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   505篇
  免费   33篇
  国内免费   52篇
  2023年   17篇
  2022年   8篇
  2021年   22篇
  2020年   14篇
  2019年   15篇
  2018年   24篇
  2017年   18篇
  2016年   26篇
  2015年   28篇
  2014年   27篇
  2013年   36篇
  2012年   26篇
  2011年   20篇
  2010年   15篇
  2009年   23篇
  2008年   18篇
  2007年   30篇
  2006年   19篇
  2005年   22篇
  2004年   15篇
  2003年   13篇
  2002年   17篇
  2001年   26篇
  2000年   27篇
  1999年   21篇
  1998年   25篇
  1997年   12篇
  1996年   6篇
  1995年   2篇
  1994年   1篇
  1993年   4篇
  1992年   4篇
  1990年   1篇
  1989年   2篇
  1988年   1篇
  1987年   1篇
  1985年   1篇
  1984年   1篇
  1983年   1篇
  1978年   1篇
排序方式: 共有590条查询结果,搜索用时 734 毫秒
61.
河流生态系统健康及其评价   总被引:103,自引:4,他引:99  
概括了河流生态系统健康概念的涵义,详细介绍了以着重藻类、无脊椎动物、鱼类为主要指示生物的河流生态系统健康的评价方法,并就其评价管理将这些方法分为预测模型法(predictive models)和多指标法(multimetrics两大类,提出了河流健康评价方法的发展方向,以及我国应尽快开展的研究工作。  相似文献   
62.
有蹄类动物营养状况的测定方法及其评价   总被引:9,自引:0,他引:9  
营养状况对于动物的生存和繁殖至关重要,是有蹄类动物种群管理的前提条件之一。文章综述了有蹄类动物营养状况的几种主要测定方法,简要介绍了这些方法的理论前提、基本模型、适用范围,并评价了它们的优缺点及如何提高其准确性的方法和途径。  相似文献   
63.
The construction of large reservoirs in South America and particularly in Brazil has intensified within the last 50 years. Built up primarily for hydroelectricity production, these artificial ecosystems now serve purposes such as: water storage for public use; fisheries and aquaculture; recreation; tourism, and irrigation. These artificial ecosystems were also built up with the purpose of enhancing the regional development. These activities produce multiple impacts among which are eutrophication, a serious problems with various ecological, economic, and social consequences. Basic studies on reservoirs have identified their main ecological characteristics and described some of their fundamental mechanisms of functioning. This information cover: factors involved in reservoir complexity, such as spatial scale, vertical and horizontal heterogeneity, temporal variation at several time scales and, in some cases, watershed/reservoir relationships, and the impact of non-point and point sources of nutrients. Numerous studies have been dedicated to describing aquatic biota, its relationship with environmental factors, and the impacts of degradation/pollution/ eutrophication on biodiversity. Research priorities have been established mainly by limnologists in response to needs identified by environmental impact assessment. Due to the pressure of multiple uses, population growth, and economic factors, aquatic scientists now face a number of questions being posed by managers and engineers. These questions relate to a wide range of practical problems, the solutions to which depend on accumulated data related to the structural complexities and function mechanisms previously referred to. These problems can be divided into three main groups: (i) eutrophication processes and their characteristics, and water quality control; (ii) impact of fisheries, aquaculture, and exotic species introduction on the biota and the water quality; (iii) sustainable development of reservoirs and optimization management of their multiple uses. Predictive capabilities required in the face of these problems should be based on modeling development and intensive use of data-bases resulting from long-term studies on reservoirs. The predictions thus made possible would involve economic factors and the costs of recovery of eutrophic reservoirs. The experience of reservoir research and management in South America and in Brazil is, probably part of a world wide tendency on reservoir research and development with the aim to optimize multiple uses. Based on the cooperation of limnologists and engineers, ecosystem management models must eventually be applied at the watershed / reservoir level, where demands on the part of managers and decision makers will certainly promote a predictive, integrated, and adaptive approach,based on fundamental research.  相似文献   
64.
我国蝗虫暴发成灾的现状及其持续控制对策   总被引:18,自引:2,他引:16  
概述并分析了我国蝗虫暴发成灾的现状及原因 ,认为全球气候变暖和区域性气候异常、人类活动的影响、监测预警技术手段落后、成灾蝗种增多和发生为害类型复杂、蝗虫研究队伍锐减及基础理论与高新技术研究滞后导致了近年来蝗虫的暴发成灾。对此 ,作者从蝗虫灾害的规律性、监测预警高新技术和综合治理技术等方面提出了实现蝗灾可持续控制的对策  相似文献   
65.
Linoleum is a floor covering consisting mainly of linseed oil, other vegetable oils, wood flour and limestone on a carrier of jute. Forbo-Krommenie B.V. commissioned the Centre of Environmental Science (CML) to carry out an Environmental Life Cycle Assessment for linoleum floors. The goal of this study was to assess the environmental performance of linoleum floors, indicating possible options for improvement, and assessing the sensitivity of the results to methodological choices. The functional unit was defined as: 2000 m2 linoleum produced in 1998, used in an office or public building over a period of 20 years. The method followed in this study is based on a nearly final draft version of the LCA guide published by CML in corporation with many others, which is an update of the guide on LCA of 1992. From the contribution analysis, the main contributing processes became clear. In addition, the sensitivity analysis by scenarios showed that the type of maintenance during use and the pigments used can have a large influence on the results. Major data gaps of the study were capital goods and unknown chemicals. Sensitivity analysis also showed that these gaps can lead to an underestimation. Based on this study, some options to improve the environmental performance of linoleum were formulated and advice for further LCA studies on linoleum was given.  相似文献   
66.
