全文获取类型
收费全文 | 4765篇 |
免费 | 302篇 |
国内免费 | 649篇 |
出版年
2023年 | 87篇 |
2022年 | 90篇 |
2021年 | 148篇 |
2020年 | 148篇 |
2019年 | 177篇 |
2018年 | 182篇 |
2017年 | 151篇 |
2016年 | 147篇 |
2015年 | 153篇 |
2014年 | 237篇 |
2013年 | 341篇 |
2012年 | 218篇 |
2011年 | 185篇 |
2010年 | 174篇 |
2009年 | 216篇 |
2008年 | 220篇 |
2007年 | 199篇 |
2006年 | 150篇 |
2005年 | 166篇 |
2004年 | 173篇 |
2003年 | 120篇 |
2002年 | 174篇 |
2001年 | 148篇 |
2000年 | 167篇 |
1999年 | 124篇 |
1998年 | 110篇 |
1997年 | 98篇 |
1996年 | 87篇 |
1995年 | 81篇 |
1994年 | 88篇 |
1993年 | 100篇 |
1992年 | 88篇 |
1991年 | 77篇 |
1990年 | 56篇 |
1989年 | 30篇 |
1988年 | 38篇 |
1987年 | 42篇 |
1986年 | 30篇 |
1985年 | 60篇 |
1984年 | 43篇 |
1983年 | 39篇 |
1982年 | 59篇 |
1981年 | 45篇 |
1980年 | 43篇 |
1979年 | 35篇 |
1978年 | 37篇 |
1977年 | 37篇 |
1976年 | 38篇 |
1975年 | 23篇 |
1973年 | 15篇 |
排序方式: 共有5716条查询结果,搜索用时 15 毫秒
971.
Raffaele Lafortezza David A. Coomes Valerie Kapos Robert M. Ewers 《Global Ecology and Biogeography》2010,19(5):741-754
Aim Few studies have attempted to assess the overall impact of fragmentation at the landscape scale. We quantify the impacts of fragmentation on plant diversity by assessing patterns of community composition in relation to a range of fragmentation measures. Location The investigation was undertaken in two regions of New Zealand – a relatively unfragmented area of lowland rain forest in south Westland and a highly fragmented montane forest on the eastern slopes of the Southern Alps. Methods We calculated an index of community similarity (Bray–Curtis) between forest plots we regarded as potentially affected by fragmentation and control forest plots located deep inside continuous forest areas. Using a multiple nonlinear regression technique that incorporates spatial autocorrelation effects, we analysed plant community composition in relation to measures of fragmentation at the patch and landscape levels. From the resulting regression equation, we predicted community composition for every forest pixel on land‐cover maps of the study areas and used these maps to calculate a landscape‐level estimate of compositional change, which we term ‘BioFrag’. BioFrag has a value of one if fragmentation has no detectable effect on communities within a landscape, and tends towards zero if fragmentation has a strong effect. Results We detected a weak, but significant, impact of fragmentation metrics operating at both the patch and landscape levels. Observed values of BioFrag ranged from 0.68 to 0.90, suggesting that patterns of fragmentation have medium to weak impacts on forest plant communities in New Zealand. BioFrag values varied in meaningful ways among landscapes and between the ground‐cover and tree and shrub communities. Main conclusions BioFrag advances methods that describe spatial patterns of forest cover by incorporating the exact spatial patterns of observed species responses to fragmentation operating at multiple spatial scales. BioFrag can be applied to any landscape and ecological community across the globe and represents a significant step towards developing a biologically relevant, landscape‐scale index of habitat fragmentation. 相似文献
972.
RIEN AERTS 《Global Change Biology》2010,16(3):1071-1081
Vast areas of (sub)arctic tundra are dominated by the ericoid dwarf shrub Empetrum hermaphroditum. Recent experimental and observational data have shown that Empetrum can be damaged heavily by recurrent extreme winter warming. In addition, summer warming leads to increased soil N availability in tundra ecosystems. In a 7‐year experiment, I investigated the recovery of subarctic Empetrum‐dominated tundra vegetation using a factorial combination of various degrees of aboveground Empetrum removal (simulating the damaging effects of extreme winter warming) and N addition (simulating one of the effects of summer warming). After 7 years no new species had established in the plots. The growth of planted Betula nana seedlings was stimulated by Empetrum removal and reduced by N addition. This Empetrum‐dominated tundra ecosystem was resilient against severe disturbances. Only when Empetrum was 100% removed did it fail to recover, and only in combination with high N supply the subordinate species (notably Eriophorum vaginatum and Rubus chamaemorus, a graminoid and a forb) could benefit. In the 50% removal treatment Empetrum recovered in 7 years when no N was supplied and the cover of the subordinate species did not change. However, when N was added Empetrum recovered faster (in 4 years) and the subordinates decreased. When Empetrum was not removed and N was added, Empetrum even increased in abundance at the expense of the subordinate species. Thus, profound changes in tundra ecosystems can only be expected when Empetrum is very heavily damaged as a result of recurrent extreme winter warming and when soil N availability is increased as a result of summer warming. These changes in species composition upon extreme disturbance events may lead to a wide variety of ecosystem feedbacks and cascade processes as this tundra system is relatively species‐poor, and can be hypothesized to have low functional redundancy. 相似文献
973.
