全文获取类型
收费全文 | 73250篇 |
免费 | 18092篇 |
国内免费 | 22篇 |
出版年
2023年 | 166篇 |
2022年 | 561篇 |
2021年 | 1326篇 |
2020年 | 2619篇 |
2019年 | 4246篇 |
2018年 | 4656篇 |
2017年 | 4837篇 |
2016年 | 5388篇 |
2015年 | 6152篇 |
2014年 | 6243篇 |
2013年 | 6961篇 |
2012年 | 5764篇 |
2011年 | 5306篇 |
2010年 | 5367篇 |
2009年 | 3961篇 |
2008年 | 3794篇 |
2007年 | 3266篇 |
2006年 | 2878篇 |
2005年 | 2743篇 |
2004年 | 2534篇 |
2003年 | 2207篇 |
2002年 | 1924篇 |
2001年 | 1563篇 |
2000年 | 1427篇 |
1999年 | 1129篇 |
1998年 | 425篇 |
1997年 | 351篇 |
1996年 | 257篇 |
1995年 | 221篇 |
1994年 | 218篇 |
1993年 | 182篇 |
1992年 | 336篇 |
1991年 | 305篇 |
1990年 | 271篇 |
1989年 | 230篇 |
1988年 | 175篇 |
1987年 | 168篇 |
1986年 | 134篇 |
1985年 | 107篇 |
1984年 | 78篇 |
1983年 | 87篇 |
1982年 | 65篇 |
1981年 | 53篇 |
1980年 | 57篇 |
1979年 | 69篇 |
1978年 | 53篇 |
1977年 | 53篇 |
1976年 | 44篇 |
1975年 | 42篇 |
1974年 | 64篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
941.
942.
943.
Ross L. Goldingay Darren G. Quin Orlando Talamo Janice Mentiplay‐Smith 《Ecological Management & Restoration》2020,21(2):131-142
Habitat preferences need to be understood if species are to be adequately managed or conserved. Habitat preferences are presumed to reflect requirements for food, shelter and breeding, as well as interactions with predators and competitors. However, one or more of these requirements may dominate. Tree‐cavity‐dependent wildlife species are one example where shelter or breeding site requirements may dominate. We installed 120 nest boxes across 40 sites to target the vulnerable Brush‐tailed Phascogale (Phascogale tapoatafa) and the non‐threatened Sugar Glider (Petaurus breviceps). The provision of shelter sites where few of quality are available may enable better resolution of habitat preferences. Over three years, we observed the Brush‐tailed Phascogale at 17 sites, whereas the Sugar Glider was observed at 39 sites. We tested four broad hypotheses (H1–H4) relating to habitat that may influence occupancy by these species. There was no influence of hollow (cavity) abundance (H1) on either species suggesting our nest boxes had satisfied their shelter requirements. There was no influence of habitat structure (canopy and tree proximity) (H2) immediately around the nest box trees. We found no influence of distance to the forest edge (H3). Variables at and away from the nest box site that appear to reflect foraging substrates (H4) were influential on the Brush‐tailed Phascogale. Sugar Glider occupancy was only influenced by a single variable at the nest box site. The lack of influence of any other variables is consistent with the very high occupancy observed, suggesting most of the forest habitat is suitable when shelter sites are available. We found no evidence that the Sugar Glider reduced site use by the Brush‐tailed Phascogale. 相似文献
944.
945.
Fumarylacetoacetate hydrolase (FAH) superfamily proteins are found ubiquitously in microbial pathways involved in the catabolism of aromatic substances. Although extensive bioinformatic data on these proteins have been acquired, confusion caused by problems with the annotation of these proteins hinders research into determining their physiological functions. Here we classify 606 FAH superfamily proteins using a maximum likelihood (ML) phylogenetic tree, comparative gene-neighbourhood patterns and in vitro enzyme assays. The FAH superfamily proteins used for the analyses are divided into five distinct subfamilies, and two of them, FPH-A and FPH-B, contain the majority of the proteins of undefined function. These subfamilies include clusters designated FPH-I and FPH-II, respectively, which include two distinct types of fumarylpyruvate hydrolase (FPH), an enzyme involved in the final step of the gentisate pathway. We determined the crystal structures of these FPH enzymes at 2.0 Å resolutions and investigate the substrate binding mode by which these types of enzymes can accommodate fumarylpyruvate as a substrate. Consequentially, we identify the molecular signatures of the two types of FPH enzymes among the broadly conserved FAH superfamily proteins. Our studies allowed us to predict the relationship of unknown FAH superfamily proteins using their sequence information. 相似文献
946.
