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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.

Estimation of properties of spatial structures from sections

Moran, R. C. C. P. January 1987 (has links)
No description available.

Understanding the brain through its spatial structure

Morrison, Will Z. 12 March 2016 (has links)
The spatial location of cells in neural tissue can be easily extracted from many imaging modalities, but the information contained in spatial relationships between cells is seldom utilized. This is because of a lack of recognition of the importance of spatial relationships to some aspects of brain function, and the reflection in spatial statistics of other types of information. The mathematical tools necessary to describe spatial relationships are also unknown to many neuroscientists, and biologists in general. We analyze two cases, and show that spatial relationships can be used to understand the role of a particular type of cell, the astrocyte, in Alzheimer's disease, and that the geometry of axons in the brain's white matter sheds light on the process of establishing connectivity between areas of the brain. Astrocytes provide nutrients for neuronal metabolism, and regulate the chemical environment of the brain, activities that require manipulation of spatial distributions (of neurotransmitters, for example). We first show, through the use of a correlation function, that inter-astrocyte forces determine the size of independent regulatory domains in the cortex. By examining the spatial distribution of astrocytes in a mouse model of Alzheimer's Disease, we determine that astrocytes are not actively transported to fight the disease, as was previously thought. The paths axons take through the white matter determine which parts of the brain are connected, and how quickly signals are transmitted. The rules that determine these paths (i.e. shortest distance) are currently unknown. By measurement of axon orientation distributions using three-point correlation functions and the statistics of axon turning and branching, we reveal that axons are restricted to growth in three directions, like a taxicab traversing city blocks, albeit in three-dimensions. We show how geometric restrictions at the small scale are related to large-scale trajectories. Finally we discuss the implications of this finding for experimental and theoretical connectomics.

Peripheralization and Spatial Structure: Factors Influencing the Sexual Behaviour of Male Japanese Monkeys / The Sociosexual Behaviour of Male Japanese Monkeys

Urban, Aleksandra 11 1900 (has links)
The present study examines the concept of peripheralization and spatial structure in the Arashiyama West troop of Japanese macaques (Macaca fuscata) , and employs an objective method to assess the existence of a central-peripheral tendency in the spatial organization of this Japanese macaque troop. By calculating each adult male's mean distance in meters from the 'alpha' male of the troop, it was possible to rank males according to their distances from the alpha male. The resultant linear gradation of males illustrates the troop's spatial organization. This study did not find evidence supporting the notion that a rigid central-peripheral structure exists in the Arashiyama West troop. Indeed, there appears to be no objective criterion whereby some males can be labelled "central" and others "peripheral", for the linear gradation of distances that defines the spatial organization of adult males can be bisected arbitrarily at any point along its continuum. In this thesis "spatial status" is defined as a gradation of the distance between each male and the alpha male, where the shortest mean distance corresponds to the highest spatial status. The predictors age, dominance rank and maternal lineage are examined for their influence in determining a male's spatial status within the troop. The isolated and cumulative effects of the aforementioned variables are also examined to explain variations in male sexual behavior. Results from this study indicate that a male's spatial status is directly influenced by the combined effects of age, rank and lineage, as well as by the isolated effects of any one of the previously mentioned variables. Furthermore, the results indicate that a male's age and spatial status explain the most variability in male mating behavior. Whether the process of peripheralization and the existence of adult male spatial status are "exaptations", that is, traits which probably were NOT selected for during the course of evolution of Macaca fuscata, cannot be proved by the data in this thesis. However, the available evidence does suggest that spatial status in particular is more likely to be an "exaptation" rather than an "adaptation", and accordingly it probably does not contribute to male fitness. / Thesis / Master of Arts (MA)

The Change of the Taipei Rapid Transit on the Value of Land Near Station Location

Tsai, Yuan-pei 20 July 2004 (has links)
From May 28, 1996, it was the beginning of a new era of internal transportation that Muzha line of Taipei Transit works. It must affect the original urban spatial structure when the new infrastructure was brought to the city. This paper bases on bid rent theory of Alonso, and research whether the spatial structure near the 47 Taipei transit stations were affected by Taipei Transit¡¦s working or not. On the other hand, there will discuss the cluster of the firms of competition with location quotient. This paper analysis the land price within 250 meters near the transit stations from 1984 to 2004, and there are 230,070 data. The part of the cluster research of firms, this paper focuses on the registered firms around the transit stations on April 2004. The research finds as following: 1. The average land price curve next to the transit station is a bid price curve. 2. The different types of transit station have different land price curve and characters. 3. The situation of firm cluster near different types of transit station is different.

