全文获取类型
收费全文 | 1895篇 |
免费 | 83篇 |
国内免费 | 5篇 |
专业分类
林业 | 52篇 |
农学 | 25篇 |
基础科学 | 6篇 |
235篇 | |
综合类 | 594篇 |
农作物 | 42篇 |
水产渔业 | 90篇 |
畜牧兽医 | 836篇 |
园艺 | 31篇 |
植物保护 | 72篇 |
出版年
2022年 | 13篇 |
2021年 | 16篇 |
2020年 | 33篇 |
2019年 | 35篇 |
2018年 | 25篇 |
2017年 | 27篇 |
2016年 | 29篇 |
2015年 | 22篇 |
2014年 | 32篇 |
2013年 | 58篇 |
2012年 | 69篇 |
2011年 | 102篇 |
2010年 | 46篇 |
2009年 | 51篇 |
2008年 | 73篇 |
2007年 | 72篇 |
2006年 | 55篇 |
2005年 | 76篇 |
2004年 | 62篇 |
2003年 | 60篇 |
2002年 | 51篇 |
2001年 | 24篇 |
2000年 | 27篇 |
1999年 | 30篇 |
1998年 | 15篇 |
1997年 | 17篇 |
1995年 | 15篇 |
1992年 | 17篇 |
1991年 | 27篇 |
1990年 | 29篇 |
1989年 | 17篇 |
1986年 | 22篇 |
1985年 | 13篇 |
1984年 | 12篇 |
1983年 | 17篇 |
1980年 | 40篇 |
1979年 | 50篇 |
1978年 | 55篇 |
1977年 | 47篇 |
1976年 | 40篇 |
1975年 | 31篇 |
1974年 | 38篇 |
1973年 | 39篇 |
1972年 | 20篇 |
1971年 | 19篇 |
1970年 | 45篇 |
1969年 | 28篇 |
1968年 | 37篇 |
1967年 | 36篇 |
1966年 | 46篇 |
排序方式: 共有1983条查询结果,搜索用时 0 毫秒
991.
Joel?HartterEmail author Sadie?J.?RyanEmail author Jane?Southworth Colin?A.?Chapman 《Landscape Ecology》2011,26(6):877-890
Kibale National Park, within the Albertine Rift, is known for its rich biodiversity. High human population density and agricultural
conversion in the surrounding landscape have created enormous resource pressure on forest fragments outside the park. Kibale
presents a complex protected forest landscape comprising intact forest inside the park, logged areas inside the park, a game
corridor with degraded forest, and forest fragments in the landscape surrounding the park. To explore the effect of these
different levels of forest management and protection over time, we assessed forest change over the previous three decades,
using both discrete and continuous data analyses of satellite imagery. Park boundaries have remained fairly intact and forest
cover has been maintained or increased inside the park, while there has been a high level of deforestation in the landscape
surrounding the park. While absolute changes in land cover are important changes in vegetation productivity, within land cover
classes are often more telling of longer term changes and future directions of change. The park has lower Normalized Difference
Vegetation Index (NDVI) values than the forest fragments outside the park and the formerly logged area—probably due to forest
regeneration and early succession stage. The corridor region has lower productivity, which is surprising given this is also
a newer regrowth region and so should be similar to the logged and forest fragments. Overall, concern can be raised for the
future trajectory of this park. Although forest cover has been maintained, forest health may be an issue, which for future
management, climate change, biodiversity, and increased human pressure may signify troubling signs. 相似文献
992.
Lundin R Barabash S Andersson H Holmström M Grigoriev A Yamauchi M Sauvaud JA Fedorov A Budnik E Thocaven JJ Winningham D Frahm R Scherrer J Sharber J Asamura K Hayakawa H Coates A Linder DR Curtis C Hsieh KC Sandel BR Grande M Carter M Reading DH Koskinen H Kallio E Riihela P Schmidt W Säles T Kozyra J Krupp N Woch J Luhmann J McKenna-Lawler S Cerulli-Irelli R Orsini S Maggi M Mura A Milillo A Roelof E Williams D Livi S Brandt P Wurz P Bochsler P 《Science (New York, N.Y.)》2004,305(5692):1933-1936
The Analyzer of Space Plasma and Energetic Atoms (ASPERA) on board the Mars Express spacecraft found that solar wind plasma and accelerated ionospheric ions may be observed all the way down to the Mars Express pericenter of 270 kilometers above the dayside planetary surface. This is very deep in the ionosphere, implying direct exposure of the martian topside atmosphere to solar wind plasma forcing. The low-altitude penetration of solar wind plasma and the energization of ionospheric plasma may be due to solar wind irregularities or perturbations, to magnetic anomalies at Mars, or both. 相似文献
993.
994.
M F Good L H Miller S Kumar I A Quakyi D Keister J H Adams B Moss J A Berzofsky R Carter 《Science (New York, N.Y.)》1988,242(4878):574-577
Current vaccine development strategies for malaria depend on widespread immunological responsiveness to candidate antigens such as the zygote surface antigens and the sporozoite coat protein, the circumsporozoite (CS) protein. Since immunological responsiveness is controlled mainly by genes mapping within the major histocompatibility complex (MHC), the humoral immune response to the zygote surface antigens and the cytotoxic T lymphocyte (CTL) response to the CS protein were examined in MHC-disparate congenic mouse strains. Only two of six strains responded to the 230-kilodalton zygote surface antigen and another two strains responded to the 48/45-kilodalton surface antigen. From two mouse strains, expressing between them five different class I MHC molecules, there was recognition of only a single CTL epitope from the CS protein, which was from a polymorphic segment of the molecule. The restricted CTL response to this protein parallels the restricted antibody response to this protein observed in humans and mice. These findings suggest that subunit malaria vaccines now being developed may be ineffective. 相似文献
995.
