首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   7524篇
  免费   1223篇
  国内免费   40篇
林业   1683篇
农学   177篇
基础科学   328篇
  2937篇
综合类   1851篇
农作物   119篇
水产渔业   826篇
畜牧兽医   484篇
园艺   130篇
植物保护   252篇
  2025年   62篇
  2024年   209篇
  2023年   187篇
  2022年   151篇
  2021年   247篇
  2020年   239篇
  2019年   304篇
  2018年   191篇
  2017年   227篇
  2016年   253篇
  2015年   199篇
  2014年   324篇
  2013年   404篇
  2012年   500篇
  2011年   517篇
  2010年   427篇
  2009年   464篇
  2008年   471篇
  2007年   514篇
  2006年   468篇
  2005年   330篇
  2004年   327篇
  2003年   295篇
  2002年   221篇
  2001年   218篇
  2000年   204篇
  1999年   108篇
  1998年   118篇
  1997年   108篇
  1996年   102篇
  1995年   92篇
  1994年   123篇
  1993年   52篇
  1992年   52篇
  1991年   25篇
  1990年   18篇
  1989年   13篇
  1988年   6篇
  1987年   5篇
  1986年   5篇
  1985年   1篇
  1984年   3篇
  1981年   1篇
  1979年   1篇
  1977年   1篇
排序方式: 共有8787条查询结果,搜索用时 0 毫秒
1.
In this study, the cross‐amplification of a commercial multiplex set of 11 cattle (Bos taurus) microsatellites was tested on a panel of 35 European bison (Bison bonasus) individuals. After polymerase chain reaction optimization, all loci cross‐amplified successfully in investigated bisons. Number of alleles and observed and expected heterozygosity per locus are in the range of 2–4, 0.086–0.629 and 0.288–0.621 respectively. The availability of a heterologous set of multiplexed microsatellite markers derived from cattle opens an avenue for collecting profound genetic data for efficient conservation management strategies of the European bison.  相似文献   
2.
The establishment of large numbers of ramets from any individual coral genet can be used as a valuable source of material for the trade in ornamental marine organisms and in physiological and ecotoxicological studies. Here we discuss the rationale and the applicability of rearing ex situ large numbers of nubbins (minute fragments, down to the size of a single polyp). Nubbins taken from colonies of the branching Red Sea coral Stylophora pistillata show high survivorship under ex situ conditions and fast growth rates (5- to 6-fold increase in polyp numbers within 3 months) of the basal plates which revealed a significant correlation between tissue surface area and polyp numbers. Nubbins from branch tips and mid branches grew at a similar rate, but growth was affected by different light regimens. This approach requires further experimentation to elucidate optimal conditions.  相似文献   
3.
Abstract  Seahorses ( Hippocampus spp. ) are a major commodity fished from the shallow coastal seas of the south coast of India where there is an abundance of sea grasses, sponges and corals. They are in great demand for export as traditional medicines, curios and aquarium fish. Organised fishing and trade of seahorses exists in India along the Palk Bay and Gulf of Mannar coasts. At the Palk Bay coast, seahorses are targeted by divers along with sea cucumbers ( Holothuria spp. ) and gastropods (e.g. Murex spp., Xancus pyrum Hornell). In the Gulf of Mannar, most of the seahorses are landed as bycatch of shrimp trawling. Seahorses are also fished from Kerala as a bycatch of trawling, although no organised fishery and trade exists. Five species of seahorses were identified from the Palk Bay coast, whereas only two species were obtained from Kerala. Most seahorses from India are exported to Singapore, Hong Kong, Malaysia and the United Arab Emirates. The volume of dried seahorse trade from India was estimated to be 9.75 t as derived from catch data in 2001, which was much higher than official statistics of 4.34 t during 2001–2002, suggesting the major part of the exports might be through non-conventional means and goes undeclared. Some aspects of the impact of large-scale fishing and trade on conservation of these seahorses are discussed.  相似文献   
4.
5.
