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   112篇
  免费   17篇
  国内免费   12篇
  2023年   1篇
  2022年   3篇
  2021年   6篇
  2020年   4篇
  2019年   3篇
  2018年   4篇
  2017年   8篇
  2016年   7篇
  2015年   8篇
  2014年   4篇
  2013年   11篇
  2012年   4篇
  2011年   9篇
  2010年   7篇
  2009年   10篇
  2008年   6篇
  2007年   7篇
  2006年   5篇
  2005年   2篇
  2004年   2篇
  2003年   4篇
  2002年   1篇
  2001年   3篇
  2000年   1篇
  1999年   3篇
  1998年   2篇
  1997年   2篇
  1995年   1篇
  1994年   6篇
  1992年   1篇
  1991年   1篇
  1985年   1篇
  1984年   2篇
  1979年   1篇
  1976年   1篇
排序方式: 共有141条查询结果,搜索用时 15 毫秒
51.
河北侏罗-白垩纪界线最古老的恐爪龙类足迹   总被引:1,自引:0,他引:1  
记述中国河北省赤城县倪家沟化石点一组恐龙行迹,并命名一新属新种--中国猛龙足迹(Menglongipus sinensis ichnogen.et ichnosp.nov.).足迹来自土城子组,位于侏罗-白孚纪界线.相邻的义县组曾发现最古老的恐爪龙类骨骼化石,而中国猛龙足迹的发现表明恐爪龙类早在义县组之前便出现在该地区.中国猛龙足迹的造迹者体长约65cm,非常接近于基十的近鸟类.此外,为四川伶盗龙足迹(Velociraptorichnus sichuanensis)的模式标本提供更多细节,并讨论驰龙类足迹型与其他兽脚类足迹之间的联系.  相似文献   
52.
Based on known data sets and maximum entropy distribution data of fern and lycopod species registered in the Yucatán Peninsula, track and parsimony analyses were undertaken to evaluate the contribution of these groups to the establishment of biogeographical relationships of the peninsula with other areas. The resulting generalized tracks clearly agree with the geological origin of the peninsula and the previously recognized relationship with the Greater Antilles is not supported for ferns and lycopods. Instead, a Central American generalized track connects the Yucatán Peninsula with south‐eastern México and Central America. Floristically, the peninsula harbours 66 species of ferns and lycopods. Seven are registered for the first time in the Yucatán Peninsula and one is a new species for México. These species do not follow the latitudinal pattern expected if ecological factors, such as humidity and rainfall, were the most important in determining their distributions. Groups of areas recognized with parsimony analysis of endemicity could not be defined as provinces as a result of the lack of endemic species. Nevertheless, a regionalization scheme based on maximum entropy distribution data and supported by track analyses is proposed. Two separate districts are recognized within the Yucatán Peninsula: arid/dry Yucatán in the north and El Petén (humid) in the south. © 2009 The Linnean Society of London, Biological Journal of the Linnean Society, 2009, 98 , 775–786.  相似文献   
53.
运用CLIMEX软件和ArcGIS软件,对落叶松大小蠹Dendroctonus simplex Leconte在中国的适生区进行了预测.结果表明:落叶松大小蠹在我国有很高的适生性,适生范围涉及30个省、市、区,其中黑龙江、吉林、内蒙古、新疆东部、西藏中部和青海为高度适生区.  相似文献   
54.
