全文获取类型
收费全文 | 79346篇 |
免费 | 8604篇 |
国内免费 | 45篇 |
出版年
2022年 | 799篇 |
2021年 | 1432篇 |
2020年 | 1045篇 |
2019年 | 1184篇 |
2018年 | 1628篇 |
2017年 | 1423篇 |
2016年 | 2318篇 |
2015年 | 3493篇 |
2014年 | 3613篇 |
2013年 | 4608篇 |
2012年 | 4908篇 |
2011年 | 4434篇 |
2010年 | 3002篇 |
2009年 | 2968篇 |
2008年 | 3349篇 |
2007年 | 3236篇 |
2006年 | 3076篇 |
2005年 | 3216篇 |
2004年 | 3108篇 |
2003年 | 2712篇 |
2002年 | 2274篇 |
2001年 | 1747篇 |
2000年 | 1612篇 |
1999年 | 1552篇 |
1998年 | 998篇 |
1997年 | 918篇 |
1996年 | 1019篇 |
1995年 | 841篇 |
1994年 | 850篇 |
1993年 | 816篇 |
1992年 | 1265篇 |
1991年 | 1160篇 |
1990年 | 1096篇 |
1989年 | 1068篇 |
1988年 | 1087篇 |
1987年 | 978篇 |
1986年 | 922篇 |
1985年 | 914篇 |
1984年 | 855篇 |
1983年 | 712篇 |
1982年 | 592篇 |
1981年 | 587篇 |
1980年 | 578篇 |
1979年 | 675篇 |
1978年 | 597篇 |
1977年 | 518篇 |
1976年 | 423篇 |
1975年 | 508篇 |
1974年 | 473篇 |
1973年 | 499篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
101.
102.
103.
Eliane S. Meier Thomas C. Edwards Jr Felix Kienast Matthias Dobbertin Niklaus E. Zimmermann 《Journal of Biogeography》2011,38(2):371-382
Aim During recent and future climate change, shifts in large‐scale species ranges are expected due to the hypothesized major role of climatic factors in regulating species distributions. The stress‐gradient hypothesis suggests that biotic interactions may act as major constraints on species distributions under more favourable growing conditions, while climatic constraints may dominate under unfavourable conditions. We tested this hypothesis for one focal tree species having three major competitors using broad‐scale environmental data. We evaluated the variation of species co‐occurrence patterns in climate space and estimated the influence of these patterns on the distribution of the focal species for current and projected future climates. Location Europe. Methods We used ICP Forest Level 1 data as well as climatic, topographic and edaphic variables. First, correlations between the relative abundance of European beech (Fagus sylvatica) and three major competitor species (Picea abies, Pinus sylvestris and Quercus robur) were analysed in environmental space, and then projected to geographic space. Second, a sensitivity analysis was performed using generalized additive models (GAM) to evaluate where and how much the predicted F. sylvatica distribution varied under current and future climates if potential competitor species were included or excluded. We evaluated if these areas coincide with current species co‐occurrence patterns. Results Correlation analyses supported the stress‐gradient hypothesis: towards favourable growing conditions of F. sylvatica, its abundance was strongly linked to the abundance of its competitors, while this link weakened towards unfavourable growing conditions, with stronger correlations in the south and at low elevations than in the north and at high elevations. The sensitivity analysis showed a potential spatial segregation of species with changing climate and a pronounced shift of zones where co‐occurrence patterns may play a major role. Main conclusions Our results demonstrate the importance of species co‐occurrence patterns for calibrating improved species distribution models for use in projections of climate effects. The correlation approach is able to localize European areas where inclusion of biotic predictors is effective. The climate‐induced spatial segregation of the major tree species could have ecological and economic consequences. 相似文献
104.
S. E. Franklin L. Young D. Watson A. Cigan T. Meyer L. A. Bulla Jr. 《Molecular genetics and genomics : MGG》1997,256(5):517-524
Various subspecies of the gram-positive bacterium Bacillus thuringiensis are known to produce a wide array of insecticidal crystal proteins (ICPs) upon sporulation. These ICPs act primarily on the brush border of midgut epithelial cells of susceptible larvae. Recently, a protein of 210?kDa, isolated from the midgut of Manduca sexta, has been demonstrated to bind the Cry1Ab toxin produced by B. thuringiensis subsp. berliner and is therefore postulated to be involved in mediating the toxicity of Cry1Ab. The cDNA encoding the 210?kDa protein, termed BT-R1 (Bacillus thuringiensis receptor-1), was recently cloned, and shows limited homology to the cadherin superfamily of proteins. Quite naturally, there is a great deal of interest in the characterization of BT-R 1 , the gene encoding the 210?kDa Cry1Ab binding protein. The studies presented here involve the use of various restriction fragments prepared from the cDNA encoding BT-R1 as probes of Southern blots bearing M. sexta genomic DNA cleaved with a variety of restriction endonucleases. These Southern blot data reveal that there are two discrete regions within the M. sexta genome which encode sequences homologous to BT-R1. On the basis of the signal intensities seen on Southern blots, it appears that only one of these genes encodes BT-R1, whereas the other is a closely related homologue. 相似文献
105.
Jeroen M. G. Stevens Hilde Vervaecke Han de Vries Linda van Elsacker 《International journal of primatology》2007,28(6):1417-1430
Bonobos have a reputation as a female-dominated and egalitarian species. We examined the 2 aspects of dominance in 6 captive
bonobo groups. Females do not consistently evoke submission from all males in all contexts. Though females occupy the highest-ranking
positions in the dominance hierarchy, there are in each group males that obtain rather high ranks and are able to dominate
≥1 female. Thus female dominance is not complete and hierarchies can be better described as nonexclusive female dominance.
We studied egalitarianism by measuring linearity and steepness of dominance hierarchies. The hierarchies of all groups are
highly linear. Hierarchies among males are steeper than among females. On average, male bonobos are more despotic than females,
but females too can have despotic relations, both with other females and with males. Hence one can call bonobos in captivity
semidespotic rather than egalitarian. 相似文献
106.
107.
108.
Martin T. R. Kuiper Marijke Holtrop Hans de Vries 《Molecular & general genetics : MGG》1988,213(2-3):519-528
109.
Hydrobiologia - In shallow lakes, macrophytes have important effects on food webs, community structure and nutrient dynamics. For this reason they play a significant role in the restoration of... 相似文献
110.