首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   73篇
  免费   10篇
  国内免费   8篇
  2023年   1篇
  2022年   2篇
  2021年   4篇
  2020年   2篇
  2019年   1篇
  2018年   6篇
  2017年   5篇
  2015年   6篇
  2014年   5篇
  2013年   5篇
  2012年   3篇
  2011年   2篇
  2010年   4篇
  2009年   5篇
  2008年   3篇
  2007年   4篇
  2006年   4篇
  2005年   1篇
  2004年   3篇
  2003年   3篇
  2002年   2篇
  2001年   3篇
  2000年   2篇
  1999年   1篇
  1997年   1篇
  1994年   2篇
  1992年   2篇
  1991年   1篇
  1987年   2篇
  1982年   2篇
  1981年   1篇
  1979年   1篇
  1977年   2篇
排序方式: 共有91条查询结果,搜索用时 78 毫秒
61.
62.
63.
A myelolipoma was surgically removed from the abdomen of a 15-year-old female cottontop marmoset (Saguinus oedipus). Because of its close adherence to the right kidney, a unilateral nephrectomy was performed. The post-surgical recovery was uneventful. A myelolipoma is a circumscribed mass of bone marrow elements embedded in mature adipose tissue. These masses have been reported in the adrenal glands, paravertebral tissue, intrathoracic tissues, and mesentery of man. A similar condition has been observed in the liver and spleen of both captive and wild felidae. This report documents the first observation of this tumor-like condition in nonhuman primates with the possible exception of a subcutaneous myeloliposarcoma in a potto.  相似文献   
64.
A surface trawl survey (2001-2005) in Penobscot Bay, Maine, U.S.A., and the nearshore Gulf of Maine waters was conducted to investigate early marine dynamics of a hatchery-dependent Atlantic salmon Salmo salar population from a severely modified river system. Data generated were used to evaluate the effect of stocking location and time on migration success and to describe the migratory pathways and environments that post-smolts traverse. Significant differences in early migration success were detected among different stocking groups, but subsequent marine survival was independent of stocking group. While the post-smolt population was primarily composed of hatchery origin smolt-stocked fish, other life-stage stocking strategies (i.e. parr stocking) represented a higher proportion of the population than previously assumed. Catch distribution suggests evidence of an initial marine migratory pathway out of the dynamic Penobscot Bay environment. The hypothesized benefits of a predator refuge based on the co-occurring species complex is considered minimal for emigrating post-smolts given a mismatch in the size overlap among species and low abundance of other co-occurring diadromous populations. These data can be used to modify current management actions to optimize S. salar recovery and inform future research agendas.  相似文献   
65.
Urban ores can be classified into two types—those made up of items that were in the possession of consumers, such as end‐of‐life home appliances, and those that were not, such as manufacturing wastes. The dephosphorization slag generated at some steelmaking plants is an example of a manufacturing waste. In this study, the potential for this slag to become a new phosphorous resource is evaluated in terms of total material requirement (TMR). To do this, we compare two types of TMR—natural ore TMR (NO‐TMR) and urban ore TMR (UO‐TMR)—defined respectively as the TMR to obtain or recycle 1 kilogram of phosphoric acid from natural phosphate ore or dephosphorization slag. In the dephosphorization slag process, the slag is magnetically separated into a phosphorous‐rich (PR) phase and an iron‐rich (IR) phase, and the phosphorous in the PR phase is subsequently converted to phosphoric acid. We included case studies that considered generation of potentially useful by‐products, such as phosphogypsum and the IR phase. The effect of declines in natural phosphate ore grade on the potential recyclability of phosphoric acid from dephosphorization slag was also explored. In addition, the effect of changes in the slag's contribution to the UO‐TMR of phosphoric acid was considered. We found that, in many scenarios, the UO‐TMR for phosphoric acid was lower than its NO‐TMR. This indicates that dephosphorization slag has the potential to be a new phosphorous resource. We also found that recycling of the IR phase by recharging it to the blast furnace plays an important role in improving the potential feasibility of slag as a new phosphorous resource. The value of the slag stream, relative to the value of the pig iron stream, is also a key parameter, with low relative values improving the potential for production of phosphoric acid. Evaluating the UO‐TMR of materials recovered from manufacturing waste streams promises to be a useful tool for assessing the potential of these waste streams to serve as urban ores.  相似文献   
66.
