全文获取类型
收费全文 | 2270篇 |
免费 | 238篇 |
国内免费 | 437篇 |
出版年
2024年 | 12篇 |
2023年 | 63篇 |
2022年 | 51篇 |
2021年 | 79篇 |
2020年 | 104篇 |
2019年 | 116篇 |
2018年 | 89篇 |
2017年 | 107篇 |
2016年 | 119篇 |
2015年 | 101篇 |
2014年 | 103篇 |
2013年 | 159篇 |
2012年 | 99篇 |
2011年 | 128篇 |
2010年 | 99篇 |
2009年 | 122篇 |
2008年 | 149篇 |
2007年 | 109篇 |
2006年 | 102篇 |
2005年 | 110篇 |
2004年 | 90篇 |
2003年 | 83篇 |
2002年 | 79篇 |
2001年 | 81篇 |
2000年 | 56篇 |
1999年 | 60篇 |
1998年 | 58篇 |
1997年 | 40篇 |
1996年 | 49篇 |
1995年 | 45篇 |
1994年 | 35篇 |
1993年 | 35篇 |
1992年 | 32篇 |
1991年 | 25篇 |
1990年 | 24篇 |
1989年 | 22篇 |
1988年 | 10篇 |
1987年 | 18篇 |
1986年 | 21篇 |
1985年 | 8篇 |
1984年 | 12篇 |
1983年 | 4篇 |
1982年 | 10篇 |
1981年 | 2篇 |
1980年 | 3篇 |
1979年 | 7篇 |
1977年 | 5篇 |
1976年 | 5篇 |
1971年 | 1篇 |
1958年 | 1篇 |
排序方式: 共有2945条查询结果,搜索用时 665 毫秒
91.
Michael Klockmann Uta Schröder Fajes Karajoli Klaus Fischer 《Entomologia Experimentalis et Applicata》2016,158(1):60-68
Temperature is one of the most important ecological factors affecting species survival and distributions. Therefore, global climate change, involving increases in mean surface temperature and the occurrence of extreme weather events, may pose a substantial challenge to biodiversity. Whereas tropical ectotherms are believed to be very sensitive to climate change, temperate‐zone species may actually benefit from higher temperatures. However, as in temperate zones large parts of the year are unsuitable for growth and reproduction, seasonal time constraints may complicate matters. Against this background we here investigate the impact of simulated climate change, involving increased mean temperatures and heat waves, across developmental pathways of the butterfly Lycaena tityrus (Poda) (Lepidoptera: Lycaenidae). Increased temperatures speeded up development but decreased pupal mass as expected. However, we found no evidence for detrimental effects of increased temperatures or even simulated heat waves. Furthermore, patterns did not differ between indirectly and directly developing individuals, which are assumed to be more time constrained. Our findings support the notion that not all species will be detrimentally affected by climate change, and suggest that species attributes may be more important than potential time constraints imposed by different developmental pathways. 相似文献
92.
《Revue de Micropaléontologie》2016,59(3):233-237
A spherical coccosphere and two collapsed coccospheres composed of monomorphic rhombic coccoliths were encountered in 2005 in the Java upwelling system of the SE Indian Ocean, while a further two specimens with elongate coccospheres were recently found in the Gulf of Mexico. All of the specimens were collected from the lower photic zone (75–160 m). The coccoliths possess a proximal flange, a slightly flared wall with a serrated distal margin, and a relatively plain central area structure comprised only of overlapping laths. The taxon appears to be an undescribed species of the Syracosphaera nodosa group, so we describe it herein as Syracosphaera rhombica sp. nov. 相似文献
93.
94.
Tatiana A. Trifonova Ekaterina Y. Alkhutova 《International journal of phytoremediation》2016,18(12):1209-1220
This study considered the possibility of using plant community phytomass for the assessment of soil pollution with heavy metals (HM) from industrial wastes. The three-year-long field experiment was run under the regional natural meadow vegetation; the polymetallic galvanic slime was used as an industrial waste contaminant. It is shown that soil contamination primarily causes decrease of phytomass in the growing phytocenosis. The vegetation experiments determined nonlinear dependence of cultivated and wild plant biomass on the level of soil contamination; it is described by the equations of logistic and Gaussian regression. In the absence of permanent contaminants, the soil is self-cleaned over time. It reproduces phytomass mainly due to the productivity increase of the most pollution-tolerant species in the remaining phytocenosis. This phenomenon is defined as environmental hysteresis. Soil pollution by industrial waste leads to the loss of plant biodiversity. The research shows that the study of the HM impact on ecosystems is expedient given the consideration of the “soil–phytocenosis–pollutant” complex in the “dose–response” aspect. The reaction of phytocenosis on HM showing decline in phytomass leads to serious limitations in the choice of accumulating plants, because the adsorbed HM are rejected through phytomass. 相似文献
95.
