全文获取类型
收费全文 | 1024篇 |
免费 | 97篇 |
国内免费 | 1篇 |
专业分类
1122篇 |
出版年
2023年 | 3篇 |
2022年 | 8篇 |
2021年 | 15篇 |
2020年 | 15篇 |
2019年 | 17篇 |
2018年 | 17篇 |
2017年 | 18篇 |
2016年 | 34篇 |
2015年 | 50篇 |
2014年 | 58篇 |
2013年 | 62篇 |
2012年 | 83篇 |
2011年 | 81篇 |
2010年 | 58篇 |
2009年 | 55篇 |
2008年 | 66篇 |
2007年 | 63篇 |
2006年 | 46篇 |
2005年 | 59篇 |
2004年 | 60篇 |
2003年 | 52篇 |
2002年 | 65篇 |
2001年 | 19篇 |
2000年 | 22篇 |
1999年 | 7篇 |
1998年 | 13篇 |
1997年 | 11篇 |
1996年 | 11篇 |
1995年 | 5篇 |
1994年 | 8篇 |
1993年 | 5篇 |
1992年 | 5篇 |
1991年 | 2篇 |
1990年 | 4篇 |
1989年 | 1篇 |
1988年 | 2篇 |
1987年 | 1篇 |
1986年 | 2篇 |
1985年 | 1篇 |
1984年 | 2篇 |
1983年 | 1篇 |
1982年 | 4篇 |
1981年 | 1篇 |
1980年 | 3篇 |
1979年 | 1篇 |
1978年 | 1篇 |
1977年 | 2篇 |
1975年 | 1篇 |
1974年 | 2篇 |
排序方式: 共有1122条查询结果,搜索用时 0 毫秒
1.
Aino Juslén Juha Pykälä Saija Kuusela Lauri Kaila Jaakko Kullberg Jaakko Mattila Jyrki Muona Sanna Saari Pedro Cardoso 《Biodiversity and Conservation》2016,25(3):569-585
For the first time ever, the International Union for Conservation of Nature Red List Index for habitat types was calculated for an entire country, Finland. The RLIs were based on species threat assessments from 2000 and 2010 and included habitat definitions for all 10,131 species of 12 organism groups. The RLIs were bootstrapped to track statistically significant changes. The RLI changes of species grouped by habitats were negative for all habitat types except for forests and rural biotopes which showed a stable trend. Trends of beetles and true bugs were positive in rural and forest habitats. Other 16 observed trends of species group and habitat combinations were negative. Several trends observed were in accordance with studies focusing on particular taxa and habitats, and drivers for their change. This study demonstrates the usefulness of the RLI as a tool for observing habitat change based on species threat assessment data. 相似文献
2.
3.
Radioimmunoassays were developed for the basement membrane components, 7 S collagen and fragments of the noncollagenous protein laminin, which allowed quantitative analysis of as little as 0.1–0.4 ng of these proteins. Similar materials could be detected by these assays in serum, in kidney digests, and in the medium of cell cultures obtained from mice and rats. Distinct changes in the amounts of antigen in serum and kidney were observed during aging and in mice inoculated with a basement membrane tumor. 相似文献
4.
The recovery of L. Vesijärvi following sewage diversion 总被引:2,自引:2,他引:0
Juha Keto 《Hydrobiologia》1982,86(1-2):195-199
For 60 years the sewage of the City of Lahti was discharged into L. Vesijärvi. The eutrophication of the lake was observed as early as the 1920s but in the 1960s the pollution became obvious. The sewage was completely diverted in 1976. As a result of the diversion the bacteriological defects were eliminated within one year, restoring the recreational value of the lake. The recovery of water quality was rapid during the first two years. The oxygen content increased markedly but hypolimnetic oxygen depletion persisted. The phosphorus content decreased about 60% and the nitrogen content about 30%. After that period the recovery slowed down. The biomass of phytoplankton decreased but the abundance of heterocystous blue green algae increased causing blooms in July and August. The primary production has lately been reduced despite the lack of improvement in chemical water quality. Hypolimnetic aeration was started in order to accelerate the recovery. The results during winter stagnation have been encouraging. 相似文献
5.
6.
