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Fundamental Challenge (fundamental + challenge)
Selected AbstractsEvaluating a general sediment transport model for linear incisions under field conditionsEARTH SURFACE PROCESSES AND LANDFORMS, Issue 14 2009T. Vanwalleghem Abstract Prediction of sediment transport in concentrated overland flow remains a fundamental challenge in soil erosion assessment. Sediment transport is often modelled as a non-linear function of shear stress. These relations are mostly derived from river channels or flume experiments. Here, new data of active incisions that occurred as a response to a single runoff event is presented. This allowed to complement and revisit a field-based sediment transport-shear stress equation. The results demonstrate the general applicability of the proposed relation for predicting sediment transport in linear incisions, ranging from rills to gullies, in field conditions. These findings have important implications for erosion modelling. Copyright © 2009 John Wiley & Sons, Ltd. [source] Scales of association: hierarchical linear models and the measurement of ecological systemsECOLOGY LETTERS, Issue 6 2007Sean M. McMahon Abstract A fundamental challenge to understanding patterns in ecological systems lies in employing methods that can analyse, test and draw inference from measured associations between variables across scales. Hierarchical linear models (HLM) use advanced estimation algorithms to measure regression relationships and variance,covariance parameters in hierarchically structured data. Although hierarchical models have occasionally been used in the analysis of ecological data, their full potential to describe scales of association, diagnose variance explained, and to partition uncertainty has not been employed. In this paper we argue that the use of the HLM framework can enable significantly improved inference about ecological processes across levels of organization. After briefly describing the principals behind HLM, we give two examples that demonstrate a protocol for building hierarchical models and answering questions about the relationships between variables at multiple scales. The first example employs maximum likelihood methods to construct a two-level linear model predicting herbivore damage to a perennial plant at the individual- and patch-scale; the second example uses Bayesian estimation techniques to develop a three-level logistic model of plant flowering probability across individual plants, microsites and populations. HLM model development and diagnostics illustrate the importance of incorporating scale when modelling associations in ecological systems and offer a sophisticated yet accessible method for studies of populations, communities and ecosystems. We suggest that a greater coupling of hierarchical study designs and hierarchical analysis will yield significant insights on how ecological processes operate across scales. [source] Molecular Origins of the Mechanical Behavior of Hybrid GlassesADVANCED FUNCTIONAL MATERIALS, Issue 17 2010Mark S. Oliver Abstract Hybrid organic-inorganic glasses exhibit unique electro-optical properties along with excellent thermal stability. Their inherently mechanically fragile nature, however, which derives from the oxide component of the hybrid glass network together with the presence of terminal groups that reduce network connectivity, remains a fundamental challenge for their integration in nanoscience and energy technologies. We report on a combined synthesis and computational strategy to elucidate the effect of molecular structure on mechanical properties of hybrid glass films. We first demonstrate the importance of rigidity percolation to elastic behavior. Secondly, using a novel application of graph theory, we reveal the complex 3-D fracture path at the molecular scale and show that fracture energy in brittle hybrid glasses is fundamentally governed by the bond percolation properties of the network. The computational tools and scaling laws presented provide a robust predictive capability for guiding precursor selection and molecular network design of advanced hybrid organic-inorganic materials. [source] Do dams and levees impact nitrogen cycling?GLOBAL CHANGE BIOLOGY, Issue 8 2005Simulating the effects of flood alterations on floodplain denitrification Abstract A fundamental challenge in understanding the global nitrogen cycle is the quantification of denitrification on large heterogeneous landscapes. Because floodplains are important sites for denitrification and nitrogen retention, we developed a generalized floodplain biogeochemical model to determine whether dams and flood-control levees affect floodplain denitrification by altering floodplain inundation. We combined a statistical model of floodplain topography with a model of hydrology and nitrogen biogeochemistry to simulate floods of different magnitude. The model predicted substantial decreases in NO3 -N processing on floodplains whose overbank floods have been altered by levees and upstream dams. Our simulations suggest that dams may reduce nitrate processing more than setback levees. Levees increased areal floodplain denitrification rates, but this effect was offset by a reduction in the area inundated. Scenarios that involved a levee also resulted in more variability in N processing among replicate floodplains. Nitrate loss occurred rapidly and completely in our model floodplains. As a consequence, total flood volume and the initial mass of nitrate reaching a floodplain may provide reasonable estimates of total N processing on floodplains during floods. This finding suggests that quantifying the impact of dams and levees on floodplain denitrification may be possible using recent advances in remote sensing of floodplain topography and flood stage. Furthermore, when considering flooding over the long-term, the cumulative N processed by frequent smaller floods was estimated to be quite large relative to that processed by larger, less frequent floods. Our results suggest that floodplain denitrification may be greatly influenced by the pervasive anthropogenic flood-control measures that currently exist on most majors river floodplains throughout the world, and may have the potential to be impacted by future changes in flood probabilities that will likely occur as a result of climate shifts. [source] Wicked Problems, Knowledge Challenges, and