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排序方式: 共有79条查询结果,搜索用时 15 毫秒
1.
Summary Maximum penetrometer pressure was measured on artificial soil aggregates of finite size (2–29 mm) using blunt probes (total
cone angle 60°) driven at 3 mm min−1. Maximum penetrometer pressure increased asymptotically with increase in dimensionless aggregate radius,b/a, wherea andb are the probe and aggregate radii, respectively. A theory was developed for penetration of blunt probes into soil aggregates
of finite size. The theory assumed that plastic failure occurs out to a radius,R, and that beyond this only elastic straining occurs. This theory can be applied to estimate the radial and tangential stresses
adjacent to a blunt probe. The estimated radial and tangential stresses increased with increase in dimensionless aggregate
radius,b/a. The radius of the plastic front,R, around the probe is predicted to increase with increased aggregate size. The results also demonstrate the effect of soil
shear cohesion and internal friction angle onR. The results are discussed with reference to root penetration. 相似文献
2.
Evolution of dispersal and life history interact to drive accelerating spread of an invasive species
T. Alex Perkins Benjamin L. Phillips Marissa L. Baskett Alan Hastings 《Ecology letters》2013,16(8):1079-1087
Populations on the edge of an expanding range are subject to unique evolutionary pressures acting on their life‐history and dispersal traits. Empirical evidence and theory suggest that traits there can evolve rapidly enough to interact with ecological dynamics, potentially giving rise to accelerating spread. Nevertheless, which of several evolutionary mechanisms drive this interaction between evolution and spread remains an open question. We propose an integrated theoretical framework for partitioning the contributions of different evolutionary mechanisms to accelerating spread, and we apply this model to invasive cane toads in northern Australia. In doing so, we identify a previously unrecognised evolutionary process that involves an interaction between life‐history and dispersal evolution during range shift. In roughly equal parts, life‐history evolution, dispersal evolution and their interaction led to a doubling of distance spread by cane toads in our model, highlighting the potential importance of multiple evolutionary processes in the dynamics of range expansion. 相似文献
3.
A basic protein has been purified from sheep brain. The purified protein sedimented in the analytical centrifuge at 56,000 r.p.m. as an homogenous product. This protein induced an allergic encephalitis when injected into guinea pigs. Some physiochemical properties of the protein were studied: the sedimentation coefficient was 1.52 and the molecular weight was 20,000 +/- 2,000, as estimated by electrophoresis in acrylamide gels containing SDS and urea; the specific extinction coefficient (see article) was 6.01 +/- 0.20. The aminoacid composition of the molecule was determined and its most prominent aspects are a high content of arginine and lysine, the presence of a single tryptophan, the total absence of cysteine and cystine and a blocked N-terminal residue. All these properties are very close to those of human and bovine encephalitogenic proteins. 相似文献
4.
具时滞的人口模型的行波解 总被引:9,自引:5,他引:4
研究具时滞的人口模型的行波解存在性问题,利用[5]中的方法,鹕行波解的存在性问题转化为寻找上下解的问题。 相似文献
5.
When organisms perform a single task, selection leads to phenotypes that maximize performance at that task. When organisms need to perform multiple tasks, a trade‐off arises because no phenotype can optimize all tasks. Recent work addressed this question, and assumed that the performance at each task decays with distance in trait space from the best phenotype at that task. Under this assumption, the best‐fitness solutions (termed the Pareto front) lie on simple low‐dimensional shapes in trait space: line segments, triangles and other polygons. The vertices of these polygons are specialists at a single task. Here, we generalize this finding, by considering performance functions of general form, not necessarily functions that decay monotonically with distance from their peak. We find that, except for performance functions with highly eccentric contours, simple shapes in phenotype space are still found, but with mildly curving edges instead of straight ones. In a wide range of systems, complex data on multiple quantitative traits, which might be expected to fill a high‐dimensional phenotype space, is predicted instead to collapse onto low‐dimensional shapes; phenotypes near the vertices of these shapes are predicted to be specialists, and can thus suggest which tasks may be at play. 相似文献
6.