Lovisa Wennerström Linda Laikre Nils Ryman Fred M. Utter Nurul Izza Ab Ghani Carl André Jacquelin DeFaveri Daniel Johansson Lena Kautsky Juha Merilä Natalia Mikhailova Ricardo Pereyra Annica Sandström Amber G. F. Teacher Roman Wenne Anti Vasemägi Małgorzata Zbawicka Kerstin Johannesson Craig R. Primmer 《Biodiversity and Conservation》2013,22(13-14):3045-3065
Information on spatial and temporal patterns of genetic diversity is a prerequisite to understanding the demography of populations, and is fundamental to successful management and conservation of species. In the sea, it has been observed that oceanographic and other physical forces can constitute barriers to gene flow that may result in similar population genetic structures in different species. Such similarities among species would greatly simplify management of genetic biodiversity. Here, we tested for shared genetic patterns in a complex marine area, the Baltic Sea. We assessed spatial patterns of intraspecific genetic diversity and differentiation in seven ecologically important species of the Baltic ecosystem—Atlantic herring (Clupea harengus), northern pike (Esox lucius), European whitefish (Coregonus lavaretus), three-spined stickleback (Gasterosteus aculeatus), nine-spined stickleback (Pungitius pungitius), blue mussel (Mytilus spp.), and bladderwrack (Fucus vesiculosus). We used nuclear genetic data of putatively neutral microsatellite and SNP loci from samples collected from seven regions throughout the Baltic Sea, and reference samples from North Atlantic areas. Overall, patterns of genetic diversity and differentiation among sampling regions were unique for each species, although all six species with Atlantic samples indicated strong resistence to Atlantic-Baltic gene-flow. Major genetic barriers were not shared among species within the Baltic Sea; most species show genetic heterogeneity, but significant isolation by distance was only detected in pike and whitefish. These species-specific patterns of genetic structure preclude generalizations and emphasize the need to undertake genetic surveys for species separately, and to design management plans taking into consideration the specific structures of each species. 相似文献
7.
Ahola T Kujala P Tuittila M Blom T Laakkonen P Hinkkanen A Auvinen P 《Journal of virology》2000,74(15):6725-6733
The membrane-associated alphavirus RNA replication complex contains four virus-encoded subunits, the nonstructural proteins nsP1 to nsP4. Semliki Forest virus (SFV) nsP1 is hydrophobically modified by palmitoylation of cysteines 418 to 420. Here we show that Sindbis virus nsP1 is also palmitoylated on the same site (cysteine 420). When mutations preventing nsP1 palmitoylation were introduced into the genomes of these two alphaviruses, the mutant viruses remained viable and replicated to high titers, although their growth was slightly delayed. The subcellular distribution of palmitoylation-defective nsP1 was altered in the mutant: it no longer localized to filopodial extensions, and a fraction of it was soluble. The ultrastructure of the alphavirus replication sites appeared normal, and the localization of the other nonstructural proteins was unaltered in the mutants. In both wild-type- and mutant-virus-infected cells, SFV nsP3 and nsP4 could be extracted from membranes only by alkaline solutions whereas the nsP2-membrane association was looser. Thus, the membrane binding properties of the alphavirus RNA replication complex were not determined by the palmitoylation of nsP1. The nsP1 palmitoylation-defective alphaviruses produced normal plaques in several cell types, but failed to give rise to plaques in HeLa cells, although they induced normal apoptosis of these cells. The SFV mutant was apathogenic in mice: it caused blood viremia, but no infectious virus was detected in the brain. 相似文献
8.
Louise Ilum Sørensen Juha Mikola Minna-Maarit Kytöviita Johan Olofsson 《Ecosystems》2009,12(5):830-842
Mammal grazing is composed of three mechanisms—removal of foliar tissue (defoliation), return of nutrients via dung and urine
(fertilization), and trampling. To evaluate the relative role of these mechanisms in the effect of reindeer grazing on soil
biota in northern grasslands, we subjected experimental plots in a sub-arctic alpine meadow to defoliation, fertilization
(using NPK-solution), simulated trampling, and their factorial combinations once a year from 2002 to 2004 and measured the
response of plants and decomposers (including microbes, nematodes, collembolans, and enchytraeids) in 2004. Trampling affected
both plant and decomposer communities: the coverage of the moss Pleurozium schreberi and the sedge Carex vaginata, as well as the abundance of collembolans and enchytraeids were reduced in trampled plots. Trampling and fertilization also
interacted significantly, with fertilization increasing the abundance of bacteria and bacterial-feeding and omnivorous nematodes
in trampled plots only, and trampling decreasing fungal biomass in non-fertilized plots only. Defoliation had no overall effects
on plants or decomposers. Nematode genera were not affected by the experimental treatments, but nematode and plant communities
were significantly associated, and all decomposer biota, except collembolans, were strongly affected by the spatial heterogeneity
of the study site. Our results indicate that trampling may have larger and defoliation and fertilization smaller roles than
anticipated in explaining reindeer grazing effects in sub-arctic grasslands. However, even the effects of trampling seem to
be outweighed by the spatial heterogeneity of decomposer abundances. This suggests that in sub-arctic grasslands spatial variation
in abiotic factors can be a more important factor than grazing in controlling soil biota abundances.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.
