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   1273篇
  免费   339篇
  国内免费   327篇
安全科学   103篇
废物处理   27篇
环保管理   81篇
综合类   651篇
基础理论   819篇
污染及防治   163篇
评价与监测   64篇
社会与环境   12篇
灾害及防治   19篇
  2024年   1篇
  2023年   15篇
  2022年   29篇
  2021年   30篇
  2020年   28篇
  2019年   36篇
  2018年   112篇
  2017年   86篇
  2016年   96篇
  2015年   103篇
  2014年   127篇
  2013年   158篇
  2012年   130篇
  2011年   136篇
  2010年   107篇
  2009年   88篇
  2008年   73篇
  2007年   63篇
  2006年   50篇
  2005年   47篇
  2004年   47篇
  2003年   43篇
  2002年   32篇
  2001年   33篇
  2000年   61篇
  1999年   27篇
  1998年   20篇
  1997年   19篇
  1996年   21篇
  1995年   18篇
  1994年   16篇
  1993年   16篇
  1992年   19篇
  1991年   6篇
  1990年   3篇
  1989年   9篇
  1988年   6篇
  1987年   5篇
  1985年   3篇
  1984年   3篇
  1983年   6篇
  1982年   3篇
  1981年   1篇
  1980年   1篇
  1979年   1篇
  1978年   2篇
  1975年   1篇
  1974年   1篇
  1971年   1篇
排序方式: 共有1939条查询结果,搜索用时 656 毫秒
901.
Abstract: Fragmentation of animal and plant populations typically leads to genetic erosion and increased probability of extirpation. Although these effects can usually be reversed by re‐establishing gene flow between population fragments, managers sometimes fail to do so due to fears of outbreeding depression (OD). Rapid development of OD is due primarily to adaptive differentiation from selection or fixation of chromosomal variants. Fixed chromosomal variants can be detected empirically. We used an extended form of the breeders’ equation to predict the probability of OD due to adaptive differentiation between recently isolated population fragments as a function of intensity of selection, genetic diversity, effective population sizes, and generations of isolation. Empirical data indicated that populations in similar environments had not developed OD even after thousands of generations of isolation. To predict the probability of OD, we developed a decision tree that was based on the four variables from the breeders’ equation, taxonomic status, and gene flow within the last 500 years. The predicted probability of OD in crosses between two populations is elevated when the populations have at least one of the following characteristics: are distinct species, have fixed chromosomal differences, exchanged no genes in the last 500 years, or inhabit different environments. Conversely, the predicted probability of OD in crosses between two populations of the same species is low for populations with the same karyotype, isolated for <500 years, and that occupy similar environments. In the former case, we recommend crossing be avoided or tried on a limited, experimental basis. In the latter case, crossing can be carried out with low probability of OD. We used crosses with known results to test the decision tree and found that it correctly identified cases where OD occurred. Current concerns about OD in recently fragmented populations are almost certainly excessive.  相似文献   
902.
Abstract: Even under the most optimistic scenarios, during the next century human‐caused climate change will threaten many wild populations and species. The most useful conservation response is to enlarge and link protected areas to support range shifts by plants and animals. To prioritize land for reserves and linkages, some scientists attempt to chain together four highly uncertain models (emission scenarios, global air–ocean circulation, regional circulation, and biotic response). This approach has high risk of error propagation and compounding and produces outputs at a coarser scale than conservation decisions. Instead, we advocate identifying land facets—recurring landscape units with uniform topographic and soil attributes—and designing reserves and linkages for diversity and interspersion of these units. This coarse‐filter approach would conserve the arenas of biological activity, rather than the temporary occupants of those arenas. Integrative, context‐sensitive variables, such as insolation and topographic wetness, are useful for defining land facets. Classification procedures such as k‐means or fuzzy clustering are a good way to define land facets because they can analyze millions of pixels and are insensitive to case order. In regions lacking useful soil maps, river systems or riparian plants can indicate important facets. Conservation planners should set higher representation targets for rare and distinctive facets. High interspersion of land facets can promote ecological processes, evolutionary interaction, and range shift. Relevant studies suggest land‐facet diversity is a good surrogate for today's biodiversity, but fails to conserve some species. To minimize such failures, a reserve design based on land facets should complement, rather than replace, other approaches. Designs based on land facets are not biased toward data‐rich areas and can be applied where no maps of land cover exist.  相似文献   
903.
