全文获取类型
收费全文 | 5994篇 |
免费 | 449篇 |
国内免费 | 3篇 |
出版年
2023年 | 28篇 |
2022年 | 23篇 |
2021年 | 67篇 |
2020年 | 53篇 |
2019年 | 62篇 |
2018年 | 166篇 |
2017年 | 125篇 |
2016年 | 185篇 |
2015年 | 329篇 |
2014年 | 336篇 |
2013年 | 430篇 |
2012年 | 504篇 |
2011年 | 482篇 |
2010年 | 320篇 |
2009年 | 239篇 |
2008年 | 397篇 |
2007年 | 397篇 |
2006年 | 382篇 |
2005年 | 377篇 |
2004年 | 322篇 |
2003年 | 309篇 |
2002年 | 271篇 |
2001年 | 41篇 |
2000年 | 46篇 |
1999年 | 48篇 |
1998年 | 39篇 |
1997年 | 35篇 |
1996年 | 26篇 |
1995年 | 24篇 |
1994年 | 16篇 |
1993年 | 27篇 |
1992年 | 23篇 |
1991年 | 16篇 |
1990年 | 26篇 |
1989年 | 15篇 |
1988年 | 22篇 |
1987年 | 14篇 |
1986年 | 25篇 |
1985年 | 12篇 |
1984年 | 19篇 |
1983年 | 11篇 |
1982年 | 10篇 |
1981年 | 10篇 |
1979年 | 13篇 |
1978年 | 15篇 |
1977年 | 9篇 |
1974年 | 10篇 |
1973年 | 9篇 |
1969年 | 7篇 |
1968年 | 9篇 |
排序方式: 共有6446条查询结果,搜索用时 62 毫秒
41.
42.
Dorothee Hodapp Irene T. Roca Dario Fiorentino Cristina Garilao Kristin Kaschner Kathleen Kesner-Reyes Birgit Schneider Joachim Segschneider Ádám T. Kocsis Wolfgang Kiessling Thomas Brey Rainer Froese 《Global Change Biology》2023,29(12):3304-3317
Driven by climate change, marine biodiversity is undergoing a phase of rapid change that has proven to be even faster than changes observed in terrestrial ecosystems. Understanding how these changes in species composition will affect future marine life is crucial for conservation management, especially due to increasing demands for marine natural resources. Here, we analyse predictions of a multiparameter habitat suitability model covering the global projected ranges of >33,500 marine species from climate model projections under three CO2 emission scenarios (RCP2.6, RCP4.5, RCP8.5) up to the year 2100. Our results show that the core habitat area will decline for many species, resulting in a net loss of 50% of the core habitat area for almost half of all marine species in 2100 under the high-emission scenario RCP8.5. As an additional consequence of the continuing distributional reorganization of marine life, gaps around the equator will appear for 8% (RCP2.6), 24% (RCP4.5), and 88% (RCP8.5) of marine species with cross-equatorial ranges. For many more species, continuous distributional ranges will be disrupted, thus reducing effective population size. In addition, high invasion rates in higher latitudes and polar regions will lead to substantial changes in the ecosystem and food web structure, particularly regarding the introduction of new predators. Overall, our study highlights that the degree of spatial and structural reorganization of marine life with ensued consequences for ecosystem functionality and conservation efforts will critically depend on the realized greenhouse gas emission pathway. 相似文献
43.
Migration is ubiquitous and can strongly shape food webs and ecosystems. Less familiar, however, is that the majority of life cycle, seasonal and diel migrations in nature are partial migrations: only a fraction of the population migrates while the other individuals remain in their resident ecosystem. Here, we demonstrate different impacts of partial migration rendering it fundamental to our understanding of the significance of migration for food web and ecosystem dynamics. First, partial migration affects the spatiotemporal distribution of individuals and the food web and ecosystem-level processes they drive differently than expected under full migration. Second, whether an individual migrates or not is regularly correlated with morphological, physiological, and/or behavioural traits that shape its food-web and ecosystem-level impacts. Third, food web and ecosystem dynamics can drive the fraction of the population migrating, enabling the potential for feedbacks between the causes and consequences of migration within and across ecosystems. These impacts, individually and in combination, can yield unintuitive effects of migration and drive the dynamics, diversity and functions of ecosystems. By presenting the first full integration of partial migration and trophic (meta-)community and (meta-)ecosystem ecology, we provide a roadmap for studying how migration affects and is affected by ecosystem dynamics in a changing world. 相似文献
44.
