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   7309篇
  免费   865篇
  国内免费   2篇
  2021年   85篇
  2020年   60篇
  2019年   72篇
  2018年   68篇
  2017年   87篇
  2016年   126篇
  2015年   210篇
  2014年   253篇
  2013年   301篇
  2012年   386篇
  2011年   367篇
  2010年   228篇
  2009年   209篇
  2008年   295篇
  2007年   367篇
  2006年   305篇
  2005年   260篇
  2004年   281篇
  2003年   251篇
  2002年   229篇
  2001年   200篇
  2000年   177篇
  1999年   134篇
  1998年   105篇
  1997年   86篇
  1996年   95篇
  1995年   97篇
  1994年   95篇
  1993年   80篇
  1992年   149篇
  1991年   101篇
  1990年   132篇
  1989年   117篇
  1988年   126篇
  1987年   111篇
  1986年   88篇
  1985年   90篇
  1984年   112篇
  1983年   84篇
  1982年   67篇
  1981年   85篇
  1980年   67篇
  1979年   84篇
  1978年   61篇
  1977年   54篇
  1976年   60篇
  1975年   53篇
  1974年   78篇
  1972年   51篇
  1968年   52篇
排序方式: 共有8176条查询结果,搜索用时 146 毫秒
31.
32.
33.
34.
Small Wind Turbines (SWTs) are a growing micro-generation industry with over 870,000 installed units worldwide. No research has focussed on public attitudes towards SWTs, despite evidence the perception of such attitudes are key to planning outcomes and can be a barrier to installations. Here we present the results of a UK wide mail survey investigating public attitudes towards SWTs. Just over half of our respondents, who were predominantly older, white males, felt that SWTs were acceptable across a range of settings, with those on road signs being most accepted and least accepted in hedgerows and gardens. Concern about climate change positively influenced how respondents felt about SWTs. Respondent comments highlight visual impacts and perceptions of the efficiency of this technology are particularly important to this sector of the UK public. Taking this into careful consideration, alongside avoiding locating SWTs in contentious settings such as hedgerows and gardens where possible, may help to minimise public opposition to proposed installations.  相似文献   
35.
Both the size of founding populations (propagule size) and environmental suitability are known to influence whether a species newly introduced to a location will establish a self-sustaining population. However, these two factors do not operate independently: it is the interaction between propagule size and environmental suitability that determines the probability an introduced population will establish. Here I use the example of dung beetle introductions to Australia to illustrate the importance of this interaction. I first describe equations that model establishment success jointly as a function of propagule size and environmental suitability. I then show how these equations provide insight into the different outcomes observed in two dung beetle species widely introduced to Australia. In one species, variation in propagule size had relatively little influence on establishment success due to large variation in environmental suitability, leading to an essentially bimodal outcome: sites were either very suitable for establishment and introductions succeeded, or sites were unsuitable and introductions failed regardless of propagule size. For the second species, there was much less variation among locations in environmental suitability, leading to propagule size having a strong influence on establishment success. These examples highlight how the interplay between environmental suitability and founding population size is central to determining the probability an introduced species will establish.  相似文献   
36.
37.
During years with good winter rainfall, ephemeral plants can contribute considerably to primary production in the upper Sonoran Desert. This study was designed to compare ephemeral community productivity and production efficiency within the different microhabitats created by trees, shrubs and open spaces in a Sonoran Desert ecosystem. Ephemerals were periodically harvested in the different habitats and dried for biomass and caloric determinations. The caloric data were compared to total solar input into the different microhabitats in order to determine production efficiencies of each stand. Productivity rates were also determined for the total community for the sample period, winters of 1972-74. Both productivity and production efficiency were highest under the shade of Cercidium. The highest production efficiency occurring in the shaded sites was 5.03% which approaches the theoretical maximum photosynthetic efficiency level. The production efficiency of the entire ephemeral community for the whole growing season was only 0.17% during an optimal year, demonstrating the influence of the environment in the interspaces on productivity and solar conversion in the desert.  相似文献   
38.
39.
40.
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