全文获取类型
收费全文 | 6019篇 |
免费 | 597篇 |
国内免费 | 300篇 |
出版年
2024年 | 16篇 |
2023年 | 91篇 |
2022年 | 93篇 |
2021年 | 136篇 |
2020年 | 190篇 |
2019年 | 257篇 |
2018年 | 218篇 |
2017年 | 208篇 |
2016年 | 210篇 |
2015年 | 221篇 |
2014年 | 304篇 |
2013年 | 379篇 |
2012年 | 224篇 |
2011年 | 232篇 |
2010年 | 196篇 |
2009年 | 296篇 |
2008年 | 299篇 |
2007年 | 321篇 |
2006年 | 303篇 |
2005年 | 273篇 |
2004年 | 231篇 |
2003年 | 205篇 |
2002年 | 195篇 |
2001年 | 157篇 |
2000年 | 154篇 |
1999年 | 127篇 |
1998年 | 118篇 |
1997年 | 98篇 |
1996年 | 97篇 |
1995年 | 85篇 |
1994年 | 77篇 |
1993年 | 88篇 |
1992年 | 87篇 |
1991年 | 97篇 |
1990年 | 70篇 |
1989年 | 62篇 |
1988年 | 56篇 |
1987年 | 39篇 |
1986年 | 54篇 |
1985年 | 67篇 |
1984年 | 54篇 |
1983年 | 37篇 |
1982年 | 44篇 |
1981年 | 38篇 |
1980年 | 29篇 |
1979年 | 21篇 |
1978年 | 15篇 |
1976年 | 12篇 |
1973年 | 11篇 |
1972年 | 7篇 |
排序方式: 共有6916条查询结果,搜索用时 31 毫秒
101.
At two sites, one with a 4-year-old (4-Y) secondary vegetation and the other with a 20-year-old (20-Y) vegetation, the influence of burning slashed vegetation on crop performance was studied during three seasons. In the first season after clearing, also the influence on weed growth was studied. At both sites, burning significantly decreased the number of weed seedlings. The lowest number of seedlings was found on the burnt plots of the 20-Y site. Burning increased yield and nutrient uptake significantly in the first and second season after clearing. In the third season after burning, only at the 4-Y site a significantly higher yield and nutrient uptake were found. At the 20-Y site the effect had disappeared. Calculations of efficiency of utilization of absorbed N, P and K indicated that P was the least available nutrient, also after burning. At both sites three consecutive crops absorbed approximately 40% of P applied in ash, while the cumulative recovery of K was at least 36% at the 4-Y site and at least 59% at the 20-Y site. On non-burnt plots, yields were not lower in the third season than in the first season after clearing, thus indicating that the inherent soil fertility did not decrease. Hence, yield decline on the burnt plots could be ascribed to ash depletion. It was concluded that in the local shifting cultivation system, the combination of ash depletion and infestation of weeds are the main reasons for abandoning the fields. 相似文献
102.
NMR imaging of root water distribution in intact Vicia faba L plants in elevated atmospheric CO2 总被引:1,自引:1,他引:0
The effect of elevated atmospheric CO2 on water distribution in the intact roots of Vicia faba L. bean seedlings grown in natural soil was studied noninvasively with proton (1H) nuclear magnetic resonance (NMR) imaging. Exposure of 24-d-old plants to atmospheric CO2-enriched air at 650 cm3 m?3 produced significant increases in water imaged in upper roots, hypogeal cotyledons and lower stems in response to a short-term drying-stress cycle. Above ground, drying produced negligible stem shrinkage and stomatal resistance was unchanged. In contrast, the same drying cycle caused significant depletion of water imaged in the same upper root structures in control plants subject to ambient CO2 (350 m3 m?3), and stem shrinkage and increased stomatal resistance. The results suggest that inhibition of transpiration caused by elevated CO2 does not necessarily result in attenuation of water transport from lower root structures. Inhibition of water loss from upper roots and lower stem in elevated CO2 environments may be a mitigating factor in assessing deleterious effects of greenhouse changes on crops during periods of dry climate. 相似文献
103.
