全文获取类型
收费全文 | 6596篇 |
免费 | 405篇 |
国内免费 | 797篇 |
专业分类
林业 | 942篇 |
农学 | 483篇 |
基础科学 | 695篇 |
1310篇 | |
综合类 | 3305篇 |
农作物 | 124篇 |
水产渔业 | 228篇 |
畜牧兽医 | 376篇 |
园艺 | 118篇 |
植物保护 | 217篇 |
出版年
2024年 | 62篇 |
2023年 | 161篇 |
2022年 | 269篇 |
2021年 | 271篇 |
2020年 | 249篇 |
2019年 | 307篇 |
2018年 | 186篇 |
2017年 | 302篇 |
2016年 | 328篇 |
2015年 | 271篇 |
2014年 | 393篇 |
2013年 | 367篇 |
2012年 | 495篇 |
2011年 | 501篇 |
2010年 | 412篇 |
2009年 | 348篇 |
2008年 | 313篇 |
2007年 | 383篇 |
2006年 | 360篇 |
2005年 | 285篇 |
2004年 | 230篇 |
2003年 | 205篇 |
2002年 | 128篇 |
2001年 | 122篇 |
2000年 | 105篇 |
1999年 | 84篇 |
1998年 | 97篇 |
1997年 | 74篇 |
1996年 | 56篇 |
1995年 | 71篇 |
1994年 | 64篇 |
1993年 | 56篇 |
1992年 | 44篇 |
1991年 | 40篇 |
1990年 | 47篇 |
1989年 | 44篇 |
1988年 | 22篇 |
1987年 | 17篇 |
1986年 | 7篇 |
1985年 | 3篇 |
1984年 | 3篇 |
1983年 | 2篇 |
1982年 | 1篇 |
1981年 | 5篇 |
1978年 | 3篇 |
1977年 | 1篇 |
1976年 | 1篇 |
1973年 | 1篇 |
1956年 | 2篇 |
排序方式: 共有7798条查询结果,搜索用时 31 毫秒
991.
GIS在森林资源管理中的应用 总被引:1,自引:0,他引:1
以东北林业大学帽儿山实验林场为例,利用GIS软件Arcview3.3、数据库软件Foxpro、图像处理软件Photoshop等工具,建立了帽儿山林场森林资源信息管理系统:并详细介绍了该系统的相关功能。 相似文献
992.
基于森林资源二类调查数据的森林景观分类研究 总被引:16,自引:1,他引:15
以吉林省汪清林业局金沟岭林场为例 ,提出一种基于森林资源二类调查数据的景观要素分类方法。选取优势树种 (组 )、海拔、坡度、坡向和太阳辐射强度作为分类因子 ,在GIS技术支持下 ,按分类因子分割林场森林景观空间为 975 7个空间基本单元 ,建立起量化的、与植被和环境因子相关的森林景观空间基本单元数据库 ,在此基础上采用聚类分析方法把所有基本单元聚合为不同相似水平上的景观要素类型 ,结合生态特征和经营需要对聚类结果进行分析并最终确定了 7种景观要素类型 ,生成了景观要素类型斑块图。斑块特征和空间格局分析结果表明 :该林区森林景观中以小斑块要素比例最大 ,中、小斑块数占总斑块数的 90 %。针阔混交林具有最大的面积、平均斑块面积、斑块周长、最大生境面积和内部生境指数 ,是该地区受人为干扰相对较小、仍需要进行保护的景观要素类型。在森林经营中 ,应注意保护大型斑块的景观要素类型 相似文献
993.
Christina M. Cianfrani S. Mažeika P. Sullivan W. Cully Hession Mary C. Watzin 《水产资源保护:海洋与淡水生态系统》2009,19(2):147-156
- 1. Stream classification systems are widely used in stream management and restoration. Whereas the principal morphological types of these classification systems are increasingly recognized for their ecological connections, the roles of intermediate and mixed morphologies are still poorly understood, yet may be biologically significant.
- 2. Twenty‐five stream reaches in north‐western Vermont were classified by channel morphology to determine whether fish community diversity differed among pool‐riffle, mixed (i.e. pool‐riffle/cascade, pool‐riffle/other) and forced pool‐riffle stream morphological groups. Stream reach surveys included cross‐sectional surveys, longitudinal profiles, bed substrate characterization, and fish surveys.
- 3. Three fish community diversity measures were calculated: (1) species richness (S); (2) Shannon–Weaver Index (H′); and (3) Simpson's Index (1/D). Multivariate analysis of covariance (MANCOVA) followed by analysis of variance (ANOVA) were used to explore potential differences in fish diversity among stream morphological groups. Fish diversity was significantly different for all three community diversity measures (P?0.05), with pool‐riffle/cascade morphology consistently exhibiting the greatest fish diversity and forced pool‐riffle the lowest.
