全文获取类型
收费全文 | 3441篇 |
免费 | 334篇 |
国内免费 | 104篇 |
专业分类
林业 | 821篇 |
农学 | 255篇 |
基础科学 | 141篇 |
520篇 | |
综合类 | 1456篇 |
农作物 | 18篇 |
水产渔业 | 274篇 |
畜牧兽医 | 95篇 |
园艺 | 129篇 |
植物保护 | 170篇 |
出版年
2024年 | 9篇 |
2023年 | 67篇 |
2022年 | 94篇 |
2021年 | 96篇 |
2020年 | 110篇 |
2019年 | 131篇 |
2018年 | 85篇 |
2017年 | 142篇 |
2016年 | 137篇 |
2015年 | 149篇 |
2014年 | 215篇 |
2013年 | 264篇 |
2012年 | 384篇 |
2011年 | 335篇 |
2010年 | 250篇 |
2009年 | 230篇 |
2008年 | 194篇 |
2007年 | 201篇 |
2006年 | 206篇 |
2005年 | 148篇 |
2004年 | 113篇 |
2003年 | 82篇 |
2002年 | 57篇 |
2001年 | 40篇 |
2000年 | 39篇 |
1999年 | 23篇 |
1998年 | 22篇 |
1997年 | 17篇 |
1996年 | 6篇 |
1995年 | 7篇 |
1994年 | 9篇 |
1993年 | 4篇 |
1992年 | 4篇 |
1991年 | 2篇 |
1989年 | 1篇 |
1988年 | 4篇 |
1987年 | 1篇 |
1986年 | 1篇 |
排序方式: 共有3879条查询结果,搜索用时 31 毫秒
1.
Jordana Ramalho 《Asia Pacific viewpoint》2019,60(1):24-36
In the Philippines, calls for creating ‘global’, ‘sustainable’ and ‘resilient’ cities are placing urban poor communities in increasingly precarious positions. These communities have long been the targets of urban development and ‘modernisation’ efforts; more recently the erasure of informal settlements from Philippine cities is being bolstered at the behest of climate change adaptation and disaster risk management (DRM) agendas. In Metro Cebu, flood management has been at the heart of DRM and broader urban development discussions, and is serving as justification for the demolition and displacement of informal settler communities in areas classed as ‘danger zones’. Using Kusno's (2010) interpretation of the ‘exemplary centre’ as a point of departure, this paper interrogates the relationship between DRM, worlding aspirations (Roy and Ong, 2011) and market‐oriented urbanisation in Cebu, and considers the socio‐spatial implications of these intersecting processes for urban poor communities. Through analysing the contradictions inherent in framings of certain bodies and spaces as being ‘of risk’ or ‘at risk’ over others, I argue that the epistemologies of modernity, disaster risk and resilience endorsed and propagated by the state are facilitating processes of displacement and dispossession that serve elite commercial interests under the auspices of disaster resilience and pro‐poor development. 相似文献
2.
Trout and charr, members of the salmonid family, have high conservation value but are also susceptible to anthropogenic threats in part due to the specificity of their habitat requirements. Understanding historical and future threats facing these species is necessary to promote their recovery. Of freshwater trout and charr in the Canadian Rocky Mountain region, westslope cutthroat trout (Oncorhynchus clarkii lewisi), bull trout (Salvelinus confluentus; a charr species) and Athabasca rainbow trout (Oncorhynchus mykiss) are of conservation concern. And indeed, range contractions and declining populations are evident throughout much of their ranges. Range contraction was most evident in the southern Alberta designatable unit (DU) of westslope cutthroat trout. Diminished populations were also evident in the downstream watersheds of the Alberta bull trout range, and throughout the Athabasca rainbow trout range. We assessed historical and future threats to evaluate the relative importance of individual threats to each DU and compare their impact among species. Individual threats fall into the broad categories of angling, non-native species and genes, habitat loss and alteration, and climate change. Severity of each threat varies by DU and reflects the interaction between species’ biology and the location of the DU. Severity of threats facing each DU has changed over time, reflecting extirpation of native populations, changes in management and industry best practices, expansion of non-native species and progressing climate change. The overall threat impact for each DU indicates a high probability of substantial and continuing declines and calls for immediate action. 相似文献
3.
