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   2063篇
  免费   178篇
  国内免费   97篇
林业   151篇
农学   104篇
基础科学   71篇
  287篇
综合类   800篇
农作物   78篇
水产渔业   95篇
畜牧兽医   571篇
园艺   44篇
植物保护   137篇
  2024年   13篇
  2023年   56篇
  2022年   83篇
  2021年   88篇
  2020年   70篇
  2019年   96篇
  2018年   57篇
  2017年   75篇
  2016年   95篇
  2015年   69篇
  2014年   112篇
  2013年   132篇
  2012年   152篇
  2011年   137篇
  2010年   112篇
  2009年   136篇
  2008年   106篇
  2007年   150篇
  2006年   118篇
  2005年   89篇
  2004年   81篇
  2003年   61篇
  2002年   39篇
  2001年   47篇
  2000年   28篇
  1999年   20篇
  1998年   23篇
  1997年   18篇
  1996年   11篇
  1995年   6篇
  1994年   13篇
  1993年   11篇
  1992年   2篇
  1991年   11篇
  1990年   4篇
  1989年   2篇
  1988年   2篇
  1987年   4篇
  1985年   1篇
  1984年   2篇
  1983年   1篇
  1982年   1篇
  1981年   1篇
  1979年   1篇
  1975年   1篇
  1974年   1篇
排序方式: 共有2338条查询结果,搜索用时 31 毫秒
131.
Temporal variation in Lake Balaton and its fish populations   总被引:4,自引:1,他引:3  
Abstract— Dramatic changes have occurred in Lake Balaton in the last 40–45 years. Variations in water quality have resulted in a decrease in fish and invertebrate diversity. Fish assemblages have changed in response to human-induced eutrophication, environmental perturbations, introduction of exotic fish species, over-fishing and species-habitat interactions. Fish biomass and productivity first increased during the early 1970s with enhanced primary production, then decreased with the reduction of exploitation since the late 1970s and induced intra- and interspecific competition. Previous stable populations began to oscillate, and the majority of stocks became fragile by the mid-1980s. Extinction of native fish and invasion of new alien species alternate. Alien species have transformed the food webs, causing long-lasting ecological changes. Reduced and non-selective exploitation has resulted in higher fish density and lower production rates. Recent studies have revealed differences in the carrying capacity of fish between the littoral zone and the open water areas.  相似文献   
132.
建立山羊亚硝酸盐中毒模型,用棉签分别蘸取口液、尿液立即滴加偶氮试剂均显粉红色,中毒后10~20min显浅粉红色;60min显深粉红色,随中毒症状加重,棉签显色相应加深,甚致变为棕红色。同时棉签显色加深和血液褐变程度加重与MHb/Hb比值升高同步。此种诊断方法是诊断亚硝酸盐中毒的动物可靠、灵敏和快速的诊断方法。  相似文献   
133.
Heartworm disease caused by Dirofilaria immitis affects canine and feline hosts, with infections occasionally being reported in humans. Studies have shown that both dirofilarial antigens and those derived from its bacterial endosymbiont Wolbachia, interact with the host organism during canine, feline and human infections and participate in the development of the pathology and in the regulation of the host’s immune response. Both innate and acquired immune responses are observed and the development of the acquired response may depend on the host and, or on its parasitological status. This review aims at illustrating current research on the role of both D. immitis and Wolbachia, in the immunology and immunopathology of dirofilariosis.  相似文献   
134.
Summary While the issue of invasive alien species has important biological components, economic factors such as global trade deserve much greater attention for several reasons. First, virtually all of our planet’s ecosystems have a strong and increasing anthropogenic component that is being fed by increasing globalisation of the economy. Second, people are designing the kinds of ecosystems they find productive or congenial, incorporating species from all parts of the world through quicker and more efficient means of transportation. And third, growing travel and trade, coupled with weakening customs and quarantine controls, mean that people are both intentionally and inadvertently introducing alien species that may become invasive. The great increase in the introduction of alien species that people are importing for economic, aesthetic, accidental, or even psychological reasons is leading to more species invading native ecosystems, with disastrous results: they become invasive alien species (IAS) that have significant deleterious effects on both ecosystems and economies. This paper examines some of the important non-biological dimensions of the IAS problem, including historical, economic, cultural, linguistic, health, sociological, management, legal, military, ethical, and political dimensions. These are addressed in terms of the causes, consequences, and responses to the problem of IAS. These dimensions of IAS are fundamental, and successfully addressing the problem will call for greater collaboration between different economic sectors and among a wide range of disciplines. The Convention on Biological Diversity, the negotiations of the World Trade Organisation, and many other international agreements offer important opportunities for addressing the complex global problems of IAS through improved international cooperation.  相似文献   
135.
