全文获取类型
收费全文 | 2361篇 |
免费 | 168篇 |
国内免费 | 222篇 |
专业分类
2751篇 |
出版年
2024年 | 5篇 |
2023年 | 32篇 |
2022年 | 35篇 |
2021年 | 54篇 |
2020年 | 58篇 |
2019年 | 75篇 |
2018年 | 51篇 |
2017年 | 121篇 |
2016年 | 155篇 |
2015年 | 135篇 |
2014年 | 112篇 |
2013年 | 135篇 |
2012年 | 88篇 |
2011年 | 93篇 |
2010年 | 82篇 |
2009年 | 164篇 |
2008年 | 160篇 |
2007年 | 166篇 |
2006年 | 134篇 |
2005年 | 104篇 |
2004年 | 83篇 |
2003年 | 86篇 |
2002年 | 60篇 |
2001年 | 58篇 |
2000年 | 51篇 |
1999年 | 43篇 |
1998年 | 39篇 |
1997年 | 37篇 |
1996年 | 26篇 |
1995年 | 30篇 |
1994年 | 32篇 |
1993年 | 25篇 |
1992年 | 34篇 |
1991年 | 18篇 |
1990年 | 27篇 |
1989年 | 23篇 |
1988年 | 16篇 |
1987年 | 20篇 |
1986年 | 8篇 |
1985年 | 16篇 |
1984年 | 12篇 |
1983年 | 3篇 |
1982年 | 10篇 |
1981年 | 5篇 |
1980年 | 7篇 |
1979年 | 4篇 |
1978年 | 5篇 |
1977年 | 4篇 |
1976年 | 4篇 |
1972年 | 2篇 |
排序方式: 共有2751条查询结果,搜索用时 15 毫秒
121.
The depletion of biodiversity is a major environmental challenge of the present time, with the agricultural sector being a significant contributor to the loss. Therefore, farmers' decisions and practices are pivotal in safeguarding biodiversity at the farm level. Accordingly, this study aimed to unravel the key factors influencing farmers' intention to conserve biodiversity (ICB), and in doing so, it adopted the protection motivation theory (PMT) as the theoretical framework. The study's statistical population comprised 412 farmers residing in Bavi County in the southwestern region of Iran. The research employed a questionnaire as the data collection instrument, and the research hypotheses were tested using structural equation modeling (SEM). The findings indicated that the respondents held a perception of high costs associated with implementing biodiversity conservation behaviors. They believed that such measures would not be easy to implement, and they would incur both monetary and non-monetary costs. Furthermore, the findings of the study revealed that the variables of SE and ICB among the studied farmers were lower than the theoretical median of 3, indicating that farmers may not have a strong desire and self-efficacy to engage in biodiversity conservation practices. Through the application of SEM, the study determined that the PMT accounts for 65.2% of farmers’ ICB. Additionally, perceived severity and self-efficacy were found to have the strongest effects on farmers’ ICB (as the first and second determinants), highlighting the importance of these variables in predicting farmers’ conservation behaviors. Overall, it can be concluded that these results have significant implications for policymakers in the agricultural sector, providing valuable insights into how to promote biodiversity conservation practices among farmers. 相似文献
122.
Muyiwa Ajoke Akindolire Ajay Kumar Collins Njie Ateba 《Saudi Journal of Biological Sciences》2018,25(7):1348-1355
Food borne diseases are a major public health concern worldwide. Staphylococcus aureus is one of the potential food borne pathogens which causes nosocomial and community acquired infections. In the present study, 74 representative strains of S. aureus isolated and characterized in previous study from different milk samples were subjected to random amplified polymorphic DNA (RAPD) polymerase chain reaction (PCR) and enterobacterial repetitive intergenic consensus (ERIC)-PCR to generate fingerprints to determine the genetic relationships of the isolated strains. A total of 20 RAPD patterns were generated and the number of amplified fragments obtained ranged from 0 to 8 with molecular weight ranging from 250 to 2000 bp. A dendrogram based on fingerprinting pattern grouped isolates into twelve major clusters (I–XII). In the case of ERIC-PCR 9 banding patterns were obtained with amplicons ranging from 1 to 8 and band sizes ranging from 250 to 2000 bp. A total of four major clusters (I–IV) were observed in the dendrogram based on ERIC fingerprints. The discrete banding patterns obtained both from ERIC-PCR and RAPD-PCR showed remarkably the genetic diversity of S. aureus. The findings of this study indicate that raw, bulk and pasteurized milk in the North-West Province was contaminated with toxigenic and multi-drug resistant S. aureus strains. This emphasizes the need to implement appropriate control measures to reduce contamination as well as the spread of virulent S. aureus strains to reduce the burden of disease in humans. 相似文献
123.
