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   2126篇
  免费   245篇
  国内免费   670篇
安全科学   74篇
废物处理   35篇
环保管理   249篇
综合类   1111篇
基础理论   1144篇
污染及防治   179篇
评价与监测   77篇
社会与环境   162篇
灾害及防治   10篇
  2024年   8篇
  2023年   61篇
  2022年   105篇
  2021年   107篇
  2020年   137篇
  2019年   100篇
  2018年   88篇
  2017年   105篇
  2016年   115篇
  2015年   141篇
  2014年   123篇
  2013年   186篇
  2012年   163篇
  2011年   196篇
  2010年   148篇
  2009年   146篇
  2008年   142篇
  2007年   182篇
  2006年   118篇
  2005年   119篇
  2004年   88篇
  2003年   77篇
  2002年   56篇
  2001年   46篇
  2000年   47篇
  1999年   32篇
  1998年   31篇
  1997年   30篇
  1996年   30篇
  1995年   22篇
  1994年   21篇
  1993年   17篇
  1992年   12篇
  1991年   6篇
  1990年   7篇
  1989年   4篇
  1988年   4篇
  1987年   4篇
  1986年   5篇
  1984年   6篇
  1982年   2篇
  1981年   1篇
  1976年   1篇
  1973年   1篇
  1972年   1篇
排序方式: 共有3041条查询结果,搜索用时 453 毫秒
201.
Abstract:  The World Conservation Union (IUCN) Red List Index (RLI) is used to measure global trends in the status of biodiversity. We examined how the index might be used to measure the trend in the status of indigenous breeding birds in British Columbia between 1992 and 2006. We followed the RLI method described by Butchart et al. (2004, 2007) as closely as possible . Because IUCN Red List assessments at the regional level are not available in British Columbia, we used NatureServe S (subnational) ranking data. We calculated three index trend lines. The first two of these allowed us to compare an index based on our original data to one based on data that had been retrospectively corrected; the latter produced a smooth, flat line. A third trend line, based on the corrected data but excluding species new to province since 1947, produced a gently sloping downward trend. Ongoing immigration of bird species in and out of British Columbia added to the complexity of interpreting our regional RLI-type index, especially because our S-rank data did not incorporate transboundary "rescue" effects. Because the RLI is scaled so that the maximum value is based on a state in which all species are simultaneously ranked as least concern, it may exaggerate the highest potential status of intrinsically vulnerable species. A simpler, more intuitive graphic allows reporting that is less dependent on context. We believe the RLI approach holds useful innovation for an indicator of change in biodiversity within jurisdictional boundaries.  相似文献   
202.
The population density and relative abundance of some bird species reach the highest values in the communities of habitats with a high degree of anthropogenic transformation. The species diversity of birds shows an inverse correlation with the index of anthropogenic impact (in grades) calculated with regard to a number of parameters. Relevant correlations change periodically, with the correlation coefficient reaching the highest values in the nesting period and decreasing to a minimum during the flight period.  相似文献   
203.
To reveal the effect of forest fires on the avifauna of the Northern Amur Region, bird assemblages of primary dark conifer-deciduous forests, wild rosemary larch forests, and secondary larch-birch forests have been studied in the Komsomol’skii and Norskii nature reserves. It has been shown that the replacement of primary dark conifer-deciduous forests by secondary forests is accompanied by a significant decrease in bird population density, with some species being lost and the composition of the dominant species group being changed. On the other hand, fires lead to increasing patchiness of the environment, which can sometimes provide for an increase in biological diversity.  相似文献   
204.
The relation among inbreeding, heterozygosity, and fitness has been studied primarily among outbred populations, and little is known about these phenomena in endangered populations. Most researchers conclude that the relation between coefficient of inbreeding estimated from pedigrees and fitness traits (inbreeding‐fitness correlations) better reflects inbreeding depression than the relation between marker heterozygosity and fitness traits (heterozygosity‐fitness correlations). However, it has been suggested recently that heterozygosity‐fitness correlations should only be expected when inbreeding generates extensive identity disequilibrium (correlations in heterozygosity and homozygosity across loci throughout the genome). We tested this hypothesis in Mohor gazelle (Gazella dama mhorr) and Iberian lynx (Lynx pardinus). For Mohor gazelle, we calculated the inbreeding coefficient and measured heterozygosity at 17 microsatellite loci. For Iberian lynx, we measured heterozygosity at 36 microsatellite loci. In both species we estimated semen quality, a phenotypic trait directly related to fitness that is controlled by many loci and is affected by inbreeding depression. Both species showed evidence of extensive identity disequilibrium, and in both species heterozygosity was associated with semen quality. In the Iberian lynx the low proportion of normal sperm associated with low levels of heterozygosity was so extreme that it is likely to limit the fertility of males. In Mohor gazelle, although heterozygosity was associated with semen quality, inbreeding coefficient was not. This result suggests that when coefficient of inbreeding is calculated on the basis of a genealogy that begins after a long history of inbreeding, the coefficient of inbreeding fails to capture previous demographic information because it is a poor estimator of accumulated individual inbreeding. We conclude that among highly endangered species with extensive identity disequilibrium, examination of heterozygosity‐fitness correlations may be an effective way to detect inbreeding depression, whereas inbreeding‐fitness correlations may be poor indicators of inbreeding depression if the pedigree does not accurately reflect the history of inbreeding. Correlaciones Heterocigosidad‐ Adaptabilidad y Depresión Endogámica en Dos Especies de Mamíferos Críticamente en Peligro  相似文献   
205.
