首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   392篇
  免费   31篇
  国内免费   111篇
  2024年   5篇
  2023年   19篇
  2022年   26篇
  2021年   33篇
  2020年   21篇
  2019年   32篇
  2018年   25篇
  2017年   26篇
  2016年   22篇
  2015年   23篇
  2014年   10篇
  2013年   24篇
  2012年   14篇
  2011年   21篇
  2010年   13篇
  2009年   25篇
  2008年   24篇
  2007年   24篇
  2006年   15篇
  2005年   15篇
  2004年   13篇
  2003年   15篇
  2002年   10篇
  2001年   8篇
  2000年   6篇
  1999年   10篇
  1998年   6篇
  1997年   4篇
  1996年   5篇
  1995年   7篇
  1994年   2篇
  1993年   5篇
  1992年   2篇
  1991年   4篇
  1990年   6篇
  1989年   1篇
  1987年   1篇
  1986年   1篇
  1985年   2篇
  1984年   3篇
  1982年   1篇
  1980年   1篇
  1978年   2篇
  1973年   1篇
  1971年   1篇
排序方式: 共有534条查询结果,搜索用时 14 毫秒
1.
We have compared the species richness of medicinal plants and the differential patterns of use amongst settlements in the Andean communities of Northwest Argentina which have differing levels of isolation. About 259 ethnoespecies, belonging to 74 plant families, were included, representing between 70 and 80% of the total estimate. The results indicate that Coronopus didymus is the most relevant and important species. The method of use of medicinal plants and the ailments treated by rural doctors compared to those of the layperson is different. Native and exotic plants are used differently according to the body system treated. There are some relationships between internal and external use and body systems and recipes. The greater medicinal species richness found in the less isolated locations is due to external enriching cultural influences.  相似文献   
2.
As we confront the current environmental crisis, determining the biophysical base (e.g., materials, energy, land, and water) of nations has become paramount. With advanced economies benefiting from the import of resource-intensive primary goods originating from poorer parts of the world, especially emerging nations, these are dilapidating their natural capital. Brazil is one of such emerging economies, whose mining and farming activities, propping up its export-led economic growth, exert great pressure on the environment. In particular, farming has been shown to have one of the world's greatest environmental impacts, especially as a consequence of land use associated with cattle ranching. Since a nation-wide evaluation of land-use types across the whole sectorial spectrum of the country's economy is still lacking, we used the most recently available Input–Output Economic Model for Brazil and the Ecological Footprint method to identify those economic sectors with the greatest potential for appropriating portions of the natural world.Our results show that: (i) the biggest chunk of Brazil's Ecological Footprint is due to its Carbon Footprint and, in particular, emissions from cattle; (ii) only a few economic sectors exhibit high Ecological Footprint values, chiefly those belonging to livestock farming and energy production based on fossil fuels; (iii) excluding the soybeans and slaughter sectors, export-oriented sectors have below-average Ecological Footprint values; and (iv) the percentage of Brazil's Ecological Footprint due to household consumption (excluding imports) is three times bigger than that attributable to exports, with sectors belonging to livestock farming contributing the most to such disparity.These results underscore that the environmental impact of the Brazilian economy can be drastically reduced by tackling the emission-intensive production processes of a few sectors only and disincentivizing the domestic consumption of a narrow range of products, especially with respect to the livestock segment.  相似文献   
3.
Interaction networks are widely used as tools to understand plant–pollinator communities, and to examine potential threats to plant diversity and food security if the ecosystem service provided by pollinating animals declines. However, most networks to date are based on recording visits to flowers, rather than recording clearly defined effective pollination events. Here we provide the first networks that explicitly incorporate measures of pollinator effectiveness (PE) from pollen deposition on stigmas per visit, and pollinator importance (PI) as the product of PE and visit frequency. These more informative networks, here produced for a low diversity heathland habitat, reveal that plant–pollinator interactions are more specialized than shown in most previous studies. At the studied site, the specialization index was lower for the visitation network than the PE network, which was in turn lower than for the PI network. Our study shows that collecting PE data is feasible for community-level studies in low diversity communities and that including information about PE can change the structure of interaction networks. This could have important consequences for our understanding of threats to pollination systems.  相似文献   
4.