The maintenance and gain of enterprise value have something to do with human capital. Human capital impels the realization of enterprise value through their inter-relationship, depending on given organization environment. This article systemically analyses tier value chain, paratactic value chain and network value chain, which are formed in the course of the creating value by human capital, then educes the adjustors of the management and forms according to the environment.  相似文献   
136.
As the development of knowledge economic , human resource has been the core of the world economic .So searching the reasonable evaluating and managing methods becomes one of the hot issues which have been concerned by people . Traditional evaluating methods of human resource are simple and direct , but they greatly depend on people's subjective judgement and experience . In order to overcome the default , this paper puts forward the method of synthetically evaluation on rough sets, which takes advantage of the rough sets theory, the concept of information entropy and relative analysis to filter the original indexes, then to standardize the importance degree of every index and get the weight of every index. Synthetically valuation model and corresponding arithmetic are summed up. Finally the sample illuminates the validity and feasibility of this method.  相似文献   
137.
An experiment is conducted on the impact of green roof on Indoor Human Thermal Comfort. The result shows that the temperatures of the building walls and indoor air fall for certain degrees, and that is good. But unfortunately, the relative humidity of the indoor air rises. Totally speaking, the whole effect is changed from feeling warm to basically acceptable. When the requirement of the indoor environment is high, it is impossible to be satisfied by exploiting green roof solely, other methods, such as using night ventilation, electric fans or air-conditioners, should be taken at the same time.  相似文献   
138.
人肥胖基因在大肠杆菌中的表达及生物学活性检测   总被引:3,自引:0,他引:3  
肥胖基因(ob)编码的瘦蛋白可以反映机体脂肪含量信息,在发育、繁殖、造血及体重调节等方面具有重要功能。利用PCR方法扩增去除信号肽的人肥胖基因cDNA序列,并将位于基因5!端的CCC转变为大肠杆菌(Escherichiacoli)常用密码子CCG,扩增片段经测序证实后,克隆到原核表达载体pET5a,重组质粒转化大肠杆菌BL21(DE3),经0.1mmol/LIPTG诱导,获得分子量约为16kD的特异蛋白表达带,表达量最高占菌体蛋白总含量的55%。重组蛋白纯化后,注射昆明白小白鼠,小白鼠体重明显下降,说明纯化的重组蛋白具有生物学活性。  相似文献   
139.
在GIS支持下,利用云南小江流域两期土地利用图形数据叠加分析,得到土地利用变化图和土地利用转移矩阵。结果发现,1982—2003年全流域610.12km2的土地利用面积发生变化,占流域总面积的59%。土地利用的类型主要是由未利用地向耕地和林地以及林地向耕地的转化,流域综合土地利用变化动态度达2.95%。土地利用空间格局的基本构型由以大斑块为主体,散布型斑块镶嵌其中转变为以中等斑块为主体,散布型斑块镶嵌其中的构型;区域土地利用的斑块数由1014个增加到1889个,平均斑块面积由1.02km2/个减少到0.55km2/个;流域土地利用空间格局指数中多样性指数增长4.14%,均匀度指数增长4.14%,破碎度指数增长84.6%,优势度指数降低1.6%;未利用地、林地的分离度分别增加114.58%和69.64%,而耕地的分离度减少45.86%;流域耕地、建设用地重心向东北方向偏移,而未利用地、林地重心向南东、南西方向偏移。引入社会、经济数据分析,人口增长、经济发展是流域耕地增加进而导致系列土地利用变化的主要人文驱动力,而生态环境保护等宏观政策是流域林地变化的主要驱动力。  相似文献   
140.
有机农业的环境效益评估   总被引:9,自引:0,他引:9  
常规农业所带来的环境破坏影响越来越被人们所认识。有机农业作为一种新型农业发展模式 ,以保持农业可持续发展和环境影响最小化为其基本原则 ,近 10 a在全球得到迅速发展和推广。与常规农业相比 ,有机农业的环境破坏风险较低。介绍了有机农业的发展现状 ,并就有机农业可能影响到的水环境、土壤环境、大气环境、生物多样性以及人类健康等方面进行探讨 ,以全面评估有机农业的环境效益  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