全文获取类型
收费全文 | 1581篇 |
免费 | 188篇 |
出版年
2021年 | 20篇 |
2019年 | 19篇 |
2018年 | 20篇 |
2017年 | 21篇 |
2016年 | 15篇 |
2015年 | 32篇 |
2014年 | 43篇 |
2013年 | 62篇 |
2012年 | 67篇 |
2011年 | 65篇 |
2010年 | 44篇 |
2009年 | 37篇 |
2008年 | 44篇 |
2007年 | 49篇 |
2006年 | 52篇 |
2005年 | 41篇 |
2004年 | 55篇 |
2003年 | 48篇 |
2002年 | 54篇 |
2001年 | 55篇 |
2000年 | 43篇 |
1999年 | 55篇 |
1998年 | 31篇 |
1997年 | 17篇 |
1996年 | 23篇 |
1995年 | 19篇 |
1994年 | 20篇 |
1993年 | 25篇 |
1992年 | 47篇 |
1991年 | 38篇 |
1990年 | 39篇 |
1989年 | 39篇 |
1988年 | 29篇 |
1987年 | 27篇 |
1986年 | 29篇 |
1985年 | 25篇 |
1984年 | 27篇 |
1983年 | 27篇 |
1982年 | 17篇 |
1981年 | 18篇 |
1979年 | 23篇 |
1978年 | 16篇 |
1977年 | 13篇 |
1975年 | 16篇 |
1973年 | 19篇 |
1972年 | 14篇 |
1971年 | 13篇 |
1970年 | 17篇 |
1969年 | 14篇 |
1966年 | 15篇 |
排序方式: 共有1769条查询结果,搜索用时 31 毫秒
41.
42.
43.
Lynn V. Dicks Hugh L. Wright Joscelyne E. Ashpole James Hutchison Caitlin G. McCormack Barbara Livoreil Klaus Peter Zulka William J. Sutherland 《Biodiversity and Conservation》2016,25(7):1383-1399
This paper documents an exercise to synthesize and assess the best available scientific knowledge on the effectiveness of different farm practices at enhancing natural pest regulation in agriculture. It demonstrates a novel combination of three approaches to evidence synthesis—systematic literature search, collated synopsis and evidence assessment using an expert panel. These approaches follow a logical sequence moving from a large volume of disparate evidence to a simple, easily understandable answer for use in policy or practice. The example of natural pest regulation in agriculture was selected as a case study within two independent science-policy interface projects, one European and one British. A third funder, a private business, supported the final stage to translate the synthesized findings into a useful, simplified output for agronomists. As a whole, the case study showcases how a network of scientific knowledge holders and knowledge users can work together to improve the use of science in policy and practice. The process identified five practices with good evidence of a benefit to natural pest regulation, with the most beneficial being ‘Combine trap and repellent crops in a push–pull system’. It highlights knowledge gaps, or potential research priorities, by showing practices considered important by stakeholders for which there is not enough evidence to make an assessment of effects on natural pest regulation, including ‘Alter the timing of pesticide application.’ Finally, the process identifies several important practices where the volume of evidence of effects on natural pest regulation was too large (>300 experimental studies) to be summarised with the resources available, and for which focused systematic reviews may be the best approach. These very well studied practices include ‘Reduce tillage’ and ‘Plant more than one crop per field’. 相似文献
44.
Newly hatched Laspeyresia pomonella larvae were attracted to only two of six synthetic stereo-isomers of the acyclic sesquiterpene farnesene. These were (E,E)-α-farnesene and (Z,E)-α-farnesene which together comprise a natural attractant for the insect. Two other α-isomers and two β-isomers had no influence on larval behaviour. The activity of isomers is correlated with their molecular shape. 相似文献
45.
C. Simone Sutherland Joshua Yukich Ron Goeree Fabrizio Tediosi 《PLoS neglected tropical diseases》2015,9(2)
Human African trypanosomiasis (HAT) is a disease caused by infection with the parasite Trypanosoma brucei gambiense or T. b. rhodesiense. It is transmitted to humans via the tsetse fly. Approximately 70 million people worldwide were at risk of infection in 1995, and approximately 20,000 people across Africa are infected with HAT. The objective of this review was to identify existing economic evaluations in order to summarise cost-effective interventions to reduce, control, or eliminate the burden of HAT. The studies included in the review were compared and critically appraised in order to determine if there were existing standardised methods that could be used for economic evaluation of HAT interventions or if innovative methodological approaches are warranted. A search strategy was developed using keywords and was implemented in January 2014 in several databases. The search returned a total of 2,283 articles. After two levels of screening, a total of seven economic evaluations were included and underwent critical appraisal using the Scottish Intercollegiate Guidelines Network (SIGN) Methodology Checklist 6: Economic Evaluations. Results from the existing studies focused on the cost-effectiveness of interventions for the control and reduction of disease transmission. Modelling was a common method to forecast long-term results, and publications focused on interventions by category, such as case detection, diagnostics, drug treatments, and vector control. Most interventions were considered cost-effective based on the thresholds described; however, the current treatment, nifurtomix-eflornithine combination therapy (NECT), has not been evaluated for cost-effectiveness, and considerations for cost-effective strategies for elimination have yet to be completed. Overall, the current evidence highlights the main components that play a role in control; however, economic evaluations of HAT elimination strategies are needed to assist national decision makers, stakeholders, and key funders. These analyses would be of use, as HAT is currently being prioritized as a neglected tropical disease (NTD) to reach elimination by 2020. 相似文献
46.