Polycentric development and transport network in China's megaregions

Song, Ge 08 June 2015 (has links)
China’s mega-regions, in addition to cities and metropolitan areas, have become the engines for economic development, and the target areas for regional and national policies. Reflecting upon China’s current path of regional urbanization, the proposed research examines a fundamental issue for China’s megaregional development: the impact of transport network development on the spatial pattern of China’s megaregions. Using the multiple national Censuses (1982, 1990, 2000, 2010) and the transport network GIS data in the corresponding years, this research 1) constructs measures of megaregional spatial patterns, 2) assesses the spatial trajectory of megaregional growth based on the differentiated growth rates of metropolitan cities, 3) computes indicators of megaregional transport network connectivity and accessibility, 4) examines the impacts of transportation infrastructure on megaregional growth trajectory. This research helps understand the spatial structure of China’s megaregions with newly constructed quantitative measures of polycentric spatial development, as well as the intra-megaregion and inter-megaregion variation of transport network in China. It also clarifies the link between transport infrastructure and megaregional spatial structure in China’s unique context by providing quantitative evaluation of the implications of transport investment for the spatial pattern in Chinese megaregions. Finally it enriches the megaregional solutions to China’s vision of economic, social and environmental sustainability.

Territoriality and Spatial Structure in the Green Anole, Anolis carolinensis

Weber, William D, Jr. 16 December 2016 (has links)
Anolis carolinensis has been a model organism for ecology and evolutionary biology since the seventies, yet there are still understudied aspects of their ecology. A five-year study has provided microsatellite genotypes to be used in building a pedigree and assess relatedness, enabling us to evaluate the spatial distribution of an urban population of A. carolinensis. Results indicate no correlation between a male’s size and the distance others keep from it; however, males belonging in the heavyweight morph are dictating the spatial distribution in this population. In addition, juvenile dispersal of male offspring and partial philopatry of female offspring are key in this dynamic, where a single heavyweight male will actively defend a small area that contains multiple females, some of which are be daughters, and multiple unrelated males, most likely sneaker males.

Mathematical models of microbial evolution : cooperative systems

Pawlowska, Bogna Julia January 2016 (has links)
Microbes usually live in large communities, where they interact with other organisms and species. These interactions include cooperation, when individuals facilitate each others growth and reproduction. Such cooperation has been for instance observed within pathogens in the process of infection. Therefore, given the number and the frequency of infectious diseases, understanding the nature and the dynamics of microbial cooperation may be a crucial step in modern medicine. Microbes often secrete costly enzymes which extracellularly metabolise resources available in the environment. This external metabolism is a form of ’public good cooperation’, in which individuals invest their energy in producing ’public goods’, available to other organisms. To study this phenomenon we deploy mathematical models which are based on biologically relevant assumptions. Our models not only aim to capture the dynamics of studied microbial communities, but also to remove the natural complexity arising in the empirical studies and thus to provide a mechanistic understanding of their results. We first recover and explain the recent empirical finding, about mixed strain infections, showing that an addition of a low virulent strain which does not produce public goods (termed ’cheat’) may counter-intuitively enhance the total population virulence. What drives this result turns out to be an interaction of two different cooperative traits and the presence of spatial structure. Next we study the competition between the strains that do and do not produce public goods. Our results depend on environmental conditions, such as resource concentration and population density, but they are also determined by the degree of spatial structure - the ecological trait which so far has been treated only as a binary variable. Finally, we identify some environmental threats for the external metabolism feeding strategy, and we examine its competitiveness in comparison to ’internal metabolism’, in which the costly enzymes are private.

