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   2569篇
  免费   152篇
  国内免费   302篇
林业   296篇
农学   82篇
基础科学   60篇
  558篇
综合类   1253篇
农作物   14篇
水产渔业   484篇
畜牧兽医   106篇
园艺   14篇
植物保护   156篇
  2024年   12篇
  2023年   48篇
  2022年   70篇
  2021年   85篇
  2020年   78篇
  2019年   92篇
  2018年   69篇
  2017年   114篇
  2016年   156篇
  2015年   140篇
  2014年   168篇
  2013年   199篇
  2012年   291篇
  2011年   272篇
  2010年   227篇
  2009年   182篇
  2008年   153篇
  2007年   151篇
  2006年   115篇
  2005年   70篇
  2004年   57篇
  2003年   41篇
  2002年   33篇
  2001年   37篇
  2000年   19篇
  1999年   23篇
  1998年   18篇
  1997年   23篇
  1996年   16篇
  1995年   8篇
  1994年   11篇
  1993年   9篇
  1992年   14篇
  1991年   4篇
  1990年   5篇
  1989年   10篇
  1988年   1篇
  1987年   2篇
排序方式: 共有3023条查询结果,搜索用时 15 毫秒
91.
对于具有耦合关系的流域复杂系统,从演化过程、驱动机制和政策效应等方面分析生态-农业耦合效应,是实现流域生态文明的方法论。论文采用系统耦合测度模型,对洱海流域系统耦合度及动态耦合过程进行分析,揭示流域生态文明下洱海流域生态农业政策的效应。结果表明:①生态农业措施对洱海流域生态-农业系统协调耦合产生积极作用。②1994-2015年,洱海流域生态-农业系统耦合度由0.48提升到0.75,接近0.8的协调耦合阈值,表明生态-农业系统由磨合阶段进入高水平耦合阶段。③耦合态势可分为磨合消耗、快速消耗、快速协调与磨合协调四个阶段,既揭示了初期生态农业政策的短期效益,又预示出中远期流域农业面源污染治理的困境。  相似文献   
92.
2004年6月-2007年5月,采集柴窝堡湖高白鲑标本79尾,对其年龄与生长状况及高白鲑在柴窝堡湖移殖效应进行了分析。研究表明:柴窝堡湖高白鲑1龄平均逆算体长L1=11.7cm、2龄平均逆算体长L2=26.1cm;体长与体重的回归方程为W=0.013474L^309069;高白鲑在柴窝堡湖生长正常,能够形成捕捞群体,但由于柴窝堡湖现有的渔业捕捞方式,影响了高白鲑在柴窝堡湖渔业移殖效应。  相似文献   
93.
试论结合场地特征的杭州湖滨和北山路景区规划设计   总被引:2,自引:0,他引:2  
高强  柴明良 《山东林业科技》2010,40(5):73-75,37
探讨了风景园林师如何正确理解场地特征,在保留场地原有景观典型性的基础上,营造出最适宜这片土地的地域性景观。杭州是一个历史悠久的山水风景城市,西湖风景区既有得天独厚的自然景观,又有大量的历史人文景观及传统街区。杭州西湖风景区中的湖滨景区和北山路景区是探索场地特征营造的典型案例。  相似文献   
94.
95.
环太湖地区休闲农业旅游的发展探析   总被引:2,自引:0,他引:2  
齐震 《安徽农业科学》2008,36(16):6902-6903
综述了环太湖地区丰富的农村人文资源,探讨了其开发经营休闲农业的优劣势,并提出了建议对策。  相似文献   
96.
为了防止南湖水质进一步恶化,改善与提高南湖水环境质量,针对南湖水环境污染现状,提出了集中治理,分散治理及综合治理3种方案,并对这些方案进行了优化分析。  相似文献   
97.
Geographical, macro- and microhabitat distribution of and mate recognition among color morphs of a complex of Lake Victoria cichlids was studied. The distribution of most morphs was mutually exclusive but two morphs overlapped extensively and differed only in microhabitat requirements. Coloration differences between them were inherited, and wild fishes were homozygous in this respect. Positive morph assortative female mate choice was demonstrated. Over the largest part of their common geographical range intermediate phenotypes are absent and the two morphs are to be considered two biological species. However, in populations at one end of the range intermediate phenotypes with continuously varying male coloration make up the majority of the individuals. It is suggested that the status of color morphs as phenotypes of one species or as separate biological species is a function of environmental conditions that vary considerably between the localities. The data support the hypothesis that polymorphisms can be a step preceding speciation in cichlid fish.  相似文献   
98.
Abstract— The geographical and macro- and microhabitat distribution of and mate recognition among color morphs of a complex of Lake Victoria cichlids were studied. The distribution of most morphs was mutually exclusive, but two morphs overlapped extensively and differed only in microhabitat requirements. Coloration differences between them were inherited, and wild fishes were homozygous in this respect. Positive morph-assortative female mate choice was demonstrated. Over the largest part of their common geographical range, intermediate phenotypes are absent and the two morphs are to be considered two biological species. However, in populations at one end of the range, intermediate phenotypes with continuously varying male coloration make up the majority of the individuals. It is suggested that the status of color morphs as phenotypes of one species or as separate biological species is a function of environmental conditions that vary considerably between the localities. The data support the hypothesis that polymorphisms can be a step preceding speciation in cichlid fish.  相似文献   
99.
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   
100.
家鹑与野生日本鸣鹑群体微卫星DNA标记的遗传学分析   总被引:14,自引:6,他引:8  
选择10个微卫星标记位点,合成引物对微山湖野生日本鸣鹑和家鹌鹑各40只进行PCR扩增,变性聚丙烯酰胺凝胶电泳检测分型,并对其品种内和品种间的遗传变异进行群体遗传学分析,从分子水平上揭示这两个鹌鹁群体的遗传结构关系、亲缘关系和遗传分化程度,为鹌鹑的遗传资源研究提供新资料。  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