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   1279篇
  免费   107篇
  2023年   7篇
  2021年   35篇
  2020年   20篇
  2019年   28篇
  2018年   18篇
  2017年   32篇
  2016年   57篇
  2015年   70篇
  2014年   86篇
  2013年   93篇
  2012年   108篇
  2011年   99篇
  2010年   59篇
  2009年   53篇
  2008年   68篇
  2007年   66篇
  2006年   65篇
  2005年   55篇
  2004年   54篇
  2003年   53篇
  2002年   50篇
  2001年   15篇
  2000年   10篇
  1999年   13篇
  1998年   15篇
  1997年   6篇
  1996年   5篇
  1995年   9篇
  1994年   11篇
  1993年   8篇
  1992年   6篇
  1990年   6篇
  1989年   3篇
  1988年   4篇
  1987年   5篇
  1986年   5篇
  1985年   8篇
  1984年   4篇
  1983年   6篇
  1982年   8篇
  1981年   7篇
  1980年   4篇
  1979年   8篇
  1978年   5篇
  1977年   4篇
  1975年   6篇
  1974年   6篇
  1972年   3篇
  1971年   3篇
  1964年   4篇
排序方式: 共有1386条查询结果,搜索用时 15 毫秒
101.
The concept of ecosystem services (ESs) is a relatively new scientific methodology, offering a possible approach to the prevention of ecological problems caused by human action and to the resolution of conflicts arising from land-use questions. Since ESs were launched as a major conceptual tool in the Millennium Ecosystem Assessment (MA, 2005), interest in them has been increasing. Despite the scientific as well as economic and political enthusiasm for the ES approach, only few case studies have as yet been published. We studied the interface between ESs and landscape planning in Forest Lapland, in northern Finland. In the article, we present a methodology and various databases which can be used in applied research on ESs. We classify the ESs offered by various biotopes of the study area, and examine the effects of different land-use forms on the provision of ESs. On the basis of our results, we suggest possible uses of the European CORINE land cover database in case studies.  相似文献   
102.
Many successful invasions have taken place in systems where harmful disturbance has changed habitat conditions. However, less attention has been paid to the role of habitat restoration, which modifies habitats and thus also has the potential to facilitate invasions. We examined whether in‐stream habitat restorations have the potential to either facilitate or resist invasion by two widely introduced non‐native stream salmonids, Salvelinus fontinalis Mitchill and Oncorhynchus mykiss Walbaum, in Finland. A physical habitat simulation system was used to calculate whether the habitat area for the target species increased or decreased following the restorations. For comparison, we also reported results for four native stream fish species. The simulations showed that the restored streams provided the highest amount of usable habitat area for the native species, particularly for Salmo salar L. and Gottus gobio L. However, it was interesting to note that the restorations significantly increased habitat quality for the two non‐native species, especially at low flows. Nevertheless, the non‐native species had the lowest amount of usable habitat area overall. The modeling results indicated that not only habitat destruction but also habitat restoration could contribute to the spread of non‐native species. Fisheries and wildlife managers should be aware of the possibility, when restoring habitats in order to preserve native ecosystems, that non‐native species could manage to gain a foothold in restored habitats and use them as population sources for further spread. Knowing the widespread negative effect of non‐native species, this risk should not be underestimated.  相似文献   
103.
We introduce a Bayesian theoretical formulation of the statistical learning problem concerning the genetic structure of populations. The two key concepts in our derivation are exchangeability in its various forms and random allocation models. Implications of our results to empirical investigation of the population structure are discussed.  相似文献   
104.
BACKGROUND: In severe, medically unresponsive congenital hyperinsulinism (CHI), the histological differentiation of focal versus diffuse disease is vital, since the surgical management is completely different. Genetic analysis may help in the differential diagnosis, as focal CHI is associated with a paternal germline ABCC8 or KCNJ11 mutation and a focal loss of maternal chromosome 11p15, whereas a maternal mutation, or homozygous/compound heterozygous ABCC8 and KCNJ11 mutations predict diffuse-type disease. However, genotyping usually takes too long to be helpful in the absence of a founder mutation. METHODS: In 4 patients, a rapid genetic analysis of the ABBC8 and KCNJ11 genes was performed within 2 weeks on request prior to the decision of pancreatic surgery. RESULTS: Two patients had no mutations, rendering the genetic analysis non-informative. Peroperative multiple biopsies showed diffuse disease. One patient had a paternal KCNJ11 mutation and focal disease confirmed by positron emission tomography scan and biopsies. One patient had a de novo heterozygous ABBC8 mutation and unexplained diffuse disease confirmed by positron emission tomography scan and biopsies. CONCLUSION: A rapid analysis of the entire ABBC8 and KCNJ11 genes should not stand alone in the preoperative assessment of patients with CHI, except for the case of maternal, or homozygous/compound heterozygous disease-causing mutations.  相似文献   
105.