Collaborative Capacity Builders in Network SettingsPUBLIC ADMINISTRATION REVIEW, Issue 2 2008Edward P. Weber Networks have assumed a place of prominence in the literature on public and private governing structures. The many positive attributes of networks are often featured,the capacity to solve problems, govern shared resources, create learning opportunities, and address shared goals,and a literature focused on the challenges networks pose for managers seeking to realize these network attributes is developing. The authors share an interest in understanding the potential of networks to govern complex public, or "wicked," problems. A fundamental challenge to effectively managing any public problem in a networked setting is the transfer, receipt and integration of knowledge across participants. When knowledge is viewed pragmatically, the challenge is particularly acute. This perspective, the authors argue, presents a challenge to the network literature to consider the mind-set of the managers,or collaborative capacity-builders,who are working to achieve solutions to wicked problems. This mind-set guides network managers as they apply their skills, strategies, and tools in order to foster the transfer, receipt, and integration of knowledge across the network and, ultimately, to build long-term collaborative problem-solving capacity. [source] The Origins of Institutional Crises in Latin AmericaAMERICAN JOURNAL OF POLITICAL SCIENCE, Issue 3 2010Gretchen Helmke Institutional instability and interbranch crises pose a fundamental challenge to democracies in Latin America and the developing world more generally. Combining a standard game theoretic model of crisis bargaining with a unique dataset on courts, executives, and legislatures for 18 Latin American countries between 1985 and 2008, the article develops a strategic account of how interbranch crises emerge and evolve. In addition to providing the first systematic picture of the frequency, type, and location of interbranch crises for the region, the article demonstrates that the decision to initiate an interbranch crisis is influenced by the allocation of institutional powers, public support for the targeted branch, and the expectations of success based on recent experiences. Building on these results, the article identifies several novel directions for future research on institutional instability. [source] Postneoliberalism and its MalcontentsANTIPODE, Issue 2010Jamie Peck Abstract:, The onset of the global financial crisis in 2008 has been widely interpreted as a fundamental challenge to, if not crisis of, neoliberal governance. Here, we explore some of the near-term and longer-run consequences of the economic crisis for processes of neoliberalization, asking whether we have been witnessing the terminal unraveling of neoliberalism as a form of social, political, and economic regulation. In many ways a creature of crisis, could neoliberalism now be falling to a crisis of its own making? Answering this question is impossible, we argue, without an adequate understanding of the nature of neoliberalization and its evolving sociospatial manifestations. These are more than definitional niceties. The prospects and potential of efforts to move genuinely beyond neoliberalism must also be considered in this light. [source] Oriented Nanostructures for Energy Conversion and StorageCHEMSUSCHEM CHEMISTRY AND SUSTAINABILITY, ENERGY & MATERIALS, Issue 8-9 2008Jun Liu Dr. Abstract Recently, the role of nanostructured materials in addressing the challenges in energy and natural resources has attracted wide attention. In particular, oriented nanostructures demonstrate promising properties for energy harvesting, conversion, and storage. In this Review, we highlight the synthesis and application of oriented nanostructures in a few key areas of energy technologies, namely photovoltaics, batteries, supercapacitors, and thermoelectrics. Although the applications differ from field to field, a common fundamental challenge is to improve the generation and transport of electrons and ions. We highlight the role of high surface area to maximize the surface activity and discuss the importance of optimum dimension and architecture, controlled pore channels, and alignment of the nanocrystalline phase to optimize the transport of electrons and ions. Finally, we discuss the challenges in attaining integrated architectures to achieve the desired performance. Brief background information is provided for the relevant technologies, but the emphasis is focused mainly on the nanoscale effects of mostly inorganic-based materials and devices. [source] A landscape theory for food web architectureECOLOGY LETTERS, Issue 8 2008Neil Rooney Abstract Ecologists have long searched for structures and processes that impart stability in nature. In particular, food web ecology has held promise in tackling this issue. Empirical patterns in food webs have consistently shown that the distributions of species and interactions in nature are more likely to be stable than randomly constructed systems with the same number of species and interactions. Food web ecology still faces two fundamental challenges, however. First, the quantity and quality of food web data required to document both the species richness and the interaction strengths among all species within food webs is largely prohibitive. Second, where food webs have been well documented, spatial and temporal variation in food web structure has been ignored. Conversely, research that has addressed spatial and temporal variation in ecosystems has generally ignored the full complexity of food web architecture. Here, we incorporate empirical patterns, largely from macroecology and behavioural ecology, into a spatially implicit food web structure to construct a simple landscape theory of food web architecture. Such an approach both captures important architectural features of food webs and allows for an exploration of food web structure across a range of spatial scales. Finally, we demonstrated that food webs are hierarchically organized along the spatial and temporal niche axes of species and their utilization of food resources in ways that stabilize ecosystems. [source] Accounting for Corruption: Economic Structure, Democracy, and TradeINTERNATIONAL STUDIES QUARTERLY, Issue 1 2000Wayne Sandholtz Though corruption poses fundamental challenges to both democratic governance