Abstract: Species occurrence in a habitat patch depends on local habitat and the amount of that habitat in the wider landscape. We used predictions from empirical landscape studies to set quantitative conservation criteria and targets in a multispecies and multiscale conservation planning effort. We used regression analyses to compare species richness and occurrence of five red‐listed lichens on 50 ancient oaks (Quercus robur; 120–140 cm in diameter) with the density of ancient oaks in circles of varying radius from each individual oak. Species richness and the occurrence of three of the five species were best explained by increasing density of oaks within 0.5 km; one species was best explained by the density of oaks within 2 km, and another was best predicted by the density of oaks within 5 km. The minimum numbers of ancient oaks required for “successful conservation” was defined as the number of oaks required to obtain a predicted local occurrence of 50% for all species included or a predicted local occurrence of 80% for all species included. These numbers of oaks were calculated for two relevant landscape scales (1 km2 and 13 km2) that corresponded to various species responses, in such a way that calculations also accounted for local number of oaks. Ten and seven of the 50 ancient oaks surveyed were situated in landscapes that already fulfilled criteria for successful conservation when the 50% and 80% criteria, respectively, were used to define the level of successful conservation. For cost‐efficient conservation, oak stands in the landscapes most suitable for successful conservation should be prioritized for conservation and management (e.g., grazing and planting of new oaks) at the expense of oak stands situated elsewhere.  相似文献   
904.
针对非常规突发事件的链式演化特征,为实现云应急环境下云应急管理体系内资源的智能化配置和敏捷高效的按需服务,对接入云端的应急资源、服务进行面向应急管理应用的分类,构建以"感知-决策-处置"为主线的云应急体系的按需服务机制;运用X列表(Bill of X,BOX)模型,分析云应急体系结构,提出面向云应急的X列表体系结构模型,建立列表要素间的语义描述框架和关联关系模型;设计了云应急模式下基于X列表的云应急平台的架构,以期为应急管理活动提供参考和依据。  相似文献   
905.
于2018年11月1日至2019年1月31日(OP2018-2019)和2019年11月1日至2020年1月20日(OP2019-2020)在青岛对PM1进行了连续两年秋冬季逐日采集.将观测期划分为4个空气质量等级(Ⅰ级、Ⅱ级、Ⅲ级和Ⅳ级),分析了PM1中金属元素浓度特征及来源,评估了不同人群的非致癌风险(Zn、Pb、Mn、Cu和V)和致癌风险(As、Cr、Ni、Cd和Co).结果表明,与OP2018-2019相比,OP2019-2020期间不同空气质量等级下金属元素总浓度变化与Ca、K和Al浓度变化有关,受扬尘源和生物质燃烧源影响较大.与OP2018-2019相比,OP2019-2020期间不同空气质量等级下V浓度分别下降19.0%、60.5%、82.7%和77.5%.推测与船舶国内排放控制区域(DECA)政策实施有关,青岛周边海域船舶改换燃油品质,导致V浓度大幅降低.由富集因子、比值法和气流后向轨迹结果进一步表明V浓度变化主要受DECA政策影响.然而,实施DECA政策后,V/Ni值作为判断区域内受船舶源影响的限值,需进一步探究.由健康风险评估结果表明,OP2018-2019和OP2019-2020期间非致癌元素Mn的危险系数范围为0.07~1.22,建议加强管控含Mn污染源的排放.OP2018-2019和OP2019-2020期间不同空气质量下As、Cd的终生致癌风险概率(ILCR)值低于10-4,但高于10-6,表明存在致癌概率,但仍可接受.OP2018-2019期间空气质量为Ⅳ级时,Cr的ILCR值高于10-4,存在致癌风险.  相似文献   
906.
