全文获取类型
收费全文 | 957篇 |
免费 | 47篇 |
国内免费 | 65篇 |
专业分类
林业 | 33篇 |
农学 | 55篇 |
基础科学 | 23篇 |
209篇 | |
综合类 | 624篇 |
农作物 | 50篇 |
水产渔业 | 6篇 |
畜牧兽医 | 29篇 |
园艺 | 18篇 |
植物保护 | 22篇 |
出版年
2024年 | 9篇 |
2023年 | 18篇 |
2022年 | 17篇 |
2021年 | 31篇 |
2020年 | 44篇 |
2019年 | 42篇 |
2018年 | 24篇 |
2017年 | 52篇 |
2016年 | 67篇 |
2015年 | 59篇 |
2014年 | 70篇 |
2013年 | 60篇 |
2012年 | 99篇 |
2011年 | 74篇 |
2010年 | 75篇 |
2009年 | 67篇 |
2008年 | 47篇 |
2007年 | 54篇 |
2006年 | 21篇 |
2005年 | 21篇 |
2004年 | 11篇 |
2003年 | 16篇 |
2002年 | 9篇 |
2001年 | 12篇 |
2000年 | 12篇 |
1999年 | 9篇 |
1998年 | 11篇 |
1997年 | 4篇 |
1996年 | 5篇 |
1995年 | 8篇 |
1994年 | 4篇 |
1993年 | 5篇 |
1992年 | 5篇 |
1991年 | 2篇 |
1990年 | 2篇 |
1989年 | 1篇 |
1988年 | 2篇 |
排序方式: 共有1069条查询结果,搜索用时 15 毫秒
61.
Dennis L. Murray Douglas W. Smith Curtis Mack Joe Fontaine Michael Jiminez Thomas J. Meier James Holyan 《Biological conservation》2010,143(11):2514-2524
There is substantial interest in how mortality rates affect animal populations, but mechanisms explaining when and under what circumstances particular causes of death incur demographic responses are far from clear. In theory, small or expanding populations should experience additive mortality from anthropogenic causes of death, but whether such effects are homogenous across a population or expressed only in certain high-risk individuals is open for debate. We used competing risks models to analyze mortality patterns among radio-collared wolves (Canis lupus, n = 711) from three populations in northwestern United States (1982-2004), and evaluated the degree to which anthropogenic mortality was additive vs. compensatory to natural demographic processes. Almost 80% (n = 320) of wolves dying of known fates were killed by anthropogenic causes (legal control, illegal killing, harvest in Canada, vehicle collision), and additive effects of anthropogenic mortality were most pronounced in northwestern Montana where wolf exposure to humans and livestock was high compared to either the Greater Yellowstone Area or central Idaho, where anthropogenic risk was lower. In contrast, risk from natural hazards was lower in northwestern Montana than in the other areas, implying some degree of compensatory mortality from anthropogenic risk. Animals recruited to the study following human-wolf conflict had markedly higher anthropogenic risk than those recruited for standard monitoring purposes, and juvenile wolves as well as dispersers, succumbed to higher anthropogenic risk. Multivariate models revealed that increasing wolf population density promoted higher anthropogenic risk and reduced natural risk, indicating that partially-compensatory effects of anthropogenic mortality actually became increasingly additive with population density. The observed compensatory mortality and hazard heterogeneity in our study implies that demographic responses to mortality risk may be complex and more subtle than previously thought; the density-dependent effect of anthropogenic mortality portends a stabilizing influence of humans on recovering wolf populations. We conclude that future assessment of the role of anthropogenic mortality should include individual-based hazard estimation as a complement to traditional population-level approaches. 相似文献
62.
63.
64.
There exists the problems of low technology, low quality of components and lacked process in the traditional houses. This article states the experience and lesson in the development of Chongqing Longhu Garden, and the efforts of developers in the industrialized housing. 相似文献
65.
66.
几种利用方式下酸性硫酸盐土的环境风险及其连锁效应 总被引:2,自引:0,他引:2
研究并比较了不同利用方式下酸性硫酸盐土酸形态和铝形态的结构特征,分析了硫形态与酸形态、酸形态与铝形态之间的相关性。研究发现,酸性硫酸盐土的不同利用类型中,土壤酸度由大到小依次为:鱼塘塘基、荒稻田>稻田>荒旱地>改良稻田>蔗田>红树林迹地;活性铝含量由高到低依次为:荒旱地>鱼塘塘基、荒稻田>稻田>改良稻田>蔗田>红树林迹地。土壤酸度和铝形态含量的层间差异较为显著,多数随深度增加呈逐渐上升的趋势。酸性硫酸盐土中的硫形态—酸—铝形态之间有明显的连锁关系。 相似文献
67.
68.
我国气象灾害及其衍生灾害发生频繁,对经济社会发展和人民群众生产生活产生严重影响。农村是气象工作最广阔的领域,气象灾害防御工作对农业农村发展影响深远。本文以张家界市永定区为例,重点描述了本地区主要的气象灾害类型,分析当阶段农村气象灾害的防御现状,并从政府、资金、队伍、科技、防御、科普等六个方面,提出了气象灾害防御能力建设及为农服务的具体措施与建议。 相似文献
69.
VaR度量金融风险的几个问题 总被引:3,自引:0,他引:3
针对VaR模型中的delta-正态模型的计算,分析了其中存在的几个问题并进行了改进。结果认为,用区间估计替代点估计可以提高VaR值的有效性;在用CAPM模型计算股票资产的可能损失时,不能丢失常数项,否则计算值偏大。另鉴于传统VaR模型不能评估市场因子变化时资产损失发生的概率,现基于高维正态分布的计算方法提出了用VaR度量金融市场风险的新方法。 相似文献
70.