Buckley M Walker A Ho SY Yang Y Smith C Ashton P Oates JT Cappellini E Koon H Penkman K Elsworth B Ashford D Solazzo C Andrews P Strahler J Shapiro B Ostrom P Gandhi H Miller W Raney B Zylber MI Gilbert MT Prigodich RV Ryan M Rijsdijk KF Janoo A Collins MJ 《Science (New York, N.Y.)》2008,319(5859):33; author reply 33
We used authentication tests developed for ancient DNA to evaluate claims by Asara et al. (Reports, 13 April 2007, p. 280) of collagen peptide sequences recovered from mastodon and Tyrannosaurus rex fossils. Although the mastodon samples pass these tests, absence of amino acid composition data, lack of evidence for peptide deamidation, and association of alpha1(I) collagen sequences with amphibians rather than birds suggest that T. rex does not. 相似文献
996.
Evaluating the effectiveness of Protected Areas for conserving tropical forest butterflies of Thailand 总被引:1,自引:0,他引:1
In many cases, the designation of Protected Areas (PAs) is not based on biological information, particularly in tropical regions where such information is generally lacking. Thus it is unclear whether tropical PAs are well-placed for conserving biodiversity currently, or under future climate change. We used reserve-design software (‘Zonation’) to investigate current and future conservation value of PAs of Thailand (N = 187 PAs, covering ∼20% of Thailand) in relation to forest-cover and butterfly diversity. Currently, PAs are about 2 °C cooler than non-PAs because PAs tend to occur at higher elevation (66% of land above 1000 m is protected compared with only 6% below 250 m). Temperature is predicted to increase in Thailand in future, but PAs are predicted to remain ∼2 °C cooler than non-PAs in future. We obtained modelled distribution data for 161 butterfly species (∼12% of national butterfly fauna), and used Zonation to rank areas (∼1 km2 grid resolution) based on species richness, complementarity, and forest cover. The conservation value of PAs was approximately twice that of non-PA areas, although many highly-ranked areas are not currently protected. The species richness of PAs was projected to decline by ∼30% in future, but the relative conservation rankings of individual PAs were projected to change very little. The preponderance of PAs in montane regions makes them well-placed to support forest species shifting from areas at lower elevation that become climatically unsuitable in future. By contrast, the conservation value of low-elevation PAs may decline in future if climate conditions become unsuitable for species. 相似文献
997.
Alhazzaa R Bridle AR Nichols PD Carter CG 《Journal of agricultural and food chemistry》2011,59(15):8423-8434
The limited activity of Δ6 fatty acid desaturase (FAD6) on α-linolenic (ALA, 18:3n-3) and linoleic (LA, 18:2n-6) acids in marine fish alters the long-chain (≥C(20)) polyunsaturated fatty acid (LC-PUFA) concentration in fish muscle and liver when vegetable oils replace fish oil (FO) in aquafeeds. Echium oil (EO), rich in stearidonic acid (SDA, 18:4n-3) and γ-linoleic acid (GLA, 18:3n-6), may enhance the biosynthesis of n-3 and n-6 LC-PUFA by bypassing the rate-limiting FAD6 step. Nutritional and environmental modulation of the mechanisms in LC-PUFA biosynthesis was examined in barramundi, Lates calcarifer , a tropical euryhaline fish. Juveniles were maintained in either freshwater or seawater and fed different dietary LC-PUFA precursors present in EO or rapeseed oil (RO) and compared with FO. After 8 weeks, growth of fish fed EO was slower compared to the FO and RO treatments. Irrespective of salinity, expression of the FAD6 and elongase was up-regulated in fish fed EO and RO diets, but did not lead to significant accumulation of LC-PUFA in the neutral lipid of fish tissues as occurred in the FO treatment. However, significant concentrations of eicosapentaenoic acid (EPA, 20:5n-3) and arachidonic acid (ARA, 20:4n-6), but not docosahexaenoic acid (DHA, 22:6n-3), appeared in liver and, to a lesser extent, in muscle of fish fed EO with marked increases in the phospholipid fraction. Fish in the EO treatment had higher EPA and ARA in their liver phospholipids than fish fed FO. Endogenous conversion of dietary precursors into neutral lipid LC-PUFA appears to be limited by factors other than the initial rate-limiting step. In contrast, phospholipid LC-PUFA had higher biosynthesis, or selective retention, in barramundi fed EO rather than RO. 相似文献
998.
Zhao Y Hasjim J Li L Jane JL Hendrich S Birt DF 《Journal of agricultural and food chemistry》2011,59(17):9700-9708
This study evaluated a novel stearic acid complexed high-amylose cornstarch (SAC) for the prevention of preneoplastic lesions in the colon of azoxymethane (AOM)-treated Fisher 344 rats fed resistant starches at 50-55% of the diet for 8 weeks. Uncooked SAC (r-SAC) diet was compared with raw normal-cornstarch diet (r-CS) or raw high-amylose cornstarch diet (r-HA), and water-boiled CS (w-CS) was compared with w-HA and w-SAC, respectively. w-SAC markedly reduced mucin-depleted foci (MDF) numbers compared with w-HA or w-CS. r-HA significantly decreased aberrant crypt foci (ACF) numbers compared with r-CS or r-SAC. Increased cecum weight and decreased cecum pH were observed in the SAC or HA groups. The highest amounts of total or individual short-chain fatty acids (SCFAs) in cecum and of butyrate or propionate in feces were observed in the AOM-treated w-SAC group. This study revealed the effectiveness of a novel resistant starch in inhibiting colonic preneoplastic lesions and the importance of high-moisture cooking on the suppression of colon carcinogenesis by this resistant starch. 相似文献
999.
1000.