Chiral Separation of Uncharged Pomalidomide Enantiomers Using Carboxymethyl‐β‐Cyclodextrin: A Validated Capillary Electrophoretic Method 下载免费PDF全文
The racemic mixture of pomalidomide (POM), a second‐generation immunomodulatory uncharged drug, was separated into enantiomers by capillary zone electrophoresis for the first time. Seven different chargeable cyclodextrin (CD) derivatives were screened as complexing agents and chiral selectors, investigating the stability of the POM‐CD inclusion complexes and their enantiodiscriminating capacities. Based on preliminary experiments, carboxymethyl‐β‐CD (CM‐β‐CD) was found to be the most effective chiral selector. Factors influencing enantioseparation were systematically optimized, using an orthogonal experimental design. Optimal parameters (background electrolyte [BGE]: 50 mM Tris‐acetate buffer, pH 6.5, containing 15 mM CM‐β‐CD; capillary temperature: 20°C; voltage applied +15 kV) allowed baseline separation of POM enantiomers with a resolution as high as 4.87. The developed method was validated, in terms of sensitivity (limit of detection and limit of quantification), linearity, accuracy, repeatability, and intermediate precision. Chirality 28:199–203, 2016. © 2015 Wiley Periodicals, Inc. 相似文献
96.
Yuyuan Xie Lei Wang Xin Liu Xin Li Yuanyong Wang Bangqin Huang 《Phycological Research》2019,67(2):152-163
Microphytobenthos (MPB) and phytoplankton are important primary producers in the estuarial ecosystem, and their functions are critical to the ecosystem's biodiversity and environmental safety. The aim of this study was to compare the response of MPB and phytoplankton to the nutrient loads in a eutrophic estuary, which has seldom been studied. We used high‐performance liquid chromatography (HPLC) and CHEMTAX software to examine the biomass and taxonomic composition of both MPB and phytoplankton at Da‐yu Island (DYI) and Ji‐yu Island (JYI) in the Jiulong River Estuary from July 2010 to March 2012. The results showed that MPB chlorophyll a was low in the summer and high in the winter at both DYI and JYI, indicating a unimodal pattern. However, the phytoplankton chlorophyll a showed a mirrored pattern. Diatoms were the dominant class in both benthic and pelagic environments. Although redundancy analysis indicated that the effects of different environmental factors could not be easily separated, it is likely that phosphate and temperature were the most important factors regulating the seasonal patterns of MPB and phytoplankton diatoms, respectively. MPB and phytoplankton cyanobacteria was co‐limited by salinity and temperature. The high N/P ratio and low phosphate favored chlorophytes and cyanobacteria. Our study demonstrates the use of HPLC and CHEMTAX in an integrated survey of the spatial and temporal distribution patterns of MPB and phytoplankton in an estuarial ecosystem. The contrasting responses of MPB and phytoplankton to nutrient loads indicate the critical role of MPB in subtropical estuarial ecosystem function. The relationship between nutrients and MPB may indicate a significant contribution to carbon and nutrient cycling. 相似文献
97.
98.
99.
Chengfeng Le Shuyu Wu Chuanmin Hu Marcus W. Beck Xuchao Yang 《Global Change Biology》2019,25(10):3485-3493
Global climate change can significantly influence oceanic phytoplankton dynamics, and thus biogeochemical cycles and marine food webs. However, associative explanations based on the correlation between chlorophyll‐a concentration (Chl‐a) and climatic indices is inadequate to describe the mechanism of the connection between climate change, large‐scale atmospheric dynamics, and phytoplankton variability. Here, by analyzing multiple satellite observations of Chl‐a and atmospheric conditions from National Center for Environmental Prediction/National Center for Atmospheric Research reanalysis datasets, we show that high‐latitude atmospheric blocking events over Alaska are the primary drivers of the recent decline of Chl‐a in the eastern North Pacific transition zone. These blocking events were associated with the persistence of large‐scale atmosphere pressure fields that decreased westerly winds and southward Ekman transport over the subarctic ocean gyre. Reduced southward Ekman transport leads to reductions in nutrient availability to phytoplankton in the transition zone. The findings describe a previously unidentified climatic factor that contributed to the recent decline of phytoplankton in this region and propose a mechanism of the top‐down teleconnection between the high‐latitude atmospheric circulation anomalies and the subtropical oceanic primary productivity. The results also highlight the importance of understanding teleconnection among atmosphere–ocean interactions as a means to anticipate future climate change impacts on oceanic primary production. 相似文献
100.
Dorine Y. M. Jansen Roger Pradel Rafael Mares Claire Doutrelant Claire N. Spottiswoode Rita Covas Res Altwegg 《Population Ecology》2019,61(4):406-420
Climate models forecast increasing climatic variation and more extreme events, which could increase the variability in animal demographic rates. More variable demographic rates generally lead to lower population growth and can be detrimental to wild populations, especially if the particular demographic rates affected are those to which population growth is most sensitive. We investigated the population dynamics of a metapopulation of 25 colonies of a semi-arid bird species, the sociable weaver Philetairus socius, and how it was influenced by seasonal weather during 1993–2014. We constructed an integrated population model which estimated population sizes similar to observed population counts, and allowed us to estimate annual fecundity and recruitment. Variance in fecundity contributed most to variance in population growth, which showed no trend over time. No weather variables explained overall demographic variation at the population level. However, a separate analysis of the largest colony showed a clear decline with a high extinction probability (0.05 to 0.33) within 5 years after the study period. In this colony, juvenile survival was lower when summers were hot, and adult survival was lower when winters were cold. Rainfall was also negatively correlated with adult survival. These weather effects could be due to increased physiological demands of thermoregulation and rainfall-induced breeding activity. Our results suggest that the dynamics of the population on the whole are buffered against current weather variation, as individual colonies apparently react in different ways. However, if more and increasingly extreme weather events synchronize colony dynamics, they are likely to have negative effects. 相似文献