全文获取类型
收费全文 | 16455篇 |
免费 | 1282篇 |
国内免费 | 771篇 |
专业分类
林业 | 5370篇 |
农学 | 718篇 |
基础科学 | 231篇 |
1533篇 | |
综合类 | 5703篇 |
农作物 | 647篇 |
水产渔业 | 1330篇 |
畜牧兽医 | 1174篇 |
园艺 | 692篇 |
植物保护 | 1110篇 |
出版年
2024年 | 25篇 |
2023年 | 230篇 |
2022年 | 319篇 |
2021年 | 479篇 |
2020年 | 473篇 |
2019年 | 580篇 |
2018年 | 390篇 |
2017年 | 627篇 |
2016年 | 721篇 |
2015年 | 620篇 |
2014年 | 840篇 |
2013年 | 1002篇 |
2012年 | 1208篇 |
2011年 | 1208篇 |
2010年 | 1030篇 |
2009年 | 1012篇 |
2008年 | 890篇 |
2007年 | 960篇 |
2006年 | 859篇 |
2005年 | 728篇 |
2004年 | 544篇 |
2003年 | 480篇 |
2002年 | 380篇 |
2001年 | 403篇 |
2000年 | 309篇 |
1999年 | 283篇 |
1998年 | 258篇 |
1997年 | 205篇 |
1996年 | 208篇 |
1995年 | 216篇 |
1994年 | 180篇 |
1993年 | 167篇 |
1992年 | 176篇 |
1991年 | 141篇 |
1990年 | 126篇 |
1989年 | 86篇 |
1988年 | 68篇 |
1987年 | 30篇 |
1986年 | 12篇 |
1985年 | 8篇 |
1984年 | 1篇 |
1983年 | 3篇 |
1982年 | 5篇 |
1981年 | 5篇 |
1980年 | 1篇 |
1979年 | 1篇 |
1978年 | 4篇 |
1976年 | 2篇 |
1963年 | 1篇 |
1955年 | 4篇 |
排序方式: 共有10000条查询结果,搜索用时 62 毫秒
71.
Geographic variation in host fish use and larval metamorphosis for the endangered dwarf wedgemussel 下载免费PDF全文
Barbara St. John White C. Paola Ferreri William A. Lellis Barry J. Wicklow Jeffrey C. Cole 《水产资源保护:海洋与淡水生态系统》2017,27(5):909-918
- Host fishes play a crucial role in survival and dispersal of freshwater mussels (Unionoida), particularly rare unionids at conservation risk. Intraspecific variation in host use is not well understood for many mussels, including the endangered dwarf wedgemussel (Alasmidonta heterodon) in the USA.
- Host suitability of 33 fish species for dwarf wedgemussel glochidia (larvae) from the Delaware and Connecticut river basins was tested in laboratory experiments over 9 years. Relative suitability of three different populations of a single host fish, the tessellated darter (Etheostoma olmstedi), from locations in the Connecticut, Delaware, and Susquehanna river basins, was also tested.
- Connecticut River basin A. heterodon metamorphosed into juvenile mussels on tessellated darter, slimy sculpin (Cottus cognatus), and Atlantic salmon (Salmo salar) parr. Delaware River basin mussels metamorphosed using these three species, as well as brown trout (Salmo trutta), banded killifish (Fundulus diaphanus), mottled sculpin (Cottus bairdii), striped bass (Morone saxatilis), and shield darter (Percina peltata). Atlantic salmon, striped bass, and sculpins were highly effective hosts, frequently generating 5+ juveniles per fish (JPF) and metamorphosis success (MS; proportion of attaching larvae that successfully metamorphose) ≥ 0.4, and producing juveniles in repeated trials.
- In experiments on tessellated darters, mean JPF and MS values decreased as isolation between the mussel source (Connecticut River) and each fish source increased; mean JPF = 10.45, 6.85, 4.14, and mean MS = 0.50, 0.41, and 0.34 in Connecticut, Delaware, and Susquehanna river darters, respectively. Host suitability of individual darters was highly variable (JPF = 2–11; MS = 0.20–1.0).
