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   199篇
  免费   0篇
系统科学   4篇
现状及发展   42篇
研究方法   23篇
综合类   118篇
自然研究   12篇
  2018年   1篇
  2017年   1篇
  2016年   1篇
  2013年   1篇
  2012年   11篇
  2011年   23篇
  2010年   2篇
  2008年   9篇
  2007年   13篇
  2006年   5篇
  2005年   6篇
  2004年   8篇
  2003年   15篇
  2002年   11篇
  2001年   7篇
  2000年   3篇
  1999年   5篇
  1993年   1篇
  1992年   5篇
  1991年   3篇
  1990年   1篇
  1989年   2篇
  1988年   3篇
  1987年   4篇
  1986年   3篇
  1985年   4篇
  1984年   3篇
  1983年   4篇
  1982年   6篇
  1981年   1篇
  1980年   1篇
  1979年   5篇
  1978年   5篇
  1977年   2篇
  1976年   3篇
  1975年   1篇
  1974年   2篇
  1971年   4篇
  1970年   4篇
  1969年   1篇
  1968年   3篇
  1967年   4篇
  1966年   1篇
  1965年   1篇
排序方式: 共有199条查询结果,搜索用时 31 毫秒
111.
Averting biodiversity collapse in tropical forest protected areas   总被引:3,自引:0,他引:3  
The rapid disruption of tropical forests probably imperils global biodiversity more than any other contemporary phenomenon. With deforestation advancing quickly, protected areas are increasingly becoming final refuges for threatened species and natural ecosystem processes. However, many protected areas in the tropics are themselves vulnerable to human encroachment and other environmental stresses. As pressures mount, it is vital to know whether existing reserves can sustain their biodiversity. A critical constraint in addressing this question has been that data describing a broad array of biodiversity groups have been unavailable for a sufficiently large and representative sample of reserves. Here we present a uniquely comprehensive data set on changes over the past 20 to 30 years in 31 functional groups of species and 21 potential drivers of environmental change, for 60 protected areas stratified across the world’s major tropical regions. Our analysis reveals great variation in reserve ‘health’: about half of all reserves have been effective or performed passably, but the rest are experiencing an erosion of biodiversity that is often alarmingly widespread taxonomically and functionally. Habitat disruption, hunting and forest-product exploitation were the strongest predictors of declining reserve health. Crucially, environmental changes immediately outside reserves seemed nearly as important as those inside in determining their ecological fate, with changes inside reserves strongly mirroring those occurring around them. These findings suggest that tropical protected areas are often intimately linked ecologically to their surrounding habitats, and that a failure to stem broad-scale loss and degradation of such habitats could sharply increase the likelihood of serious biodiversity declines.  相似文献   
112.
The standard model for the origin of galactic magnetic fields is through the amplification of seed fields via dynamo or turbulent processes to the level consistent with present observations. Although other mechanisms may also operate, currents from misaligned pressure and temperature gradients (the Biermann battery process) inevitably accompany the formation of galaxies in the absence of a primordial field. Driven by geometrical asymmetries in shocks associated with the collapse of protogalactic structures, the Biermann battery is believed to generate tiny seed fields to a level of about 10(-21)?gauss (refs 7, 8). With the advent of high-power laser systems in the past two decades, a new area of research has opened in which, using simple scaling relations, astrophysical environments can effectively be reproduced in the laboratory. Here we report the results of an experiment that produced seed magnetic fields by the Biermann battery effect. We show that these results can be scaled to the intergalactic medium, where turbulence, acting on timescales of around 700 million years, can amplify the seed fields sufficiently to affect galaxy evolution.  相似文献   
113.
During progression of atherosclerosis, myeloid cells destabilize lipid-rich plaques in the arterial wall and cause their rupture, thus triggering myocardial infarction and stroke. Survivors of acute coronary syndromes have a high risk of recurrent events for unknown reasons. Here we show that the systemic response to ischaemic injury aggravates chronic atherosclerosis. After myocardial infarction or stroke, Apoe-/- mice developed larger atherosclerotic lesions with a more advanced morphology. This disease acceleration persisted over many weeks and was associated with markedly increased monocyte recruitment. Seeking the source of surplus monocytes in plaques, we found that myocardial infarction liberated haematopoietic stem and progenitor cells from bone marrow niches via sympathetic nervous system signalling. The progenitors then seeded the spleen, yielding a sustained boost in monocyte production. These observations provide new mechanistic insight into atherogenesis and provide a novel therapeutic opportunity to mitigate disease progression.  相似文献   
114.