水杉人工林树冠结构及生物生产力的研究   总被引:17,自引:2,他引:15  
研究了水杉人工林的树冠结构和林分生物生产力。结果表明,不同密度及林龄的林分树冠结构存在较大差异,随着树冠部位上升和林分密度增大,分枝角度逐渐减小;径阶大小与枝叶率成反比,与树冠重量成正比,径阶增大,树冠最大叶量层的集团上移,有效光合面积相对减少,树冠结构的变化直接影响到林分的生物量生产、分配分配和经济生物量,林分干、枝、叶的干物质累积趋势可用Richard方程描述;林龄增大,分配到主干的生物量比例  相似文献   
5.
地衣特殊性、多样性及其重要性   总被引:7,自引:0,他引:7  
陈健斌 《生物多样性》1995,3(2):113-117
本文从地衣二元性和地衣的真菌本质、地衣物种、生活环境和生命力、生长速度、代谢产物等方面阐述地衣的特殊性与多样性及其在共生进化研究、监测大气污染、测年、药用诸方面表现出的科学意义与实用价值。并在地衣的开发利用中,提出地衣多样性的保护与永续利用的建议。  相似文献   
6.
本文根据Fuzzy集理论,运用多级模型综合评判法,对巨峰系13个品种在江淮地区的经济性状表现作了评价。结果表明,江淮地区发展巨峰系葡萄,应以红伊豆三泽系红伊豆、藤稔三个早熟品种为主,还步缩小巨峰等中晚熟品种的比例。  相似文献   
7.
The seed coat morphology, investigated in taxa representative of the main European groups ofAconitum, are in good agreement with the current taxonomy of the genus. The seed coat microcharacteristics (warty epidermal cells) are very constant. There is a trend for the reduction of longitudinal wings on the edges concomitant with the development of ridges and transverse wings on the faces. Another morphological progression leads from smooth to rugulose and eventually to transverse wing-bearing seed faces. A working hypothesis suggests an ecological adaptative significance to these changes.  相似文献   
8.
经济开发区生态规划与建设研究   总被引:4,自引:0,他引:4  
陈涛  崔秋 《应用生态学报》1992,3(4):378-385
生态规划和建设是协调经济发展和生态环境关系,创造和谐的人工化环境的调控对策。木文研究了沈阳开发区生态规划和建设的理论、方法,着重探讨了开发区经济、环境协调发展的途径:1)选择理想的协调发展优化方案,2)控制发展热电生产,发展集中供热和低能耗高技术产业,3)节约水资源,提高水循环利用率,4)在生态学原理指导下进行生态管理。  相似文献   
9.
Recently, it has been indicated that systematics is of no value to the traditional Chinese medicinal studies and industry of macrofungi. As a fungal taxonomist, I use three genera of the most famous medicinal macrofungi, viz. Sanghuangporus, Ganoderma and Ophiocordyceps, as examples to rebut this opinion. Generally, the well-established systematics are helpful in using correct Latin names for fungal species in medicinal studies; furthermore, the resulting medicinal functions can be exactly matched to a particular species; finally, the particular species can be applied for permissions of utilization in traditional Chinese medicinal industry. Therefore, systematics is crucial for unlocking the potential of macrofungi as valuable resources to be developed in the traditional Chinese medicinal studies and industry, and should continue to be of interest to not just fungal taxonomists but also biochemists, pharmacists and businessmen.  相似文献   
10.
Biodiversity can be represented by different dimensions. While many diversity metrics try to capture the variation of these dimensions they also lead to a ‘fragmentation’ of the concept of biodiversity itself. Developing a unified measure that integrates all the dimensions of biodiversity is a theoretical solution for this problem, however, it remains operationally impossible. Alternatively, understanding which dimensions better represent the biodiversity of a set of communities can be a reliable way to integrate the different diversity metrics. Therefore, to achieve a holistic understand of biological diversity, we explore the concept of dimensionality. We define dimensionality of diversity as the number of complementary components of biodiversity, represented by diversity metrics, needed to describe biodiversity in an unambiguously and effective way. We provide a solution that joins two components of dimensionality – correlation and the variation – operationalized through two metrics, respectively: evenness of eigenvalues (EE) and importance values (IV). Through simulation we show that considering EE and IV together can provide information that is neglected when only EE is considered. We demonstrate how to apply this framework by investigating the dimensionality of South American small mammal communities. Our example evidenced that, for some representations of biological diversity, more attention is needed in the choice of diversity metrics necessary to effectively characterize biodiversity. We conclude by highlighting that this integrated framework provides a better understanding of dimensionality than considering only the correlation component.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号