全文获取类型
收费全文 | 5819篇 |
免费 | 2715篇 |
国内免费 | 61篇 |
专业分类
林业 | 894篇 |
农学 | 367篇 |
基础科学 | 438篇 |
2349篇 | |
综合类 | 2737篇 |
农作物 | 99篇 |
水产渔业 | 432篇 |
畜牧兽医 | 313篇 |
园艺 | 153篇 |
植物保护 | 813篇 |
出版年
2025年 | 132篇 |
2024年 | 362篇 |
2023年 | 420篇 |
2022年 | 406篇 |
2021年 | 365篇 |
2020年 | 366篇 |
2019年 | 403篇 |
2018年 | 294篇 |
2017年 | 406篇 |
2016年 | 505篇 |
2015年 | 352篇 |
2014年 | 420篇 |
2013年 | 467篇 |
2012年 | 561篇 |
2011年 | 449篇 |
2010年 | 386篇 |
2009年 | 326篇 |
2008年 | 293篇 |
2007年 | 325篇 |
2006年 | 250篇 |
2005年 | 192篇 |
2004年 | 177篇 |
2003年 | 115篇 |
2002年 | 87篇 |
2001年 | 88篇 |
2000年 | 76篇 |
1999年 | 69篇 |
1998年 | 41篇 |
1997年 | 39篇 |
1996年 | 42篇 |
1995年 | 27篇 |
1994年 | 23篇 |
1993年 | 22篇 |
1992年 | 25篇 |
1991年 | 31篇 |
1990年 | 17篇 |
1989年 | 13篇 |
1988年 | 9篇 |
1987年 | 8篇 |
1986年 | 3篇 |
1985年 | 1篇 |
1984年 | 1篇 |
1982年 | 1篇 |
排序方式: 共有8595条查询结果,搜索用时 15 毫秒
1.
In a companion paper we derived the correct analysis for gerechte designs with uncorrected errors. Here we show that this correct analysis cannot be justified by the usual randomization argument. However, when the regions are rectangular there is a randomization procedure which validates an analysis with three separate error terms. We also outline other developments in design and analysis that may be more satisfactory. 相似文献
2.
Seven variance estimators to be used under systematic sampling are evaluated in a simulation study with 270 artificial spatial populations with different levels and structure of autocorrelation. In settings without an auxiliary variable a proposed new spatial resampling estimator RHO is recommended. In setting with an auxiliary variable, an estimator based on post-stratification (PST), and one with a correction for spatial autocorrelation (DOR), generated estimates with less bias than the SRS estimator in the majority of studied settings. Only in populations with either a near zero autocorrelation at the interval of sampling, or a very strong correlation between the target and the auxiliary variable did the otherwise conservative SRS estimator perform as well as the alternatives. 相似文献
3.
4.
5.
Natural tropical forests are highly diverse and are known to contribute to forest-based services such as pollination of nearby crops. Landscape changes cause spatial and temporal bee community changes, but consequences how the community changes affect pollination is not well analyzed. This paper addresses the effects of rainforest distance and on site flower resources in agro-forests on spatial and temporal variation in pollinator communities and the consequences for coffee pollination. 相似文献
6.
Weed and nutrient management in cropping systems of semi-arid areas is a major constraint to cereal yield. Where the use of herbicides is banned or discouraged, the competitive ability of a crop is crucial to reduce weed growth and diffusion. Genotypic differences in the competitive abilities of crops are an important trait to reduce weeds, especially for plant height. However, there is contrasting information about the interactions of other management practices and genotypic traits on wheat yield and competitive ability against weeds and weed growth. The present study investigated yield and quality of durum wheat (Triticum durum Desf.) and weed growth and composition for two wheat cultivars with contrasting competitive abilities against weeds. Wheat was grown under three spatial arrangements (5-cm, 15-cm, 25-cm inter-row distance) and three sowing densities, and broadleaf weeds were either removed or not. The sowing rate did not affect the yield of these wheat cultivars or the weed growth. Reduced inter-row distance dramatically reduced weed biomass for both wheat cultivars, and increased wheat yield and nitrogen uptake in the low-competitive, high-yielding, semi-dwarf cv. ‘PR22D89’, when both weed free and with weeds. These results have direct implications for weed and nutrient management in low-input and organic cropping systems. 相似文献
7.
