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   1436篇
  免费   145篇
  国内免费   69篇
  2024年   8篇
  2023年   31篇
  2022年   41篇
  2021年   68篇
  2020年   63篇
  2019年   84篇
  2018年   65篇
  2017年   48篇
  2016年   54篇
  2015年   60篇
  2014年   74篇
  2013年   111篇
  2012年   64篇
  2011年   60篇
  2010年   71篇
  2009年   65篇
  2008年   88篇
  2007年   85篇
  2006年   58篇
  2005年   56篇
  2004年   43篇
  2003年   39篇
  2002年   40篇
  2001年   35篇
  2000年   26篇
  1999年   20篇
  1998年   20篇
  1997年   20篇
  1996年   21篇
  1995年   16篇
  1994年   11篇
  1993年   14篇
  1992年   16篇
  1991年   5篇
  1990年   7篇
  1989年   3篇
  1988年   7篇
  1987年   12篇
  1986年   6篇
  1985年   6篇
  1984年   3篇
  1983年   4篇
  1982年   4篇
  1981年   3篇
  1980年   3篇
  1979年   3篇
  1978年   3篇
  1977年   3篇
  1976年   2篇
  1974年   1篇
排序方式: 共有1650条查询结果,搜索用时 31 毫秒
991.
Phylogenetic analysis of a collection of rabies viruses that currently circulate in Canadian big brown bats (Eptesicus fuscus) identified five distinct lineages which have emerged from a common ancestor that existed over 400 years ago. Four of these lineages are regionally restricted in their range while the fifth lineage, comprising two‐thirds of all specimens, has emerged in recent times and exhibits a recent demographic expansion with rapid spread across the Canadian range of its host. Four of these viral lineages are shown to circulate in the US. To explore the role of the big brown bat host in dissemination of these viral variants, the population structure of this species was explored using both mitochondrial DNA and nuclear microsatellite markers. These data suggest the existence of three subpopulations distributed in British Columbia, mid‐western Canada (Alberta and Saskatchewan) and eastern Canada (Quebec and Ontario), respectively. We suggest that these three bat subpopulations may differ by their level of female phylopatry, which in turn affects the spread of rabies viruses. We discuss how this bat population structure has affected the historical spread of rabies virus variants across the country and the potential impact of these events on public health concerns regarding rabies.  相似文献   
992.
The negative effects of roads on wildlife in tropical rainforests are poorly understood. Road construction has high priority in Africa, while negative impacts of roads on wildlife movement often are neglected. This study aims at providing information on the effects of roads on crossing behaviour of rainforest wildlife. The probability that wildlife would cross forest roads was analysed for association with ten different factors that were linked to road presence or construction. Factors were divided into three classes: vegetation cover, topography and human influence. A trackplot survey was done in southern Cameroon, Africa. Trackplots were laid along a 32 km unpaved logging road that intersects Campo‐Ma’an National Park. Tracks of several species were found frequently (e.g. genets and porcupines); while others were found only sporadically (e.g. forest duikers and apes). The actual physical obstacles found along the road (e.g. logs, banks, etc.) were highly negatively correlated with crossing probabilities. For all wildlife species high vegetation cover was positively correlated to crossing probability. This study indicates that roads have a large impact on wildlife, and suggests which factors could be altered during road construction and maintenance in order to mitigate these impacts.  相似文献   
993.
Habitat loss and hunting pressure threaten mammal populations worldwide, generating critical time constraints on trend assessment. This study introduces a new survey method that samples continuously and non‐invasively over long time periods, obtaining estimates of abundance from vocalization rates. We present feasibility assessment methods for acoustic surveys and develop equations for estimating population size. As an illustration, we demonstrate the feasibility of acoustic surveys for African forest elephants (Loxodonta africana cyclotis). Visual surveys and vocalizations from a forest clearing in the Central African Republic were used to establish that low‐frequency elephant calling rate is a useful index of elephant numbers (linear regression P < 0.001, radj.2 = 0.58). The effective sampling area was 3.22 km2 per acoustic sensor, a dramatic increase in coverage over dung survey transects. These results support the use of acoustic surveys for estimating elephant abundance over large remote areas and in diverse habitats, using a distributed network of acoustic sensors. The abundance estimation methods presented can be applied in surveys of any species for which an acoustic abundance index and detection function have been established. This acoustic survey technique provides an opportunity to improve management and conservation of many acoustically‐active taxa whose populations are currently under‐monitored.  相似文献   
994.