206.
207.
Adaptation takes place in both private and public sectors, or as an interrelation between the two, and often under the realm of public regulation. Thispaper uses the Swedish and the Norwegian electricity grid sector, as providers of a vital public good under strict public regulation, to analyse reforms' effects on adaptive capacity in this area. The paper shows that transformational changes in both sectors during the 1990s shifted both the formal organisational structure (rules and regulations), as well as the organisational culture, in the direction of economic efficiency. These two dimensions individually reduced adaptive capacity to climate change, although differently in the two countries. However, the formal structure and organisational culture also yielded substantial influence on each other. This leads to the conclusion that when designing public regulations and implementing reforms, organisational culture must be considered in the design. Also the paper contests previous findings in the literature by showing that under given conditions there exist some substitution between the two dimensions in influencing adaptive capacity, implying that both dimensions should be included when analysing adaptation since analysing them in isolation is likely to lead to wrong conclusions.  相似文献   
208.
目前广泛使用的水质基准推导方法—物种敏感度分布法存在曲线拟合模型不确定、曲线拟合效果不佳、种内差异欠考虑、基准值不准确等诸多问题,概率物种敏感度分布法可有效解决上述问题。应用概率物种敏感度分布法构建了太湖水体中5种重金属Ag、Pb、Cd、Hg和Zn的概率物种敏感度分布曲线,在此基础上得到了保护水生生物的急性水质基准分别为1.079μg·L~(-1)、637.973μg·L~(-1)、19.465μg·L~(-1)、8.729μg·L~(-1)和105.506μg·L~(-1),慢性水质基准分别为0.108μg·L~(-1)、63.797μg·L~(-1)、1.947μg·L~(-1)、2.340μg·L~(-1)和52.753μg·L~(-1);不同类群间生物对重金属的敏感度存在差异,不同重金属对同一类群生物的毒性也存在差异;通过与国内外已有的重金属水质基准值比较,发现水质基准具有明显的区域性,目前基于国外水质基准或我国整体水域特点来制定的太湖水质标准,往往造成对太湖水生生物欠保护或过保护的状况。  相似文献   
209.
基于前向神经网络的广义环境系统评价普适模型   总被引:2,自引:1,他引:1  
为了建立由水环境、空气环境、生态环境、水资源环境、灾害环境、遥感环境、社会经济环境等不同环境系统组成的广义环境系统评价都能普适、通用的神经网络模型,针对BP神经网络因收敛速度慢、易于陷入局部极值而使实用性受限的缺陷,提出以双极性sigmoid函数作为网络隐层节点(神经元)的激活函数,而网络输出为所有隐层节点输出的线性求和的前向神经网络的广义环境系统评价模型.在设置广义环境系统指标参照值和指标值规范变换式,并对指标值进行规范变换的基础上,分别构建了适用于广义环境系统评价的任意2个指标规范值的前向神经网模型(NV-FNN(2)结构)和任意3个指标规范值的前向神经网模型(NV-FNN(3)结构).而对于指标较多的广义环境系统评价,只要将多指标分解为以上2个指标和3个指标的两种简单结构的前向神经网络的广义环境系统评价模型的组合表示即可.理论分析和实例检验结果表明:该模型对任意广义环境系统的规范指标值皆普适、通用,因而使不同环境系统的评价变得简洁、统一.规范变换和优化算法相结合的建模思想和方法对简化广义环境系统评价的多元回归、投影寻踪回归、回归支持向量机和径向基神经网络建模亦有借鉴和启迪作用.  相似文献   
210.
水环境中不同形态氮对甲芬那酸光降解影响机制的研究   总被引:2,自引:2,他引:0  
研究了紫外光照射下水环境中不同形态的无机氮(NO-3、NO-2和NH+4)对甲芬那酸(MEF)光解的影响.NO-3与NO-2均促进MEF的光解,NH+4对MEF的光解基本无影响;在NO-3、NO-2存在下添加适量异丙醇,显著抑制MEF的降解,实验表明NO-3、NO-2在光照下产生了·OH并参与对MEF的氧化降解.同时模拟研究了水体处于不同p E值下,水中存在的不同形态的无机氮对MEF光解的复合影响,其对MEF光解促进作用不是简单的叠加关系,增大p E值,MEF的光解速率先增大后减小.当NO-2和NH+4共存时,对MEF的光解主要表现为NO-2的影响;当NO-2和NO-3共存时,两者对MEF的光解存在拮抗作用.  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