome and for the analysis of the role of a UML in aquatic systems.Comment: 20 pages, 8 figure

    The influence of dispersal on a predator-prey system with two habitats

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    Dispersal between different habitats influences the dynamics and stability of populations considerably. Furthermore, these effects depend on the local interactions of a population with other species. Here, we perform a general and comprehensive study of the simplest possible system that includes dispersal and local interactions, namely a 2-patch 2-species system. We evaluate the impact of dispersal on stability and on the occurrence of bifurcations, including pattern forming bifurcations that lead to spatial heterogeneity, in 19 different classes of models with the help of the generalized modelling approach. We find that dispersal often destabilizes equilibria, but it can stabilize them if it increases population losses. If dispersal is nonrandom, i.e. if emigration or immigration rates depend on population densities, the correlation of stability with migration rates is positive in part of the models. We also find that many systems show all four types of bifurcations and that antisynchronous oscillations occur mostly with nonrandom dispersal

    Coordination, Differentiation and Fairness in a population of cooperating agents

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    In a recent paper, we analyzed the self-assembly of a complex cooperation network. The network was shown to approach a state where every agent invests the same amount of resources. Nevertheless, highly-connected agents arise that extract extraordinarily high payoffs while contributing comparably little to any of their cooperations. Here, we investigate a variant of the model, in which highly-connected agents have access to additional resources. We study analytically and numerically whether these resources are invested in existing collaborations, leading to a fairer load distribution, or in establishing new collaborations, leading to an even less fair distribution of loads and payoffs.Comment: 10 pages, 3 figure

    Learning English with a native speaker as a form of cross-cultural dialogue

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    The authors of the work examine the differences in teaching English between native and non-native teachers of English. Having analyzing TPU experience in this sphere from the student's perspective, respective strengths and weaknesses of both groups of teachers are specified and educational strategies of «mixed» teaching are suggested. В настоящей работе исследуются различия в преподавании английского языка между носителями языка и русскоязычными специалистами. Проанализировав опыт Томского политехнического университета в сфере преподавания английского языка носителями, авторы определяют плюсы и минусы и предлагают возможные варианты «смешанного» преподавания

    Collapse of an Ecological Network in Ancient Egypt

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    The dynamics of ecosystem collapse are fundamental to determining how and why biological communities change through time, as well as the potential effects of extinctions on ecosystems. Here, we integrate depictions of mammals from Egyptian antiquity with direct lines of paleontological and archeological evidence to infer local extinctions and community dynamics over a 6,000-y span. The unprecedented temporal resolution of this dataset enables examination of how the tandem effects of human population growth and climate change can disrupt mammalian communities. We show that the extinctions of mammals in Egypt were nonrandom and that destabilizing changes in community composition coincided with abrupt aridification events and the attendant collapses of some complex societies. We also show that the roles of species in a community can change over time and that persistence is predicted by measures of species sensitivity, a function of local dynamic stability. To our knowledge, our study is the first high-resolution analysis of the ecological impacts of environmental change on predator-prey networks over millennial timescales and sheds light on the historical events that have shaped modern animal communities

    Collapse of an ecological network in Ancient Egypt

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    The dynamics of ecosystem collapse are fundamental to determining how and why biological communities change through time, as well as the potential effects of extinctions on ecosystems. Here we integrate depictions of mammals from Egyptian antiquity with direct lines of paleontological and archeological evidence to infer local extinctions and community dynamics over a 6000-year span. The unprecedented temporal resolution of this data set enables examination of how the tandem effects of human population growth and climate change can disrupt mammalian communities. We show that the extinctions of mammals in Egypt were nonrandom, and that destabilizing changes in community composition coincided with abrupt aridification events and the attendant collapses of some complex societies. We also show that the roles of species in a community can change over time, and that persistence is predicted by measures of species sensitivity, a function of local dynamic stability. Our study is the first high-resolution analysis of the ecological impacts of environmental change on predator-prey networks over millennial timescales, and sheds light on the historical events that have shaped modern animal communities