and market economies, political science research has only recently begun to address corruption in a comparative context. In this article we explain variation in the perceived level of corruption (defined as the misuse of public office for private gain) across fifty countries. We propose a set of hypotheses that explain variation in corruption levels in terms of domestic political-economic structure, democratic norms, integration into the international economy, and Protestant religious affiliation. Levels of corruption, we propose, are higher: (1) the lower the average income level, (2) the greater the extent of state control of the economy, (3) the weaker are democratic norms and institutions, (4) the lower the degree of integration in the world economy, and (5) the lower the share of the population with Protestant religious affiliation. The data analysis broadly confirms our predictions: in the multivariate regression, each of the independent variables is significant in the direction we expect. [source] Structure, function and evolution of the gas exchangers: comparative perspectivesJOURNAL OF ANATOMY, Issue 4 2002J. N. Maina Abstract Over the evolutionary continuum, animals have faced similar fundamental challenges of acquiring molecular oxygen for aerobic metabolism. Under limitations and constraints imposed by factors such as phylogeny, behaviour, body size and environment, they have responded differently in founding optimal respiratory structures. A quintessence of the aphorism that ,necessity is the mother of invention', gas exchangers have been inaugurated through stiff cost,benefit analyses that have evoked transaction of trade-offs and compromises. Cogent structural,functional correlations occur in constructions of gas exchangers: within and between taxa, morphological complexity and respiratory efficiency increase with metabolic capacities and oxygen needs. Highly active, small endotherms have relatively better-refined gas exchangers compared with large, inactive ectotherms. Respiratory structures have developed from the plain cell membrane of the primeval prokaryotic unicells to complex multifunctional ones of the modern Metazoa. Regarding the respiratory medium used to extract oxygen from, animal life has had only two choices , water or air , within the biological range of temperature and pressure the only naturally occurring respirable fluids. In rarer cases, certain animals have adapted to using both media. Gills (evaginated gas exchangers) are the primordial respiratory organs: they are the archetypal water breathing organs. Lungs (invaginated gas exchangers) are the model air breathing organs. Bimodal (transitional) breathers occupy the water,air interface. Presentation and exposure of external (water/air) and internal (haemolymph/blood) respiratory media, features determined by geometric arrangement of the conduits, are important features for gas exchange efficiency: counter-current, cross-current, uniform pool and infinite pool designs have variably developed. [source] The Challenge to Restore Processes in Face of Nonlinear Dynamics,On the Crucial Role of Disturbance RegimesRESTORATION ECOLOGY, Issue 2 2007*Article first published online: 14 MAY 200, Anke Jentsch Abstract Increasingly, restoration ecologists and managers are challenged to restore ecological processes that lead to self-sustaining ecosystem dynamics. Due to changing environmental conditions, however, restoration goals need to include novel regimes beyond prior reference conditions or reference dynamics. In face of these fundamental challenges in process-based restoration ecology, disturbance ecology can offer useful insights. Here, I discuss the contribution of disturbance ecology to understanding assembly rules, ecosystem dynamics, regime shifts, and nonlinear dynamics. Using the patch and multipatch concept, all insights are organized according to two spatial and two temporal categories: "patch,event,""patch,multievent,""multipatch,event," and "multipatch,multievent." This concept implies the consideration of both spatial patterns and temporal rhythms inside and outside of a restoration site. Emerging issues, such as uncoupling of internal and external dynamics, are considered. [source] Does sympatry predict life history and morphological diversification in the Mexican livebearing fish Poeciliopsis baenschi?BIOLOGICAL JOURNAL OF THE LINNEAN SOCIETY, Issue 3 2010LAURA E. SCOTT Understanding why some species coexist and others do not remains one of the fundamental challenges of ecology. Although there is evidence to suggest that closely-related species are unlikely to occupy the same habitat because of competitive exclusion, there are many cases where closely-related species do co-occur. Research comparing sympatric and allopatric populations of co-occurring species provides a framework for understanding the role of phenotypic diversification in species coexistence. In the present study, we compare phenotypic divergence between sympatric and allopatric populations of the livebearing fish, Poeciliopsis baenschi. We focus on life-history traits and body shape, comprising two sets of integrated traits likely to diverge in response to varying selective pressures. Given that males and females can express different phenotypic traits, we also test for patterns of divergence among sexes by comparing size at maturity and sexual dimorphism in body shape between males and females in each population type. We take advantage of a natural experiment in western Mexico where, in some locations, P. baenschi co-occur with a closely-related species, Poeciliopsis turneri (sympatric populations) and, in other locations, they occur in isolation (allopatric populations). The results obtained in the present study show that sympatric populations of P. baenschi differed significantly in life-history traits and in body shape compared to their allopatric counterparts. Additionally, males and females showed different responses for size at maturity in sympatric conditions versus allopatric conditions. However, the amount of sexual dimorphism did not differ between sympatric and allopatric populations of P. baenschi. Hence, we conclude that not all traits show similar levels of phenotypic divergence in response to sympatric conditions. © 2010 The Linnean Society of London, Biological Journal of the Linnean Society, 2010, 100, 608,618. [source] | |||||||||||||