    
Soils and crops are particularly vulnerable to climate change and environmental stresses. In many agrosystems, soil biodiversity and ecosystem services provided by soils are under threat from a range of natural and human drivers. Agricultural soils are often subject to agronomic practices that disrupt soil trophic networks and make soils less productive in the long term. In this scenario, sustainable soil use aimed at improving plant/root status, growth and development plays a crucial role for enhancing the biological capacity of agricultural soils. This commentary paper is divided into the following four main sections: (i) the contentious nature of soil organic matter; (ii) soil biological quality/fertility; (iii) soil classification; and, (iv) which agricultural practices can be defined as sustainable? The published literature was analyzed within a holistic framework, with agrosystems considered as living systems where soil, vegetation, fauna and microorganisms co-evolve and are reciprocally influenced. Ultimately, this article will suggest a better stewardship of agricultural soils as a natural capital.  相似文献   
6.
Recent DNA comparisons confirm that the endangeredLake Eacham rainbow fish (Melanotaenia eachamensis Allen andCross, 1982) is a separate species, distinct from othermelanotaeniids in the area. Although the large populationformerly in Lake Eacham has not been re-established, other smallnatural populations of M. eachamensis have been identified fromLake Euramoo and Dirran Creek.Commencing with 23 captive individuals (3 14 6 I), representing three original stocks, Project Eachamensis (initiated by the Australia New Guinea Fishes Association in July 1987) has stimulated and actively encouraged the establishment of new captive populations. Approximately 15 captive breeding populations are now reported in eastern Australia -- numbers of individuals are estimated to exceed 1000; additional Bowman and Tappin populations are being maintained in North America and Europe.A recent review of this conservation programme (involving individual aquarists, hobbyist organizations and scientists as well as the aquarium industry and government) has now recommended: an alteration to the conservati on status category from endangered to vulnerable; maintenance of existing captive populations; formulation of a comprehensive recovery plan with the long-term objective of re-establishing and increasing natural populations; and encouraging public education on the risks of translocation.  相似文献   
7.
    
1. A total of 313 macroinvertebrate taxa were recorded from the 70 running‐water sites in the R iver I nV ertebrate P rediction A nd C lassification S ystem (RIVPACS III) dataset for Northern Ireland, after the application of a standardization procedure to ensure that all samples were identified to the same taxonomic level. The listing includes a small number of oligochaetes not previously reported from Ireland. The frequency of occurrence of the 313 taxa within the Northern Ireland dataset is also presented. 2. The taxonomic composition and frequency of occurrence of taxa in the Northern Ireland dataset were then compared with the 614 site dataset for Great Britain, which included 637 taxa, and a 75 site subset within Britain at a similar latitude to Northern Ireland with 333 taxa. 3. The macroinvertebrate fauna of Northern Ireland is dominated by taxa recorded at the highest frequencies of occurrence in Britain. Some notable absences are highlighted and, in particular, a small number of lotic mayflies and stoneflies which are common in Great Britain. There is documentary evidence of the introduction by man of a number of non‐insect taxa, either deliberately or by accident. 4. A detailed knowledge of the present composition of the macroinvertebrate fauna of running‐water sites in Northern Ireland, and an active research programme on the potential for new colonists to pose a threat to native species, are important factors in the future conservation of the freshwater fauna. Copyright © 2000 John Wiley & Sons, Ltd.  相似文献   
8.