Mirella Georgouli Cecilia Herraiz Eva Crosas-Molist Bruce Fanshawe Oscar Maiques Anna Perdrix Pahini Pandya Irene Rodriguez-Hernandez Kristina M. Ilieva Gaia Cantelli Panagiotis Karagiannis Silvia Mele Hoyin Lam Debra H. Josephs Xavier Matias-Guiu Rosa M. Marti Frank O. Nestle Jose L. Orgaz Victoria Sanz-Moreno 《Cell》2019,176(4):757-774.e23
7.
S. Zervoudaki T. G. Nielsen E. D. Christou I. Siokou-Frangou 《Marine Biology Research》2006,2(3):149-168
This study was carried out in the northern Aegean Sea during late summer and spring. The aim was to examine the spatial and temporal distribution of the zooplankton community across a dynamic frontal area and to investigate how the oceanographic heterogeneity structures the composition of the zooplankton assemblages. The low-salinity and cold Black Sea water coming from the Dardanelles Strait is modified by mixing with the underlying warm and saline Aegean water. These hydrological features result in a pronounced thermohaline front in the northern Aegean Sea throughout the year. In both seasons, zooplankton was collected using both 45 and 200 µm mesh plankton nets. A high abundance of zooplankton was observed in the surface layer at the stations closest to the Dardanelles Strait on the stratified side of the front. The zooplankton distribution and community structure in the northern Aegean Sea were strongly influenced by the hydrological features. The frontal structure acts as a boundary for the zooplankton community. The surface layer at the stratified stations had the lowest copepod diversity, from where it increased with depth and horizontally as the stratification weakened outside of the front. The total abundance of zooplankton collected with the 200 µm net was between two and 20 times lower than samples taken with the 45 µm net. The most pronounced differences were observed for the adults and copepodids of the small genera Oithona, Oncaea and Microsetella. Thus, to manage and understand the transfer of primary production up the food chain in the Aegean Sea, the smaller fraction of copepods should be taken into account in future investigations. 相似文献
8.
Abundance,diversity and geographic distribution of cassava mosaic disease pandemic‐associated Bemisia tabaci in Tanzania 下载免费PDF全文
L. S. Tajebe S. B. Boni D. Guastella V. Cavalieri O. S. Lund C. P. Rugumamu C. Rapisarda J. P. Legg 《Journal of Applied Entomology》2015,139(8):627-637
Bemisia tabaci (Gennadius) (Homoptera: Aleyrodidae), one of the most economically important agricultural pests worldwide, is the vector of cassava mosaic geminiviruses that cause cassava mosaic disease (CMD). In East and Central Africa, a severe CMD pandemic that spread from Uganda in the late 1980s still continues to devastate cassava crops. To assess the association of distinct B. tabaci genetic groups with the CMD pandemic, mitochondrial cytochrome oxidase I gene sequences were analysed from whiteflies collected during surveys conducted from 2010 to 2013 in Tanzania. Four genetic groups – Sub‐Saharan Africa 1 (SSA1), Mediterranean, Indian Ocean and East Africa 1, and a group of unknown whitefly species were identified. SSA1 comprised four subgroups: SSA1‐SG1, SSA1‐SG2, SSA1‐SG1/2 and SSA1‐SG3. SSA1‐SG1 was confined to the pandemic‐affected north‐western parts of Tanzania whilst SSA1‐SG2 and SSA1‐SG3 were found in the central and eastern parts not yet affected by the pandemic. The CMD pandemic front was estimated to lie in Geita Region, north‐western Tanzania, and to be spreading south‐east at a rate of ca 26 km/year. The pandemic‐associated B. tabaci SSA1‐SG1 predominated up to 180 km ahead of the CMD front indicating that changes in whitefly population characteristics precede changes in disease characteristics. 相似文献
9.
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