Author contributions LIS was involved in planning of the study, fieldwork, lab work, analysis, and wrote the article with contributions from all
other authors; JM and MMK conceived the study and contributed to the data analyses; MMK further contributed to the field work
and JM refined the final appearance of the text; JO started and was responsible for managing the field experiment, collecting
the plant data and gave advice on statistical analysis. 相似文献
9.
Robbie A. Hember Werner A. Kurz Juha M. Metsaranta T. Andy Black Robert D. Guy Nicholas C. Coops 《Global Change Biology》2012,18(6):2026-2040
To understand how environmental changes have influenced forest productivity, stemwood biomass (B) dynamics were analyzed at 1267 permanent inventory plots, covering a combined 209 ha area of unmanaged temperate‐maritime forest in southwest British Columbia, Canada. Net stemwood production (ΔB) was derived from periodic remeasurements of B collected over a 40‐year measurement period (1959–1998) in stands ranging from 20 to 150 years old. Comparison between the integrated age response of net stemwood production, ΔB(A), and the age response of stemwood biomass, B(A), suggested a 58 ± 11% increase in ΔB between the first 40 years of the chronosequence period (1859–1898) and the measurement period. To estimate extrinsic forcing on ΔB, several different candidate models were developed to remove variation explained by intrinsic factors. All models exhibited temporal bias, with positive trends in (observed minus predicted) residual ΔB ranging between of 0.40 and 0.64% yr?1. Applying the same methods to stemwood growth (G) indicated residual increases ranging from 0.43 and 0.67% yr?1. Higher trend estimates corresponded with models that included site index (SI) as a predictor, which may reflect exaggeration of the age‐decline in SI tables. Choosing a model that excluded SI, suggested that ΔB increased by 0.40 ± 0.18% yr?1, while G increased by 0.43 ± 0.12% yr?1 over the measurement period. Residual G was significantly correlated with atmospheric carbon dioxide (CO2), temperature (T), and climate moisture index (CMI). However, models driven with climate and CO2, alone, could not simultaneously explain long‐term and measurement‐period trends without additional representation of indirect effects, perhaps reflecting compound interest on direct physiological responses to environmental change. Evidence of accelerating forest regrowth highlights the value of permanent inventories to detect and understand systematic changes in forest productivity caused by environmental change. 相似文献
10.
Jussi-Pekka Usenius Pauli Vainio Juha Hernesniemi † Risto A. Kauppinen 《Journal of neurochemistry》1994,63(4):1538-1543
Abstract: We have studied 14 patients with different grades of astrocytomas using 1 H NMR spectroscopy in vivo. Typically, astrocytomas exhibited a low N -acetyl-aspartate peak, a prominent signal from choline group-containing compounds, and lactate in the 1 H NMR spectra in vivo. The uncorrected choline/creatine + phosphocreatine peak area ratios were higher in tumors than in normal brain tissue. Absolute concentration of choline-containing compounds (1.74 ± 0.09 mmol/L) in the normal brain tissue was not different in any grade of astrocytoma, but total creatine concentration in healthy brain (7.49 ± 0.30 mmol/L) was higher than that in grade IV astrocytomas (4.84 ± 0.89 mmol/L). Relaxation constants of choline-containing compounds did not differ in tumors from those determined in normal brain. Perchloric acid extracts of biopsy samples from 35 astrocytomas and 13 samples of normal temporal white matter were analyzed with 1 H NMR. Total concentration of choline-containing compounds did not differ between controls and any grade of astrocytoma when the quantification was done in vitro. It is interesting that phosphorylcholine concentration was about twofold greater in grade IV astrocytomas than in controls or other grades of astrocytomas. We conclude that high phosphorylcholine in grade IV astrocytomas may be an indicator of degree of malignancy. The proportional changes within the group of choline-containing compounds observed in vitro were not reflected in the NMR properties of choline signal in vivo. 相似文献