2016年11月,"第十九次全国环境微生物学学术研讨会"在重庆市召开。八百多名国内外与会者围绕环境微生物学前沿基础、污染物降解与微生物修复、废弃物资源化利用与环境生物技术、微生物生态、微生物资源与分布等五个主题进行了多种形式的交流讨论。会议充分展现了我国环境微生物学研究、应用进展和良好的发展前景,其中部分领域的研究工作国际领先。《微生物学通报》针对本次研讨会组织出版了本期《环境微生物学主题刊》,旨在展示我国环境微生物学最新的研究进展和成果,促进环境微生物学及相关学科的交流和发展。 相似文献
124.
125.
The induction of diapause can be adaptive for egg survival during unfavorable conditions, while direct development can be
advantageous under favorable conditions by allowing additional generations to exploit abundant resources. Therefore, the physiological
capability of a female to respond to environmental cues indicative of habitat quality by producing eggs of the appropriate
developmental phenotype should be under strong selection. Additionally, developing embryos may alter the developmental trajectory
initiated by the female in response to changing environmental conditions. In this study, we used a cross-fostering approach
to isolate the maternal effects of parental diapause history (not previously studied) and egg-laying temperature from the
influence of the incubation environment experienced by the developing embryo on the proportion of diapause eggs produced by
the striped ground cricket, Allonemobius socius. We found that an interaction between egg-incubation temperature and parental diapause history strongly affected the proportion
of diapause eggs produced and the proportion of eggs that hatched within a 16–18 day incubation period, while egg-laying temperature
and all other interactions did not. These novel results indicate that embryos can respond directly to the environmental conditions
they experience during development, but that their ability to do so is influenced by maternal effects such as parental diapause
history. The results of this study not only provide evidence, for the first time, of parental diapause history affecting diapause
proportions, but also raise additional questions about the mechanism by which environmental information is transmitted from
parent to offspring and how offspring are able to respond to conditions experienced during their own development. 相似文献
126.
Correlations between environmental factors and the distribution of amphibian and reptile species richness were investigated
in a climate transition area, Peneda-Gerês National Park (PNPG), in North-Western Portugal. Using presence-data at a local-scale
(1 × 1 km), Ecological-Niche Factor Analysis (ENFA) identified a mixture of climatic (precipitation and number of days with
fog), topographical (altitude and relief) and habitat factors (number of watercourses and water surfaces, the type of the
largest water surface and tree diversity cover), as accurate predictors of species occurrence. Three factors were common for
both taxonomic groups, and consistently presented a positive relation with species occurrence: precipitation, number of water
surfaces, and tree diversity cover; suggesting a strong coincidence in the environmental correlates that influence amphibian
and reptile species richness. Distribution patterns of observed and predicted species richness were compared using a Geographical
Information System. Overall, three high species richness areas were predicted in common for both taxonomic groups and two
additional areas for amphibians only. These areas matched with the observed species richness but suggested larger areas of
high species richness. The location of the PNPG in a biogeographic crossroad, between Euro-Siberian and Mediterranean provinces,
emphasised species richness of amphibians and reptiles and suggests a high priority conservation status for this protected
area. Most of Central-Northern Portugal is located in a climatic transition area; therefore, increased species richness should
be expected for other areas. Local scale studies for other protected areas should be planned as a framework for the development
of multi-scale conservation planning by Portuguese authorities. 相似文献
127.
This paper reports the response of isopods exposed to fallout of a municipal solid waste landfill located in central Italy. Soil samples and specimens of Armadillidium vulgare were collected at different distances from the landfill and analyzed to determine the concentrations of heavy elements such as As, Cd, Co, Cr, Cu, Ni, Pb, Sb, V and Zn. The isopod analysis was performed on unpurged and purged specimens. Analytical data indicate that the soil contents of heavy elements were quite uniform and within the respective local geochemical background. Slight enrichments of Cu and Pb were found in some soils collected within the solid waste. Purged isopods showed an accumulation of As, Co, Cr, Ni, Sb and V whose body levels decreased as the distance from the landfill increased. Cd, Cu, Pb and Zn concentrations in purged specimens were rather uniform and no significant variation trend occurred. This result probably was due to the fact that the isopods are provided with physiological mechanisms of regulation for these heavy elements. Analytical data also indicate the ability of A. vulgare to adsorb differently the heavy elements according to the following order: As > Co > Ni > Pb > V. The contents of heavy elements in unpurged specimens were higher than in purged ones. This finding suggested that the defecation has marked effects on the tissue levels of heavy elements in isopods. This study indicates that the isopods provide useful information about environmental quality in areas characterized by low and discontinuous emission of heavy elements and their low accumulation in soil. 相似文献
128.
