全文获取类型
收费全文 | 198篇 |
免费 | 38篇 |
国内免费 | 15篇 |
专业分类
安全科学 | 75篇 |
废物处理 | 1篇 |
环保管理 | 21篇 |
综合类 | 57篇 |
基础理论 | 61篇 |
污染及防治 | 13篇 |
评价与监测 | 5篇 |
社会与环境 | 3篇 |
灾害及防治 | 15篇 |
出版年
2023年 | 6篇 |
2022年 | 5篇 |
2021年 | 11篇 |
2020年 | 8篇 |
2019年 | 6篇 |
2018年 | 11篇 |
2017年 | 7篇 |
2016年 | 14篇 |
2015年 | 13篇 |
2014年 | 12篇 |
2013年 | 18篇 |
2012年 | 13篇 |
2011年 | 17篇 |
2010年 | 7篇 |
2009年 | 14篇 |
2008年 | 11篇 |
2007年 | 9篇 |
2006年 | 14篇 |
2005年 | 8篇 |
2004年 | 11篇 |
2003年 | 7篇 |
2002年 | 1篇 |
2001年 | 7篇 |
2000年 | 6篇 |
1999年 | 2篇 |
1998年 | 1篇 |
1997年 | 2篇 |
1996年 | 1篇 |
1995年 | 1篇 |
1994年 | 1篇 |
1992年 | 2篇 |
1991年 | 1篇 |
1990年 | 1篇 |
1978年 | 1篇 |
1977年 | 1篇 |
1971年 | 1篇 |
排序方式: 共有251条查询结果,搜索用时 31 毫秒
101.
JINLIANG WANG 《Conservation biology》2004,18(2):332-343
Abstract: Endangered species are commonly found in several (partially) isolated populations dispersed on different fragments of a habitat, natural reserve, or zoo. A certain level of connectivity among such populations is essential for maintaining genetic variation within and between populations to allow local and global adaptation and for preventing inbreeding depression. A rule of thumb widely accepted by the conservation community is that one migrant per generation (OMPG) into a population is the appropriate level of gene flow. This rule is based on Wright's study of his island model under a long list of simplifying assumptions. I examined the robustness of the OMPG rule to the violation of each of the many assumptions, quantifying the effect with population genetics theory. I showed that, when interpreted as one effective migrant per generation, OMPG is generally valid for real populations departing from the ideal model in the discrepancies of actual ( N ) and effective ( Ne ) population sizes and actual ( m ) and effective ( me ) migration rates. I also addressed the issue of converting the effective number of migrants ( Me = Ne me ) into the actual number of migrants ( M = Nm ) of a certain age and sex. In particular, Ne < N , a case common for natural populations, did not necessarily require M > Me to maintain a certain level of differentiation among populations. Rather, translating the elusive Me into the manageable M depends on the specific causes (e.g., biased sex ratio, reproductive skew) that lead to Ne < N . 相似文献
102.
Madeleine?BeekmanEmail author Benjamin?P.?Oldroyd 《Behavioral ecology and sociobiology》2003,54(5):480-484
Worker-reproduction is rare in queenright honey-bee colonies. When workers do lay eggs, their eggs are normally eaten by other workers presumably because they lack the queen's egg-marking signal. Workers use the absence of this queen signal to enforce the queen's reproductive monopoly by policing any worker-laid eggs. In contrast, in anarchistic colonies, the majority of the males arise from worker-laid eggs. Anarchistic worker-laid eggs escape policing because workers perceive anarchistic eggs as queen-laid. However, in this study, we show that eggs laid by queenless anarchistic workers do not escape policing and have very similar removal rates to worker-laid eggs from queenless wild-type (i.e. non-anarchistic) colonies. This suggests that, under queenless conditions, eggs laid by anarchistic workers lose their chemical protection and are therefore no longer perceived as queen-laid. Hence, the egg-marking signal seems to be only applied to eggs when queen and brood are present. This suggests that in the absence of queen and brood, the biosynthetic pathway that produces the egg-marking signal is switched off.Communicated by L. Keller 相似文献
103.
Rulon W. Clark 《Behavioral ecology and sociobiology》2005,59(2):258-261
A thorough understanding of communication requires an evaluation of both the signaler and receiver. Most analyses of prey–predator communication are incomplete because they examine only the behavior of the prey. Predators in these systems may be understudied because they are perceived as less tractable research subjects, due to their more cryptic hunting behaviors and secretive lifestyles. For example, research on interactions between rodents and rattlesnakes has focused on the behavior of rodent signalers, while responses of snakes have been virtually unexamined. Rattlesnakes are ambush predators, and capture rodents by waiting at foraging sites for long periods of time. In this study, I take advantage of the sedentary nature of this foraging strategy and use fixed videography to record natural encounters between timber rattlesnakes (Crotalus horridus) and their prey. Three different prey species were found to exhibit conspicuous visual displays to snakes, both when snakes were actively foraging, and when they were basking. After receiving displays, foraging snakes left their ambush sites and moved long distances before locating subsequent ambush sites, indicating that they responded to displays by abandoning attempts to ambush prey in the vicinity of signalers. This study represents the first quantitative analysis of the response of free-ranging snakes to signals from their prey, and elucidates a technique by which such quantitative data can be more easily obtained. 相似文献
104.
