全文获取类型
收费全文 | 5972篇 |
免费 | 524篇 |
国内免费 | 3篇 |
专业分类
6499篇 |
出版年
2023年 | 42篇 |
2022年 | 116篇 |
2021年 | 223篇 |
2020年 | 93篇 |
2019年 | 128篇 |
2018年 | 146篇 |
2017年 | 104篇 |
2016年 | 210篇 |
2015年 | 364篇 |
2014年 | 333篇 |
2013年 | 410篇 |
2012年 | 570篇 |
2011年 | 499篇 |
2010年 | 320篇 |
2009年 | 241篇 |
2008年 | 353篇 |
2007年 | 359篇 |
2006年 | 318篇 |
2005年 | 283篇 |
2004年 | 286篇 |
2003年 | 234篇 |
2002年 | 217篇 |
2001年 | 45篇 |
2000年 | 28篇 |
1999年 | 38篇 |
1998年 | 57篇 |
1997年 | 28篇 |
1996年 | 26篇 |
1995年 | 19篇 |
1994年 | 32篇 |
1993年 | 29篇 |
1992年 | 19篇 |
1991年 | 20篇 |
1990年 | 15篇 |
1989年 | 17篇 |
1988年 | 13篇 |
1987年 | 11篇 |
1986年 | 12篇 |
1985年 | 15篇 |
1984年 | 20篇 |
1983年 | 16篇 |
1982年 | 15篇 |
1981年 | 8篇 |
1980年 | 13篇 |
1979年 | 9篇 |
1978年 | 13篇 |
1977年 | 11篇 |
1976年 | 9篇 |
1974年 | 7篇 |
1971年 | 13篇 |
排序方式: 共有6499条查询结果,搜索用时 15 毫秒
51.
Bushmeat is an important resource in the livelihoods of many rural communities in sub-Saharan Africa and may be a crucial safety-net for the most vulnerable households, especially during times of economic hardship. However, little is known about the impacts of wildlife depletion on these functions. This study quantifies the role of bushmeat in diversified rural household economies in a wildlife depleted forest-farm landscape in Ghana, assessing its importance overall, as well as differentiated by the relative vulnerability of households. Using repeat socioeconomic questionnaires (N=787) among 63 households over a one-year period, the following hypotheses were tested: (a) vulnerable households harvest more bushmeat; (b) bushmeat contributes a greater proportion of household production in vulnerable households; (c) bushmeat is more important for cash income than consumption in vulnerable households; and (d) bushmeat sales are more important for vulnerable households. The bushmeat harvest value averaged less than US$1.0 per day for 89% of households and comprised less than 7% of household production value. Household wealth and gender of the household head had little effect on the importance of bushmeat. However, bushmeat harvest and sales were highest during the agricultural lean season. Overall, most harvested bushmeat (64%) was consumed, enabling households to spend 30% less on meat/fish purchases. These findings suggest that, despite heavily depleted wildlife and diversified livelihoods, bushmeat continues to have an important role in rural livelihoods by acting as a safety net for income smoothing and reducing household expenditure during times of economic hardship. 相似文献
52.
William Whittaker Laura Anselmi S?ren Rud Kristensen Yiu-Shing Lau Simon Bailey Peter Bower Katherine Checkland Rebecca Elvey Katy Rothwell Jonathan Stokes Damian Hodgson 《PLoS medicine》2016,13(9)
BackgroundHealth services across the world increasingly face pressures on the use of expensive hospital services. Better organisation and delivery of primary care has the potential to manage demand and reduce costs for hospital services, but routine primary care services are not open during evenings and weekends.Extended access (evening and weekend opening) is hypothesized to reduce pressure on hospital services from emergency department visits. However, the existing evidence-base is weak, largely focused on emergency out-of-hours services, and analysed using a before-and after-methodology without effective comparators.ConclusionsThe study found that extending access was associated with a reduction in emergency department visits in the first 12 months. The results of the research have already informed the decision by National Health Service England to extend primary care access across Greater Manchester from 2016. However, further evidence is needed to understand whether extending primary care access is cost-effective and sustainable. 相似文献
53.
