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   2815篇
  免费   194篇
  2023年   20篇
  2022年   60篇
  2021年   113篇
  2020年   84篇
  2019年   95篇
  2018年   81篇
  2017年   98篇
  2016年   151篇
  2015年   146篇
  2014年   171篇
  2013年   210篇
  2012年   222篇
  2011年   194篇
  2010年   137篇
  2009年   103篇
  2008年   142篇
  2007年   141篇
  2006年   119篇
  2005年   108篇
  2004年   95篇
  2003年   81篇
  2002年   59篇
  2001年   67篇
  2000年   48篇
  1999年   39篇
  1998年   21篇
  1997年   11篇
  1996年   14篇
  1995年   18篇
  1994年   10篇
  1993年   10篇
  1992年   15篇
  1991年   13篇
  1990年   10篇
  1989年   15篇
  1988年   5篇
  1987年   12篇
  1986年   9篇
  1985年   5篇
  1984年   7篇
  1983年   8篇
  1982年   9篇
  1981年   5篇
  1980年   3篇
  1979年   3篇
  1976年   3篇
  1973年   2篇
  1969年   2篇
  1966年   3篇
  1964年   2篇
排序方式: 共有3009条查询结果,搜索用时 15 毫秒
91.
92.
Over the last four decades the world has been losing biodiversity at an alarming rate despite the increasing number of protected areas (PAs). Certified forest management may complement the role of PAs in protecting biodiversity. Forest certification aims to promote sustainable forest management and to maintain or enhance the conservation value of certified forests. The area of forest under certified forest management has grown quickly over the past decade. Forest Stewardship Council (FSC) certification, for example, currently covers 148 million hectares, i.e., 3.7 % of the world’s forests. In spite of such increase there is, however, a dearth of information on how forest certification is related to biodiversity. In this study we assessed if FSC certification is being applied in high biodiversity areas in cork oak savannas in Portugal by comparing biodiversity values of certified and non-certified areas for birds, reptiles and amphibians. We calculated the relative species richness and irreplaceability value for each group of species in certified and non-certified areas and compared them using randomization tests. The biodiversity value of certified areas was not significantly greater than that of non-certified areas. Since FSC certification is expanding quickly in cork oak savannas it is important to consider the biodiversity value of these areas during this process. Prioritizing areas of high biodiversity value would enhance the conservation value of forest certification and facilitate integrating certification with other conservation initiatives.  相似文献   
93.
Cyanobacteria proliferation represents a problem to water treatment for human consumption supply. Species of the genus Microcystis, commonly found in continental waters, can bloom and produce microcystine and other toxic compounds associated to human and animal poisoning, depending on the nutrient levels present in the water. The use of natural products to control cyanobacteria blooms is promising since they are less harmful to the environment than compounds obtained by organic syntheses. Triterpenes, which are associated with several biological activities, have been isolated from species of the genus Maytenus (Celastraceae), commonly found in Cerrado regions of Brazil. Tingenone is a pentacyclic triterpene that has cytotoxic properties and induces growth inhibition in some microorganisms. In this study, the effects of tingenone isolated from Maytenus gonoclada at distinct concentrations (e.g., 50, 150, 500, 1,500, and 4,500 μg? L?1) on the growth rates of Microcystis novacekii was investigated. The algicide activity was verified using M. novacekii cultures growing in ASM1 culture medium at 23 °C and continuous illumination for 96 h. Growth inhibition was monitored by light microscopy and optical density (OD680 nm). The median effective concentration associated to the M. novacekii inhibition growth induced by tingenone was 12.2 μg? L?1. The result indicates that tingenone has algicide effect and can be potentially applied in water management for public supply, replacing synthetic algicides.  相似文献   
94.