Johanna Schmitt 《Evolution; international journal of organic evolution》1993,47(6):1654-1668
For plants, light availability is an important environmental factor that varies both within and between populations. Although the existence of sun and shade “ecotypes” is controversial, it is often assumed that trade-offs may exist between performance in sun and in shade. This study therefore investigated variation in reaction norms to light availability within and between two neighboring natural populations of the annual Impatiens capensis, one in full sun and the other in a forest understory. Seedlings were collected randomly from both populations and grown to maturity in a greenhouse under two light conditions: full light and 18% of full light. Selfed full-sib seed families were collected from plants from both populations grown in both parental light environments. To characterize family reaction norms, seedlings from each family were divided into the same two light treatments and individuals were scored for a variety of morphological and life-history traits. The maternal light environment had little impact on progeny reaction norms. However, the two study populations differed both qualitatively and quantitatively in plastic response to light availability (indicated by significant population x environment interactions in mixed-model ANCOVA). Much of this difference was attributable to population differences in light sensitivity of axillary meristem allocation patterns, which produced concurrent differences in reaction norms for a suite of developmentally linked traits. Within each population, different sets of traits displayed significant variation in plasticity (indicated by significant family x environment interactions). Thus, the genetic potential for evolutionary response to selection in heterogeneous light environments may differ dramatically between neighboring plant populations. Between-environment genetic correlations were largely positive in the woods population and positive or nonsignificant in the sun population; there was no evidence for performance trade-offs across environments or sun or shade “specialist” genotypes within either population. There was little evidence that population differences represented adaptive differentiation for sun or shade; rather, the results suggested the hypothesis of differential selection on patterns of meristem allocation caused by population differences in timing of mortality and intensity of competition. 相似文献
104.
The effect of sublethal levels of methylparathion (0, 1, 3, 5, 7 mg l–1) on the freshwater rotifer, Brachionus calyciflorus, during their entire life cycle was studied. Rotifers were fed on two species of unicellular algae: Nannochloris oculata and Chlorella pyrenoidosa; both algal concentrations were 5 × 105 cell ml–1.The parameters used to determine the toxicity of this compound were survival, fecundity, net reproductive rate (R)o, generation time (T), intrinsic rate of natural increase (r), reproductive value (V
x/Vo) and life expectancy at hatching (eo). All the demographic parameters studied were affected by methyl-parathion exposure on rotifers fed on both species of algae, but the toxic effect was larger when animals were fed on Chlorella pyrenoidosa; in this case, animals showed a decreased in fertility and also a delayed first reproduction. Sublethal methylparathion levels produced a reduction in most of the parameters selected, especially after exposure to 7 mg l–1, where the animals died before reproducing. 相似文献
105.
106.
Niclas U. Lundsgaard Coen Hird Kathleen A. Doody Craig E. Franklin Rebecca L. Cramp 《Global Change Biology》2023,29(14):3857-3868
Ecological carryover effects, or delayed effects of the environment on an organism's phenotype, are central predictors of individual fitness and a key issue in conservation biology. Climate change imposes increasingly variable environmental conditions that may be challenging to early life-history stages in animals with complex life histories, leading to detrimental physiological and fitness effects in later life. Yet, the latent nature of carryover effects, combined with the long temporal scales over which they can manifest, means that this phenomenon remains understudied and is often overlooked in short-term studies limited to single life-history stages. Herein, we review evidence for the physiological carryover effects induced by elevated ultraviolet radiation (UVR; 280–400 nm) as a potential contributor to recent amphibian population declines. UVR exposure causes a suite of molecular, cellular and physiological consequences known to underpin carryover effects in other taxa, but there is a lack of research linking embryonic and larval UVR exposures to fitness consequences post-metamorphosis in amphibians. We propose that the key impacts of UVR on disease-related amphibian declines are facilitated through carryover effects that bridge embryonic and larval UVR exposure with potential increased disease susceptibility post-metamorphosis. We conclude by identifying a practical direction for the study of ecological carryover effects in amphibians that could guide future ecological research in the broader field of conservation physiology. Only by addressing carryover effects can many of the mechanistic links between environmental change and population declines be elucidated. 相似文献
107.