[目的]对长三角沿江城市群生态—经济—社会系统可持续发展态势进行研究,为该区域生态经济可持续发展提供科学支持。[方法]基于能值理论构建评价指标和评价体系,分析长三角沿江城市群生态—经济—社会系统的可持续发展态势。[结果] 2017年长三角沿江城市群中29%的城市资源过度开发,外部依赖性强;53%的城市环境负载率低,29.4%的城市环境负载率高;11.7%的城市废弃物循环再生能力低;47%的城市能值交换率低,经济竞争力强;35%的城市能值货币率高,经济产出效率低。上海、苏州单位面积能值密度高,23.5%的城市较低;29.4%的城市人均能值福利低;17.6%的城市人口负载率高;41.2%的沿江城市呈可持续发展态势,58.8%的城市呈过度发展态势。[结论] 2017年沿江城市群自给能力、环境压力、生态质量呈现上游城市强、小、优向中游基本协调,下游弱、高、差的渐变态势。上海、苏南城市经济竞争力强,但产出效率低;苏中、皖中地区除省会城市外其他城市竞争力弱;皖南城市经济发展滞后且呈沿江单边孤立发展态势。上海、苏南城市社会可持续发展优,形成人才、资源强吸引同时存在用地短缺问题;苏中城市较均衡并形成弱吸引;除合肥市、铜陵市外,皖中、皖南其他城市发展滞后,缺乏有效吸引。沿江城市群整体可持续发展态势严峻。 相似文献
4.
粉红香水月季为著名的古老藤本月季,前期引种发现,粉红香水月季在上海具有良好的适应性。为进一步开发利用,本研究主要对其芳香性开展研究。采用搅拌子吸附萃取脱附结合气相色谱质谱技术提取和分析鲜花花瓣中的香气成分。结果显示,粉红香水月季的主要香气成分为3,5-二甲氧基甲苯,其次为3-乙酸叶醇酯、石竹烯、香叶醇等。香气类型为以3,5-二甲氧基甲苯为代表的茶香型,香气偏清韵。同时,3,5-二甲氧基甲苯具有独特的镇定安神效果。粉红香水月季在观赏园林和保健园林的景观营造中具广阔应用前景。 相似文献
5.
曹琪 《北京农业职业学院学报》2021,35(3):54-59
北京市昌平区通过大力实施乡村振兴战略,深入推进城乡区域协调发展,在城乡基本公共服务发展、产业发展、乡村治理、人才队伍建设等方面取得了一定进展.但同时也存在产业发展缓慢、人才力量不足等问题,为此,提出推动城乡基本公共服务均衡发展,多措并举加快乡村产业发展,创新和完善乡村治理机制,促进实用人才培养等四个方面的建议. 相似文献
6.
在生态公共服务视野下,尽管当前我国的城市森林发展较为迅速,但对其森林生态经济系统的协同机理研究仍然还是一个新课题。本文结合海南国际旅游岛建设的相关实践,从治理机制、市场机制、利益分配机制、合作机制、扶持机制5个方面,对海南城市森林进行研究,以促进海南城市森林资源的优化利用和社会经济的可持续发展。 相似文献
7.
Effect of an intensive mechanical removal effort on a population of non‐native rainbow trout Oncorhynchus mykiss in a South African headwater stream 下载免费PDF全文
Jeremy Shelton Olaf Weyl Johannes Van Der Walt Sean Marr Dean Impson Kristine Maciejewski Donovan Tye Helen Dallas Karen Esler 《水产资源保护:海洋与淡水生态系统》2017,27(5):1051-1055
- Invasions by non‐native species can compromise the conservation value of otherwise pristine headwater streams. While both developed and developing countries recognize this threat, few of the latter have suitable budgets to implement control programmes.
- This study assessed the effectiveness of a mechanical project to remove non‐native rainbow trout Oncorhynchus mykiss from a 6 km section of the upper Krom River, a small headwater stream in the Cederberg Mountains in South Africa's Cape Floristic Region (CFR).
- From October 2013 to February 2014, 354 O. mykiss were removed by angling (58%), fyke netting (28%) and gill netting (14%). This resulted in a marked reduction, but not eradication, of the O. mykiss population (fish relative abundance decreased from 0.53 ± 0.09 fish per net per night in October 2013 to 0.21 ± 0.09 fish per net per night in February 2014). Following the cessation of manual removals, the relative abundance of O. mykiss had increased to 0.56 ± 0.18 fish per net per night by March 2016, suggesting that without sustained removal effort, the population will rapidly return to its pre‐removal abundance level.
- Further work is needed to refine the methodology and test the effectiveness of mechanical removal of non‐native freshwater fish in a variety of ecological settings in the CFR. This approach holds potential for meeting the dual goals of reducing the ecological impacts of non‐native fishes and generating employment opportunities in line with the policy objectives of developing nations.