HANS J. DE BOECK FREJA E. DREESEN IVAN A. JANSSENS IVAN NIJS 《Global Change Biology》2010,16(7):1992-2000
Extreme events such as heat waves are emerging as a key facet of climate change, but to date, experiments on the impacts on plants are scarce. Experimental simulation of heat waves requires knowledge of regional heat wave characteristics, as plant responses depend heavily on meteorological conditions. We analysed nine Western European meteorological datasets, and found that heat waves occurring during the growing season in this region encompass more sunshine (+ 69%), lower precipitation (?78%) and a larger vapour pressure deficit (VPD) (+ 111%) compared with normal conditions. Possible consequences for plant responses are discussed, with emphasis on the likely seasonal variation of heat wave impacts. We explain why infrared heating (which typically increases VPD) is an appropriate technique for heat wave simulation. Finally, we advocate experiments to take into account the smaller nighttime compared with daytime temperature increases observed during heat waves, and the precipitation deficits before and during heat waves. 相似文献
974.
975.
Nakamura K Koyama Y Takahashi K Tsurui H Xiu Y Ohtsuji M Lin QS Sugawara Y Sawabe K Ohashi A Ohnuma T Arai H Nishimura H Hasegawa H Hirose S 《Journal of molecular biology》2006,363(2):345-354
Deficits in sensorimotor gating, a function to focus on the most salient stimulus, could lead to a breakdown of cognitive integrity, and could reflect the "flooding" by sensory overload and cognitive fragmentation seen in schizophrenia. Sensorimotor gating emerges at infancy, and matures during childhood. The mechanisms that underlie its development are largely unclear. Here, we screened the mouse genome, and found that tryptophan hydroxylase (TPH) is implicated in the maturation of sensorimotor gating. TPH, an enzyme involved in the biosynthesis of serotonin, proved to be required only during the weaning period for maturation of sensorimotor gating, but was dispensable for its emergence. Proper serotonin levels during development underlie the mature functional architecture for sensorimotor gating via appropriate actin polymerization. Thus, maintaining proper serotonin levels during childhood may be important for mature sensorimotor gating in adulthood. 相似文献
976.
MADELINE M. HARLEY fls 《Botanical journal of the Linnean Society. Linnean Society of London》2006,151(1):39-67
Of all monocotyledons the Arecaceae displays by far the richest fossil record, and there is an extensive literature. The earliest unequivocal fossil palm material probably dates from the early to mid Late Cretaceous (Turonian > Coniacian > Santonian). The records are geographically widespread and comprise a wide range of organs: leaves, cuticles, stems, rhizomes, roots, fruits, seeds, endocarps, rachillae, peduncles, inflorescences, individual flowers and pollen. For some of these organs records are rare while for others, such as leaves, stems and pollen, records are abundant. However, fossil material often lacks sufficient diagnostic detail to allow reasonable association with living palm taxa beyond, or even to, subfamilial level. Nevertheless, many fossil genera and numerous species have been described. A brief survey of palm fossil records is presented, and their taxonomy and morphological limitations are considered. © 2006 The Linnean Society of London, Botanical Journal of the Linnean Society , 2006, 151 , 39–67. 相似文献
977.
978.
在广东省深圳水库采集的浮游植物样品中发现团藻科1个单种属及其种:板藻属(Platydorina)、具尾板藻(Platydorina caudata Kofoid)。植物体扁平状马蹄形,微呈左旋;在马蹄形后端具3至5个胶质突起;由16或32个细胞在胶质基质中朝不同的方向排列成单层的群体,细胞球形或梨形,具2条等长鞭毛,细胞在群体两面交错排列;每个细胞具1个眼点;鞭毛基部具2个伸缩泡;色素体大型杯状,1个蛋白核位于杯状色素体的基部。无性繁殖通过亲体部分或全部细胞的反复分裂完成;有性生殖为异配生殖,形成厚壁休眠合子。 相似文献
979.
Lesliea, a newly recorded genus of Orcihdaceae from Yunnan, China, is reported. This is monotypic genus, differing from its relatives by having saccate spur, long and bifid rostellum and the obconical stipe. 相似文献
980.
We collected a specimen of Chinese Pygmy Dormouse (Typhlomys cinereus)from Nanling Nature Reserve in Guangdong Province in August 2012,analyzed the specimen’s morphological and skull indices,and compared them with the published data for other T. cinereus subspecies located throughout China. The skull of our specimen had four pairs of palatal holes:among them,the two posterior pairs merged together to form two large and long holes,and the posterior edge of the nasals was located vertically below the frontopremaxillary suture. Based on these two unique characteristics,we identified the specimen as the subspecies T. c. daloushanensis. This is the first time this species has been recorded in Guangdong Province. 相似文献