全文获取类型
收费全文 | 455篇 |
免费 | 50篇 |
出版年
2024年 | 2篇 |
2023年 | 2篇 |
2022年 | 4篇 |
2021年 | 28篇 |
2020年 | 12篇 |
2019年 | 16篇 |
2018年 | 15篇 |
2017年 | 12篇 |
2016年 | 19篇 |
2015年 | 41篇 |
2014年 | 25篇 |
2013年 | 42篇 |
2012年 | 47篇 |
2011年 | 31篇 |
2010年 | 23篇 |
2009年 | 11篇 |
2008年 | 22篇 |
2007年 | 23篇 |
2006年 | 15篇 |
2005年 | 10篇 |
2004年 | 8篇 |
2003年 | 9篇 |
2002年 | 6篇 |
2001年 | 3篇 |
2000年 | 4篇 |
1999年 | 2篇 |
1997年 | 2篇 |
1993年 | 4篇 |
1991年 | 4篇 |
1990年 | 2篇 |
1988年 | 3篇 |
1987年 | 5篇 |
1983年 | 2篇 |
1982年 | 4篇 |
1980年 | 5篇 |
1979年 | 5篇 |
1977年 | 1篇 |
1976年 | 5篇 |
1975年 | 3篇 |
1974年 | 1篇 |
1973年 | 1篇 |
1972年 | 2篇 |
1971年 | 4篇 |
1970年 | 3篇 |
1965年 | 1篇 |
1954年 | 3篇 |
1950年 | 1篇 |
1944年 | 2篇 |
1941年 | 1篇 |
1940年 | 2篇 |
排序方式: 共有505条查询结果,搜索用时 31 毫秒
81.
82.
83.
84.
The RelE and RelB proteins constitute the RNA interferase (toxin) and its cognate inhibitor (antitoxin) components of the Escherichia coli relBE toxin-antitoxin system. Despite the well-described functionality and physiological activity of this system in E. coli, no structural study was performed on the folding and stability of the protein pair in solution. Here we structurally and thermodynamically characterize the RelBE system components from E. coli in solution, both separately and in their complexed state. The RelB antitoxin, an alpha-helical protein according to circular dichroism and infrared spectroscopy, forms oligomers in solution, exhibits high thermostability with a TM of 58.5 degrees C, has a considerable heat resistance, and has high unfolding reversibility. In contrast, the RelE toxin includes a large portion of antiparallel beta-sheets, displays lower thermostability with a TM of 52.5 degrees C, and exhibits exceptional sensitivity to heat. Complex formation, accompanied by a structural transition, leads to a 12 degrees C increase in the TM and substantial heat resistance. Moreover, in vivo interaction and protein footprint experiments indicate that the C-terminal part of RelB is responsible for RelB-RelE interaction, being protease sensitive in its free state, while it becomes protected from proteolysis when complexed with RelE. Overall, our findings support the notion that RelB lacks a well-organized hydrophobic core in solution whereas RelE is a well-folded protein. Furthermore, our results support that RelB protein from E. coli is similar to ParD and CcdA antitoxins in both fold and thermodynamic properties. The differential folding state of the proteins is discussed in the context of their physiological activities. 相似文献
85.
Benjamin Cretois Emily G. Simmonds John D. C. Linnell Bram van Moorter Christer M. Rolandsen Erling J. Solberg Olav Strand Vegard Gundersen Ole Roer Jan Ketil Rd 《Ecology and evolution》2021,11(21):15191
Many publications make use of opportunistic data, such as citizen science observation data, to infer large‐scale properties of species’ distributions. However, the few publications that use opportunistic citizen science data to study animal ecology at a habitat level do so without accounting for spatial biases in opportunistic records or using methods that are difficult to generalize. In this study, we explore the biases that exist in opportunistic observations and suggest an approach to correct for them. We first examined the extent of the biases in opportunistic citizen science observations of three wild ungulate species in Norway by comparing them to data from GPS telemetry. We then quantified the extent of the biases by specifying a model of the biases. From the bias model, we sampled available locations within the species’ home range. Along with opportunistic observations, we used the corrected availability locations to estimate a resource selection function (RSF). We tested this method with simulations and empirical datasets for the three species. We compared the results of our correction method to RSFs obtained using opportunistic observations without correction and to RSFs using GPS‐telemetry data. Finally, we compared habitat suitability maps obtained using each of these models. Opportunistic observations are more affected by human access and visibility than locations derived from GPS telemetry. This has consequences for drawing inferences about species’ ecology. Models naïvely using opportunistic observations in habitat‐use studies can result in spurious inferences. However, sampling availability locations based on the spatial biases in opportunistic data improves the estimation of the species’ RSFs and predicted habitat suitability maps in some cases. This study highlights the challenges and opportunities of using opportunistic observations in habitat‐use studies. While our method is not foolproof it is a first step toward unlocking the potential of opportunistic citizen science data for habitat‐use studies. 相似文献
86.