Occurrences of harmful algal blooms (HABs) associated with ocean environments in the South China Sea 总被引:2,自引:0,他引:2
SuFen Wang DanLing Tang FangLiang He Yasuwo Fukuyo Rhodora V. Azanza 《Hydrobiologia》2008,596(1):79-93
Harmful algal blooms (HABs) occur frequently in the South China Sea (SCS), causing enormous economic losses in aquaculture.
We analyzed historical HAB records during the period from 1980 to 2003 in SCS. We found that HABs-affected areas have expanded
and the frequency of HABs varied during this period. The seasonal and annual variations, as well as causative algal species
of HABs are different among the four regions. Areas with frequent HABs include the Pearl River Estuary (China), the Manila
Bay (the Philippines), the Masinloc Bay (the Philippines), and the western coast of Sabah (Malaysia). HABs occurred frequently
during March–May in the northern region of SCS, May–July in the eastern region, July in the western region, and year-round
in the southern region. Among the species that cause HABs, Noctiluca scintillans dominated in the northern region, and Pyrodinium bahamense in the southern and eastern regions. Causative species also varied in different years for the entire SCS. Both P. bahamense and N. scintillans were the dominant species during 1980–2003. Some species not previously recorded formed blooms during 1991–2003, including
Phaeocystis globosa, Scrippsiella trochoidea, Heterosigma akashiwo, and Mesodinium rubrum. Variations in HABs are related to various regional conditions, such as a reversed monsoon wind in the entire SCS, river
discharges in the northern area, upwelling in Vietnam coastal waters during southwest winds and near Malaysia coastal waters
during northeast winds, and eutrophication from coastal aquaculture in the Pearl River estuary, Manila Bay, and Masinloc Bay.
Handling editor: D. Hamilton 相似文献
129.
As environmental problems become more and more serious all over the world, environmental efficiency evaluation has drawn increasing interests of many scholars and governments’ decision makers. Data envelopment analysis (DEA) as a non-parametric approach for evaluating the relative efficiency of a group of decision making units (DMUs) has been widely extended and applied in many areas. Among various DEA models, enhanced Russell measure (ERM) model can measure the inefficiencies from input orientation and output orientation simultaneously. In this paper, we proposed a new non-orientation DEA approach based on enhanced Russell measure for measuring the environmental efficiency of a DMU and meanwhile, provided the closest target for the evaluated DMU to efficient with less effort. At last, our approach was applied to a practical example about thermal power enterprises. The results show that our model can provide a much easier way for the inefficient enterprises to improve their efficiencies than the enhanced Russell measure method. Moreover, the provided benchmarks for enterprises through our model can be used to further rank the efficient ones. 相似文献
130.
Sylvie A. Quideau Anne C.S. McIntosh Charlotte E. Norris Emily Lloret Mathew J.B. Swallow Kirsten Hannam 《Journal of visualized experiments : JoVE》2016,(114)
Phospholipid fatty acids (PLFAs) are key components of microbial cell membranes. The analysis of PLFAs extracted from soils can provide information about the overall structure of terrestrial microbial communities. PLFA profiling has been extensively used in a range of ecosystems as a biological index of overall soil quality, and as a quantitative indicator of soil response to land management and other environmental stressors.The standard method presented here outlines four key steps: 1. lipid extraction from soil samples with a single-phase chloroform mixture, 2. fractionation using solid phase extraction columns to isolate phospholipids from other extracted lipids, 3. methanolysis of phospholipids to produce fatty acid methyl esters (FAMEs), and 4. FAME analysis by capillary gas chromatography using a flame ionization detector (GC-FID). Two standards are used, including 1,2-dinonadecanoyl-sn-glycero-3-phosphocholine (PC(19:0/19:0)) to assess the overall recovery of the extraction method, and methyl decanoate (MeC10:0) as an internal standard (ISTD) for the GC analysis. 相似文献