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   10633篇
  免费   29篇
  国内免费   9篇
安全科学   71篇
废物处理   802篇
环保管理   1478篇
综合类   1003篇
基础理论   3391篇
环境理论   3篇
污染及防治   1903篇
评价与监测   1085篇
社会与环境   930篇
灾害及防治   5篇
  2023年   10篇
  2022年   13篇
  2021年   15篇
  2020年   16篇
  2019年   13篇
  2018年   1493篇
  2017年   1393篇
  2016年   1227篇
  2015年   148篇
  2014年   46篇
  2013年   78篇
  2012年   508篇
  2011年   1399篇
  2010年   734篇
  2009年   629篇
  2008年   932篇
  2007年   1278篇
  2006年   61篇
  2005年   58篇
  2004年   76篇
  2003年   107篇
  2002年   132篇
  2001年   35篇
  2000年   22篇
  1999年   22篇
  1998年   26篇
  1997年   15篇
  1996年   20篇
  1995年   22篇
  1994年   15篇
  1993年   13篇
  1992年   10篇
  1991年   9篇
  1990年   7篇
  1989年   7篇
  1988年   8篇
  1987年   6篇
  1986年   2篇
  1985年   4篇
  1984年   20篇
  1983年   12篇
  1982年   6篇
  1981年   4篇
  1979年   3篇
  1978年   5篇
  1977年   2篇
  1974年   2篇
  1968年   1篇
  1938年   1篇
  1935年   2篇
排序方式: 共有10000条查询结果,搜索用时 12 毫秒
901.
Geographic variation in vocalizations is widespread in passerine birds, but its origins and maintenance remain unclear. In this study, we test the hypothesis that song dialect, a culturally transmitted trait, is related to the population genetic structure of the orange-tufted sunbird, Nectarinia osea. To address this, we compared mitochondrial DNA (mtDNA) sequence variation together with allele frequencies at five microsatellite loci from an urban population of sunbirds exhibiting two distinct song dialects on a microgeographic scale. Our findings reveal no association between dialect membership and genetic composition. All genetic measures, from both mitochondrial and nuclear DNA, indicate high levels of gene flow between both dialect populations. The low F ST values obtained from mtDNA and microsatellite analysis imply that the variation among dialects does not account for more than 2%, at best, of the overall genetic variation found in the entire population. These measures fall well within the range of similar measures obtained in other studies of species exhibiting vocal dialects, most of which fail to detect any dialect-based genetic differentiation. The persistence of dialects in the orange-tufted sunbird may thus best be explained by dispersal of individuals across dialect boundaries and possibly from surrounding areas, followed by postdispersal vocal matching. Because genetic structuring appears weaker than cultural structure in this species, we discuss the behavioral mechanisms underlying dialect maintenance in the presence of apparent gene flow.  相似文献   
902.
The population of small copepod species (approximately <1 mm) were investigated during an annual cycle in Disko Bay, western Greenland. The small species considered were Acartia longiremis, Pseudocalanus spp., Oithona spp., Oncaea spp., Microsetella spp., and Microcalanus spp. Most of the small species were present in the surface waters year round and numerically dominated the community, and in biomass from late summer and throughout winter. Oithona spp. was numerically the main contributor, while Pseudocalanus spp. dominated in terms of biomass. In the uppermost 50 m, maximum abundance, biomass and secondary production were observed in late September after the phytoplankton production practically had terminated and the winter initiated. The free spawning Acartia longiremis showed a strong seasonal fluctuation in biomass and egg production, in contrast to the egg carrying species Pseudocalanus spp. and Oithona spp. These had a long spawning season and maintained a more stable biomass year round. Secondary production was estimated by three different ways: (1) based on the obtained specific egg production rates, (2) a temperature dependent equation, and (3) a multilinear regression taking temperature, body weight and chlorophyll into consideration. The contribution of the small species was insignificant when compared to the large Calanus species during the spring- and post-bloom. However, during late summer and winter, where Calanus had left the upper water strata for hibernation, the small species played a crucial role in the pelagic carbon cycling.  相似文献   
903.
