全文获取类型
收费全文 | 41587篇 |
免费 | 4101篇 |
国内免费 | 3981篇 |
出版年
2024年 | 37篇 |
2023年 | 713篇 |
2022年 | 655篇 |
2021年 | 1106篇 |
2020年 | 1581篇 |
2019年 | 1804篇 |
2018年 | 1592篇 |
2017年 | 1580篇 |
2016年 | 1737篇 |
2015年 | 1749篇 |
2014年 | 2245篇 |
2013年 | 3301篇 |
2012年 | 1901篇 |
2011年 | 2188篇 |
2010年 | 1577篇 |
2009年 | 2204篇 |
2008年 | 2261篇 |
2007年 | 2343篇 |
2006年 | 2146篇 |
2005年 | 1858篇 |
2004年 | 1592篇 |
2003年 | 1551篇 |
2002年 | 1456篇 |
2001年 | 1140篇 |
2000年 | 1038篇 |
1999年 | 883篇 |
1998年 | 801篇 |
1997年 | 710篇 |
1996年 | 660篇 |
1995年 | 670篇 |
1994年 | 556篇 |
1993年 | 499篇 |
1992年 | 462篇 |
1991年 | 382篇 |
1990年 | 364篇 |
1989年 | 313篇 |
1988年 | 203篇 |
1987年 | 209篇 |
1986年 | 196篇 |
1985年 | 188篇 |
1984年 | 206篇 |
1983年 | 119篇 |
1982年 | 170篇 |
1981年 | 113篇 |
1980年 | 126篇 |
1979年 | 122篇 |
1978年 | 75篇 |
1977年 | 67篇 |
1976年 | 56篇 |
1974年 | 40篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
1.
2.
This paper introduces the vectorial Kappa (κv) that one can utilize to assess congruence between two vectorial mosaics. The vectorial Kappa extends for vectorial mosaics the approach of the so-called Cohen's Kappa index, commonly used to compare raster mosaics. By comparing both approaches, we aim to demonstrate how efficient and convenient a vector-based congruence may be when working on vectorial mosaics. 相似文献
3.
Removal of feral cats from island environments is a useful mechanism by which their ecological impact on endangered species can be reduced or ended. Nevertheless, because cats are anthropogenic in their origins, social perceptions of management practices play a large role in their implementation. Four-hundred questionnaires were delivered (386 were returned) with 100 going to each of the following: local residents; environmental workers; tourists; and, hunters. Questions explored respondents’ knowledge about island biodiversity and invasive species as well as attitudes towards cat population management methods. Habitat destruction and introduction of invasive species were considered the main threats for the conservation of island biodiversity. Most respondents considered cats to have a negative impact on biodiversity and sterilization campaigns were considered most appropriate for cat population control. Several free sterilization campaigns have been conducted in La Palma Island Biosphere Reserve in order to reduce free-ranging cats and were well received by local people. This research, which combined concepts of management, ecology and social sciences, provides valuable insights which may to be applicable on several other islands where cats and people are present and in conflict with conservation priorities. 相似文献
4.
A. A. Gontcharov 《Hydrobiologia》1996,336(1-3):93-97
A survey of the freshwater algal flora of the Primorsky Region, Russian Far East, is presented. During more than 70 years
of investigations 1659 species (including varieties and forms 2276 taxa) belonging to 346 genera of different groups of algae
have been recorded from this territory. The Primorsky Region is rich in habitats attractive to algae-rivers, lakes, reservoirs,
and ponds. The details of the distribution of algae in different types of waterbodies have been examined. The algal flora
of the region is comparable to the areas, and its taxonomical structure is typical for Eurasia. From this territory some new
species and varieties have been described — Achnanthes chankensis, Cymbella ventricosa var. hankensis, Surirella alisoviana, and others. The flora includes some rare species — Fragilaria magocsyi, Perone dimorpha, and Heterothrix monochloron, and some endemics of Asia — Navicula scutelloides var. baicalensis, N. globulifera var. nipponica, Gomphonema sphaerophorum, and Surirella tientsinensis. 相似文献
5.
The initiation of invertebrate distribution patterns in rivers occurs by choice of oviposition sites and is influenced by the evolved reproductive strategies of the individual species. Subsequent redistribution by migration or drifting establishes patterns which are then modified by environmental influences on growth and mortality. Continuity of life cycles is sustained by variations on a number of defined life history strategies combined with evolved behavioural responses. 相似文献
6.