- The results show that mussel–host fish compatibility in A. heterodon differs among Atlantic coastal rivers, and suggest that hosts including anadromous Atlantic salmon and striped bass may help sustain A. heterodon in parts of its range. Continued examination of host use variation, migratory host roles, and mussel–fish interactions in the wild is critical in conservation of A. heterodon and other vulnerable mussel species.
72.
为了解黄河陕西段鱼类种类组成及群落结构,2013-2015年调查了黄河陕西段鱼类资源。结果显示:黄河陕西段有鱼类7目15科58种,其中鲤科鱼类最多为36种,占总种类数的61.1%,较20世纪80年代减少了6种。优势种有4种,数量占渔获物总数量50.3%,生物量占65.8%。分析渔获物组成和群落多样性,黄河陕西段鱼类年龄、个体规格小型化趋势明显,不同河段鱼类群落存在明显空间差异,府谷段多样性指数最高,群落结构稳定,港口段丰富度指数最高,资源种类较多,港口和壶口的鱼类群落结构没有相似性。 相似文献
73.
Growth characteristics of the endangered thick‐shelled river mussel (Unio crassus) near the northern limit of its natural range 下载免费PDF全文
- The thick‐shelled river mussel, Unio crassus (Bivalvia: Unionoida), is one of Europe's most‐threatened mussels. Finnish populations of U. crassus lie close to the northern limit of its natural distribution. Extirpation of these populations will reduce the range of this endangered species.
- Growth characteristics of U. crassus were measured in a river running through the Helsinki metropolitan area. Shell dimensions (size‐at‐age data) and annual shell growth increments were used to reconstruct growth rate and its variation during the lifespan of individual mussels and to investigate the relationship between growth rate and longevity (age‐at‐death).
- Reconstructed growth rates compared well with size‐at‐age data conventionally used to study individual growth in natural populations, and fitted von Bertalanffy growth functions very well. Based on the same function, reconstruction and size‐at‐age methods resulted in similar estimates of growth rate.
- Shell weight explained the body size more reliably than age, suggesting that growth rate varied significantly among individuals. Comparison of individual growth histories revealed a negative correlation between age‐at‐death and growth rate, i.e. slow‐growing mussels lived longer, and vice versa.
- In comparison with populations from central and southern Europe, U. crassus populations in the north of its range grew more slowly but lived longer, a phenomenon most likely explained by latitudinal changes in ambient temperature.
- Although northern populations are expected to benefit from a relatively high number of reproductive periods and lower juvenile mortality, the individuals studied here died earlier and suffered higher mortality than expected and an elevated conservative status is identified. Any management programme should take into account the life‐history traits essential to the recognition of management units of U. crassus.
74.
Conservation implications of establishment success of the Critically Endangered Twee River redfin ‘Pseudobarbus’ erubescens (Skelton, ) in an artificial impoundment in South Africa 下载免费PDF全文
Martine S. Jordaan Johannes A. van der Walt Zanné Brink Sonja Erasmus Olaf L. F. Weyl 《水产资源保护:海洋与淡水生态系统》2017,27(4):886-889
- This study reports the first known record of breeding of the Critically Endangered Twee River redfin ‘Pseudobarbus’ erubescens in an artificial impoundment. This followed an introduction of 48 individuals into a 10 ha impoundment within the species' native range more than a decade ago.
- Sampling the impoundment using three fyke nets set overnight yielded 2838 P. erubescens, which included both juveniles and adults capable of spawning. Fork length measurements of a subsample of 250 individuals ranged from 29 to 125 mm with length cohorts indicating multiple spawning events.
- This demonstrates that this species can successfully reproduce in lentic environments and suggests that artificial impoundments could be stocked to provide refugia for P. erubescens and other highly threatened small cyprinids while conservation strategies are developed to mitigate against habitat loss resulting from alien fish invasions, increased human use of water, and from climate change in rivers.
75.
