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   102487篇
  免费   1053篇
  国内免费   900篇
  104440篇
  2024年   29篇
  2023年   35篇
  2022年   131篇
  2021年   213篇
  2020年   131篇
  2019年   180篇
  2018年   12047篇
  2017年   10818篇
  2016年   7754篇
  2015年   1157篇
  2014年   930篇
  2013年   959篇
  2012年   5083篇
  2011年   13547篇
  2010年   12472篇
  2009年   8625篇
  2008年   10366篇
  2007年   11827篇
  2006年   729篇
  2005年   959篇
  2004年   1353篇
  2003年   1353篇
  2002年   1063篇
  2001年   449篇
  2000年   344篇
  1999年   157篇
  1998年   72篇
  1997年   66篇
  1996年   37篇
  1995年   46篇
  1994年   46篇
  1993年   60篇
  1992年   77篇
  1991年   88篇
  1990年   69篇
  1989年   49篇
  1988年   52篇
  1987年   46篇
  1986年   30篇
  1985年   39篇
  1984年   34篇
  1983年   45篇
  1982年   22篇
  1981年   23篇
  1979年   21篇
  1975年   20篇
  1972年   259篇
  1971年   297篇
  1970年   22篇
  1962年   26篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
41.
Recent technological advances in Global Positioning System (GPS) telemetry have allowed the production of lightweight devices suitable for use on small mammals. We evaluated the use of GPS bugs on the European hedgehog (Erinaceus europaeus) in a series of static and field tests. Static tests were conducted in five different rural habitats, affording different degrees of obstruction to satellites. GPS bug performance was good in all habitats (fix success rate (FSR): median ≥?66.8 %; location error (LE): mean ≤?13.5 m), except woodland (FSR?=?37.7 %; LE?=?15.6 m), with performance highest in the open pasture habitat (FSR?=?100 %; LE?=?6.4 m). Field tests revealed mean FSR was high (84.6 %), with the use of nesting habitats, the probable cause of most failed fixes. Despite being more expensive, GPS bugs require less survey effort and substantially lower labour costs with unlimited longevity permitting re-use in multiple seasons. We recommend the use of GPS bugs in the spatial ecological study of any small mammal in a rural environment, providing accurate and unbiased movement data. Further performance testing is recommended before deployment on species inhabiting forested habitats where reduced FSR and high LE support the alternative use of very high frequency tracking.  相似文献   
42.
Semiochemicals are natural products occurring in plants, bacteria or animals which function as carriers of a special message. Depending on the mode of function of the semiochemicals, they are divided into pheromones that trigger a response in members of the same species and allelochemicals (kairomones, allomones) that act between individuals of different species. Semiochemicals are very important compounds that influence the behavior of plants and animals and their adaption to a changing environment. As their importance for plants, animals and the ecological system itself is huge, the synthetic access to these chemicals, their precursors and derivatives is of high interest. Beyond novel strategies for the construction of semiochemical skeletons, combinatorial methods have been implemented to synthesize medium-sized and large-sized libraries that enable diverse modifications of the active compounds. These combinatorial approaches allow the screening for more active compounds and they elucidate the mode of action of the semiochemical or of the biological target. This review summarizes the state of the art procedures for the synthesis of important skeletons appearing in semiochemicals and gives special synthetic procedures for selected examples if the procedure is suitable for a general transfer to the synthesis of derivatives. The synthetic examples are given in the context of known active phytochemicals and their function that allows an evaluation of the given procedures with respect to the fulfillment of the common structural requirements (the structural diversity and flexibility) and the importance for the regulation of biological systems. Parts of this review were given in a lecture at the BioCom 12 in Cadiz, 2012.  相似文献   
43.