γ-氨基丁酸能抑制可锐化大棕蝠听皮层神经元频率调谐   总被引:8,自引:0,他引:8  
本实验使用了 9只成年健康的大棕蝠 (Eptesicusfuscus)。采用双声刺激和多管电极电泳导入荷包牡丹碱 (bicuculline,Bic)的方法 ,研究了γ 氨基丁酸 (γ aminobutyricacid ,GABA)能抑制在锐化听皮层 (primaryauditorycortex ,AC ,即初级听皮层 )神经元频率调谐中的作用。结果发现 :正常AC神经元的频率调谐曲线表现出单峰开放式、多峰开放式和单峰封闭式 3种类型 ;用双声刺激方法研究证实 ,至AC神经元的抑制性输入能被抑制性声刺激所激活 ,且这种神经抑制有自身的最佳频率 ,根据其对兴奋反应的影响程度和系统地改变抑制性声刺激的强度 ,可在兴奋性频率调谐曲线或兴奋区的高频边或 /和低频边测出抑制性频率调谐曲线或抑制区 ;当这种抑制性输入被抑制性声刺激激活后 ,能降低阈上 10dB声强引起的兴奋反应的发放率 ,抑制效率随抑制声刺激强度的增强而加强 ;电泳GABAa受体拮抗剂荷包牡丹碱Bic后 ,可不同程度地去GABA能抑制 ,扩宽频率调谐曲线 ,使多峰调谐曲线变成单峰 ,封闭型变成开放型。表明GABA能抑制参与构成至AC神经元的抑制性输入 ,在正常情况下这种抑制有助于提高中枢听神经元的信号 /噪声比和频率分析能力 ,并锐化频率调谐。因此本结果提示 ,声音的各参量中所包含的信息从外周传入中枢后 ,随着中枢的升  相似文献   
995.
996.
旅游活动对卧龙自然保护区社区居民的经济影响   总被引:1,自引:0,他引:1  
旅游活动是提高自然保护区社区居民收入、缓解保护区与社区矛盾、促进生物多样性保护的重要途径之一.为评价旅游活动对自然保护区社区经济提升的现实效果,以卧龙自然保护区为案例,采用问卷调查的方法,通过分析保护区内旅游从业人员的经济收入、就业机会以及收入分配等,从一定程度上揭示旅游活动对该保护区社区居民的直接经济影响.结果表明,生态旅游可以给保护区的部分居民带来一定的经济收入,但这部分收入总量较低:保护区内的居民多从事一些较简单的旅游相关活动,对技能要求较高的活动则大多由来自保护区外的人员从事:旅游活动带来的经济收入不论是在当地相关从业人员之间还是在当地人与来自保护区外的人员之间的分配都表现出了一定的不公平性,居住位置、启动资金和技术技能是造成这种不公平现象的主要原因.依据以上调查及分析结果,论文提出了开发旅游项目、建立培训机制、设立专项基金以及逐步移民等相应对策.  相似文献   
997.
The common vampire bat (Desmodus rotundus) is one of three haematophagous species of bats and the only species in this genus. These New World bats prey on mammals and create significant economic impacts through transmission of rabies in areas where livestock are prevalent. Furthermore, in some portions of their range, it is not uncommon for them to prey upon humans. It is critical to the management of this species and for understanding the spread of bat rabies that detailed studies of D. rotundus population structure be conducted. To further such studies, we have characterized 12 microsatellite loci for this species.  相似文献   
998.