Tephroseris longifolia agg. is a complex group of outcrossing perennials distributed throughout Central Europe. Recent morphological study revealed six morphotypes corresponding to five previously distinguished subspecies, together with Alpine and Pannonian morphotypes of T. longifolia subsp. longifolia. The delimited morphotypes differ in relative DNA content, geographical range, and rarity. We compared ecological niches of the six morphotypes in order to assess the impact of ecological differentiation on the speciation processes within the T. longifolia agg. Further, we examined whether morphotypes with small range are more ecologically specialized than their widespread relatives. The distribution area of the aggregate includes the Alps, Apennines, Carpathians, and the Pannonian Basin. Ecological variables linked to climate, topography, soil, and vegetation were gathered from 135 circular plots recorded in 35 localities. Related variables were grouped to describe the partial ecological niches: climatic, topographic, pedological, biotic, and coenotic (based either on vascular plants or on bryophytes), each of them visualized as an envelope in the two‐dimensional nonmetric multidimensional scaling ordination space. Each partial ecological niche for a given morphotype was characterized by its position (location of the envelope centroid), breadth (surface of the envelope), and overlaps with envelopes of the other morphotypes. Mantel statistics based on Spearman correlation coefficients were used to quantify differentiation of morphotypes in ecological parameters represented by the partial ecological niches. The significant niche differentiation was confirmed for climatic, topographic, pedological, and vascular plant‐based coenotic niches. Ecological niche differentiation corresponded well to morphological and partially also to karyological differentiation. Narrowly distributed morphotypes occupied more specific habitats and had narrower ecological niches than their widespread relatives. Ecological differentiation could be considered an important driver in allopatric speciation within the T. longifolia agg. Our results demonstrate that quantification of ecological divergence is helpful in assessing evolutionary history of closely related taxa.  相似文献   
67.
Using an economic valuation approach, we assessed people's stated preferences for policy aimed at enhancing restoration of functional networks of naturally dynamic boreal forest habitats as a public good. Active landscape restoration can improve the functionality of boreal forest habitats as green infrastructure, which is essential for biodiversity conservation and delivery of multiple ecosystem services that production forests are poor at providing. In contrast, so far designation of protected areas in Fennoscandia has focused on remnant patches of near‐natural forests, and not on forest landscape restoration. We assessed citizens' preferences for forest landscape restoration in a transboundary region primarily managed for sustained‐yield wood production for the forest industry, and which hosts the transboundary Fulufjället National Parks in Sweden and Norway. We conducted a discrete choice experiment by asking Swedish and Norwegian citizens to choose among two options for the Fulufjället area, including (1) extension of passive protection on one side or both sides of the border, by additional area protection and restoration of forest naturalness, and (2) a status quo option. The scenario assumed that extension of the protected forest area would imply a compulsory tax administered bilaterally by the two countries. Just over half of the sample, in both countries expressed willingness to pay for forest landscape restoration. Alternatives that contemplated larger extensions of forest landscape restoration were assigned higher willingness‐to‐pay. Public awareness and support, combined with spatial planning, are necessary for forest landscape restoration to become a viable tool for biodiversity conservation in Scandinavia.  相似文献   
68.