    How to Overcome Poverty Traps by Education

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    The thesis addresses the question of how poverty can be overcome by education. The starting point is the following type of poverty trap: poverty forces parents to send their children to work to secure the survival of the household. Due to child labor, children do not attend school and do not form human capital. When these children are adult, the lack of education causes them being as poor as their parents have been. This produces a vicious circle that has to be broken. In an OLG growth model such a type of poverty trap is established as a locally stable steady state. Imperfect capital markets prevent the overcoming of poverty. Moreover, a positive externality of the formation of human capital on future generations is identified, which ought to be internalised. The thesis analyses two types of instruments that could generate economic growth via human capital accumulation: subsidies and land reforms. The paid subsidy or the transferred land must generate an income effect sufficiently big in size to enable the household to escape the poverty trap sustainably. The first part deals with subsidy policies. Subsidies contingent to school attendance are more cost-effective, i.e. require less resources per household to lift a household out of poverty, than the simple, uncontingent lump-sum subsidy, proposed in the literature. Conditional subsidies therefore allow a society to be educated and to be free of poverty in a shorter span of time. However, one has to distinguish between different types of conditional subsidies, since they differ with respect to cost-effectivity. A lot of aspects, like fighting corruption or inventments in the quality of schooling, enter into the optimal, multidimensional state expenditures plan. Successful tax-financed subsidy policies will only arise in democracy if constitutional rules prevent particular political failures. A major reason of failure is, among others, excessive taxation, so that households slip back into poverty. To heal these political failures a flexible majority rule and restricted rights of reelection for certain households or groups are proposed; a constitutional right to a tax exemption for protecting human capital can be justified. The second part of the thesis investigates land reforms as an alternative policy instrument. There is a dynamic connection between land reforms and the formation of human capital that has been ignored as well in the political as in the academical discussion: land reforms not only produce a more equitable distribution of land, they also, beyond this aspect, can generate a dynamic growth impulse which can leads a society out of backwardness and starts a transition towards an educated, developed industrial society. Even in a world without uncertainty, open access to land markets should temporarily not be allowed to land reform beneficiaries, as otherwise early land sales and migration can cause the failure of the land reform. A general equilibrium effect, caused by the land reform, may a wage effect in the rural labor market, so that also unsupported, landless and formerly poor households escape the poverty trap, even without receiving a plot of land. However, it is demonstrated that also the opposite effect may occur, in which case the situation of the agricultural workers get worse by a land reform. The focus in discussions on land reforms ought to be shifted: the aim of a land reform not only has to be a more equitable distribution of land, but it is especially important to target generating growth and education, as otherwise attaining a sustainable overcoming of poverty remains doubtful. Summarising, the thesis emphasises the necessity to analyse problems of poverty in a dynamic framework. Within a restricted static framework, one cannot identify and understand all the essential factors for a sustainable success of development efforts -- and the danger of deducing only temporarily successful policy implications remains

    Polymer-bound haloate(I) anions by iodine(III)-mediated oxidation of polymer-bound iodide: Synthetic utility in natural product transformations

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    A set of polymer-attached hypervalent iodate(I) complexes were prepared from polymer-bound iodide anion by ligand transfer of acetate and trifluoro acetate present in the corresponding iodine(III) reagents onto the iodide anion. The synthetic versatility of these polymer-bound reagents in terms of efficacy and ease of workup is demonstrated for selected examples in natural product synthesis and natural product derivatization. Thus, iodoacetoxylation of glycals is the initial step for the preparation of two deoxygenated disaccharides which are part of the carbohydrate units of the landomycins. In a second example, a one-pot multistep rearrangement of the decanolide decarestrictine D backbone is shown which is initiated by iodotrifluoroacylation of the olefinic double bond.Fonds der Chemischen Industri
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