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   676篇
  免费   10篇
  国内免费   5篇
安全科学   37篇
废物处理   31篇
环保管理   104篇
综合类   101篇
基础理论   147篇
污染及防治   187篇
评价与监测   51篇
社会与环境   27篇
灾害及防治   6篇
  2023年   5篇
  2022年   4篇
  2021年   12篇
  2020年   3篇
  2019年   8篇
  2018年   12篇
  2017年   21篇
  2016年   15篇
  2015年   23篇
  2014年   25篇
  2013年   50篇
  2012年   26篇
  2011年   40篇
  2010年   33篇
  2009年   31篇
  2008年   35篇
  2007年   43篇
  2006年   40篇
  2005年   31篇
  2004年   23篇
  2003年   25篇
  2002年   24篇
  2001年   10篇
  2000年   20篇
  1999年   8篇
  1998年   5篇
  1997年   4篇
  1996年   7篇
  1995年   10篇
  1994年   9篇
  1993年   14篇
  1992年   5篇
  1991年   4篇
  1990年   4篇
  1989年   3篇
  1988年   6篇
  1987年   2篇
  1985年   2篇
  1984年   9篇
  1983年   7篇
  1982年   5篇
  1981年   2篇
  1978年   3篇
  1977年   2篇
  1961年   2篇
  1959年   3篇
  1958年   2篇
  1957年   2篇
  1953年   1篇
  1935年   1篇
排序方式: 共有691条查询结果,搜索用时 6 毫秒
441.
442.
443.
Abstract

This paper summarizes information on the spatial and temporal variability of selected air toxics pollutants collected on a national basis primarily for a period encompassing 1990–2003. Spatial information on pollutant concentrations is characterized in terms of within-city and between-city variability. Temporal information is summarized as diurnal and seasonal variability and in multiyear trends. The information on variability is presented in the framework of a larger need for systematic documentation of information on air toxics pollutants to assess progress in air pollution control programs.  相似文献   
444.
We compare existing nontimber forest product extraction systems in Petén, Guatemala, and West Kalimantan, Indonesia, to identify key ecological, socioeconomic, and political factors in the design and implementation of extractive reserves. Ecological parameters include the spatial and temporal availability of harvested products and the sustainability of harvesting practices from both a population and an ecosystem perspective. Socioeconomic and political factors include the presence or absence of well-defined resource tenure rights, physical and social infrastructure, markets, and alternative land uses. We conclude that although extractive reserves can play a significant role in preserving tropical forests as a part of a broader land-use spectrum, their effectiveness is highly dependent on prevailing local ecological, socioeconomic, and political conditions. Ultimately, extractive reserves should be regarded as one component of an overall approach to the problem of tropical deforestation.  相似文献   
445.
446.
447.
Summary The responses of individually marked pikas (Ochotona princeps) to terrestrial predators were investigated in 1980 and 1981 in the Rocky Mountains of Colorado. Pikas uttered short call vocalizations in a variety of contexts: preceding or following an individual's movement, and in response to conspecifics, other nonpredaceous mammals and predators. Adult pikas apparently discriminated contexts in which predators were present by short calling more frequently and for longer duration compared with calling in nonpredator contexts. Short calls uttered by juveniles were similar in all contexts.Adults responded differently to two types of terrestrial predators: weasels and pine martens. Pikas called less frequently in response to weasels than to martens and avoided weasels more often than martens. They delayed the initiation of calling following the first sighting of a weasel more often than to martens. Weasels were determined to be more effective predators of pikas than martens, and these asymmetries in behavior and alarm vocalizations may indicate that responses reduce an individual's risk of predaton by weasels.Both male and female pikas called in response to predators, and residents called more often than nonresidents. The possible function of predator-related vocalization in pikas is discussed. It is suggested that calls to predators may function to warn local residents, which in pikas are usually closely related.  相似文献   
448.
449.
450.
Prenatal diagnosis choices were reviewed in 473 women who presented for genetic counselling prior to 11 weeks' gestation for the indication of advanced maternal age. Group A consisted of 336 patients who were unaware of a possible association between chorionic villus sampling (CVS) and limb defects. Group B consisted of 137 patients who were provided this information. Fifty-one per cent of patients in group A and 45 per cent of patients in group B chose CVS. This difference was not significant by χ2 analysis (P = 0·7). Patterns of prenatal diagnosis procedure utilization from 1987 to 1992 revealed a significant reduction in CVS utilization accompanied by a corresponding increase in amniocentesis after the association between CVS and limb defects was publicized. Referrals for CVS counselling also significantly declined. However, acceptance rates did not change for those patients who received genetic counselling. First-trimester genetic counselling, including a discussion regarding a possible association between CVS and limb defects, helps patients make informed decisions concerning prenatal diagnosis options, and, in our population, resulted in no change in CVS acceptance rates.  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