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排序方式: 共有491条查询结果,搜索用时 375 毫秒
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Studying invasion: have we missed the boat? 总被引:9,自引:0,他引:9
Invasive species, and the ensuing homogenization of the world's biota, form a global problem with consequences ranging from the decline and extirpation of native species to threats to human health. The magnitude of this issue demands a thorough understanding of the invasion process, which consists of three main stages: initial dispersal, establishment of self‐sustaining populations, and spread. To assess the relative distribution of research effort among these stages, we conducted a literature review using 873 articles published in 23 major journals over the past 10 years. Of the 873 papers, only 96 (11.0%) studied initial dispersal, and only half of these (6.2% of the total) were empirical. As the first stage in a contingent process, we argue that initial dispersal is the best stage during which to direct management efforts. In addition, initial dispersal has direct relevance for fundamental ecological questions regarding community assembly and metacommunity dynamics. In so far that answering these questions and preventing invasion are goals of ecologists, the disparity in research effort noted here suggests that ecologists need to expand their efforts to include more research on initial dispersal. 相似文献
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Ranunculus communities have been identified by the European Community Directive on Conservation of Natural and Semi-Natural Habitats
as a key habitat in need of protection. The ecological requirements and floristic composition of three Ranunculus communities were investigated in North-Eastern France. We also aimed to determine the relationships between environmental
parameters, Ranunculus abundance and species richness. R. fluitans communities clearly differed from the other communities by a higher evenness and species richness and by the occurrence of
specific species. In contrast, R. penicillatus and R. peltatus communities displayed similar floristic content. Three different sub-associations were however identified within these communities.
If alkalinity was determinant in isolating R. fluitans communities from the other stretches, the ecological ranges of the three communities strongly overlapped while considering
parameters linked with other resources or with physical constraints. Further, no significant correlations were highlighted
between environmental parameters, Ranunculus abundance and species richness. With respect to conservation, these observations may indicate the particular difficulty in
defining each Ranunculus habitat on a European scale, and identify several key points which need to be taken into account when evaluating their status. 相似文献
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Derek Mendez Thomas J. Lane Jongmin Sung Jonas Sellberg Clément Levard Herschel Watkins Aina E. Cohen Michael Soltis Shirley Sutton James Spudich Vijay Pande Daniel Ratner Sebastian Doniach 《Philosophical transactions of the Royal Society of London. Series B, Biological sciences》2014,369(1647)
Tools to study disordered systems with local structural order, such as proteins in solution, remain limited. Such understanding is essential for e.g. rational drug design. Correlated X-ray scattering (CXS) has recently attracted new interest as a way to leverage next-generation light sources to study such disordered matter. The CXS experiment measures angular correlations of the intensity caused by the scattering of X-rays from an ensemble of identical particles, with disordered orientation and position. Averaging over 15 496 snapshot images obtained by exposing a sample of silver nanoparticles in solution to a micro-focused synchrotron radiation beam, we report on experimental efforts to obtain CXS signal from an ensemble in three dimensions. A correlation function was measured at wide angles corresponding to atomic resolution that matches theoretical predictions. These preliminary results suggest that other CXS experiments on disordered ensembles—such as proteins in solution—may be feasible in the future. 相似文献
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PurposeTo assess the impact of iterative reconstructions on image quality and detectability of focal liver lesions in low-energy monochromatic images from a Fast kV-Switching Dual Energy CT (KVSCT) platform.MethodsAcquisitions on an image-quality phantom were performed using a KVSCT for three dose levels (CTDIvol:12.72/10.76/8.79 mGy). Raw data were reconstructed for five energy levels (40/50/60/70/80 keV) using Filtered Back Projection (FBP) and four levels of ASIR (ASIR30/ASIR50/ASIR70/ASIR100). Noise power spectrum (NPS) and task-based transfer function (TTF) were measured before computing a Detectability index (d′) to model the detection task of liver metastasis (LM) and hepatocellular carcinoma (HCC) as function of keV.ResultsFrom 40 to 70 keV, noise-magnitude was reduced on average by −68% ± 1% with FBP; −61% ± 3% with ASIR50 and −52% ± 6% with ASIR100. The mean spatial frequency of the NPS decreased when the energy level decreased and the iterative level increased. TTF values at 50% decreased as the energy level increased and as the percentage of ASIR increased. The detectability of both lesions increased with increasing dose level and percentage of ASIR. For the LM, d′ peaked at 70 keV for all reconstruction types, except for ASIR70 at 12.72 mGy and ASIR100, where d' peaked at 50 keV. For HCC, d’ peaked at 60 keV for FBP and ASIR30 but peaked at 50 keV for ASIR50, ASIR70 and ASIR100.ConclusionsUsing percentage of ASIR above 50% at low-energy monochromatic images could limit the increase of noise-magnitude, benefit from spatial resolution improvement and hence enhance detectability of subtle low contrast focal liver lesions such as HCC. 相似文献