In efforts such as land use change monitoring, carbon budgeting, and forecasting ecological conditions and timber supply, there is increasing demand for regional and national data layers depicting forest cover. These data layers must permit small area estimates of forest area and, most importantly, provide associated error estimates. This paper presents a model-based approach for coupling mid-resolution satellite imagery with plot-based forest inventory data to produce estimates of probability of forest and associated error at the pixel-level. The proposed Bayesian hierarchical model provides access to each pixel’s posterior predictive distribution allowing for a highly flexible analysis of pixel and multi-pixel areas of interest. The paper presents a trial using multiple dates of Landsat imagery and USDA Forest Service Forest Inventory and Analysis plot data. The results describe the spatial dependence structure within the trial site, provide pixel and multi-pixel summaries of probability of forest land use, and explore discretization schemes of the posterior predictive distributions to forest and non-forest classes. Model prediction results of a holdout set analysis suggest the proposed model provides high classification accuracy, 88%, for the trial site.
Ronald E. McRobertsEmail:
  相似文献   
904.
Summary.  Under laboratory conditions, the multicolored Asian lady beetle, Harmonia axyridis is well known as an intraguild predator of other ladybirds. However the real impact of this exotic species on native species was poorly investigated in the field. Because many ladybird species produce alkaloids as defensive compounds, we propose here a new method of intraguild predation monitoring in coccinellids based on alkaloid quantification by GC-MS. In laboratory experiments, adaline was unambiguously detected in fourth instar larvae of H. axyridis having ingested one egg or one first instar larva of Adalia bipunctata. Although prey alkaloids in the predator decreased with time, traces were still detected in pupae, exuviae and imagines of H. axyridis having ingested one prey when they were fourth instar larvae. Analysis of H. axyridis larvae collected in two potato fields shows for the first time in Europe the presence of exogenous alkaloids in 9 out of 28 individuals tested. This new method of intraguild predation detection could be used more widely to follow the interactions between predators and potential chemically defended insect preys.  相似文献   
905.
Till Tolasch 《Chemoecology》2008,18(3):177-180
Summary.   Ectinus aterrimus (L.) is a fairly common European click beetle species which develops mainly in forests. In pheromone gland extracts of female E. aterrimus, examined using gas chromatography – mass spectrometry (GC-MS), one single compound was present. This was identified as 7-methyloctyl 9-methyldecanoate by comparison with a synthetic sample. Field trapping trials revealed a highly significant attraction of male E. aterrimus towards this ester. The structure of the compound differs remarkably from the pheromones of the closely related Agriotes spp., which exclusively use terpene esters.  相似文献   
906.
In Finland, a new Government Decree on the Assessment of Soil Contamination and Remediation Needs (214/2007) came into force on 1 June 2007. According to the Decree, natural baseline concentration levels should be taken into account when assessing potential soil contamination and the need for remediation. This applies particularly in the case of toxic metallic elements, since baseline concentrations may naturally be rather high. The new decree prescribes soil screening values for 52 substances or groups of substances. The natural baseline concentrations have been taken into account in the definition of the screening values for inorganic pollutants. The Geological Survey of Finland (GTK) carried out a nation-wide geochemical mapping of till on a reconnaissance scale in 1983 and on a regional scale during 1984–1992. These surveys have provided information on natural elemental distribution in subsoils of the glacial till. However, some important trace elements in regional mapping such as arsenic are missing from the analysis, and subsoil samples do not reflect the diffuse anthropogenic input. Thus, GTK has continued the survey work by determining geochemical baselines around suburban areas. Samples have been taken from humus, topsoil and subsoil layers, and the soil parent material has covered sandy soils, glacial tills and fine-grained sediments. According to the studies, a regional difference in the levels of arsenic and some other trace elements can be clearly seen, especially in minerogenic soils. The results illustrate the importance of information on regional baseline concentrations while assessing potential soil contamination.  相似文献   
907.