John Boreman 《Environmental Biology of Fishes》1997,48(1-4):399-405
Sturgeons and paddlefish exhibit unusual combinations of morphology, habits, and life history characteristics, which make them highly vulnerable to impacts from human activities, particularly fisheries. Five North American sturgeons (shortnose, Gulf, pallid, Alabama, and green sturgeon) are listed as endangered or threatened by management authorities. Managers have instituted fishery closures for the three other species of North American sturgeons (Atlantic, white, and shovelnose) and paddlefish because of low stock abundance at some point in this century. Reproductive potential in four species I examined (Atlantic, white, and shortnose sturgeon, and paddlefish) is more sensitive to fishing mortality than it is for three other intensively-fished coastal species in North America: striped bass, winter flounder, and bluefish. The sturgeons and paddlefish are generally longer-lived than the three other coastal species, and also have an older age at full maturity, lower maximum fecundity values, and older ages at which 50% of the lifetime egg production is realized with no fishing mortality. 相似文献
7.
The ability of a number of nitrogen-containing compounds that simultaneously carry the adamantane and monoterpene moieties to inhibit Tdp1, an important enzyme of the DNA repair system, is studied. Inhibition of this enzyme has the potential to overcome chemotherapeutic resistance of some tumor types. Compound (+)-3c synthesized from 1-aminoadamantane and (+)-myrtenal, and compound 4a produced from 2-aminoadamantane and citronellal were found to be most potent as they inhibited Tdp1 with IC50 values of 6 and 3.5 µM, respectively. These compounds proved to have low cytotoxicity in colon HCT-116 and lung A-549 human tumor cell lines (CC50 > 50 µM). It was demonstrated that compound 4a at 10 µM enhanced cytotoxicity of topotecan, a topoisomerase 1 poison in clinical use, against HCT-116 more than fivefold and to a lesser extent of 1.5 increase in potency for A-549. 相似文献
8.
Honghong Yang Yuanyuan Zhang Zhihai Wang Shixun Zhong Guohua Hu Wenqi Zuo 《Bioelectromagnetics》2020,41(3):219-229
To investigate the possible mechanisms for biological effects of 1,800 MHz mobile radiofrequency radiation (RFR), the radiation-specific absorption rate was applied at 2 and 4 W/kg, and the exposure mode was 5 min on and 10 min off (conversation mode). Exposure time was 24 h short-term exposure. Following exposure, to detect cell DNA damage, cell apoptosis, and reactive oxygen species (ROS) generation, the Comet assay test, flow cytometry, DAPI (4′,6-diamidino-2-phenylindole dihydrochloride) staining, and a fluorescent probe were used, respectively. Our experiments revealed that mobile phone RFR did not cause DNA damage in marginal cells, and the rate of cell apoptosis did not increase (P > 0.05). However, the production of ROS in the 4 W/kg exposure group was greater than that in the control group (P < 0.05). In conclusion, these results suggest that mobile phone energy was insufficient to cause cell DNA damage and cell apoptosis following short-term exposure, but the cumulative effect of mobile phone radiation still requires further confirmation. Activation of the ROS system plays a significant role in the biological effects of RFR. Bioelectromagnetics. © 2020 The Authors. Bioelectromagnetics published by Wiley Periodicals, Inc. 相似文献
9.
《Current biology : CB》2020,30(7):1280-1286.e2
10.
Paul M. Severns 《Journal of Insect Conservation》2008,12(6):651-661
Fender’s blue butterfly is an endangered species restricted to fragmented, grassland remnants that are becoming increasingly
dominated by tall, invasive grasses in western Oregon, USA. I performed a removal experiment to assess the impacts of structural
degradation accompanying the invasion of Arrhenatherum elatius, tall oat grass, on butterfly fitness and fitness related behaviors. Clipping of A. elatius to native grass sward height resulted in 2.5–5 times as many eggs laid per leaf of host plant. Both male and female butterflies
basked more frequently in areas removed of A. elatius inflorescences and upon encountering the treatment edge butterflies had a high rate of return into a large area removed of
the grass inflorescences. Although butterfly behavior appeared to be affected by the change in sward height on the treatment
edge, there was no evidence for the edge causing a disproportionate egg load. Invasion and dominance by A. elatius appeared to diminish host plant apparency which may result in overloading of eggs on conspicuous host plants, increased incidence
of emigration, and a decrease in the likelihood of colonization because female butterflies appeared indifferent to larval
resources beneath A. elatius inflorescences. Dominance of natural shortgrass prairies by tall stature grasses like A. elatius may be an insidious form of habitat degradation for grassland Lepidoptera worldwide, but it may go largely unnoticed because
larval and adult resources can persist under the unnaturally tall grass canopy. 相似文献