This study investigated the effects of multiple cognitive tasks on human performance. Twenty-four students at North Carolina A&;T State University participated in the study. The primary task was auditory signal change perception and the secondary task was a computational task. Results showed that participants' performance in a single task was statistically significantly different from their performance in combined tasks: (a) algebra problems (algebra problem primary and auditory perception secondary); (b) auditory perception tasks (auditory perception primary and algebra problems secondary); and (c) mean false-alarm score in auditory perception (auditory detection primary and algebra problems secondary). Using signal detection theory (SDT), participants' performance measured in terms of sensitivity was calculated as ?0.54 for combined tasks (algebra problems the primary task) and ?0.53 auditory perceptions the primary task. During auditory perception tasks alone, SDT was found to be 2.51. Performance was 83% in a single task compared to 17% when combined tasks. 相似文献
105.
The noise included in pipeline pressure signal is a small noise whose energy takes a small proportion of pressure signal and is concentrated on high frequency components. However, it will influence pipeline leakage identification and even cause false alarms. Thus, a small-noise reduction method based on EMD (SNR-EMD) is proposed to remove small noise from pressure signal. EMD is applied for extracting the mean envelope of the signal. Then, small fluctuations around the mean envelope are considered to be small noises. Meanwhile, end effect of SNR-EMD is restrained by extrema mirror extension (EME). The results of simulation studies with SNR-EMD show that the larger the noisy signal's signal-to-noise ratio (SNR) is, the better noise reduction effect becomes. And SNR-EMD considered as a low-pass filter removes or reduces the high frequency components. Furthermore, superiorities of SNR-EMD are verified by comparison studies with wavelet packet transform (WPT) and singular value decomposition (SVD). Finally, a case study of leakage identification shows that SNR-EMD can improve the performance of leakage identification and reduce the possibility of false alarms, which makes much easier and further effective to distinguish the leakage mode from other modes after removing noise from pressure signal. 相似文献
106.
目的总结分析振动试验信号循环全过程的数学模型及传递关系。方法根据信号传递过程中的作用,将振动设备划分为五类关键设备,并且将信号流在信号转换、信号可测及运动可控等方面的传递关系进行分析汇总,最后给出以控制仪为回路节点的两类开环故障的影响分析。结果通过对信号流循环过程的分析研究,总结了闭环反馈控制过程各环节中的信号传递关系,分析了各关键设备性能对振动试验信号传递的影响,以便于试验人员更好地开展振动试验。结论信号流循环的正常运转是振动试验顺利开展的基础,深入了解信号传递关系和影响因素,可以高质量地开展振动试验。 相似文献
107.
Earlier studies on fault diagnosis of the pipeline and pump unit systems (PPU) relied mainly on independent equipment analyses, which usually lead to false alarms because of the loss of information fusion. The aim of this study is to utilize the status coupling relationship to improve fault detection sensitivity and reduce false alarm rate. A real-time status identification of related equipment step is added between capturing abnormal signals and listing out diagnosis results. For example, when the pipeline pressure fluctuation is found abnormal, a status analysis of pump units is performed immediately, if the pump units are proven to be operational normally, then the pipeline leak alarm is acknowledged valid. The logical reasoning algorithm is used to capture abnormal conditions of pipeline pressures. The pump unit faults are captured by combining information from multiple sources. Field applications show that the proposed method significantly improves the PPU fault detection capability on fault detection sensitive and accuracy. 相似文献
108.
Robinson L. Contiero Fabiano A. Rios Denis F. Biffe Jamil Constantin Rubem S. de Oliveira Jr 《Journal of environmental science and health. Part. B》2013,48(10):803-809
AbstractThe aim of this study was to evaluate the effect of time of the day and their associated climatic conditions on spray deposition of two 2,4-D formulations, as well as the influence on weed control. The experiment was installed in the field in complete randomized design. Treatments were arranged in factorial design 8?×?2, with 20 repetitions. First factor corresponded to different application time (1:00, 4:00, 7:00, 10:00, 13:00, 16:00, 19:00, and 22:00) with their respective climatic conditions. The second factor consisted of two formulations of 2,4-D applied at 776?g a.e. ha?1 (2,4-D amine and 2,4-D choline salt with Colex-D? Technology) + glyphosate (816?g a.e. ha?1). There was more spray deposition when 2,4-D choline formulation was used, and such differences were more evident for applications performed under adverse climatic conditions. More spray deposition was found in applications performed at times of day with more favorable temperature and humidity of the air conditions. Only the initial control of the evaluated species was affected by the time of application. 相似文献
109.
110.
Hans Lazarus 《International journal of occupational safety and ergonomics》2013,19(3):233-250
Recognition of acoustic signals when perception is subject to interference from noise has already been extensively studied. In this study the influence of hearing protectors (HP) (plugs, muffs) and hearing loss on signal recognition is examined. Different spectrums and levels of the noise are also included. The test results are shown as the masked threshold for the signals heard and identified. In the case of normally hearing subjects a frequency-independent HP (plug) improves hearing performance, while frequency-dependent HP (muffs) tends to worsen it, especially with low-frequency noise. Hearing losses even worsen hearing performance when plugs are worn. Design suggestions are made to optimise signal recognition. Minimum signal-to-noise ratio and the use of HP are discussed. 相似文献