Many parasites with complex life cycles increase the chances of reaching a final host by adapting strategies to manipulate their intermediate host's appearance, condition or behaviour. The acanthocephalan parasite Pomphorhynchus laevis uses freshwater amphipods as intermediate hosts before reaching sexual maturity in predatory fish. We performed a series of choice experiments with infected and uninfected Gammarus pulex in order to distinguish between the effects of visual and olfactory predator cues on parasite-induced changes in host behaviour. When both visual and olfactory cues, as well as only olfactory cues were offered, infected and uninfected G. pulex showed significantly different preferences for the predator or the non-predator side. Uninfected individuals significantly avoided predator odours while infected individuals significantly preferred the side with predator odours. When only visual contact with a predator was allowed, infected and uninfected gammarids behaved similarly and had no significant preference. Thus, we believe we show for the first time that P. laevis increases its chance to reach a final host by olfactory-triggered manipulation of the anti-predator behaviour of its intermediate host.  相似文献   
106.
107.
Enolase occurs as a cytoplasmic and a surface-associated protein in bacteria. Enolases of the bacterial pathogens Streptococcus pyogenes, Streptococcus pneumoniae and Staphylococcus aureus, as well as of the commensal lactic acid bacteria, Lactobacillus crispatus and Lactobacillus johnsonii, were purified as His(6)-fusion proteins from recombinant Escherichia coli. The fusion proteins were compared for putative virulence-associated functions, i.e., binding of human plasminogen, enhancement of plasminogen activation by human plasminogen activators, as well as binding to immobilized laminin, fibronectin and collagens. The individual enolases showed varying efficiencies in these functions. In particular, highly and equally effective interactions with plasminogen and laminin were seen with lactobacillar and staphylococcal enolases.  相似文献   
108.
Laccases are able to initiate nuclear amination of p-hydroquinones with primary aromatic amines, resulting in the formation of the corresponding monoaminated and diaminated quinones. Two laccase catalyzed reactions are compared with established synthetic routes to aminoquinones, showing that formation of products from laccase catalyzed reaction is comparable with reaction using sodium iodate as oxidant. Advantages and disadvantages of laccase catalyzed amination are discussed. It is concluded that laccase catalysis is less suitable than sodium iodate oxidation for the amination of simple p-hydroquinones with simple amines.  相似文献   
109.
Reactive oxygen species, such as myeloperoxidase-derived hypochlorite, induce oxidative stress and DNA injury. The subsequent activation of the DNA-damage-poly(ADP-ribose) polymerase (PARP) pathway has been implicated in the pathogenesis of various diseases, including ischemia-reperfusion injury, circulatory shock, diabetic complications, and atherosclerosis. We investigated the effect of PARP inhibition on the impaired endothelium-dependent vasorelaxation induced by hypochlorite. In organ bath experiments for isometric tension, we investigated the endothelium-dependent and endothelium-independent vasorelaxation of isolated rat aortic rings using cumulative concentrations of acetylcholine and sodium nitro-prusside. Endothelial dysfunction was induced by exposing rings to hypochlorite (100-400 microM). In the treatment group, rings were preincubated with the PARP inhibitor INO-1001. DNA strand breaks were assessed by the TUNEL method. Immunohistochemistry was performed for 4-hydroxynonenal (a marker of lipid peroxidation), nitrotyrosine (a marker of nitrosative stress), and poly(ADP-ribose) (an enzymatic product of PARP). Exposure to hypochlorite resulted in a dose-dependent impairment of endothelium-dependent vasorelaxation of aortic rings, which was significantly improved by PARP inhibition, whereas the endothelium-independent vasorelaxation remained unaffected. In the hypochlorite groups we found increased DNA breakage, lipidperoxidation, and enhanced nitrotyrosine formation. The hypochloride-induced activation of PARP was prevented by INO-1001. Our results demonstrate that PARP activation contributes to the pathogenesis of hypochlorite-induced endothelial dysfunction, which can be prevented by PARP inhibitors.  相似文献   
110.
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