近二十年来上海不同城市空间尺度绿地的生态效益   总被引:1,自引:0,他引:1  
凌焕然  王伟  樊正球  王祥荣 《生态学报》2011,31(19):5607-5615
基于CITYgreen模型,通过对1987、2000、2004和2008年4a的TM影像分析,从上海城市空间大尺度(上海市外环内整体)、中尺度(上海市外环内各行政区)和小尺度(上海市外环内各镇及街道)计算绿地年固碳和空气污染物去除两项生态效益,其中大尺度绿地的生态效益为:1987年为16722.58万元,2000年为8546.36万元,2004年为8161.48万元,2008年为968.47万元。继而利用基础统计学以及ArcGIS地统计分析扩展模块对生态效益进行分析,得出结论:(1)大尺度上分析,1987年至2008年,由于受到城市建设用地拓展的影响,城市绿地生态效益总体上呈下降趋势;(2)中尺度上分析,不同行政区绿地生态效益的浮动范围与变化趋势均不相同,中心城区外围的浦东区、宝山区、普陀区等一直处于较高水平且变动幅度较大,而中心城区一直处于较低状态;(3)小尺度上分析,基于克里格插值的曲面拟合分析表明中心城区部分区域出现回升。旨在通过模型拟合分析,完善上海市绿地生态效益研究,并为特大型城市绿地规划建设提供科学依据。  相似文献   
55.
目的对2 229份住院患者标本进行分离培养,并对非发酵菌分离株进行药敏试验,了解呼吸道与非呼吸道主要分离株的耐药率差异,以便指导临床合理使用抗菌药物。方法收集2008年8月至2010年10月分离自患者呼吸道、非呼吸道标本的革兰阴性非发酵菌,采用VITEK-32全自动微生物鉴定药敏仪进行细菌鉴定,药敏测定采用K-B法,并比较呼吸道与非呼吸道标本所检出的主要革兰阴性非发酵菌对抗菌药物的耐药率差异。结果共检出非发酵菌556株,其中呼吸道标本366株,非呼吸道标本190株,前3位非发酵菌分离株分别为铜绿假单胞菌、鲍曼不动杆菌和嗜麦芽窄食单胞菌。非发酵菌菌株ESBLs、AmpC、MBL、同产ESBLs+AmpC和同产ESBLs+MBL检出率分别为17.62%、13.67%、14.39、12.23%和6.12%,其中呼吸道标本分离株铜绿假单胞菌的ESBLs、AmpC检出率明显高于非呼吸道标本(均P<0.05),鲍曼不动杆菌的ESBLs、AmpC检出率明显低于非呼吸道标本(均P<0.05);呼吸道、非呼吸道标本的铜绿假单胞菌、鲍曼不动杆菌和嗜麦芽窄食单胞菌分离株对多种抗生素的耐药率不同,差异均有统计学意义(均P<0.05)。结论呼吸道与非呼吸道标本分离的主要非发酵菌分离株对同一抗菌药物耐药率不同,治疗不同部位非发酵菌引起的感染,要考虑由于感染部位不同而产生的耐药性以及药物有效浓度的差异,根据药敏结果合理使用抗菌药物。  相似文献   
56.
It is well recognized that human movement in the spatial and temporal dimensions has direct influence on disease transmission1-3. An infectious disease typically spreads via contact between infected and susceptible individuals in their overlapped activity spaces. Therefore, daily mobility-activity information can be used as an indicator to measure exposures to risk factors of infection. However, a major difficulty and thus the reason for paucity of studies of infectious disease transmission at the micro scale arise from the lack of detailed individual mobility data. Previously in transportation and tourism research detailed space-time activity data often relied on the time-space diary technique, which requires subjects to actively record their activities in time and space. This is highly demanding for the participants and collaboration from the participants greatly affects the quality of data4.Modern technologies such as GPS and mobile communications have made possible the automatic collection of trajectory data. The data collected, however, is not ideal for modeling human space-time activities, limited by the accuracies of existing devices. There is also no readily available tool for efficient processing of the data for human behavior study. We present here a suite of methods and an integrated ArcGIS desktop-based visual interface for the pre-processing and spatiotemporal analyses of trajectory data. We provide examples of how such processing may be used to model human space-time activities, especially with error-rich pedestrian trajectory data, that could be useful in public health studies such as infectious disease transmission modeling.The procedure presented includes pre-processing, trajectory segmentation, activity space characterization, density estimation and visualization, and a few other exploratory analysis methods. Pre-processing is the cleaning of noisy raw trajectory data. We introduce an interactive visual pre-processing interface as well as an automatic module. Trajectory segmentation5 involves the identification of indoor and outdoor parts from pre-processed space-time tracks. Again, both interactive visual segmentation and automatic segmentation are supported. Segmented space-time tracks are then analyzed to derive characteristics of one''s activity space such as activity radius etc. Density estimation and visualization are used to examine large amount of trajectory data to model hot spots and interactions. We demonstrate both density surface mapping6 and density volume rendering7. We also include a couple of other exploratory data analyses (EDA) and visualizations tools, such as Google Earth animation support and connection analysis. The suite of analytical as well as visual methods presented in this paper may be applied to any trajectory data for space-time activity studies.  相似文献   
57.