Tom Pinceel Wouter Hawinkel Enya Wynants Luc Brendonck Bram Vanschoenwinkel 《Hydrobiologia》2017,798(1):141-149
Along with the development of nanotechnology, an increase in production and application of nanosized magnetite (Fe3O4) is expected. Though magnetite is considered relatively safe, information concerning potential hazards of synthetic magnetite nanoparticles with unique physico-chemical characteristics to aquatic organisms is still limited. In this study, we evaluated the toxicity of nanosized (27.2 ± 9.8 nm) and bulk (144.2 ± 67.7 nm) magnetite particles to different life stages of the aquatic crustacean Daphnia magna. In addition, phytotoxicity of the magnetite was evaluated using duckweed Lemna minor. The study did not reveal any statistically significant differences between the biological effects of nanosized and bulk magnetite particles. Both forms of magnetite induced very low toxicity (EC50 > 100 ppm) to D. magna and L. minor in the standard acute assays. However, it was demonstrated that at acutely subtoxic magnetite concentrations (10 and 100 ppm), the number of neonates hatched from D. magna ephippia was decreased. Moreover, short-term (48 h) exposure of neonate daphnids to these concentrations may significantly affect the long-term survival and reproductive potential of daphnids. These results indicate that substantial contamination of aquatic ecosystems by magnetite may disrupt the stability of cladoceran populations. 相似文献
87.
Joachim Mariën Benny Borremans Sophie Gryseels Bram Vanden Broecke Beate Becker-Ziaja Rhodes Makundi Apia Massawe Jonas Reijniers Herwig Leirs 《EcoHealth》2017,14(3):463-473
Infectious diseases of wildlife are typically studied using data on antibody and pathogen levels. In order to interpret these data, it is necessary to know the course of antibodies and pathogen levels after infection. Such data are typically collected using experimental infection studies in which host individuals are inoculated in the laboratory and sampled over an extended period, but because laboratory conditions are controlled and much less variable than natural conditions, the immune response and pathogen dynamics may differ. Here, we compared Morogoro arenavirus infection patterns between naturally and experimentally infected multimammate mice (Mastomys natalensis). Longitudinal samples were collected during three months of bi-weekly trapping in Morogoro, Tanzania, and antibody titer and viral RNA presence were determined. The time of infection was estimated from these data using a recently developed Bayesian approach, which allowed us to assess whether the natural temporal patterns match the previously observed patterns in the laboratory. A good match was found for 52% of naturally infected individuals, while most of the mismatches can be explained by the presence of chronically infected individuals (35%), maternal antibodies (10%), and an antibody detection limit (25%). These results suggest that while laboratory data are useful for interpreting field samples, there can still be differences due to conditions that were not tested in the laboratory. 相似文献
88.