- Understanding population size and trend is critical information in species management and conservation. To enable accurate population trend estimates, consistent robust monitoring of a species is essential, particularly for a species such as the New Zealand (NZ) sea lion, Phocarctos hookeri , which has experienced an almost continuous decline in pup production since the late 1990s.
- This research examines the pup production estimates for all known breeding sites for this species, and using a stage‐structured matrix population model, estimates population size and trend between 1995 and 2015.
- Overall, it is estimated that 2,316 pups were born in 2015, a decrease of 13% since 1995 and a 27% decline since the highest pup production estimate in 1998. This decline has been driven by the significant decline of 48% at the main breeding area, the Auckland Islands since 1998.
- Using the stage‐structured matrix population model a total species population size of 11,767 sea lions (95% CrI: 10,790–12,923) was estimated. This is the lowest population size of any sea lion species. Trend data for the Auckland Islands indicated that pup and population numbers have decreased at 1.9% yr?1 in the last 20 yr, while total species population decline is 0.6% yr?1.
- Estimates of population trends for this species have been hindered by inconsistent monitoring at most breeding sites. This study strengthens the growing field of research highlighting the need for consistent long‐term monitoring for the conservation management of endangered species.
76.
【目的】探索太白山锐齿槲栎林林下草本植物组成和多样性与环境因子的关系,为该区域森林保护、经营管理提供理论指导。【方法】基于太白山锐齿槲栎林林下草本植物野外调查及室内试验数据,采用CCA和偏CCA排序法分析环境因子对太白山锐齿槲栎林林下草本植物组成的影响,采用相关分析和多元逐步回归分析探索环境因子对其草本植物多样性的影响。【结果】环境因子能显著解释太白山锐齿槲栎林林下草本植物组成变异,海拔、坡向、坡度、pH、枯落物厚度和有机质、碱解N、有效P、有效K含量及郁闭度、灌木盖度11个环境因子共解释了物种组成变异的39.05%。其中,郁闭度、土壤有效P含量、pH值和海拔是影响林下草本植物组成的主要环境因子。海拔、郁闭度、有效P和有效K含量能够显著影响太白山锐齿槲栎林林下草本植物的多样性,海拔较高、郁闭度较大、土壤有效P和有效K含量较高的生境具有较好的草本植物多样性。【结论】在太白山锐齿槲栎林经营管理中,可通过适量施用P肥和K肥来促进林下草本植物发育,提高物种多样性。 相似文献
77.
78.
中国竹类多样性及其重要价值 总被引:5,自引:5,他引:0
竹类属于禾本科Gramineae(Poaceae)、竹亚科Bambusoideae植物,全世界约1 500多种。中国竹类的自然分布广阔,横跨热带、亚热带和温带地区,涵盖包括台湾、香港在内的21个有竹子分布的省、自治区、直辖市和特区。中国是世界上竹类多样性最丰富的国家,也是世界上栽培竹类最丰富的国家,共有竹类植物753种(含22种引进种),约占世界总数的50%以上。同时,中国竹类在起源、遗传进化、内部构造、生态及林型等方面均具有十分丰富的多样性。中国竹类从宏观到微观所展现出的多样性特质,是大自然留给人类的宝贵遗产,对于人类认识自然、保护自然、从而更好的利用竹类资源服务于人类的长远利益,具有重要价值。文章从起源、生态、林型、物种、形态、遗传等方面概述了中国竹类的多样性,为进一步探讨竹类多样性提供基础信息。 相似文献
79.
80.
Iosif Vasiu Roman Dąbrowski Asta Tvarijonaviciute 《Reproduction in domestic animals》2021,56(2):208-230
Mastitis is a common reproductive disorder in bitches, reaching a prevalence of 0.71%. Mastitis has a wide range of forms, from asymptomatic to severe gangrenous mastitis that can lead to septic shock and death of the bitch and nurslings. However, most of the time it is overlooked, undiagnosed or mistreated. The present systematic review was performed to revise and summarize the existing knowledge related to this disorder, including diagnosis, treatment and prevention. 相似文献