全文获取类型
收费全文 | 2150篇 |
免费 | 185篇 |
国内免费 | 274篇 |
出版年
2023年 | 49篇 |
2022年 | 67篇 |
2021年 | 80篇 |
2020年 | 86篇 |
2019年 | 115篇 |
2018年 | 85篇 |
2017年 | 78篇 |
2016年 | 69篇 |
2015年 | 73篇 |
2014年 | 92篇 |
2013年 | 130篇 |
2012年 | 68篇 |
2011年 | 85篇 |
2010年 | 69篇 |
2009年 | 109篇 |
2008年 | 118篇 |
2007年 | 146篇 |
2006年 | 106篇 |
2005年 | 100篇 |
2004年 | 89篇 |
2003年 | 79篇 |
2002年 | 84篇 |
2001年 | 58篇 |
2000年 | 61篇 |
1999年 | 46篇 |
1998年 | 45篇 |
1997年 | 57篇 |
1996年 | 41篇 |
1995年 | 29篇 |
1994年 | 34篇 |
1993年 | 27篇 |
1992年 | 33篇 |
1991年 | 30篇 |
1990年 | 13篇 |
1989年 | 32篇 |
1988年 | 16篇 |
1987年 | 22篇 |
1986年 | 12篇 |
1985年 | 18篇 |
1984年 | 10篇 |
1983年 | 3篇 |
1982年 | 11篇 |
1981年 | 8篇 |
1980年 | 4篇 |
1979年 | 4篇 |
1978年 | 7篇 |
1976年 | 3篇 |
1975年 | 2篇 |
1974年 | 3篇 |
1973年 | 1篇 |
排序方式: 共有2609条查询结果,搜索用时 20 毫秒
1.
2.
Building resource-conserving and environmental-friendly society (referred to as “two-oriented society”, TOS) is an important way proposed by the Chinese government to achieve sustainable development. In this paper, a pilot city of constructing TOS in China-Wuhan is taken as a case to evaluate the performance of TOS from 2005 to 2012. Treating the indicators of TOS as multi-dimensional vectors, this paper proposes a methodological framework by integrating the methods of vector angle and Euclidean distance to measure the angle and distance between the vector of annual status of TOS and the vector of planning target of TOS. Based on this, the paper employs coordination (the angle between the two vectors) and effectiveness (the distance between the two vectors) to describe the performance of TOS and its subsystems, including economic development (ED), social and people's well-being (SW), resource consumption (RC), resource recycling (RR), environmental quality (EQ) and pollution control (PC). Moreover, grey relational analysis approach is used to analyse the core factors influencing TOS construction. Results reveal that the performance of TOS in Wuhan is continually improved in the study period while the performance of its subsystems shows several differences: (1) both coordination and effectiveness of TOS, ED and SW keep on improving, while those of EQ fluctuate; (2) the effectiveness of both RR and PC shows a downward trend while the coordination of them fluctuates; (3) the actual value of RC indicators reaches the planning targets. Based on the analysis of determinants, we suggest that strengthening technological ability and adding investment are extremely important to improve the performance of RC and PC. For the sake of improving RR, it is critical to provide more government public expenditure and encourage financial institutions to provide more loans to stimulate and support the businesses. Moreover, increasing the amount of R&D funding and maintaining steady external economic environment are proved to be effective to improve all subsystems of TOS. 相似文献
3.
D.J. BULLOCK 《Biological journal of the Linnean Society. Linnean Society of London》1995,56(S1):119-126
A preliminary analysis of the use by bats of National Trust properties revealed that all 14 resident species have been recorded. Comparison with a national database of records for Britain managed by English Nature (NCC Bat database) showed that the scarcer species are better represented roosting in buildings on Trust properties than in other buildings. In addition, survey of Trust properties has shown that a high proportion of buildings are used by bats. Bats provide unusual wildlife links between buildings, gardens and countryside through which the Trust can promote nature conservation, and resolve conflicts between building or landscape restoration and the bat habitat requirements. The role of the County Bat Groups in survey and promotion of bats on Trust properties is becoming increasingly important, especially if we are able to identify key properties for bats. 相似文献
4.
Xuan He Abishek Chandrashekar Roland Zahn Frank Wegmann Jingyou Yu Noe B. Mercado Katherine McMahan Amanda J. Martinot Cesar Piedra-Mora Sidney Beecy Sarah Ducat Ronnie Chamanza Sietske Rosendahl Huber Marjolein van Heerden Leslie van der Fits Erica N. Borducchi Michelle Lifton Jinyan Liu Dan H. Barouch 《Cell》2021,184(13):3467-3473.e11
5.
The potential for and constraints on the evolution of compensatory ability in Asclepias syriaca 总被引:2,自引:0,他引:2
To investigate the potential for and constraints on the evolution of compensatory ability, we performed a greenhouse experiment
using Asclepias syriaca in which foliar damage and soil nutrient concentration were manipulated. Under low nutrient conditions, significant genetic
variation was detected for allocation patterns and for compensatory ability. Furthermore, resource allocation to storage was
positively, genetically correlated both with compensatory ability and biomass when damaged, the last two being positively,
genetically correlated with each other. Thus, in the low nutrient environment, compensatory ability via resource allocation
to storage provided greater biomass when damaged. A negative genetic correlation between compensatory ability and plant biomass
when undamaged suggests that this mechanism entailed an allocation cost, which would constrain the evolution of greater compensatory
ability when nutrients are limited. Under high nutrient conditions, neither compensatory ability nor allocation patterns predicted
biomass when damaged, even though genetic variation in compensatory ability existed. Instead, plant biomass when undamaged
predicted biomass when damaged. The differences in outcomes between the two nutrient treatments highlight the importance of
considering the possible range of environmental conditions that a genotype may experience. Furthermore, traits that conferred
compensatory ability did not necessarily contribute to biomass when damaged, demonstrating that it is critical to examine
both compensatory ability and biomass when damaged to determine whether selection by herbivores can favor the evolution of
increased compensation.