Carles Ibáñez Nuno Caiola José Barquín Oscar Belmar Xavier Benito-Granell Frederic Casals Siobhan Fennessy Jocelyne Hughes Margaret Palmer Josep Peñuelas Estela Romero Jordi Sardans Michael Williams 《Global Change Biology》2023,29(5):1248-1266
Trends and ecological consequences of phosphorus (P) decline and increasing nitrogen (N) to phosphorus (N:P) ratios in rivers and estuaries are reviewed and discussed. Results suggest that re-oligotrophication is a dominant trend in rivers and estuaries of high-income countries in the last two–three decades, while in low-income countries widespread eutrophication occurs. The decline in P is well documented in hundreds of rivers of United States and the European Union, but the biotic response of rivers and estuaries besides phytoplankton decline such as trends in phytoplankton composition, changes in primary production, ecosystem shifts, cascading effects, changes in ecosystem metabolism, etc., have not been sufficiently monitored and investigated, neither the effects of N:P imbalance. N:P imbalance has significant ecological effects that need to be further investigated. There is a growing number of cases in which phytoplankton biomass have been shown to decrease due to re-oligotrophication, but the potential regime shift from phytoplankton to macrophyte dominance described in shallow lakes has been documented only in a few rivers and estuaries yet. The main reasons why regime shifts are rarely described in rivers and estuaries are, from one hand the scarcity of data on macrophyte cover trends, and from the other hand physical factors such as peak flows or high turbidity that could prevent a general spread of submerged macrophytes as observed in shallow lakes. Moreover, re-oligotrophication effects on rivers may be different compared to lakes (e.g., lower dominance of macrophytes) or estuaries (e.g., limitation of primary production by N instead of P) or may be dependent on river/estuary type. We conclude that river and estuary re-oligotrophication effects are complex, diverse and still little known, and in some cases are equivalent to those described in shallow lakes, but the regime shift is more likely to occur in mid to high-order rivers and shallow estuaries. 相似文献
108.
Within estuarine and coastal ecosystems globally, extensive habitat degradation and loss threaten critical ecosystem functions and necessitate widescale restoration efforts. There is abundant evidence that ecological processes and species interactions can vary with habitat characteristics, which has important implications for the design and implementation of restoration efforts aimed at enhancing specific ecosystem functions and services. We conducted an experiment examining how habitat characteristics (presence; edge vs. interior) influence the communities of resident fish and mobile invertebrates on restored oyster (Crassostrea virginica) reefs. Similar to previous studies, we found that restored reefs altered community composition and augmented total abundance and biomass relative to unstructured sand habitat. Community composition and biomass also differed between the edge and interior of individual reefs as a result of species-specific patterns over small spatial scales. These patterns were only weakly linked to oyster density, suggesting that other factors that vary between edge and interior (e.g. predator access or species interactions) are likely more important for community structure on oyster reefs. Fine-scale information on resident species' use of oyster reefs will help facilitate restoration by allowing decision makers to optimize the amount of edge versus interior habitat. To improve the prediction of faunal use and benefits from habitat restoration, we recommend investigations into the mechanisms shaping edge and interior preferences on oyster reefs. 相似文献
109.
St Lucia Lake on the north coast of Natal, South Africa, has an area of 325 km2 and is the largest estuarine complex in Africa. It consists of a 20 km tidal channel, averagingca. 400 m in width, linking the sea with the non-tidal lake which is H-shaped with a maximum length ofca. 40 km and width ofca. 20 km. Except during flood periods the depth of the lake does not exceed 2 m. The salinity gradient depends on evaporation,
the configuration of the mouth and on the input of fresh water from four rivers which discharge into the northern and western
areas of the lake. If fresh water input is high, the lake and much of the channel may be fresh. An intermediate stage features
a normal salinity gradient while a third stage shows a reversed salinity gradient with salinities in excess of 100‰ in the
upper reaches of the system. Changing salinities have marked effects on the biota. Aquatic macrophytes show cycles of appearance
and disappearance depending on salinity tolerance and the presence of dormant stages. The resident benthic faunal species
go through cycles of range expansion and contraction depending on prevailing salinities and recolonisation by dispersal phases.
To date salinities in the southern part of the lake have approached, but not exceeded, lethal levels and this has therefore
acted as a reservoir area. Catchment degradation and water abstraction are anticipated to exacerbate future salinity extremes.
This has resulted in concern for the long term viability of this Ramsar site which has major southern African populations
of hippopotamus and crocodile, provides breeding sites for South African Red Data water bird species and plays an important
nursery role for marine fish and penaeid prawns. 相似文献
110.
For more than 20 years scientists of the ‘Food-chain studies’ Group of the former Limnological Institute have been studying
interactions within the pelagic food web. Purpose of research was to explain the structure and dynamics of the zooplankton
and fish communities in lakes and reservoirs in relation to biotic and abiotic environmental factors. A so-called multi-species
approach was used, in which all common and abundant species within a specific ecosystem were studied on the individual and
population level with the same degree of detail. The recent results and the scientific approach used are evaluated and the
main gaps in knowledge about food-web dynamics in shallow eutrophic lakes are identified and discussed. It is concluded that
instead of the purely functional approach used so far, future studies should also include evolutionary aspects which determine
the success of an organism in a given environment and that more attention should be paid to central questions in ‘community
ecology’.
This paper is based on a lecture given by the first author for the Netherlands Society of Aquatic Ecology on May 12th, 1992,
in Amsterdam, The Netherlands. 相似文献