Ronan Becheler Constance Xhaard Etienne K. Klein Katherine J. Hayden Pascal Frey Stéphane De Mita Fabien Halkett 《Ecology and evolution》2016,6(18):6625-6632
The genetic consequences of range expansions have generally been investigated at wide geographical and temporal scales, long after the colonization event. A unique ecological system enabled us to both monitor the colonization dynamics and decipher the genetic footprints of expansion over a very short time period. Each year an epidemic of the poplar rust (Melampsora larici‐populina) expands clonally and linearly along the Durance River, in the Alps. The colonization dynamics observed in 2004 showed two phases with different genetic outcomes. Upstream, fast colonization maintained high genetic diversity. Downstream, the colonization wave progressively faltered, diversity eroded, and differentiation increased, as expected under recurrent founder events. In line with the high dispersal abilities of rust pathogens, we provide evidence for leapfrog dispersal of clones. Our results thus emphasize the importance of colonization dynamics in shaping spatial genetic structure in the face of high gene flow. 相似文献
54.
55.
56.
57.
58.
Matthew A. Kolmann Karly E. Cohen Katherine E. Bemis Adam P. Summers Frances J. Irish L. Patricia Hernandez 《Evolution & development》2019,21(5):247-262
Tooth replacement in piranhas is unusual: all teeth on one side of the head are lost as a unit, then replaced simultaneously. We used histology and microCT to examine tooth‐replacement modes across carnivorous piranhas and their herbivorous pacu cousins (Serrasalmidae) and then mapped replacement patterns onto a molecular phylogeny. Pacu teeth develop and are replaced in a manner like piranhas. For serrasalmids, unilateral tooth replacement is not an “all or nothing” phenomenon; we demonstrate that both sides of the jaws have developing tooth rows within them, albeit with one side more mineralized than the other. All serrasalmids (except one) share unilateral tooth replacement, so this is not an adaptation for carnivory. All serrasalmids have interlocking teeth; piranhas interdigitate lateral tooth cusps with adjacent teeth, forming a singular saw‐like blade, whereas lateral cusps in pacus clasp together. For serrasalmids to have an interlocking dentition, their teeth need to develop and erupt at the same time. We propose that interlocking mechanisms prevent tooth loss and ensure continued functionality of the feeding apparatus. Serrasalmid dentitions are ubiquitously heterodont, having incisiform and molariform dentitions reminiscent of mammals. Finally, we propose that simultaneous tooth replacement be considered as a synapomorphy for the family. 相似文献
59.
60.
Daniel Medina Myra C. Hughey Jenifer B. Walke Matthew H. Becker Katherine Pontarelli Shan Sun Brian Badgley Lisa K. Belden 《Environmental microbiology》2019,21(8):2905-2920
Amphibian population declines caused by the fungus Batrachochytrium dendrobatidis (Bd) have prompted studies on the bacterial community that resides on amphibian skin. However, studies addressing the fungal portion of these symbiont communities have lagged behind. Using ITS1 amplicon sequencing, we examined the fungal portion of the skin microbiome of temperate and tropical amphibian species currently coexisting with Bd in nature. We assessed cooccurrence patterns between bacterial and fungal OTUs using a subset of samples for which bacterial 16S rRNA gene amplicon data were also available. We determined that fungal communities were dominated by members of the phyla Ascomycota and Basidiomycota, and also by Chytridiomycota in the most aquatic amphibian species. Alpha diversity of the fungal communities differed across host species, and fungal community structure differed across species and regions. However, we did not find a correlation between fungal diversity/community structure and Bd infection, though we did identify significant correlations between Bd and specific OTUs. Moreover, positive bacterial–fungal cooccurrences suggest that positive interactions between these organisms occur in the skin microbiome. Understanding the ecology of amphibian skin fungi, and their interactions with bacteria will complement our knowledge of the factors influencing community assembly and the overall function of these symbiont communities. 相似文献