Systematic geochemical mapping based on sampling and analysis of surface soils (0–2 cm) has been carried out in several Norwegian cities. The soils in the oldest parts of the cities are contaminated with metals (especially Pb) and polycyclic aromatic hydrocarbons (PAH). Due to the fact that children are often in contact with surface soil, it was realized that special focus had to be directed towards soils in day-care centers and playgrounds. The first mapping and remedy program was initiated in Trondheim in 1996. Here, the importance of copper–chromium–arsenic (CCA)-pressure-impregnated wood in playing equipment as a pollution source for soils was documented, and a process was started with the aim to ban this product. Soils from day-care centers in the inner city of Bergen were polluted to a degree that required remediation in 45% out of 87 centers, mainly due to high concentrations of Pb and benzo(a)pyrene. In Oslo, 38% of 700 day-care centers needed remediation due to soil pollution by Pb, BaP, Cd, Hg, Ni and PCB. Removal of CCA-impregnated wood was necessary in more than half of the day-care centers The Norwegian parliament has decided to investigate all outdoor playing areas in day-care centers, playgrounds and schoolyards in Norway, starting in 2007 with day-care centers in the ten largest cities and five most important industrial areas. The Norwegian Institute of Public Health has developed quality criteria for soils in day-care centers and playgrounds for As, Cd, Cr6+, Hg, Ni, Pb, zinc, PAHsum16, benzo(a)pyrene, and PCBsum7. The Geological Survey of Norway has developed guidance for mapping of soil pollution (sampling, chemical analysis and reporting) in day-care centers. Especially the sampling strategy has been developed in the period 1996–2007, and the preferred sampling strategy is to collect at least 10 samples of surface soil (0–2 cm) from (1) “original soil” on the site, (2) artificial man-made “hills”, and (3) soils used for growing vegetables. A total number of 2,000 day-care-centres are to be investigated, and necessary remediation should be completed before the end of 2010.  相似文献   
908.
909.
Summary.  We conducted a series of experiments with the white-spotted longicorn beetle Anoplophora malasiaca (Thomson), and its host plant, Citrus unshiu, to examine the origin of the sesquiterpene hydrocarbons (SHCs; including β-elemene, β-caryophyllene, α-humulene, α-farnesene, and several unidentified compounds) that are contained in the elytra of the beetles and act as an attractant. In the laboratory, mechanically wounded citrus branches, as well as those fed upon by A. malasiaca, attracted males more frequently than intact branches. Solid phase microextraction (SPME) and subsequent analyses by gas chromatography (GC) analyses detected measurable SHCs from the air around both mechanically wounded and beetle-infested branches, as well as trace amounts from intact branches. The SHCs were also detected for a certain time from beetles that had fed on the citrus branches, but the amounts decreased rapidly after they were removed from the host. This decrease generally corresponded with a reduction of attractiveness of the beetles in a behavioural assay. Isolated females acquired the SHCs after exposure to, but not upon contact with, other females that had fed on C. unshiu branches. We hypothesize that the citrus SHCs are adsorbed in, retained on, and released from the wax layer of the beetle elytra. Since these compounds are released from branches when beetles feed, they may indirectly signal the presence of beetles to others in the field. The high response rate to SHCs by males is likely representing mate searching behavior. The SHCs act as kairomones with a releaser effect in the communication system of A. malasiaca. Hiroe Yasui, Toshiharu Akino, Midori Fukaya: These authors contributed equally to this article.  相似文献   
910.
Parental care is a costly part of reproduction. Hence, natural selection should favor males which avoid caring for unrelated young. However, the decision to abandon or reduce care requires cues which are evaluated to give information on potential reproductive value of the offspring. The prediction that male sand gobies, Pomatoschistus minutus, care for foreign eggs as long as they were spawned in their own nest and at least some of such cues are fulfilled was tested. Egg-guarding males that had recently taken part in a spawning event were given a clutch of eggs that was sired either by themselves or another male, in either their own or another male’s aquarium. Males that had not taken part in a spawning event were used as controls and were given eggs sired by another male. We measured the amount of filial cannibalism and nest building. Control group males did not care for eggs and ate them all before rebuilding the nest. In the other treatments, there were no significant effects of paternity, though males moved to another male’s aquarium increased their clutch area threshold and completely consumed larger clutches than males that were not moved. There was no intermediate response in any treatment in the form of increased partial filial cannibalism or less well-constructed nests. Our results suggest that egg-guarding males cannot distinguish between eggs sired by themselves and those sired by other males but are able to react to cues indicating paternity state. Males do not adopt eggs to attract females in P. minutus.  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