In this study, nuclear magnetic resonance techniques coupled with multivariate data analysis were used for the metabolic profiling of mycelia and fruiting bodies of the entomopathogenic fungi, Cordyceps bassiana according to developmental stages. A direct extraction method using two deuterated solvents of D2O and CDCl3 was used to investigate the relative levels of identified metabolites in each extraction condition in the mycelium and fruiting body formation stages. There was a clear separation among mycelia and fruiting bodies with various developmental stages in partial least-squares discriminant analysis (PLS-DA) derived score plots. During the transition from mycelia to fruiting bodies, the major metabolic change observed was the conversion of glucose to mannitol, and beauvericin to phenylalanine and 1-hydroxyisovaleric acid. In the developmental stages of fruiting bodies studied, there was a clear separation between stage 3 and the other stages in PLS-DA derived score plots. Nineteen compounds including 13 amino acids, 2 nucleosides, 3 organic acids, and glucose showed the highest levels in stage 3 fruiting bodies. The flavonoid content in the fruiting bodies showed similar levels during stages 1, 2, and 3, whereas the level at stage 4 was significantly decreased compared to the other stages. Results suggest that the fruiting body of C. bassiana is richer in natural resources at stage 3 compared to the other fruiting body stages due to its high abundance of compounds including total flavonoids. The metabolome information acquired in this study can be useful criteria for the quality control of commercial use of C. bassiana.  相似文献   
44.
We examined the structure of the arthropod community among deciduous and coniferous dead woods along the process of wood decay. We collected dead wood‐dwelling arthropods from April 2010 to October 2011 by using a vacuum aspirator and an electric chain saw in three areas (Mt. Woonak, Mt. Wolchul, Mt. Jingang) in Korea. We identified them to species levels and classified them into functional groups. We collected 8792 arthropods (5 classes, 20 orders, 58 families, and 93 species). The species richness and abundance of arthropods increased with the progress of decay in dead woods. The evenness index seemed to be shown at a lower value at late decay stage than at early‐ and mid‐decay stages. The diversity index (H′) in conifers was lower than that in deciduous dead woods at the early decay stage but this situation was reversed at the late decay stage. Arthropod communities of functional groups, except the xylophagous insects, did not differ in the variables, but the proportion of xylophagous insects increased as the decay stages progressed. The abundance of arthropods and xylophagous was statistically significantly different. The patterns generated by non‐metric multidimensional scaling in the overall arthropod community composition revealed that the species composition between study areas were significantly different. We confirmed that dead woods play very important roles as arthropods' habitats. Thus, we suggest that the role of dead woods should be emphasized in the management of forest ecosystems.  相似文献   
45.
46.
47.
ObjectiveThe objective of this study was to determine whether dietary black soldier fly (Hermetia illucens, HI) larvae oil (HILO) could serve as an alternative fat source to soybean oil (SBO) in laying hen diets.MethodsWe randomly assigned 25-week-old Hy-line Brown laying hens (n = 144) to receive (n = 6 hens/group; eight replicates) a control or an experimental diet in which SBO was replaced with 50% (50HILO) or 100% HILO (100HILO).ResultsDietary HILO did not negatively affect body weight or productive performance during the study. The eggs also had similar quality parameters, proximate composition, and cholesterol levels. However, the yolk color index was significantly higher (p<0.01) in the 100HILO than in the other groups. Dietary HILO significantly altered the composition of fatty acids (FAs) in abdominal fat and eggs. Total saturated fatty acids (SFAs) and total polyunsaturated FAs (PUFAs) were significantly increased and decreased in the 50HILO and 100HILO groups, respectively, compared with those in the control group (p<0.001 and p<0.0001, respectively). Specifically, the medium-chain FAs lauric and myristic acids were remarkably increased in the abdominal fat of laying hens fed HILO (p<0.0001), whereas only myristic acid increased in eggs (p<0.0001). Undesirable heavy metal (aluminum, fluorine, arsenic, lead, mercury, and cadmium) concentrations were below permissible limits in eggs.ConclusionWe considered that HILO could be an alternative dietary fat to SBO for laying hens with maintained productive performance and good egg quality.  相似文献   
48.
49.
50.
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