Population growth and human development result in biodiversity loss and biological homogenization not only in developed countries, but increasingly in the less developed countries as well. In those countries, where urbanization and agricultural intensification occur at a faster rate than in developed countries, habitat degradation appears to be the leading cause of wildlife loss. During the breeding seasons of 2002–2005 we conducted road surveys across five biomes of Argentina to detect variations in raptor community attributes as potential indicators of broad scale habitat degradation. Abundance of individuals, richness and diversity of species were calculated to assess the effects of habitat transformation and patch size on these community attributes. Raptor communities strongly varied in relation to habitat transformations, with lower abundance of individuals, richness and diversity of species in more transformed landscapes. Small patches of natural vegetation and locations in which natural and cultivated lands where interspersed showed lower richness and diversity of raptors than large patches. Fragmentation was the main cause of reductions in abundance of individuals. Although the relative contribution of our two estimates of habitat degradation to abundance, richness and diversity of raptors varied among biomes, these community attributes proved useful as predictors of habitat degradation. This was especially true in habitats where raptor communities are more complex although overall patterns remained constant across biomes, from forests to deserts. Taking into account current trends of habitat transformation (drastic increments in monocultures, urban areas, and habitat patchiness), the conservation of raptor communities in these biomes could be seriously compromised. In terms of species-specific responses of raptors to habitat degradation, a rapid process of homogenization can be expected, resulting in only a few winner species within a general scenario of losers.  相似文献   
9.
    
Macrotermes termitaria are conspicuous features of savannah ecosystems in the Sudanian and Sahelian zones of West Africa. The mounds, alive or abandoned, are a major source of heterogeneity in the landscape. The purpose of the present study was to assess the impact of termitaria on tree community in a state forest of the Sudanian regional centre (Tiogo forest, Burkina Faso), under controlled burning and grazing experiments. A comparative inventory was carried out in a split-plot experiment (16 subplots of 2500 m2): 8 subplots where fire regime and grazing were controlled and 8 subplots exposed to grazing and with annual prescribed fire since 1992. All tree individuals (≥1.5 m) were recorded, both on termitaria and outside and their basal area at stump level was measured. A total of 61 observed (or 65.7 ± 2.4 estimated) tree species were recorded on 28 Macrotermes subhyalinus mounds (54 observed species or 60.8 ± 3.3 estimated), the immediate surroundings (44 observed and 59.0 ± 0.0 estimated species) and the rest of subplots (56 observed and 63.6 ± 0.0 estimated). Specific density was higher on mounds in comparison with the surroundings (P < 0.05). Results showed that termitaria played a key role in maintaining higher species diversity as compared to their surroundings (P < 0.05). Differences in species diversity between termitaria and immediate surroundings appeared more pronounced in disturbed plots (submitted to both fire and grazing). Some species, such as Tamarindus indica, Boscia senegalensis, Cadaba farinosa, Capparis sepiaria and Maerua angolensis are found solely on termitaria. Besides, the density of trees was significantly higher on termitaria compared to the surrounding (P < 0.05), as well as total basal area per unit of 100 m2 area (P < 0.05). We concluded that Macrotermes termitaria play an important role as a source of heterogeneity in this Sudanian savannah woodland ecosystem. This role is particularly important in ecosystems under stresses. Termitaria acted as refuge for tree vegetation. The density and dynamics of M. subhyalinus termitaria should, therefore, be taken into account in the global strategy of the forest resources management and conservation.  相似文献   
10.
    
The US Department of Agriculture's Farm and Ranch Irrigation Survey collects the most detailed and comprehensive data on US irrigation practices. Yet, because the data are only easily available in cross‐tab form, data are rarely used for statistical analysis of irrigator behavior. Using data from Arizona and New Mexico, this study illustrates how statistical measures of association can be used test hypotheses about how farm size (measured by sales class) affects (i) use of water management information, (ii) investment in irrigation improvements, and (iii) participation in conservation programs. Parametric (Cochran–Armitage trend test) and nonparametric (Goodman–Kruskall gamma) methods yielded similar results. Reliance on low‐cost, general information was common among all size classes, while larger operations relied more on private, tailored information. Larger operations were more likely to use directly provided data (e.g. media and Internet reports) than smaller operators, who relied more on information provided by intermediaries. Smaller farms were less likely to investigate irrigation improvements, use management‐intensive methods for irrigation scheduling, or participate in cost‐share programs to encourage adoption of improved irrigation practices. Adoption of scientific irrigation scheduling methods was low for all groups, but especially low for small‐scale irrigators. There appear to be significant barriers to information acquisition, use of management‐intensive irrigation practices, and participation in conservation programs among smaller‐scale irrigators. Copyright © 2012 John Wiley & Sons, Ltd.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号