Temperature is a key factor in determining the structure and performance of fungal assemblages on decomposing plant litter in streams. However, little is known of how temperature affects interspecific relationships among fungi. We compared the growth of four aquatic hyphomycetes co-occurring in temperate streams, in monocultures and all species combinations when exposed to five temperatures from 11 to 27 °C. In monocultures, maximum growth rates of Heliscus submersus, Lunulospora curvula and Varicosporium elodeae occurred at 27 °C whereas Articulospora tetracladia had the lowest growth rate. At 27 °C, the increase in species diversity had no effect on the growth of V. elodeae, increased the growth of H. submersus and L. curvula, and decreased the growth of A. tetracladia. Results suggest that within a species' optimal temperature range the growth of that species increases with higher fungal diversity, while outside this range growth decreases with diversity.  相似文献   
95.
Molecular typing is an important tool in the investigation of methicillin resistant Staphylococcus aureus (MRSA) outbreaks and in following the evolution of MRSA. The staphylococcal cassette chromosome mec (SCCmec) contains a hypervariable region with a variable number of 40 bp repeats named direct repeat units (dru). The dru region has been suggested as a supplementary typing method for MRSA and an international nomenclature exists. The purpose of this study was to investigate the diversity and variability of the dru region in a diverse collection of MRSA. We studied 302 MRSA isolates harbouring SCCmec types I to VI. The isolates represented a broad genetic background based on Staphylococcal protein A (spa) typing and multilocus sequence typing (MLST) and included 68 isolates (68 patients) from an outbreak with t024-ST8-IVa and 26 isolates from the same patient. Sequencing identified 53 dru types (dt) in 283 isolates, while eighteen isolates contained no dru repeats and one isolate resisted sequencing. The most common dru type, dt10a, was present in 53% of the sequenced isolates and was found in all SCCmec types, except type II. Seven (10%) of the 68 epidemiologically related patients had isolates with dru type variants indicating that dru typing is not useful as a first line epidemiological typing tool. However, MRSA isolates cultured from a single patient over a three year period exhibited a single dru type. The finding of dt10a in most SCCmec types suggests that dru and mecA originate from the same Staphylococcus species.  相似文献   
96.
Carvedilol has beneficial effects on cardiac function in patients with heart failure but its effect on ovariectomy-induced myocardial contractile dysfunction remains unclear. Estrogen deficiency induces myocardial contractile dysfunction and increases cardiovascular disease risk in postmenopausal women. Our aim was to investigate whether carvedilol, a beta receptor blocker, would prevent ovariectomy-induced myocardial contractile dysfunction. Female rats (8 weeks old) that underwent bilateral ovariectomy were randomly assigned to receive daily treatment with carvedilol (OVX+CAR, 20 mg/kg), placebo (OVX) and SHAM for 58 days. Left ventricle papillary muscle was mounted for isometric tension recordings. The inotropic response to Ca2+ (0.62 to 3.75 mM) and isoproterenol (Iso 10−8 to 10−2 M) were assessed. Expression of calcium handling proteins was measured by western blot analysis. Carvedilol treatment in the OVX animals: prevented weight gain and slight hypertrophy, restored the reduced positive inotropic responses to Ca2+ and isoproterenol, prevented the reduction in SERCA2a expression, abolished the increase in superoxide anion production, normalized the increase in p22phox expression, and decreased serum angiotensin converting enzyme (ACE) activity. This study demonstrated that myocardial contractile dysfunction and SERCA2a down regulation were prevented by carvedilol treatment. Superoxide anion production and NADPH oxidase seem to be involved in this response.  相似文献   
97.

