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   264篇
  免费   11篇
  国内免费   4篇
  2021年   4篇
  2020年   4篇
  2016年   4篇
  2015年   5篇
  2014年   6篇
  2013年   7篇
  2012年   6篇
  2011年   8篇
  2010年   5篇
  2009年   12篇
  2008年   14篇
  2007年   7篇
  2006年   4篇
  2005年   7篇
  2004年   9篇
  2003年   6篇
  2001年   3篇
  2000年   5篇
  1999年   6篇
  1998年   6篇
  1997年   3篇
  1996年   4篇
  1995年   3篇
  1991年   3篇
  1990年   4篇
  1955年   6篇
  1954年   6篇
  1952年   4篇
  1950年   3篇
  1949年   2篇
  1945年   2篇
  1943年   2篇
  1942年   4篇
  1928年   2篇
  1927年   3篇
  1925年   3篇
  1919年   2篇
  1917年   2篇
  1916年   2篇
  1913年   2篇
  1909年   3篇
  1908年   2篇
  1907年   2篇
  1905年   3篇
  1904年   4篇
  1903年   4篇
  1902年   2篇
  1901年   2篇
  1900年   3篇
  1899年   4篇
排序方式: 共有279条查询结果,搜索用时 15 毫秒
21.
22.
23.
Little is known about the evolutionary history of most complex multi‐trophic insect communities. Widespread species from different trophic levels might evolve in parallel, showing similar spatial patterns and either congruent temporal patterns (Contemporary Host‐tracking) or later divergence in higher trophic levels (Delayed Host‐tracking). Alternatively, host shifts by natural enemies among communities centred on different host resources could disrupt any common community phylogeographic pattern. We examined these alternative models using two Megastigmus parasitoid morphospecies associated with oak cynipid galls sampled throughout their Western Palaearctic distributions. Based on existing host cynipid data, a parallel evolution model predicts that eastern regions of the Western Palaearctic should contain ancestral populations with range expansions across Europe about 1.6 million years ago and deeper species‐level divergence at both 8–9 and 4–5 million years ago. Sequence data from mitochondrial cytochrome b and multiple nuclear genes showed similar phylogenetic patterns and revealed cryptic genetic species within both morphospecies, indicating greater diversity in these communities than previously thought. Phylogeographic divergence was apparent in most cryptic species between relatively stable, diverse, putatively ancestral populations in Asia Minor and the Middle East, and genetically depauperate, rapidly expanding populations in Europe, paralleling patterns in host gallwasp species. Mitochondrial and nuclear data also suggested that Europe may have been colonized multiple times from eastern source populations since the late Miocene. Temporal patterns of lineage divergence were congruent within and across trophic levels, supporting the Contemporary Host‐tracking Hypothesis for community evolution.  相似文献   
24.
1. Rapid expansion and intensification of anthropogenic activities in the 20th century has caused profound changes in freshwater assemblages. Unfortunately, knowledge of the extent and causes of species loss (SL) is limited due to the lack of reliable historical data. An unusual data set allows us to compare changes in the most sensitive of aquatic insect orders, the Plecoptera, at some 170 locations in the Czech Republic between two time periods, 1955–1960 and 2006–2010. Historical data (1890–1911) on assemblages of six lowland rivers allow us to infer even earlier changes. 2. Regional stonefly diversity decreased in the first half of the 20th century. Streams at lower altitudes lost a substantial number of species, which were never recovered. In the second half of the century, large‐scale anthropogenic pressure caused SL in all habitats, leading to a dissimilarity of contemporary and previous assemblages. The greatest changes were found at sites affected by organic pollution and a mixture of organic pollution and channelisation or impoundment. Colonisation of new habitats was observed in only three of the 80 species evaluated. 3. Species of moderate habitat specialisation and tolerance to organic pollution were most likely to be lost. Those with narrow specialisations in protected habitats were present in both historical and contemporary collections. 4. Contemporary assemblages are the consequence of more than a 100 years of anthropogenic impacts. In particular, streams at lower altitude and draining intensively exploited landscapes host a mere fragment of the original species complement. Most stonefly species are less frequently present than before, although their assemblages remain almost intact in near‐natural mountain streams. Our analyses demonstrate dramatic restriction of species ranges and, in some cases, apparent changes in altitudinal preference throughout the area.  相似文献   
25.
26.
27.
The objective of enterprise modeling is to develop a repository regarding organizational elements and functions that maps information objectives with business functions. This is accomplished through an exhaustive process that analyzes and models the business to a level of detail sufficient to enable selection of appropriate technologies and the design of specific information systems. In this paper, enterprise modeling is utilized to provide a baseline reference for the successful integration of an FMS in an aerospace electronics manufacturing facility.  相似文献   
28.
29.
30.
Gastrectomy     
  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