How tree species identity and diversity affect light transmittance to the understory in mature temperate forests
下载免费PDF全文
![点击此处可从《Ecology and evolution》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Bram K. Sercu Lander Baeten Frieke van Coillie An Martel Luc Lens Kris Verheyen Dries Bonte 《Ecology and evolution》2017,7(24):10861-10870
Light is a key resource for plant growth and is of particular importance in forest ecosystems, because of the strong vertical structure leading to successive light interception from canopy to forest floor. Tree species differ in the quantity and heterogeneity of light they transmit. We expect decreases in both the quantity and spatial heterogeneity of light transmittance in mixed stands relative to monocultures, due to complementarity effects and niche filling. We tested the degree to which tree species identity and diversity affected, via differences in tree and shrub cover, the spatiotemporal variation in light availability before, during, and after leaf expansion. Plots with different combinations of three tree species with contrasting light transmittance were selected to obtain a diversity gradient from monocultures to three species mixtures. Light transmittance to the forest floor was measured with hemispherical photography. Increased tree diversity led to increased canopy packing and decreased spatial light heterogeneity at the forest floor in all of the time periods. During leaf expansion, light transmittance did differ between the different tree species and timing of leaf expansion might thus be an important source of variation in light regimes for understory plant species. Although light transmittance at the canopy level after leaf expansion was not measured directly, it most likely differed between tree species and decreased in mixtures due to canopy packing. A complementary shrub layer led, however, to similar light levels at the forest floor in all species combinations in our plots. Synthesis. We find that a complementary shrub layer exploits the higher light availability in particular tree species combinations. Resources at the forest floor are thus ultimately determined by the combined effect of the tree and shrub layer. Mixing species led to less heterogeneity in the amount of light, reducing abiotic niche variability. 相似文献
89.
Hans van Calster Richard Chevalier Bram van Wyngene Frédéric Archaux Kris Verheyen Martin Hermy 《应用植被学》2008,11(2):251-260
Questions: How do changes in forest management, i.e. in disturbance type and frequency, influence species diversity, abundance and composition of the seed bank? How does the relationship between seed bank and vegetation change? What are the implications for seed bank dynamics? Location: An ancient Quercus petraea — Carpinus betulus forest in conversion from coppice‐with‐standards to regular Quercus high forest near Montargis, France. Methods: Seed bank and vegetation were sampled in six replicated stand types, forming a chronosequence along the conversion pathway. The stand types represented mid‐successional stages of stands in transition from coppice‐with‐standards (to high forest (16 plots) and early‐ and mid‐successional high forest stands (32 plots). Results: Seed bank density and species richness decreased with time since last disturbance. Adjusting for seed density effects obscured species richness differences between stand types, but species of later seres were nested subsets of earlier seres, implying concomitant shifts in species richness and composition with time since disturbance. Later seres were characterized by species with low seed weight and high seed longevity. Seed banks of early seres were more similar to vegetation than to later seres. Conclusions: Abandonment of the coppice‐with‐standards regime altered the seed bank characteristics, as well as its relationship with vegetation. Longer management cycles under high forest yield impoverished seed banks. For their persistence, seed bank species will increasingly rely on management of permanently open areas in the forest landscape. Thus, revegetation at the beginning of new high‐forest cycles may increasingly depend on inflow from seed sources. 相似文献
90.
Ann Hulsmans Bram Vanschoenwinkel Chris Pyke Bruce J. Riddoch Luc Brendonck 《Ecosystems》2008,11(1):89-100
Ecological and evolutionary processes in temporary rock pools operate within constraints imposed by their hydrologic regimes.
These shallow pools flood when seasonal rains accumulate on impermeable substrates. Despite the ecological importance of hydrologic
conditions for these ecosystems, we typically lack tools and empirical data required to understand the implications of hydrologic
variability and climate change for biotic populations and communities in these habitats. In this study, we developed a hydrologic
model to simulate rock pool hydrologic regimes based on rainfall, evapotranspiration, and basin geometry. The model was used
to investigate long-term patterns of seasonal and inter-annual variation in hydroregime. In addition, hydrologic conditions
associated with potential climate change scenarios were simulated and evaluated with respect to the biological requirements
of the anostracan Branchipodopsis wolfi. The model’s output for daily inundation matched with field observations with an overall accuracy of 85% and correctly estimated
complete hydroperiods with an overall accuracy of 70%. Simulations indicate large variation in individual hydroperiods (76–115%)
as well as in the number of hydroperiods per year (19–23%). Furthermore, this study suggests that climate change may significantly
alter the rock pool hydroregime. These findings confirm the hydrologic sensitivity of these ephemeral habitats to precipitation
patterns, and their potential sensitivity to future climate change. Modelling indicates that the suitability of average inundation
conditions for B. wolfi deteriorates significantly under future climate predictions. High levels of spatial and temporal variation in hydrologic
conditions are dominant features of these habitats and an essential consideration for understanding population and community-level
ecological processes. 相似文献