Objective: To assess the quality of life (QOL) in severely obese subjects before and after Lap‐Band gastric restrictive surgery and identify factors that may influence change. Research Methods and Procedures: All patients, over a 3‐year period, attending for preoperative assessment (n = 459) or annual review after surgery (n = 641) have completed the Short Form‐36 (SF‐36) health survey. Eight domain and physical component summary (PCS) and mental component summary (MCS) scores were calculated. Scores were analyzed in groups based on time after surgery and compared with community normal (CN) values. Paired preoperative and 1‐year scores (n = 218) data were used to find predictors of QOL change. Results: All preoperative mean scores (n = 459) were lower than CN values, with greater impairment in the PCS (36.8 ± 9.5 vs. CN: 51.3 ± 8.3, p < 0.001) than in the MCS (45.7 ± 8.2 vs. CN: 48.8 ± 9.5, p < 0.001) scores. After 1 year, scores were closer to CN scores (PCS: 52.4 ± 8.2 and MCS: 48.4 ± 7.7), and these remained closer for 4 years. Preoperative obesity comorbidity, especially physical disability, was the best predictor of poor preoperative SF‐36 scores and of improvement in scores at 1 year. The percentage of excess weight loss at 1 year (46 ± 16%) was of little predictive value of improved QOL. Discussion: Severely obese subjects have poor health‐related QOL as measured by the SF‐36 health survey. Lap‐Band surgery for this group has provided a dramatic and sustained improvement in all measures of the SF‐36. Improvement is greater in those with greater preoperative disability, and the extent of weight loss is not a good predictor of improved QOL.  相似文献   
999.
Defining the habitats of fishes and associated fauna on outer continental shelves is problematic given the paucity of data on the actual types and distributions of seafloor habitats. However many regions have good data on the distributions of fishes from resource surveys or catch statistics because of the economic importance of the fisheries. Fish distribution data (species or communities) have been used as a proxy for the distribution of habitats to develop precautionary conservation strategies for habitat protection (e.g., marine protected areas, fishing gear restrictions). In this study we assessed the relationships between the distributions of fish communities and species derived from trawl survey data with the spatial distribution of sediment types determined by sampling and acoustic reflectance derived from multibeam sonar surveys in Stellwagen Bank National Marine Sanctuary. Fish communities were correlated with reflectance values but all communities did not occur in unique sediment types. This suggests that use of community distributions as proxies for habitats should include the caveat that a greater number of communities within an area could indicate a greater range of habitat types. Single species distributions showed relationships between abundance and reflectance values. Trawl catches with low abundances had wide variations in reflectance values while those with high abundances had narrower ranges indicating habitat affinities. Significant non-random frequency-dependent relationships were observed for 17 of 20 species although only 12 of 20 species had significant relationships based on rank correlation. These results suggest that species distributions based on trawl survey data can be used as proxies for the distribution of seafloor habitats. Species with known habitat associations can be used to infer habitat requirements of co-occurring species and can be used to identify a range of habitat types.  相似文献   
1000.
狶科化石一新种在内蒙古上始新统的发现   总被引:1,自引:1,他引:0  
记述了在我国内蒙古沙拉木仑地区晚始新世乌兰戈楚组中发现的科化石———内蒙古短面 (新种 ) (Brachyhyopsneimongolensissp .nov .)。新种的主要特征是 :下颌骨深 ,前臼齿部分更缩短 ,下缘明显斜向前上方 ,颊齿主尖高锐 ,脊形 ,上臼齿梯形 ,已形成原脊 ,原尖位置靠后 ,M3具次尖 ,后齿带弱。研究表明Eoentelodon和Brachyhyops在下颌骨具骨质突起的刍形 ,前部缩短 ,下颌骨联合达p3,p3与p4彼此紧靠 ,下臼齿的下前尖与下后尖明显分开 ,以及上颊齿的形态上都很接近。Eoentelodon为Brachyhyops的后出同物异名  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