Sandrine Villechanoux Monique Garnier Frédéric Laigret Joël Renaudin Joseph-Marie Bové 《Current microbiology》1993,26(3):161-166
We have recently cloned three DNA fragments (In-2.6, In-1.0, and In-0.6) of the noncultured, bacterial-like organism (BLO) associated with citrus greening disease. Nucleotide sequence determination has shown that fragment In-2.6 is part of therplKAJL-rpoBC gene cluster, a well-known operon in eubacteria. The DNA fragment upstream of and partially overlapping with In-2.6 could be isolated and was shown to be thenusG gene. InEscherichia coli, nusG is also immediately upstream ofrplKAJL-rpoBC. Fragment In-1.0 carries the gene for a bacteriophage type DNA polymerase. Fragment In-0.6 could not be identified.When In-2.6 was used, at high stringency, as a probe to detect greening BLO strains in infected plants, hybridization was obtained with all Asian strains tested, but not with the African strain examined. At lower stringencies, In-2.6 was able to detect also the African strain. The implications of these reults in the taxonomical position of the greening BLO are discussed. 相似文献
45.
Karine Cahier Damien Piel Rubén Barcia-Cruz David Goudenège K. Mathias Wegner Marc Monot Jesús L. Romalde Frédérique Le Roux 《Environmental microbiology》2023,25(8):1424-1438
Phages depend on their bacterial hosts to replicate. The habitat, density and genetic diversity of host populations are therefore key factors in phage ecology, but our ability to explore their biology depends on the isolation of a diverse and representative collection of phages from different sources. Here, we compared two populations of marine bacterial hosts and their phages collected during a time series sampling program in an oyster farm. The population of Vibrio crassostreae, a species associated specifically to oysters, was genetically structured into clades of near clonal strains, leading to the isolation of closely related phages forming large modules in phage–bacterial infection networks. For Vibrio chagasii, which blooms in the water column, a lower number of closely related hosts and a higher diversity of isolated phages resulted in small modules in the phage–bacterial infection network. Over time, phage load was correlated with V. chagasii abundance, indicating a role of host blooms in driving phage abundance. Genetic experiments further demonstrated that these phage blooms can generate epigenetic and genetic variability that can counteract host defence systems. These results highlight the importance of considering both the environmental dynamics and the genetic structure of the host when interpreting phage–bacteria networks. 相似文献
46.
Emanuel A. Fronhofer Dov Corenblit Jhelam N. Deshpande Lynn Govaert Philippe Huneman Frédérique Viard Philippe Jarne Sara Puijalon 《Ecology letters》2023,26(Z1):S91-S108
Eco-evolutionary dynamics, or eco-evolution for short, are often thought to involve rapid demography (ecology) and equally rapid heritable phenotypic changes (evolution) leading to novel, emergent system behaviours. We argue that this focus on contemporary dynamics is too narrow: Eco-evolution should be extended, first, beyond pure demography to include all environmental dimensions and, second, to include slow eco-evolution which unfolds over thousands or millions of years. This extension allows us to conceptualise biological systems as occupying a two-dimensional time space along axes that capture the speed of ecology and evolution. Using Hutchinson's analogy: Time is the ‘theatre’ in which ecology and evolution are two interacting ‘players’. Eco-evolutionary systems are therefore dynamic: We identify modulators of ecological and evolutionary rates, like temperature or sensitivity to mutation, which can change the speed of ecology and evolution, and hence impact eco-evolution. Environmental change may synchronise the speed of ecology and evolution via these rate modulators, increasing the occurrence of eco-evolution and emergent system behaviours. This represents substantial challenges for prediction, especially in the context of global change. Our perspective attempts to integrate ecology and evolution across disciplines, from gene-regulatory networks to geomorphology and across timescales, from today to deep time. 相似文献
47.
48.
A major difference between the divergence patterns within the lines-1 families in mice and voles 总被引:3,自引:0,他引:3
Vanlerberghe F; Bonhomme F; Hutchison CA d; Edgell MH 《Molecular biology and evolution》1993,10(4):719-731
L1 retroposons are represented in mice by subfamilies of interspersed
sequences of varied abundance. Previous analyses have indicated that
subfamilies are generated by duplicative transposition of a small number of
members of the L1 family, the progeny of which then become a major
component of the murine L1 population, and are not due to any active
processes generating homology within preexisting groups of elements in a
particular species. In mice, more than a third of the L1 elements belong to
a clade that became active approximately 5 Mya and whose elements are >
or = 95% identical. We have collected sequence information from 13 L1
elements isolated from two species of voles (Rodentia: Microtinae: Microtus
and Arvicola) and have found that divergence within the vole L1 population
is quite different from that in mice, in that there is no abundant
subfamily of homologous elements. Individual L1 elements from voles are
very divergent from one another and belong to a clade that began a period
of elevated duplicative transposition approximately 13 Mya. Sequence
analyses of portions of these divergent L1 elements (approximately 250 bp
each) gave no evidence for concerted evolution having acted on the vole L1
elements since the split of the two vole lineages approximately 3.5 Mya;
that is, the observed interspecific divergence (6.7%-24.7%) is not larger
than the intraspecific divergence (7.9%-27.2%), and phylogenetic analyses
showed no clustering into Arvicola and Microtus clades.