The response of biological communities to spatial and temporal changes in a regenerating coastal dune forest along the north-east coast of South Africa

Ott, Theresia 30 January 2013 (has links)
Ecological restoration that aims to reinstate indigenous processes may be constrained by regional and local conditions, especially those that drive dispersal and colonisation. Local conditions can be managed, while regional conditions cannot. The management of costly rehabilitation programmes is considered best practice when scientifically informed. My thesis documents the responses of biological communities to a range of local conditions developing in coastal dune habitats in response to ecological restoration. Here, landscape-level (spatial structure of patches of tree canopies) local conditions were distinguished from site-level (topography, soil nutrient content, woody plant community richness, microclimatic variables) local conditions. The spatial structure of tree canopies varied over time and differed between the mining lease site and the relatively undisturbed benchmark site prior to (1937- 1970), and after (1990-2006) mining. For example, approximately 20% of the mining lease site and 40% of the benchmark site was covered by tree canopies prior to mining. However, after mining and rehabilitation, the structure of tree canopies began to converge towards that of the benchmark. The topographic profile of coastal dunes was less heterogeneous after mining and rehabilitation than before. Aspect, elevation and gradient of dune slopes were also different and had shifted in space. Variability in the structure of tree canopies could not explain variability in species richness, forest-associated species richness and proportion of benchmark species for the millipede, dung beetle and bird communities ashad been expected. However, species composition did change (though idiosyncratically) with age-related changes in soil nutrient availability and tree species diversity. Temperature, relative humidity and light intensity varied with dune topography, but soil nutrient content (C: N ratio and pH) was better accounted for by the age of the regenerating forest than by dune topography. Similarly, analysis of covariance suggested that tree canopy density, woody plant richness and millipede species richness only responded to the aspect, elevation and gradient of restored coastal dunes when age was taken into account as a covariate. However, the response of keeled millipedes to dune topography, regardless of regeneration age, suggested that the microclimatic variability brought about by topographic heterogeneity may provide these specialists with suitable microhabitats. Throughout my thesis, the age of regenerating patches of indigenous canopies was often more important as an explanatory variable than habitat conditions per se. Age itself is not a determinant of biological communities, but merely the axis along which habitat conditions change with succession, and later, patch dynamics. Therefore, as elsewhere, my thesis has highlighted age as a useful proxy for the response of biological communities to local conditions. It seems that managing local variables such as those considered in my assessment is not an avenue through which to enhance restoration. After kick-starting initial conditions, best practice rehabilitation management should therefore focus on minimizing external disturbances rather than interfering with natural processes. / Thesis (PhD)--University of Pretoria, 2013. / Zoology and Entomology / unrestricted

Location Choice, Linkages and the Spatial Economy: Essays on Theory, Evidence and Heterodox Assessment

Bieri, David S. 23 September 2010 (has links)
The essays in this dissertation represent theoretical and empirical contributions to urban economics and regional science, focusing on the growing importance of nonmarket interactions. There is increasing evidence that the process of globalization is rendering the world "spiky" rather than "flat". Nonmarket interactions, such as knowledge spillovers, innovation or amenity-based externalities, play a central role in this process. As economic activity is not evenly spread across space, a detailed understanding of the economic linkages between regions is key to the design of effective public policy. This is particularly important in the context of economic linkages between regions or cities, highlighting the key adjustment mechanisms -- via both market and nonmarket transactions -- and their long-run implications for incomes, the cost of living, and the spatial distribution of population. Both the neoclassically-grounded field of urban economics and the rapidly expanding New Economic Geography (NEG) literature pioneered by Krugman offer a variety of models and (not infrequently competing) predictions about the factors and processes that shape the spatial structure of the economy. At the same time, the dialogue between qualitative and quantitative discourses in regional science has been marred by an increasingly embittered dispute over methodology. While acutely pronounced in economics, this development has re-shaped large parts of its sister disciplines as well, particularly sociology and geography. Across the board, proponents of quantitative science methodology increasingly likened themselves to their natural science counterparts, whereas qualitative methods had become the last bastion of "true social scientists". Today, these so-called "science wars" have rendered "qualitative" and "quantitative" analysis into almost mutually exclusive concepts. / Ph. D.

Sélection et polymorphisme chez des grenouilles mimétiques du Pérou (Dendrobatidae)

