Research in the Aloha system

The Aloha system was studied and developed and extended to advanced forms of computer communications networks. Theoretical and simulation studies of Aloha type radio channels for use in packet switched communications networks were performed. Improved versions of the Aloha communications techniques and their extensions were tested experimentally. A packet radio repeater suitable for use with the Aloha system operational network was developed. General studies of the organization of multiprocessor systems centered on the development of the BCC 500 computer were concluded

Effects of wastewater treatment plant effluent inputs on planktonic metabolic rates and microbial community composition in the Baltic Sea

The Baltic Sea is the world's largest area suffering from eutrophication-driven hypoxia. Low oxygen levels are threatening its biodiversity and ecosystem functioning. The main causes for eutrophication-driven hypoxia are high nutrient loadings and global warming. Wastewater treatment plants (WWTP) contribute to eutrophication as they are important sources of nitrogen to coastal areas. Here, we evaluated the effects of wastewater treatment plant effluent inputs on Baltic Sea planktonic communities in four experiments. We tested for effects of effluent inputs on chlorophyll <i>a</i> content, bacterial community composition, and metabolic rates: gross primary production (GPP), net community production (NCP), community respiration (CR) and bacterial production (BP). Nitrogen-rich dissolved organic matter (DOM) inputs from effluents increased bacterial production and decreased primary production and community respiration. Nutrient amendments and seasonally variable environmental conditions lead to lower alpha-diversity and shifts in bacterial community composition (e.g. increased abundance of a few cyanobacterial populations in the summer experiment), concomitant with changes in metabolic rates. An increase in BP and decrease in CR could be caused by high lability of the DOM that can support secondary bacterial production, without an increase in respiration. Increases in bacterial production and simultaneous decreases of primary production lead to more carbon being consumed in the microbial loop, and may shift the ecosystem towards heterotrophy

Vision and revision: wavefront sensing from the image domain

An ideal telescope with no optical aberrations can achieve a resolution and contrast limited by the wave nature of light, such that the finest detail that can be resolved is of the order of the angle subtended by one wavelength over the diameter of the telescope. For telescopes operating close to this ideal case, however, it is rare that the full performance of the diffraction limit is achieved, as small optical imperfections cause speckles to appear in the image. These are difficult to calibrate, as they are often caused by thermal and mechanical variations in the optical path which vary slowly with time. The quasi-static speckles that they impose can mimic the real signal of a faint star or planet orbiting the primary target, and these therefore impose the principal limitation on the angular resolution and contrast of instruments designed to detect exoplanets and faint companions. These aberrations can be corrected by active optics, where a wavefront sensor is used to used to reconstruct a map of the distortions which can then be compensated for by a deformable mirror, but there is a problem with this also: differrential aberrations between the wavefront sensor and science camera are not detected. In this thesis, I will discuss a successful laboratory implementation of a recently-proposed technique for reconstructing a wavefront map using only the image taken with the science camera, which can be used to calibrate this non-common path error. This approach, known as the asymmetric pupil Fourier wavefront sensor, requires that the pupil not be centrosymmetric, which is easily achieved with a mask, with segment tilting, or with judiciously placed spiders to support the secondary mirror, and represents a promising way forward for characterizing and correcting segment misalignments on future missions such as the James Webb Space Telescope

Overestimation of Basis Set Superposition Error in Molecular Dimer Correlation Potential Energy Calculations.

It is important to remove the basis set superposition error (BSSE) from the intermolecular potential energy calculations with the counterpoise (CP) correction especially for weak intermolecular interactions. However, it has been still questioned whether the counterpoise (CP) correction overestimates the real BSSE. To answer this question is the purpose of this research. In this study, the overestimation of basis set superposition error (BSSE) by the counterpoise (CP) method has been analyzed by an approach called localized orbital pair correlation . This approach is able to eliminate the overestimation of basis set superposition error (BSSE) for the second order potential energies. Prototype calculations of various molecular dimers have been performed with different geometry configurations and various basis set functions. All results have shown that with the counterpoise (CP) method the overestimation of basis set superposition error (BSSE) at the second order electron correlation potential energy calculations is small and it decreases rapidly as the interaction separation increases. Fortunately, the overestimation of basis set superposition error (BSSE) around the potential energy minimum distance is small enough so that the calculated potential energy at the second order electron correlation calculations are quite accurate with the full counterpoise (FCP) correction

Regional Rarity: Consequences of Dispersal Limitation and Strategies for Reintroduction

Understanding factors that limit species distributions within restored habitats has been a longstanding goal of restoration ecology, particularly with respect to differences between rare and widespread species. Numerous mechanisms have been proposed to contribute to rarity, including species traits, propagule limitation, ecological requirements, and population level processes such as demographic and environmental stochasticity. Yet, our understanding of species rarity remains limited since few studies have examined how these factors act in concert. This dissertation examines how these mechanisms may interact to influence the distribution and population establishment of plant species in restored xeric glades in the Missouri Ozarks: USA). First, I used a trait analysis of species in restored glades to determine whether traits related to niche breadth, persistence, or dispersal ability are related to species distributions. The results from this study suggest that several traits may interact to allow species to become widespread in this restored xeric habitat, including habitat specialization of open canopy environments, perennial or clonal life histories, and adaptations to arid conditions such as short plant height. To follow up these observations, I experimentally tested the degree to which propagule limitation, competitive environment: competitors present, burned, or removed), and abiotic stress: precipitation of typical dry, average and wet years) differentially limit the seedling establishment of 32 rare versus widespread species. I found that rare species established as well as widespread species in all treatments, suggesting that dispersal, rather than differences in stress tolerance or competitive ability, is the predominant limiting force for species distributions. This suggests that propagule introductions may be sufficient to increase rare species distributions. To examine how the success of propagule introduction might be influenced by propagule pressure: i.e., a single large versus several small introductions) versus habitat management during introduction: i.e., creating favorable conditions for seedling establishment), I conducted a meta-analysis with demographic modeling. My results indicate that habitat management during introduction has a much greater effect on long-term population establishment compared to propagule pressure. In summary, this dissertation demonstrates the importance of dispersal for limiting species distributions in restored habitats and also provides recommendations on how to best reintroduce rare species

Expert perspectives on global biodiversity loss and its drivers and impacts on people

Despite substantial progress in understanding global biodiversity loss, major taxonomic and geographic knowledge gaps remain. Decision makers often rely on expert judgement to fill knowledge gaps, but are rarely able to engage with sufficiently large and diverse groups of specialists. To improve understanding of the perspectives of thousands of biodiversity experts worldwide, we conducted a survey and asked experts to focus on the taxa and freshwater, terrestrial, or marine ecosystem with which they are most familiar. We found several points of overwhelming consensus (for instance, multiple drivers of biodiversity loss interact synergistically) and important demographic and geographic differences in specialists’ perspectives and estimates. Experts from groups that are underrepresented in biodiversity science, including women and those from the Global South, recommended different priorities for conservation solutions, with less emphasis on acquiring new protected areas, and provided higher estimates of biodiversity loss and its impacts. This may in part be because they disproportionately study the most highly threatened taxa and habitats