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   41685篇
  免费   3210篇
  国内免费   7篇
  2023年   308篇
  2022年   431篇
  2021年   898篇
  2020年   695篇
  2019年   867篇
  2018年   1251篇
  2017年   1208篇
  2016年   1496篇
  2015年   1759篇
  2014年   1898篇
  2013年   2713篇
  2012年   3389篇
  2011年   3363篇
  2010年   1991篇
  2009年   1626篇
  2008年   2489篇
  2007年   2431篇
  2006年   2323篇
  2005年   1990篇
  2004年   1967篇
  2003年   1806篇
  2002年   1630篇
  2001年   651篇
  2000年   695篇
  1999年   540篇
  1998年   380篇
  1997年   268篇
  1996年   254篇
  1995年   261篇
  1994年   183篇
  1993年   197篇
  1992年   288篇
  1991年   234篇
  1990年   206篇
  1989年   178篇
  1988年   147篇
  1987年   169篇
  1986年   142篇
  1985年   119篇
  1984年   166篇
  1983年   107篇
  1982年   126篇
  1981年   107篇
  1980年   101篇
  1979年   91篇
  1978年   83篇
  1977年   66篇
  1976年   60篇
  1975年   68篇
  1974年   80篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Endangered dholes (Cuon alpinus) are restricted to small and declining populations in Southeast Asia, and little is known about how their ecology differs within the region. We used DNA-confirmed scats and prey surveys to determine the seasonal diet and prey selection of dholes in 2 different landscapes that dominate Southeast Asia: closed evergreen forests in hilly terrain in northern Laos, and open deciduous forests in relatively flat terrain in eastern Cambodia. On both sites, muntjac (Muntiacus spp.; 20–28 kg) was the dominant prey item and was selectively consumed over other ungulates in all seasons. Our findings differ from previous conclusions, based largely on studies from India, that the preferred prey weight range of dholes was either 40–60 kg or 130–190 kg. Other important prey were sambar (Rusa unicolor) in Laos, and wild pig (Sus scrofa) and banteng (Bos javanicus) in Cambodia. Seasonal differences in overall diet occurred in Laos, but not Cambodia, primarily because of an increase in livestock consumption. The mean number of dhole scats in group defecation sites was higher in Cambodia (5.9 ± 0.5 [SE]) than Laos (2.4 ± 0.2), suggesting pack sizes were larger in Cambodia. Our results suggest that regardless of land cover type, prey diversity, or pack size, the management of muntjac will be important for conserving dhole populations in Southeast Asia. In Laos, we recommend that local villagers remove livestock from the protected area during the hot-dry season to reduce livestock predation by dholes. © 2020 The Authors. The Journal of Wildlife Management published by Wiley Periodicals LLC on behalf of The Wildlife Society.  相似文献   
992.
993.
Two new species of Ophiorrhiza collected from Mt Hamiguitan Range Wildlife Sanctuary in Davao Oriental, Philippines, are herein described and illustrated. Ophiorrhiza erythropilosa is the third Philippine species possessing involucral bracts and is further characterized by its predominantly villose stems that appear red due to strikingly red-violet trichomes, subpersistent bifid stipules, linear involucral bracts without prominent midrib, and 4 mm long urceolate corolla that is villose outside and lightly puberulous inside. Ophiorrhiza hamiguitanensis is characterized by its coriaceous, lanceolate leaves with attenuate apex and base, brochidodromous venation, subpersistent bilobed stipules with slightly recurved acute tips, presence of 1.5–2.5 mm long linear ensiform bracts, heterostylous flowers, clavate calyces and broadly obcordate capsules.  相似文献   
994.
995.
996.
Biological Trace Element Research - We report cadmium (Cd) concentrations in muscle, liver, and blood of striped marlin (Kajikia audax) and blue marlin (Makaira nigricans), caught in the southwest...  相似文献   
997.
Biological Trace Element Research - The aim of the present study was to determine changes occurring in the erythrocyte concentrations of arsenic (As), cadmium (Cd) and lead (Pb) in highly trained...  相似文献   
998.
999.
Biological Trace Element Research - Metabolic diseases or injuries damage bone structure and self-renewal capacity. Trace elements and hydroxyapatite crystals are important in the development of...  相似文献   
1000.
Clinical prediction models play a key role in risk stratification, therapy assignment and many other fields of medical decision making. Before they can enter clinical practice, their usefulness has to be demonstrated using systematic validation. Methods to assess their predictive performance have been proposed for continuous, binary, and time-to-event outcomes, but the literature on validation methods for discrete time-to-event models with competing risks is sparse. The present paper tries to fill this gap and proposes new methodology to quantify discrimination, calibration, and prediction error (PE) for discrete time-to-event outcomes in the presence of competing risks. In our case study, the goal was to predict the risk of ventilator-associated pneumonia (VAP) attributed to Pseudomonas aeruginosa in intensive care units (ICUs). Competing events are extubation, death, and VAP due to other bacteria. The aim of this application is to validate complex prediction models developed in previous work on more recently available validation data.  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