- 4. These results suggest that fish community diversity is significantly associated with distinct channel morphologies. Generally, pool‐riffle/cascade and pool‐riffle/other stream morphological groups supported habitats that fostered greater species diversity than more homogeneous and uniform pool‐riffle reaches. The observed patterns of diversity are likely to be the result of habitat patches created by variations in flow and other physical characteristics in reaches of mixed morphologies.
- 5. These results support fish sampling schemes that incorporate morphological heterogeneity, such as proportional‐distance designation. Sampling strategies that focus on homogeneous reaches may underestimate diversity, and misrepresent stream condition when fish community data are used in indices of biological integrity (IBIs). Reaches of mixed stream morphologies should be recognized as areas of biological importance in stream and catchment management and in conservation efforts.
994.
Cassidy D Peterson Carolyn N Belcher Dana M Bethea William B Driggers III Bryan S Frazier Robert J Latour 《Fish and Fisheries》2017,18(5):845-859
Relative abundance of many shark species in the Atlantic is assessed by compiling data from several independently conducted, but somewhat spatially limited surveys. Although these localized surveys annually sample the same populations, resulting trends in yearly indices often conflict with one another, thereby hindering interpretation of abundance patterns at broad spatial scales. We used delta‐lognormal generalized linear models (GLMs) to generate indices of abundance for seven Atlantic coastal shark species from six fishery‐independent surveys along the US east coast and Gulf of Mexico from 1975 to 2014. These indices were further analysed using dynamic factor analysis (DFA) to produce simplified, broad‐scale common trends in relative abundance over the entire sampled distribution. Effects of drivers including the North Atlantic Oscillation index, the Atlantic Multidecadal Oscillation index, annually averaged sea surface temperature and species landings were evaluated within the DFA model. The two decadal oscillations and species landings were shown to affect shark distribution along south‐east US coast. Estimated common trends of relative abundance for all large coastal shark species showed similar decreasing patterns into the early 1990s, periods of sustained low index values thereafter and recent indications of recovery. Small coastal shark species exhibited more regional variability in their estimated common trends, such that two common trends were required to adequately describe patterns in relative abundance throughout the Gulf of Mexico and Atlantic. Overall, all species’ (except the Gulf of Mexico blacknose shark) time series concluded with an increasing trend, suggestive of initial recovery from past exploitation. 相似文献
995.
针对服装样片纸样的形状与结构特点,提出一种由曲率确定型值点分布的非均匀B样条曲线拟合识别方法,并用其样条小波进行型值点压缩和优化,以达到在给定误差下型值点数目尽量少的目的.通过分析比较和实例检验表明,这种算法识别服装纸样方便可靠,为服装计算机辅助设计(CAD)中样片的编辑设计提供一种智能高效的输入技术. 相似文献
996.
997.
功能型酸奶独特的风味及保健功能,受到越来越多消费群体的青睐,有较好的消费市场和发展前景,也有较大的研究价值。本文在国内外研究基础上,对功能型酸奶常见的两种分类方式(按添加成分和消费者群体分类)、不同的保健功能(解酒、抗氧化、通便、清热解毒和对砷毒性的缓解作用等),及目前在国内外市场销售的酸奶进行总结,为功能型酸奶的研发及工业化的生产提供理论基础。 相似文献
998.
Kevin N
T Norman 《Pest management science》2000,56(2):154-158
The persistence of methyl bromide (MB) in white rice, brown rice, sultanas, raisins, pumpkin seeds, brazil nuts, groundnuts and walnuts was monitored following laboratory fumigations at different concentration time products and temperatures. A sensitive automated headspace gas chromatographic method with a limit of quantitation of 0.005 mg kg−1 was used to determine MB residues. Nuts and seeds sorbed the highest amounts of MB followed by rice and dried fruit. Measurable MB residues were shown to persist in nuts and seeds for 10 weeks and in dried fruit for four weeks following a fumigation with recommended concentration time products. The data presented suggest MB residues may persist in unprocessed nuts, seeds or dried fruit at the point of consumption using recommended fumigation practices, or particularly following overdosing or multiple fumigations. Crown copyright © 2000 Reproduced with the permission of Her Majesty's Stationery Office 相似文献
999.
1000.
为了探讨高光谱图像技术在小麦种子分类识别中应用的可行性,采集了河南地区主要种植的7个小麦品种的种子高光谱图像及900~1 700nm范围的光谱信息,建立了主成分分析法(PCA)-支持向量机(SVM)分类模型。运用PCA对光谱数据进行降维处理,结合SVM模型比较了不同实验条件下小麦种子分类准确率以及在最佳条件下3个、4个和6个品种种子的分类准确率。结果显示,3个品种间种子分类准确率除个别外平均达到95%以上,4个品种间种子分类准确率在80%左右,6个品种间种子分类准确率在66%左右。这说明充分利用光谱信息可以对3个或4个小麦品种进行多籽粒分类。 相似文献