Received: 2 April 1999 / Accepted: 21 September 1999 相似文献
6.
Summary The control of phytopathogenic fungi in agriculture requires sometimes excessive chemical treatments because, in many cases, reliable monitoring systems in order to assess the real risk for the cultivations are not available.Therefore, the likelihood to assess the presence and the amount of phytophatogenic fungus propagules in the air with a spore-trap has been investigated in order to evaluate their correlation with the climate and the disease occurrence. Four phytopathogenic fungi, namelyVenturia inequalis, Phytophthora infestans, Cercospora beticola andDiaporthe phaseolorum var.caulivora were taken into consideration.Practical use of airborne spore monitoring in a disease control strategy proved to be useful only forVenturia inequalis andPhytophthora infestans causal agent of apple scab and tomato late blight respectively. For the latter, the utilisation of an aerobiological monitoring system for the disease prediction can be assumed. 相似文献
7.
Michael Kleyer 《植被学杂志》1999,10(5):697-708
Abstract. In this study, plant functional types are understood as groups of plants with similar biological traits displaying significant optima or maxima on a gradient plane of resource supply and disturbance intensity. The biological traits refer to expansion, vegetative regeneration, generative reproduction, dispersal and seed bank longevity. 129 vegetation samples were taken in an agricultural landscape in southwestern Germany, covering a wide range of terrestrial vegetation types – but with the exception of forests and wetlands. For each site, also soil data were recorded. Mean daily soil moisture was estimated with a simple model. Soil moisture, balanced nitrogen supply and available phosphorus were combined into a factor ‘resource supply’. In addition, disturbance intensity was estimated for each site. This factor was based on (1) frequency of disturbance, (2) disturbance depth below or above the soil surface, and (3) proportion of the area affected by a discrete disturbance event. 30 plant groups with similar biological characteristics resulted from a cluster analysis, based on a compilation of 19 biological traits for a regional species pool. Logistic regression on a gradient plane of disturbance intensity and resource supply yielded response curves for 28 groups. The dependent variable was defined as the probability of encountering all members of a group in a sample. 17 groups display a significant response curve on the gradient plane. Plants with a potential for long- range dispersal are concentrated on sites with low or high disturbance intensities (e.g. fallow land, fields, lawns). On sites with medium disturbance intensity (e.g. meadows) and low to medium resource supply, small-range dispersal predominates. There are no distinct trends concerning seed bank longevity. The potential for vertical and lateral expansion increases with decreasing disturbance intensity. Only at medium disturbance intensities does vertical expansion correlate positively with resource supply. Rapid detachment of daughter individuals occurs more often on productive sites than on less productive sites. Diversity of groups with similar biological traits is highest on sites with medium disturbance intensities. 相似文献
8.
Feeding of an Iberian stream cyprinid assemblage: seasonality of resource use in a highly variable environment 总被引:1,自引:0,他引:1
M. F. Magalhães 《Oecologia》1993,96(2):253-260
Food resource use by seven cyprinids from an Iberian stream was analysed over 9 months. Differences in food resource use were found both between species and within species between seasons. Plant material was a more important food for carp, nase, goldfish and barbel than for gudgeon, roach and chub, irrespective of the season. Chironomid larvae were the staple animal food for the former five species throughout the year. Roach and chub, especially the latter, displayed large seasonal variations in prey use, with chironomid larvae only being important during autumn. Ephemerellid nymphs and ephemeropteran imagos dominated the animal diet of chub during spring and summer, respectively. Dipteran adults and Formicidae were the most important prey for roach during spring and summer, with other common prey being ephemerellid nymphs and hydropsychid larvae. Food resource overlap among the three dominant species (roach, barbel and chub) displayed a large seasonal variation. High overlaps were observed during autumn when these species used the same resources. During summer overlaps were much lower with each species specialising on different prey. The remaining less abundant species had large diet overlaps amongst themselves and with barbel, over all seasons. It is suggested that morphological constraints, habitat partitioning and temporal changes in food resource limitation may be involved in producing these patterns of food resource use. 相似文献
9.
Resource scarcity poses an increasing threat to the supply security of modern economies. Some grand challenges ahead are the limits to agricultural expansion and the geologic scarcity of metals. To better understand the drivers behind land and metal depletion, footprint-type indicators are gaining importance. Such indicators, however, fail to differentiate between vastly different degrees of resource availability across regions. Using crop suitability areas and metal reserve base data, we calculate scarcity-weighted land and metal footprints for the major economies with the EXIOBASE global multi-regional input-output model. Scarcity-weighting causes a significant reordering of the global rankings of countries for both land and metal footprints. Land scarcity focuses mostly on cereals (∼54% from the total agricultural land used) and oil crops (∼15%), the former being notably affected by water scarcity issues in Asia and the Middle East. Metal scarcity focuses on copper ores (∼69%) and iron (∼11%), the former being a globally scarce metal impacting multiple economies. The large impact of scarcity-weighting suggests that, while non-weighted resource footprints are a valid proxy of resource use, these are not always aligned with further implications of resource depletion and supply security. In this sense, scarcity-weighting can offer an initial overview of those countries where analyses at finer scales may be more valuable. Our results also show that international trade is a major driver of land and metal depletion in some developing regions. This highlights the intersection of environmental justice and globalization, as the burden of resource depletion often falls into poorer regions which critically rely on exports. 相似文献
10.
Population Ecology - 相似文献