47.
48.
David Finlay Satish Patel Lorna M Dickson Natalia Shpiro Rodolfo Marquez Chris J Rhodes Calum Sutherland 《BMC molecular biology》2004,5(1):15-13
Background
Hepatic expression of several gene products involved in glucose metabolism, including phosphoenolpyruvate carboxykinase (PEPCK), glucose-6-phosphatase (G6Pase) and insulin-like growth factor binding protein-1 (IGFBP-1), is rapidly and completely inhibited by insulin. This inhibition is mediated through the regulation of a DNA element present in each of these gene promoters, that we call the Thymine-rich Insulin Response Element (TIRE). The insulin signalling pathway that results in the inhibition of these gene promoters requires the activation of phosphatidylinositol 3-kinase (PI 3-kinase). However, the molecules that connect PI 3-kinase to these gene promoters are not yet fully defined. Glycogen Synthase Kinase 3 (GSK-3) is inhibited following activation of PI 3-kinase. We have shown previously that inhibitors of GSK-3 reduce the activity of two TIRE-containing gene promoters (PEPCK and G6Pase), whose products are required for gluconeogenesis. 相似文献49.
Aim: To study genotypic diversity of isolates of Brochothrix thermosphacta recovered from meat, poultry and fish. Methods and Results: A total of 27 bacteria isolated from 19 samples of meat, poultry and fish were identified phenotypically and genotypically using PCR amplification of 16S‐23S rDNA intergenic transcribed spacer (ITS‐PCR), repetitive sequence‐based PCR (rep‐PCR) and 16S rDNA sequencing. Using ITS‐PCR, all bacteria showed the same DNA profile as the reference strains of Br. thermosphacta, allowing typing of the isolates at species level. Using 16S rDNA sequencing, all isolates were identified, at genus and species level, as Br. thermosphacta. Identification as Br. campestris was observed with a lower, but very close, level of similarity. Rep‐PCR was more discriminatory than ITS‐PCR and allowed differentiation of four subgroups among the isolates. Conclusion: Minor genotypic differences among Br. thermosphacta strains from meat, poultry and fish were observed. Significance and Impact of the Study: A rudimentary exploration of genotypic differences of Br. thermosphacta from meat, poultry and fish resulted in preliminary confirmation of the suitability of ITS‐PCR for typing Br. thermosphacta and confirmed the value of rep‐PCR fingerprinting to discriminate between Br. thermosphacta strains. 相似文献
50.
Brooke E. Crowley Laurie R. Godfrey Richard J. Bankoff George H. Perry Brendan J. Culleton Douglas J. Kennett Michael R. Sutherland Karen E. Samonds David A. Burney 《Ecography》2017,40(8):901-912
Researchers are divided about the relative importance of people versus climate in triggering the Late Holocene extinctions of the endemic large‐bodied fauna on the island of Madagascar. Specifically, a dramatic and synchronous decline in arboreal pollen and increase in grass pollen ca 1000 yr ago has been alternatively interpreted as evidence for aridification, increased human activity, or both. As aridification and anthropogenic deforestation can have similar effects on vegetation, resolving which of these factors (if either) led to the demise of the megafauna on Madagascar has remained a challenge. We use stable nitrogen isotope (δ15N) values from radiocarbon‐dated subfossil vertebrates to disentangle the relative importance of natural and human‐induced changes. If increasing aridity were responsible for megafaunal decline, then we would expect an island‐wide increase in δ15N values culminating in the highest values at the time of proposed maximum drought at ca 1000 yr ago. Alternatively, if climate were relatively stable and anthropogenic habitat alteration explains the palynological signal, then we would anticipate little or no change in habitat moisture, and no systematic, directional change in δ15N values over time. After accounting for the confounding influences of diet, geographic region, and coastal proximity, we find no change in δ15N values over the past 10 000 yr, and no support for a period of marked, geographically widespread aridification culminating 900–950 yr ago. Instead, increases in grasses at around that time may signal a transition in human land use to a more dedicated agro‐pastoralist lifestyle, when megafaunal populations were already in decline. Land use changes ca 1000 yr ago would have simply accelerated the inevitable loss of Madagascar's megafauna. 相似文献