相似文献
49.
In vitro influence of zinc and magnesium on the deformability of red blood cells artificially hardened by heating 总被引:2,自引:0,他引:2
Collette Dupuy-Fons Jean-Frédéric Brun Claire Mallart Joseph Carvajal Michèle Fussellier Lucette Bardet André Orsetti 《Biological trace element research》1995,47(1-3):247-255
Trace elements have been shown to improve red blood cell (RBC) deformability: zinc in sickle cell disease and magnesium in
an in vitro model of chemically rigidified erythrocytes. In this study, we investigated the effect and the influence of incubation
time of zinc or magnesium on an in vitro model of rigidified RBCs by heating. Erythrocyte rigidity was determined by viscosimetry
at high shear rate by a falling ball viscosimeter MT 90.
In the first part of the study, six normal volunteers participated. Viscosimetry was performed on native blood before and
after heating the sample for 10 min at 50°C. Therefore, increasing concentrations of zinc gluconate (final concentration:
0.5–4 g/L) or isotonic NaCl as control medium were added to the sample. Heating induced a twofold increase in all indices
of RBC rigidity (p<0.05). At all these concentrations of zinc, a highly significant, dose-related fluidifying effect was observed (40–70%):
this effect was immediately obtained and did not change over 60 min. Even at the highest concentration, recovery was not complete.
In the second part of the study, we studied magnesium’s effects on blood. In a first protocol, whole blood was rigidified
by heating at 56°C for 10 min, and the correcting effect of 5 min of incubation at 37°C of RBCs in 150 mmol/L NaCl, MgSO4, magnesium acetate, and magnesium gluconate was investigated. In a second protocol, the same incubation with NaCl and magnesium
salts was made on blood that had not been previously heated. In a third protocol, the correcting effect of magnesium gluconate
on heated red blood cells was tested at four concentrations (75, 150, 225, and 300 mmol/L) over 1 h, for evaluating the effects
of both concentration and time. Erythrocyte rigidity by heating is corrected by the three salts employed in protocol 1 (compared
to sodium). In protocol 2, the deformability of normal (nonheated) red cells is not modified by magnesium. In protocol 3,
no marked modification over 1 h is observed. The correcting effect is not complete for 75 mmol/L Mg, but remains the same
at the three other concentrations.
This study shows that zinc and magnesium at supraphysiological concentration are able to reverse RBC’s rigidification induced
by heating, but that magnesium does not modify the flexibility of normal RBCs. This article suggests that zinc and magnesium
may be studied in vivo as potential pharmacologic tools for improving hemorheologic disturbances. 相似文献
50.
Jean-Frédéric Brun Rosine Guintrand-Hugret Colette Fons Joseph Carvajal Christine Fedou Michelle Fussellier Lucette Bardet André Orsetti 《Biological trace element research》1995,47(1-3):385-391
Zinc improves both insulin secretion and insulin sensitivity, and exerts insulin-like effects. We investigated its acute effects on the parameters of glucose assimilation determined with the minimal model technique from frequent sampling intravenous glucose tolerance test (FSIVGTT) in seven healthy volunteers. FSIVGTTs (0.5 g/kg of glucose, followed by 2 U insulin iv injection at 19 min) were performed after the subjects had taken 20 mg zinc gluconate twice (the evening before and 30 min before the beginning of the test) or placebo pills (simple blind randomized protocol). Glucose assimilation was analyzed by calculating Kg (slope of the exponential decrease in glycemia), glucose effectiveness Sg (i.e., ability of glucose itself to increase its own disposal independent of insulin response), and SI (insulin sensitivity, i.e. the effect of increases in insulinemia on glucose disposal). The two latter parameters were calculated by fitting the experimental data with the two equations of Bergman’s “minimal model”. Zinc increased Kg (p<0.05) and Sg (p<0.05), whereas SI and insulin first-phase secretion did not significantly increase. This study suggests that zinc improves glucose assimilation, as evidenced by the increase in Kg, and that this improvement results mainly from an increase in glucose effectiveness (insulin-like effect), rather than an action on insulin response or insulin sensitivity. 相似文献