Chouteau, Mathieu 06 1900 (has links)
La diversification des signaux aposématiques dans un cadre de mimétisme müllérien est un phénomène intrigant. Alors que la théorie relative à l'aposématisme et au mimétisme suggère l'évolution vers un signal aposématique unique, d'impressionnantes variations peuvent être observées entre les populations, et cela à petite échelle spatiale. Il a été supposé que la variation spatiale des pressions de sélection engendrées par différents prédateurs puisse être à l'origine de ce phénomène. Afin de tester cette hypothèse, nous avons étudié la transition entre deux systèmes géographiques caractérisés par des patrons aposématiques distincts chez des grenouilles mimétiques et toxiques du nord du Pérou (Dendrobatidae) en combinant les outils de génétique des populations aux outils écologiques. Dans chacun de ces systèmes, Ranitomeya imitator vit en sympatrie avec R. ventrimaculata ou R. variabilis. Il s'agit du principal exemple empirique suggérant que dans un cadre de mimétisme müllérien, il n'y a pas convergence des signaux aposématiques des deux espèces, mais plutôt convergence unidirectionnelle où R. imitator, étant polymorphe, imite des espèces monomorphes avec lesquelles elle est sympatrique. Premièrement, les résultats réfutent les prémisses qui suggèrent que R. imitator converge vers le signal aposématique d’une autre espèce. La haute similarité génétique entre les espèces modèles suggère qu'elles ont divergé plus récemment que les populations de R. imitator ou qu'elles sont encore connectées par du flux génique. Ces résultats indiquent que ces espèces ont été identifiées à tort comme des espèces différentes. De fait, l'identification de l'espèce imitatrice basée sur la variabilité phénotypique est invalidée dans ce système puisque R. imitator et R. variabilis/ventrimaculata démontrent la même variabilité. Deuxièmement, nos résultats démontrent que la prédation varie spatialement, autant en intensité qu'en direction, créant ainsi un paysage hétérogène de pressions de sélection. Ainsi, de fortes pressions de prédation stabilisatrice permettent le maintien de l'organisation géographique de différents signaux aposématiques et expliquent l'uniformité de ces signaux ainsi que les relations mimétiques. Par contre, le relâchement temporaire des pressions de prédation permet l'apparition de nouveaux phénotypes aposématiques via les processus évolutifs neutres, conduisant à un haut polymorphisme au niveau de ces populations. L'interaction de ces modes sélectifs nous a permis de démontrer pour la première fois comment la théorie évolutive de Wright (shifting balance theory) permet la diversification adaptative dans un système naturel. Pour conclure, cette étude a permis de mettre en évidence à quel point les systèmes de mimétisme müllérien peuvent être dynamiques. L'alternance spatiale entre les processus évolutifs neutres et la sélection naturelle permet l'émergence de nouveaux phénotypes aposématiques à une échelle locale, ainsi que l'apparition d'une organisation géographique des signaux d'avertissement et des relations de mimétisme müllérien. / The diversification of aposematic signals in Müllerian mimicry systems is a puzzling phenomenon. Although aposematism and mimicry are expected to promote uniformity in warning signals, impressive variations may be observed among populations at relatively small spatial scales. It has been suggested that spatial variation in selective pressures caused by predators might be responsible for this phenomenon. In order to test this hypothesis, we studied the transition in the aposematic signals of the mimetic poison-dart frogs of Northern Peru (Dendrobatidae) between two geographical systems characterized by distinct aposematic signals. This was done by combining population genetics and ecological tools. In both systems, Ranitomeya imitator is sympatric with either R. ventrimaculata or R. variabilis. This system is recognized as a rare example in support of the hypothesis for advergence: R. imitator, which is polymorphic, is believed to mimic distinct sympatric monomorphic model species. However, our results do not support the hypothesis for advergence of the aposematic signals by R. imitator. The genetic similarity between the model species suggests that they have diverged more recently than R. imitator populations or that they are still connected by gene flow. These results indicate that these species were misidentified as being different species. As such, the identification of the mimetic species based on phenotypic variability is invalidated in this system, since R. imitator is as variable as R. variabilis/ventrimaculata. Also, our results demonstrate that predation pressure is spatially variable, in both intensity and direction, thus creating a heterogeneous selective landscape. As such, strong and directed stabilizing selective pressures maintain the geographic organisation of aposematic signals and explain phenotypic uniformity and mimetic relationships. Relaxation of these selective pressures enables for the appearance of novel aposematic phenotypes and promotes high phenotypic variability via neutral evolutionary processes. The interaction between these selective regimes has enabled us to demonstrate, and this for the first time, how the Wright’s shifting balance theory of evolution may promote adaptive diversification in a natural system. In conclusion, this study highlights just how dynamic Müllerian mimicry systems can be. The interplay between neutral evolutionary processes and natural selection enables for the predominance of novel aposematic phenotypes at a local scale, and the geographical organisation of warning signals and Müllerian relationships.

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