SUMMARY 1. Mathematical functions developed in long‐term laboratory experiments at different constant temperatures were combined with daily water temperatures for 1991–93 in eight Austrian streams and rivers to simulate the complex life histories and reproductive capacities of two freshwater amphipods: Gammarus fossarum and G. roeseli. The functions describe brood development times, hatching success, times taken to reach sexual maturity, growth, and fecundity. The sex ratio was assumed to be 0.5 and an autumn–winter reproductive resting period was based on observations of six river populations. Simulations included summer‐cold mountain streams, summer‐warm lowland rivers, watercourses fed by groundwater or influenced by heated effluents, and varying amplitudes of change within each year. 2. A fortran 77 computer program calculated growth from birth to sexual maturity of first‐generation females born on the first day of each calendar month in 1991, and the numbers of offspring successfully released from the maternal broodpouch in successive broods. At the 1991–93 regimes of temperature, individual G. fossarum released 127–208 offspring and G. roeseli released 120–169 in seven or eight successive broods during life spans of less than 2 years in six rivers. Life spans extended into a third year in the relatively cool River Salzach (mean temperature 7.5 °C). They were not completed in the very cold River Steyr (mean 5.6, range 2.5–7.9 °C), where G. fossarum produced five broods (totalling 120 offspring) and G. roeseli only two broods (totalling 28 offspring) in the 3‐year period. Except in the Steyr, some offspring grew rapidly to maturity and produced several second‐generation broods during the simulation period; in the warmest rivers some third‐generation broods were also produced. Birth dates, early or late in the year, influenced the subsequent production of broods and young, depending on temperature regimes in particular rivers. Total numbers of offspring produced by the second and third generations represent the theoretical reproductive capacities of G. fossarum and G. roeseli. Minimum and maximum estimates mostly ranged from 100 to 17 300, were larger for G. fossarum except in the warmest river (March), where temperatures rose above 20 °C for 56–78 days in summer, and largest (maximum 37 600) in the River Voeckla heated by discharge from a power‐station (mean 11.5 °C). Results from the simulations agree with preliminary assessments of relative abundances for G. fossarum and G. roeseli in several of the study rivers, but in some one or both species appear to be absent. On a wider scale, the present study confirms that G. fossarum is potentially more successful than G. roeseli in cool rivers but indicates that neither species is likely to maintain viable populations in cold rivers strongly influenced by snow and ice‐melt. 3. The potential impacts of future river warming by increases of 1, 2 and 3 °C, due to climate change, vary according to river site, date of fertilisation, the extent of temperature increase, and the species of Gammarus. For Austrian rivers with mean temperatures in the range c. 7–10 °C, future warming would result in modest changes in the life histories and reproductive capacities of both G. fossarum and G. roeseli; the former would find improved temperature conditions in watercourses that are currently very cold throughout the year, and both would find warm rivers less tolerable. 4. The high potential reproductive capacity of gammarids, with rapid production of numerous successive broods when sexual maturity is finally achieved, indicates adaptation to high mortality during the relatively long period of growth to sexual maturity, and provides scope for an opportunistic strategy of emigration from centres of population abundance to colonise new territory when conditions are favourable. Rapid expansion of populations is desirable to combat the effects of environmental catastrophes, both frequent and short‐term floods and droughts, and more long‐term climatic changes that have occurred several times in glacial–interglacial periods during the current Ice Age.  相似文献   
69.
Was human fighting always there, as old as our species? Or is it a late cultural invention, emerging after the transition to agriculture and the rise of the state, which began, respectively, only around ten thousand and five thousand years ago? Viewed against the life span of our species, Homo sapiens, stretching back 150,000–200,000 years, let alone the roughly two million years of our genus Homo, this is the tip of the iceberg. We now have a temporal frame and plenty of empirical evidence for the “state of nature” that Thomas Hobbes and Jean‐Jacque Rousseau discussed in the abstract and described in diametrically opposed terms. All human populations during the Pleistocene, until about 12,000 years ago, were hunter‐gatherers, or foragers, of the simple, mobile sort that lacked accumulated resources. Studying such human populations that survived until recently or still survive in remote corners of the world, anthropology should have been uniquely positioned to answer the question of aboriginal human fighting or lack thereof. Yet access to, and the interpretation of, that information has been intrinsically problematic. The main problem has been the “contact paradox.” Prestate societies have no written records of their own. Therefore, documenting them requires contact with literate state societies that necessarily affects the former and potentially changes their behavior, including fighting.  相似文献   
70.
天然芪类化合物的研究概况   总被引:9,自引:0,他引:9  
本文就天然芪类化合物的生物合成途径、结构类型、理化性质,分布及生物活性作一简要综述。  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号