Natural killer (NK) cells have an important role in the control of viral infections, recognizing virally infected cells through a variety of activating and inhibitory receptors. Epidemiological and functional studies have recently suggested that NK cells can also contribute to the control of HIV-1 infection through recognition of virally infected cells by both activating and inhibitory killer immunoglobulin-like receptors (KIRs). However, it remains unknown whether NK cells can directly mediate antiviral immune pressure in vivo in humans. Here we describe KIR-associated amino-acid polymorphisms in the HIV-1 sequence of chronically infected individuals, on a population level. We show that these KIR-associated HIV-1 sequence polymorphisms can enhance the binding of inhibitory KIRs to HIV-1-infected CD4(+) T cells, and reduce the antiviral activity of KIR-positive NK cells. These data demonstrate that KIR-positive NK cells can place immunological pressure on HIV-1, and that the virus can evade such NK-cell-mediated immune pressure by selecting for sequence polymorphisms, as was previously described for virus-specific T cells and neutralizing antibodies. NK cells might therefore have a previously underappreciated role in contributing to viral evolution.  相似文献   
115.
Radio telemetry was used to study habitat use of breeding male sage grouse ( Centrocercus urophasianus ) at a lek in northeastern Utah during 1983 and 1984. Objectives were to determine if grouse day-use areas differed significantly in sagebrush characteristics from adjacent nonuse areas and to establish a simplified method for use by land managers in identifying grouse use areas. We determined that male grouse used areas of greatest sagebrush height and cover. Our methods provide a means for land managers to identify habitat associated with a lek that is suitable for male sage grouse day use in the event sagebrush alteration is planned within 3 km of a lek.  相似文献   
116.
R B Murphy 《Experientia》1978,34(2):188-189
Insulin is unaffected by pressures of 48,000 b in the solid state at room temperature, as elucidated both by spectroscopic measurements and bioassay. Its compression curve is reversible. The presence of water does not alter this property. Of a number of other proteins investigated insulin appears to be unique with regard to its pressure stability. The relative rigidity of the molecule combined with its small size may account for some of these properties.  相似文献   
117.
This paper reviews the visioning literature along five key dimensions: analysis of an organization's current situation, assessment of the external environment, identification of desired future vision, connection of the future to the present state, and testing the vision. It then describes one particular participative visioning methodology, which has largely been used in the public sector, the CHOICES approach. The paper then analyzes the CHOICES approach along the five key dimensions identified earlier. Gaps in the approach are identified and a revised methodology is outlined. The paper ends with a summary of results from a pilot study which tested the revised methodology.  相似文献   
118.
The spotted bat ( Euderma maculatum ) has been virtually unknown in Oregon despite the existence of potential habitat in many areas of the state. In 2002 and 2003 we searched for spotted bats along the John Day, Deschutes, and Crooked Rivers and at a remote dry canyon southeast of the city of Bend in central Oregon. The species was documented through the use of mist-nets, a bat detector, and recognition of audible spotted bat calls. Spotted bats were found at 11 locations in 6 Oregon counties. Nightly activity patterns of spotted bats were unpredictable. Spotted bats were found in 78% of search areas but on only 48% of survey nights. We observed spotted bats foraging above fields and low upland slopes adjacent to rivers and creeks and along the rims of cliffs. Estimated flying heights of spotted bats ranged from 3 m to 50 m aboveground. The species was difficult to capture and was captured only after considerable experimentation with methods and materials. Three spotted bats were captured toward the end of the project in 2003 and accounted for only 0.5% of all bats captured during the study. Although we attached radio transmitters to 2 spotted bats, we found no roost locations. We believe additional spotted bat surveys in Oregon are warranted, especially in higher-elevation habitats, but recommend that to increase their effectiveness, surveys accommodate the unique foraging behavior of the species.  相似文献   
119.
Evidence is presented indicating that Lichanura roseofusca and Lichanura trivirgata are conspecific. Data include the report of an intermediate specimen from El Arco, Baja California Norte, a site midway between the previously known peninsular ranges of the two species; captive hybridization provides additional support for the conclusion.  相似文献   
120.
Genetically effective and adult population sizes of the Apache silverspot butterfly ( Speyeria nokomiis apacheana ) from the Great Basin of North America were tracked from 1991 to 1997. Speyeria nokomis inhabits mesic habitats in the Great Basin, and its recent declines in abundance have raised concerns about its future persistence. Adult population size estimates determined from mark-recapture methods ranged from 100 to 1900 individuals from 1993 through 1997. Genetically effective population sizes based on temporal changes in allozyme frequencies ranged from less tha 1 individual to 20 individuals between 1991 and 1995. Despite these small annual estimates, it is likely that gene flow resulted in overestimation of genetically effective population sizes. These reusuts, combined with observations in the field, suggest that suitable but vacan habitat patches be maintained for potential recolonization by S. n. apacheana.  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