The environmental performance of a color computer monitor is investigated by implementing a Life Cycle Assessment. The goal of this study is to collect LCI data of foreground systems, to identify hot spots, and to introduce life cycle thinking at the product design stage. Secondary data are used in the background system, and site-specific data are collected in the foreground system. Results show that the use phase is the most contributing phase. The operating mode and the energy saving mode during the overall use phase contribute to the total by 59% and by 9.9%, respectively. In the production phase, the cathode ray tube assembly process and the printed circuit board assembly process are the most contributing processes. The sensitivity analysis on the use pattern scenario shows that the contribution ratio of the use phase ranges from 32% to 84%. Even in the home use case, which is the best case scenario, the use phase is one of the most contributing processes to the environmental performance of the color computer monitor. There is no significant difference in the choice of the impact assessment methodologies for identifying the improvement opportunities. For the external use of Life Cycle Assessment in a short-run product for the market, it is recommended that Life Cycle Assessment should be carried out in parallel with the product design stage. It is also necessary to have a pre-existing, in-house database for a product group in order to accelerate life cycle procedures.  相似文献   
67.
An integrated impact assessment model is used to calculate the impact per tonne of SO2, NOx, fine particles, and NMVOC emitted from different source countries on human health, acidification, eutrophication, and the man-made environment (crop yield and building materials). Indicators on the endpoint level are used to measure the effects resulting from a marginal change in emission levels. While the assessment of impacts on ecosystems and the man-made environment is limited to Europe, damage factors for health effects are also derived for Asia and South America. For Europe, emission scenarios for the years 1990 and 2010 are considered to analyse the influence of changing background conditions on the resulting impacts. Results show that there is a significant variation in the damage resulting from a unit emission for some of the impact categories, both between countries and between base years. Depending on the scope of the study and the information available from the life cycle inventory, results from the paper can be used to consider site dependent conditions in life cycle impact assessment as a complement to the current site-independent (or global) approach.  相似文献   
68.
Global Scenarios: Background Review for the Millennium Ecosystem Assessment   总被引:1,自引:0,他引:1  
Paul D. Raskin 《Ecosystems》2005,8(2):133-142
The long-range outlook for the world’s ecosystems depends on the course taken by global development in the coming decades. Current global trends and ecological dynamics are consistent with very different outcomes, defined by alternative assumptions about the technological, economic, demographic, geopolitical, and social aspects of development and the ways in which institutions, personal and public values, and natural systems may be expected to respond to historically novel stressors. Recent advances in scenario analysis have addressed the dual methodological challenge of exploring these uncertainties in an organized way and determining what would be needed to make the transition to sustainability. This paper reviews global scenario research, setting current efforts in a historical context. It focuses on seven recent studies that are comprehensive, regionally disaggregated, and narratively rich—and thus of greatest relevance to the Millennium Ecosystem Assessment (MA). It summarizes their social visions and the level of quantitative detail used in these exercises. Taken together, this suite of global scenario studies provides a useful platform for the MA by offering insight into the complex factors that drive ecosystem change, estimating the magnitude of regional pressures on ecosystems, sounding the alert on critical uncertainties that could undermine sustainable development, and understanding the importance of institutions and values. But these studies are only a point of departure. The integration of changing ecosystem conditions into global development scenarios, as both effects and causes, is at the cutting edge of scenario analysis. The paper concludes by identifying directions for this research program and suggesting ways that the MA can contribute to this effort.  相似文献   
69.
Protein aggregation is a widespread phenomenon with important implications in many scientific areas. Although amyloid formation is typically considered as detrimental, functional amyloids that perform physiological roles have been identified in all kingdoms of life. Despite their functional and pathological relevance, the structural details of the majority of molecular species involved in the amyloidogenic process remains elusive. Here, we explore the application of AlphaFold, a highly accurate protein structure predictor, in the field of protein aggregation. While we envision a straightforward application of AlphaFold in assisting the design of globular proteins with improved solubility for biomedical and industrial purposes, the use of this algorithm for predicting the structure of aggregated species seems far from trivial. First, in amyloid diseases, the presence of multiple amyloid polymorphs and the heterogeneity of aggregation intermediates challenges the “one sequence, one structure” paradigm, inherent to sequence-based predictions. Second, aberrant aggregation is not the subject of positive selective pressure, precluding the use of evolutionary-based approaches, which are the core of the AlphaFold pipeline. Instead, amyloid polymorphism seems to be constrained by the need for a defined structure-activity relationship in functional amyloids. They may thus provide a starting point for the application of AlphaFold in the amyloid landscape.  相似文献   
70.
The shortage of data for emissions from agricultural tractors contributes to LCA results on environmental load from modern crop production possibly having high error levels and high uncertainties. The first part of this work describes measurements and calculations made in order to obtain operation-specific agricultural emission data. Calculations are based on emission data measured on a standard 70 kW tractor of a widely available make. In the second part, results from an LCI on wheat production based on traditionally used emission data are calculated and compared with results obtained when using the emission data for specific working operations derived in part one. One conclusion of the study is that the emission values, when related to the energy in the used fuel, show very large variations between different driving operations. Another conclusion is that the use of the new data results in a marked reduction of the total air emissions produced in the wheat production chain, especially for CO and HC, but also for NOx and SO2.  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