8.
Interactions of baseflow habitat constraints: Macroinvertebrate drift,stream temperature,and physical habitat for anadromous salmon in the Calapooia River,Oregon 下载免费PDF全文
Robert J. Danehy Robert E. Bilby Sara Owen Steven D. Duke Alex Farrand 《水产资源保护:海洋与淡水生态系统》2017,27(3):653-662
- The Calapooia River in western Oregon supports a small winter steelhead trout (Oncorhynchus mykiss) population and historically supported spring Chinook salmon (Oncorhynchus tshawytscha). Early timber harvesting removed the riparian forest, and log transportation practices simplified the channel. Those disturbance legacies continue to affect fish habitat by limiting shade and channel complexity, complicating conservation efforts.
- To evaluate juvenile salmonid rearing potential, macroinvertebrate drift, thermal regime and physical habitat were measured at eight sites in 24 km of the upper river during late summer baseflow.
- Overall physical habitat was simple, with few functioning instream structures or pools. During the 22‐day drift study, flows declined and maximum site stream temperatures ranged from 23.1°C at the lower end to 16.4°C 24 km upstream.
- Macroinvertebrate drift concentrations ranged from 0.7–13.7 ind. m?3 with biomasses from 0.02–1.23 mg m?3. Drift concentration biomass was higher upstream (P = 0.006) than downstream and declined overall (P < 0.001) during the study. Drift biomass was dominated by five taxon groups – Baetis tricaudatus, Calineuria californica, Hesperoperla pacifica, Simulium spp., and Chironomidae, which were 65% of total biomass. During twilight, total biomass and biomass of B. tricaudatus, Simulium spp., and Chironomidae (both larvae and adults) were higher.
- Total drift declined dramatically over the study period owing to decreases in drift concentration and a 58% decline in discharge, greatly reducing overall drift and available food resources for juvenile‐rearing salmonids.
- The upper catchment, both with cooler temperatures and higher food availability, provided the best conditions for juvenile anadromous salmonids to survive late summer conditions. Conservation consequences of climate change‐induced alterations in flow and temperature may further affect habitat quality for juvenile salmonids in this catchment in the coming decades.
9.
2011年7月—2012年4月对武汉市汉阳地区五个湖泊(后官湖,三角湖,南太子湖,墨水湖和龙阳湖)的水质及沉水植物进行了季节性调查,以了解湖泊富营养化现状及其对沉水植物的影响。结果显示:后官湖为中营养,而三角湖、南太子湖、墨水湖和龙阳湖为重度富营养,其中龙阳湖污染最重。共采集沉水植物7种,隶属于5科5属,主要种类为金鱼藻(Ceratophyllum demersum)、苦草(Vallisneria spiralis)和菹草(Potamogeton crispus)。沉水植物主要分布在后官湖沿岸带,其它四个湖泊中沉水植物已严重退化,仅在部分水域偶见。后官湖沉水植物盖度和生物量最大值出现在10月,分别为67.5%和5.58 kg/m~2,1月份较低,分别为29.3%和1.88 kg/m~2,优势种存在明显的季节更替。结果表明,汉阳地区湖泊沉水植物退化与富营养化引起的水下光照下降和高密度的水产养殖有关。 相似文献
10.
Enhancing capacity for freshwater conservation at the genetic level: a demonstration using three stream macroinvertebrates 下载免费PDF全文
- Species diversity is declining more rapidly in freshwater ecosystems than in any other, but the consequences for genetic diversity, and hence evolutionary potential, are poorly understood. In part this reflects limited use and development of modern molecular tools and genetic approaches to address conservation questions in rivers, lakes and wetlands. As widespread, diverse and functionally important organisms, freshwater macroinvertebrates are ideal candidates for genetic approaches to reveal, for example, the conservation consequences of demographic histories and past disturbances. However, the availability of microsatellite markers for this group is very limited.
- Using next generation sequencing, microsatellite markers were developed for Isoperla grammatica (Poda, 1761), Amphinemura sulcicollis (Stephens, 1836) and Baetis rhodani (Pictet, 1843) to enable conservation genetic investigations of these widespread invertebrate species. Fifty‐two robust microsatellite loci were developed (18, 21 and 13 per species), all with high levels of allelic diversity (7–27, 3–16, 5–13 alleles per loci, respectively).
- These tools will allow assessment of genetic structure, dispersal and demographic resilience in these model species as a function of environmental change and variation, thereby aiding freshwater monitoring and conservation. The authors urge further capacity building to support genetic applications to the conservation biology of other aquatic organisms.