MapX在空间数据绑定与林业专题图制作中的应用探讨 总被引:2,自引:0,他引:2
从空间地图数据与属性数据的关联、专题图的生成各方法,就MapX在林业地理信息系统开发中空间数据绑定与专题图制作中的技术等作了简要探讨。 相似文献
8.
对茶梢蛾幼虫的空间分布型及抽样技术的研究结果表明,幼虫为普通聚集分布,其基本成分是面积为一株油茶大小的个体群,个体间相互吸引。调查时以棋盘式取样为佳。根据空间分布型分析,计算出序贯抽样模型为:最适理论抽样数模型为: 相似文献
9.
Spatial distribution of weeds in arable crops: are current sampling and analytical methods appropriate? 总被引:5,自引:0,他引:5
This paper reviews the literature concerning the spatial distribution of weeds; highlighting the limitations of our current sampling and analytical methodologies, and suggesting how these inadequacies can be addressed. Most research studies have used discrete sampling, i.e. weeds are counted within a quadrat, on a grid basis. Few have mapped weeds at a whole-field scale, either with a resolution appropriate to spraying operations or key ecological processes. Statistical analyses used to describe the data can be divided into two main types, spatially implicit (also at the scale of the sampling unit) or spatially explicit, in which the location of individuals is included in the analyses. Spatially implicit methods can be strongly affected by quadrat size and mean density and are of doubtful benefit. More attention is required to address sampling resolution issues for spatially explicit methods. Our understanding of the formation and dynamics of spatial pattern, as well as predicting the consequences of site-specific management, can be improved with models. Unfortunately, most models consider only newly expanding patches and appear incapable of predicting spatial distributions when an area has been fully invaded. More detailed biological information is required if models are to become more realistic and informative. We also need to ensure that we understand the spatial processes in the context of the whole field environment, to optimize the success of site-specific weed management in the longer term. 相似文献
10.
《水产资源保护:海洋与淡水生态系统》2018,28(1):216-230
- Many species and populations of marine megafauna are undergoing substantial declines, while many are also very poorly understood. Even basic information on species presence is unknown for tens of thousands of kilometres of coastline, particularly in the developing world, which is a major hurdle to their conservation.
- Rapid ecological assessment is a valuable tool used to identify and prioritize areas for conservation; however, this approach has never been clearly applied to marine cetaceans. Here a rapid assessment protocol is outlined that will generate broad‐scale, quantitative, baseline data on cetacean communities and potential threats, that can be conducted rapidly and cost‐effectively across whole countries, or regions.
- The rapid assessment was conducted in Tanzania, East Africa, and integrated collection of data on cetaceans from visual, acoustic, and interview surveys with existing information from multiple sources, to provide low resolution data on cetacean community relative abundance, diversity, and threats. Four principal threats were evaluated and compared spatially using a qualitative scale: cetacean mortality in fishing gear (particularly gillnets); cetacean hunting, consumption or use by humans; shipping related collision risk and noise disturbance; and dynamite fishing.
- Ninety‐one groups of 11 species of marine mammal were detected during field surveys. Potentially the most important area for cetaceans was the Pemba Channel, a deep, high‐current waterway between Pemba Island and mainland Africa, where by far the highest relative cetacean diversity and high relative abundance were recorded, but which is also subject to threats from fishing.
- A rapid assessment approach can be applied in data deficient areas to quickly provide information on cetaceans that can be used by governments and managers for marine spatial planning, management of developments, and to target research activities into the most important locations.