The potential distribution of Bletilla striata and its climate characteristics was predicted with MaxEnt model and ArcGIS. The data was based on the information of geographic distribution of the plant and its dominant climate factors. The results showed that the model built by dominant climate factors had an ideal forecasting effect, which was tested by the ROC curve (AUC > 09). The model revealed that the potential distribution areas were mainly in the south of the Qinling Mountains and Huai river. The major suitable provinces for the plant were mainly restricted to Yunnan, Hubei, Sichuan, Hunan and Jiangxi. A jackknife test in MaxEnt suggested that Min temperature of Apr(tmin4), Min temperature of Oct (tmin10), Temperature annual range (bio7) and Precipitation of Nov (prc11) were the most important climate variables affecting the potential distribution of the plant. The results can provide some theoretical basis for the resources protection, germplasm collections and reasonable development and utilization of wild Bletilla striata.  相似文献   
58.
Several new Early Cretaceous tracksites from the Lower Cretaceous Xiagou Formation of Gansu Province (China) with tracks of large sauropods and ornithopods are described. Previously reported bird tracks were missing due to human negligence. The studied specimens are preserved as impressions and shallow and deep natural track casts. These dinosaur tracks are first reported from the Jiuquan area in the Changma Basin, matching well with the skeletal record of diverse non-avian dinosaur-bird faunas of this region. Moreover, they add new data to the dinosaur ichnofaunas of the Lanzhou-Minhe Basin (Gansu Province) and indicate a wide distribution of dinosaur-bird assemblages in the Early Cretaceous. Regarding morphology, sauropod, and ornithopod tracks from the Lanzhou-Minhe Basin and the Jiuquan area are very similar to each other. Titanosauriform trackmakers are assumed for the sauropod tracks and possibly iguanodontids have left the large, tridactyl ornithopod tracks. Of particular interest are well-preserved, deep natural track casts of large ornithopods and sauropods preserving ridges and grooves as well as striation marks on the lateral sides of the casts that allow the reconstruction of complex pathways of the foot within the substrate. One particular sauropod pes–manus track cast even indicates lateral and vertical sliding within the sediment because of the presence of “double impressions of digits” on the bottom.  相似文献   
59.
Groundwater pollution risk assessment is a useful tool to prevent and control groundwater pollution, but its quantitative research is still relatively immature. In this study, a geographic information system based LVF model for a semiquantitative assessment of groundwater pollution risk is established, based on the groundwater pollution source load (L), groundwater vulnerability (V), and groundwater function value (F). Further, the pollution source load is characterized by seven specific pollution sources, the vulnerability is calculated by the modified DRASTIC method, and the groundwater function value is evaluated with the water quality and quantity. The model was used to assess the risk of shallow groundwater pollution in Shenyang city, Northeast China, with an area of 8,263 km2. The results show that highest risk areas account for 16.3% of the study area and are mainly distributed in the east of the study area. High risk areas are significantly affected by a shallow buried depth of groundwater and many types of harmful pollutants from industrial, agricultural, and domestic pollution sources. The evaluation results reflect the risk of groundwater pollution in the Shenyang area, which is relevant to the management and sustainable use of groundwater resources in the area.  相似文献   
60.
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