Background

Most research on Ocean Acidification (OA) has largely focused on the process of calcification and the physiological trade-offs employed by calcifying organisms to support the building of calcium carbonate structures. However, there is growing evidence that OA can also impact upon other key biological processes such as survival, growth and behaviour. On wave-swept rocky shores the ability of gastropods to self-right after dislodgement, and rapidly return to normal orientation, reduces the risk of predation.

Methodology/Principal Findings

The impacts of OA on this self-righting behaviour and other important parameters such as growth, survival, shell dissolution and shell deposition in Concholepas concholepas (loco) were investigated under contrasting pCO2 levels. Although no impacts of OA on either growth or net shell calcification were found, the results did show that OA can significantly affect self-righting behaviour during the early ontogeny of this species with significantly faster righting times recorded for individuals of C. concholepas reared under increased average pCO2 concentrations (± SE) (716±12 and 1036±14 µatm CO2) compared to those reared at concentrations equivalent to those presently found in the surface ocean (388±8 µatm CO2). When loco were also exposed to the predatory crab Acanthocyclus hassleri, righting times were again increased by exposure to elevated CO2, although self-righting times were generally twice as fast as those observed in the absence of the crab.

Conclusions and Significance

These results suggest that self-righting in the early ontogeny of C. concholepas will be positively affected by pCO2 levels expected by the end of the 21st century and beginning of the next one. However, as the rate of self-righting is an adaptive trait evolved to reduce lethal predatory attacks, our result also suggest that OA may disrupt prey responses to predators in nature.  相似文献   
98.

Background

A growing literature suggests that low birth weight increases the risk of poor health outcomes in adulthood. We tested this hypothesis among young adults living in São Paulo State, Brazil.

Methods and Findings

To identify the effects of low birth weight on young adulthood outcomes, a medical assessment of 297 individuals born between 1977 and 1989 was conducted at a primary care unit in São Paulo State, Brazil. We analyzed body mass index (BMI), waist-hip ratio, blood pressure, fasting glucose and total cholesterol levels using linear and logistic regressions. Low birth was negatively associated with BMI (β = −2.0, 95% CI: −3.69, −0.27, p = 0.02), fasting glucose levels (β = −1.9, 95% CI: −3.9, −0.07, p = 0.05), waist-hip ratio (β = −0.03, 95% CI: −0.07, −0.01, p = 0.10), systolic blood pressure (β = −3.32, 95% CI: −7.60, 0.96, p = 0.12), and total cholesterol levels (β = −3.19, 95% CI: −16.43, 10.05, p = 0.636). Low birth weight was also associated with lower odds of young adults being overweight and obese, but neither association was statistically significant. Weight gain in the first 12 months of life was associated with higher adult BMI (β = 0.79, 95% CI: −0.0455, 1.623, p = 0.064) and blood pressure (β = 2.79, 95% CI: 0.22, 5.35, p = 0.034). No associations were found between low birth weight and early life (catch-up) growth.

Conclusions

Low birth weight was not associated with poor health outcomes among young adults in Brazil. These results appear inconsistent with the original Barker hypothesis, but will need to be corroborated in larger samples with longer follow-ups to allow a more general evaluation of the validity of the hypothesis in low and middle income countries.  相似文献   
99.
We have constructed a set of plasmids that allow efficient expression of both N- and C-terminal fusions of proteins of interest to fluorescent proteins mCherry, Citrine, CFP and GFP in the Gram-positive pathogen Streptococcus pneumoniae. In order to improve expression of the fluorescent fusions to levels that allow their detection by fluorescence microscopy, we have introduced a 10 amino acid tag, named i-tag, at the N-terminal end of the fluorescent proteins. This caused increased expression due to improved translation efficiency and did not interfere with the protein localization in pneumococcal bacteria. Localizing fluorescent derivatives of FtsZ, Wzd and Wze in dividing bacteria validated the developed tools. The availability of the new plasmids described in this work should greatly facilitate studies of protein localization in an important clinical pathogen.  相似文献   
100.
The pinewood nematode, Bursaphelenchus xylophilus, is one of the greatest threats to coniferous forests worldwide, causing severe ecological damage and economic loss. The biology of B. xylophilus is similar to that of its closest relative, B. mucronatus, as both species share food resources and insect vectors, and have very similar morphological characteristics, although little pathogenicity to conifers has been associated with B. mucronatus. Using both nuclear and mitochondrial DNA markers, we show that B. xylophilus and B. mucronatus form distinct phylogenetic groups with contrasting phylogeographic patterns. B. xylophilus presents lower levels of intraspecific diversity than B. mucronatus, as expected for a species that evolved relatively recently through geographical or reproductive isolation. Genetic diversity was particularly low in recently colonised areas, such as in southwestern Europe. By contrast, B. mucronatus displays high levels of genetic diversity and two well-differentiated clades in both mitochondrial and nuclear DNA phylogenies. The lack of correlation between genetic and geographic distances in B. mucronatus suggests intense gene flow among distant regions, a phenomenon that may have remained unnoticed due to the reduced pathogenicity of the species. Overall, our findings suggest that B. xylophilus and B. mucronatus have different demographic histories despite their morphological resemblance and ecological overlap. These results suggest that Bursaphelenchus species are a valuable model for understanding the dispersion of invasive species and the risks posed to native biodiversity and ecosystems.  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