Paul Daly Mao Peng Hugh D. Mitchell Young-Mo Kim Charles Ansong Heather Brewer Peter de Gijsel Mary S. Lipton Lye Meng Markillie Carrie D. Nicora Galya Orr Ad Wiebenga Kristiina S. Hildén Mirjam A. Kabel Scott E. Baker Miia R. Mäkelä Ronald P. de Vries 《Environmental microbiology》2020,22(3):1154-1166
947.
Samuel Barton James Jenkins Angus Buckling C.-Elisa Schaum Nicholas Smirnoff John A. Raven Gabriel Yvon‐Durocher 《Ecology letters》2020,23(4):722-733
The efficiency of carbon sequestration by the biological pump could decline in the coming decades because respiration tends to increase more with temperature than photosynthesis. Despite these differences in the short‐term temperature sensitivities of photosynthesis and respiration, it remains unknown whether the long‐term impacts of global warming on metabolic rates of phytoplankton can be modulated by evolutionary adaptation. We found that respiration was consistently more temperature dependent than photosynthesis across 18 diverse marine phytoplankton, resulting in universal declines in the rate of carbon fixation with short‐term increases in temperature. Long‐term experimental evolution under high temperature reversed the short‐term stimulation of metabolic rates, resulting in increased rates of carbon fixation. Our findings suggest that thermal adaptation may therefore have an ameliorating impact on the efficiency of phytoplankton as primary mediators of the biological carbon pump. 相似文献
948.
Hao Wang Huiying Liu Guangmin Cao Zhiyuan Ma Yikang Li Fawei Zhang Xia Zhao Xinquan Zhao Lin Jiang Nathan J. Sanders Aime T. Classen Jin‐Sheng He 《Ecology letters》2020,23(4):701-710
Satellite data indicate significant advancement in alpine spring phenology over decades of climate warming, but corresponding field evidence is scarce. It is also unknown whether this advancement results from an earlier shift of phenological events, or enhancement of plant growth under unchanged phenological pattern. By analyzing a 35‐year dataset of seasonal biomass dynamics of a Tibetan alpine grassland, we show that climate change promoted both earlier phenology and faster growth, without changing annual biomass production. Biomass production increased in spring due to a warming‐induced earlier onset of plant growth, but decreased in autumn due mainly to increased water stress. Plants grew faster but the fast‐growing period shortened during the mid‐growing season. These findings provide the first in situ evidence of long‐term changes in growth patterns in alpine grassland plant communities, and suggest that earlier phenology and faster growth will jointly contribute to plant growth in a warming climate. 相似文献
949.
Lihua Zhang Fenghui Yuan Junhong Bai Hongtao Duan Xueying Gu Longyu Hou Yao Huang Mingan Yang Jin‐Sheng He Zhenhua Zhang Lijun Yu Changchun Song David A. Lipson Donatella Zona Walter Oechel Ivan A. Janssens Xiaofeng Xu 《Ecology letters》2020,23(5):821-830
Grassland ecosystems account for more than 10% of the global CH4 sink in soils. A 4‐year field experiment found that addition of P alone did not affect CH4 uptake and experimental addition of N alone significantly suppressed CH4 uptake, whereas concurrent N and P additions suppressed CH4 uptake to a lesser degree. A meta‐analysis including 382 data points in global grasslands corroborated these findings. Global extrapolation with an empirical modelling approach estimated that contemporary N addition suppresses CH4 sink in global grassland by 11.4% and concurrent N and P deposition alleviates this suppression to 5.8%. The P alleviation of N‐suppressed CH4 sink is primarily attributed to substrate competition, defined as the competition between ammonium and CH4 for the methane mono‐oxygenase enzyme. The N and P impacts on CH4 uptake indicate that projected increases in N and P depositions might substantially affect CH4 uptake and alter the global CH4 cycle. 相似文献
950.
Juan F. Espínola‐Novelo M. Teresa Gonzlez Aldo S. Pacheco Jos L. Luque Marcelo E. Oliva 《Ecology letters》2020,23(4):631-641
Parasite communities are similar to free‐living communities; decay of similarity over geographic distance, theory of island biogeography, species–area relationships and nestedness have been documented in both communities. Ecological succession has been studied in free‐living communities but has rarely been examined in parasite communities. We use seriation with replication to test the hypothesis that succession of parasite community structure is deterministic, thus developing throughout consecutive changes along the fish ontogeny, via a seriated pattern. 12 306 marine fishes (95 species) were studied. In 40 species, a seriated pattern was detected; 25 had a tendency towards a seriated pattern, and for 31 species, succession was at random. Age‐classes for each host species explained deterministic successional patterns for whole parasite communities and ectoparasites. Richness and number of age‐classes explained this pattern for endoparasites. Seriated successional pattern was evident for parasite communities of long‐lived marine fish, indicating that parasite communities follow sequential changes over time, like many free‐living communities. 相似文献