Fujise H  Annoura T  Sasawatari S  Ikeda T  Ueda K 《Chemosphere》2002,46(9-10):1505-1511
Endocrine disrupters such as sex hormone-like chemicals and the non-physiological ligands for aryl hydrocarbon receptor (AhR) exert many adverse biological effects. The ligands for AhR disturb gene expression downstream of the gene induced by estrogen receptor at a very low concentration. Thus, transepithelial transport and cellular accumulation of cortisol (COR) and estrogen as congeners of sex hormone-like chemicals, and 3,3,4,4-tetrachlorobiphenyl (TeCB) as one of the ligands for AhR were examined in a monolayer of porcine kidney cells transfected with human P-glycoprotein (LLC-COL). The net basal-to-apical transport of COR increased in LLC-COL compared to that in the wild type cells (LLC-PK1) the same as in vinblastine, whereas the net transport of estradiol (EST) was not detected in either cell group. Though the diffusion transports of EST for both directions, basal-to-apical and apical-to-basal, were higher than that of COR, cellular accumulation of EST was higher than that of COR. Transepithelial transport of TeCB was very low and the net basal-to-apical transport was not detected, while it was highly accumulated in the epithelial cells. The accumulation was slightly higher in LLC-COL than in LLC-PK1 at high dose.  相似文献   
907.
中小企业“4+1”安全管理与监管模式研究   总被引:7,自引:5,他引:2  
在对全国中小企业安全生产状况调查的基础上,分析了中小企业的安全生产现状其数量多、绝大多数在乡镇街道、生产安全事故多发、职业病危害严重、从业人员素质低、生产力水平低;政府针对中小企业的安全监管人员数量少;乡镇街道的安全生产中介服务机构数量少,服务内容单一。因此,提出了中小企业安全生产监督管理“4+1”模式,其中“4”是指中小企业、政府安全监管部门、中介机构和社会监督部门的安全生产管理、监管、服务、监督体系,“1”是指提高监管效能的运行保障机制。该模式从宏观上为政府监管中小企业提出了一种新的思路,从微观上为中小企业、政府安全监管部门、中介服务机构、社会监督部门的安全生产工作提出了新的方法。  相似文献   
908.
The presence of Escherichia coli in recreational and potable waters is a major concern to the general public as elevated levels of E. coli suggest the presence of pathogenic bacteria and viruses. Unfortunately, traditional microbial techniques do not allow specific identification of the source of E. coli. This reduces the ability to target management practices that reduce bacterial contamination. In the Finger Lakes region of western New York, USA, wildlife resides in relatively high densities on watersheds dominated by people and dairy farms, and as a result, the sources of fecal degradation of potable and recreational waters are often unknown. In the Conesus Lake watershed, the sources of microbial contamination were assessed using Rep-PCR molecular tools, a method of amplifying repetitive DNA sequences found throughout the E. coli genome to produce distinct fingerprints for a given ecotype. Molecular fingerprints of E. coli isolated from regional populations of cattle, humans, geese and deer were compared to E. coli isolated from stream water samples. Canonical discriminant function analysis indicated that the DNA fingerprints of the original source group isolates were correctly predicted 90.2% of the time. Since land use in the sub-watersheds was dominated by dairy and cash crop farms, it was expected that the majority of E. coli isolated would be identified as cows; however, an unexpectedly high percentage of isolates were identified as wildlife (geese and deer). Geese were the dominant source of E. coli (44.7-73.7% of the total sources) in four sub-watersheds followed by cows (10.5-21.1%), deer (10.5-18.4%), humans (5.3-12.9%) and unidentifiable sources (0.0-11.8%). Management practices intended to decrease the number of cattle or the amount of manure spread in a sub-watershed were reflected in a decrease of E. coli ecotypes associated with dairy cows.  相似文献   
909.
阐述了活性污泥 1号模型 (ASM1)入流中 7个含碳有机物、4个含氮物质、碱度等四大类 13种组分的分析方法 ,并用这些方法测定了西安市北石桥、电子村两个排放口的城市污水水质。实验结果表明快速可生物降解有机物 (SS)采用间歇实验法、慢速可生物降解有机物 (XS)采用测定BOD5间接计算法既操作简单又准确可靠 ,能为利用活性污泥 1号模型进行污水处理厂的设计、模拟及管理提供入流组分分析依据  相似文献   
910.
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