全文获取类型
收费全文 | 184119篇 |
免费 | 8880篇 |
国内免费 | 848篇 |
出版年
2021年 | 1440篇 |
2020年 | 940篇 |
2019年 | 1242篇 |
2018年 | 13044篇 |
2017年 | 11744篇 |
2016年 | 9493篇 |
2015年 | 4448篇 |
2014年 | 4661篇 |
2013年 | 5527篇 |
2012年 | 11167篇 |
2011年 | 19280篇 |
2010年 | 15961篇 |
2009年 | 12092篇 |
2008年 | 15233篇 |
2007年 | 16774篇 |
2006年 | 5729篇 |
2005年 | 5796篇 |
2004年 | 6049篇 |
2003年 | 5909篇 |
2002年 | 5377篇 |
2001年 | 1199篇 |
2000年 | 862篇 |
1999年 | 985篇 |
1998年 | 1228篇 |
1997年 | 832篇 |
1996年 | 748篇 |
1995年 | 766篇 |
1994年 | 661篇 |
1993年 | 763篇 |
1992年 | 678篇 |
1991年 | 595篇 |
1990年 | 570篇 |
1989年 | 550篇 |
1988年 | 523篇 |
1987年 | 480篇 |
1986年 | 434篇 |
1985年 | 608篇 |
1984年 | 687篇 |
1983年 | 634篇 |
1982年 | 721篇 |
1981年 | 657篇 |
1980年 | 659篇 |
1979年 | 386篇 |
1978年 | 465篇 |
1977年 | 395篇 |
1976年 | 397篇 |
1974年 | 351篇 |
1973年 | 343篇 |
1972年 | 456篇 |
1971年 | 457篇 |
排序方式: 共有10000条查询结果,搜索用时 46 毫秒
1.
Jakub Horák 《Journal of Insect Conservation》2017,21(1):1-5
Insect and veteran trees are important parts of ecosystems and are usually included in ecological studies of forest management. The loss of veteran trees in woodlands and open landscapes would lead to the loss of saproxylic organisms—an important part of biodiversity. Hence, the persistence of many specialized insects depends on the presence of veteran trees scattered in woodlands (e.g. ancient wood pastures, game parks or protected areas), cities, towns and villages (e.g. avenues, parks or chateau parks) or open landscapes (e.g. fishpond dams, solitary trees or fruit orchards). Veteran tree conditions could be fairly well described by three components—diameter, age and microhabitats present. The problem is that diameter belongs to the most studied characteristics, while age and microhabitats, which can be quite complicated to measure, are much less studied. This paper illustrates that, due to this unbalanced use of indicators of veteran-tree conditions, we are still missing some important information on saproxylic species ecology—and sometimes only large trees might be studied, rather than real veterans. Although we already know that veteran trees are essential habitat for a range of saproxylic organisms, there are still gaps in our knowledge of the specific conditions that veteran trees provide. It is vital that these are quantified and understood so that this information can be used to conserve veteran trees and their associated species. 相似文献
2.
Tom P. Moorhouse Cecilia A. L. Dahlsj? Sandra E. Baker Neil C. D'Cruze David W. Macdonald 《PloS one》2015,10(10)
Tourism accounts for 9% of global GDP and comprises 1.1 billion tourist arrivals per annum. Visits to wildlife tourist attractions (WTAs) may account for 20–40% of global tourism, but no studies have audited the diversity of WTAs and their impacts on the conservation status and welfare of subject animals. We scored these impacts for 24 types of WTA, visited by 3.6–6 million tourists per year, and compared our scores to tourists’ feedback on TripAdvisor. Six WTA types (impacting 1,500–13,000 individual animals) had net positive conservation/welfare impacts, but 14 (120,000–340,000 individuals) had negative conservation impacts and 18 (230,000–550,000 individuals) had negative welfare impacts. Despite these figures only 7.8% of all tourist feedback on these WTAs was negative due to conservation/welfare concerns. We demonstrate that WTAs have substantial negative effects that are unrecognised by the majority of tourists, suggesting an urgent need for tourist education and regulation of WTAs worldwide. 相似文献
3.
4.
5.
6.
7.
8.
9.
10.
A nonplanar distortion of the heme ofc-type cytochromes is conserved in the proteins isolated from diverse species based upon a comprehensive analysis of available highresolution X-ray crystal structures. This distortion is induced through the cysteine thioether linkages between the porphyrin pyrrole groups and the polypeptide and results in an asymmetric pyrrole distortion. This asymmetry in the heme distortion is also conserved. For other heme proteins which lack these covalent bonds, nearly planar porphyrins are observed. Resonance Raman evidence indicates that nonplanar distortion of porphyrins containing metals, like iron, with large core sizes (?2.00 Å) is energetically unfavorable and can occur only in the presence of significant environmental perturbations. Further, energy minimization and dynamics calculations on the ferric form of yeast iso-1-cytochromec, starting from the crystallographic coordinates and using a molecular mechanics force field which accurately reproduces nonplanar distortions in metalloporphyrins, suggest that this distortion is indeed maintained by the protein tertiary structure. It is proposed that this protein-linked heme distortion modulates electron transfer function through modification of redox potentials of the porphyrin ring and the protein binding properties ofc-type cytochromes. 相似文献