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   187篇
  免费   6篇
  国内免费   16篇
  209篇
  2023年   4篇
  2022年   2篇
  2021年   5篇
  2020年   7篇
  2019年   11篇
  2018年   8篇
  2017年   8篇
  2016年   5篇
  2015年   3篇
  2014年   2篇
  2013年   10篇
  2012年   2篇
  2011年   4篇
  2010年   2篇
  2009年   16篇
  2008年   12篇
  2007年   8篇
  2006年   17篇
  2005年   6篇
  2004年   9篇
  2003年   5篇
  2002年   4篇
  2001年   4篇
  2000年   4篇
  1999年   16篇
  1998年   3篇
  1997年   3篇
  1996年   1篇
  1995年   2篇
  1994年   2篇
  1993年   2篇
  1992年   1篇
  1991年   2篇
  1990年   4篇
  1989年   1篇
  1988年   3篇
  1987年   1篇
  1986年   2篇
  1984年   1篇
  1981年   1篇
  1980年   1篇
  1979年   1篇
  1978年   3篇
  1952年   1篇
排序方式: 共有209条查询结果,搜索用时 15 毫秒
61.
Disentangling the complexities that influence animal space use poses substantial challenges based on decision trade-offs and constraints imposed on animals. Optimal decisions suggest that the spatial complexity of home-range shapes should be inversely related to energy conservation and fitness. Hence, the most beneficial shape should be the circle. We evaluated whether shape complexity (i.e., an index of circularity) of home ranges was influenced by two extrinsic (spatial heterogeneity, preferred habitat [i.e., deciduous forest]), and three intrinsic (sex, season [breeding, non-breeding], intensity of use) factors, with intensity of use indexed as contours containing core and peripheral areas. We estimated utilization distributions of 39 radiomarked adult American black bears (Ursus americanus), a habitat generalist, using fixed-kernel techniques and estimated 50% (core area) and 95% (peripheral area) contours. We fit a set of 47 models using linear modeling and ranked models using small-sample Akaike Information Criterion. Coefficients for the best model were the intrinsic factors intensity of use (reference category = core; 0.118; 95% CL = 0.064–0.173), sex (reference = female; 0.105; 95% CL = 0.043–0.167), and intercept (0.229; 95% CL = 0.186–0.272). Shape complexity was less for core areas than peripheral areas and less for females than males. Considering complex resource selection patterns within a fragmented landscape, both sexes appeared to use energy-maximizing strategies, although the increase in shape complexity for males may be an allometric relationship based on size dimorphism. Our approach supported the phenomenon of optimality as manifested through home-range shape complexity, but we suggest that assessment of this phenomenon for habitat specialists may yield different results, including the potential importance of intrinsic factors based on more restrictive limiting factors.  相似文献   
62.
Study-abroad programs are a ‘globalizing project’ developed at the intersection of educational institutions seeking new income sources and the ideologies that the world is comprised of a mosaic of cultures and that ‘intercultural experience’ is valuable. While research on study-abroad programs often focuses on their effect on the students’ ‘global competence’ and language proficiency, this article focuses on the space, the space of host family in particular, produced in study-abroad programs. Study-abroad programs here are perceived in a wide sense to include various programs for students to go to another country to study.

Study-abroad guidebooks and researchers portray the host family's life as constituting a quintessential ‘native life’, in which study-abroad students should be immersed. However, based on ethnographic fieldwork in Aotearoa/New Zealand, this article argues that the distinction between ‘cultural difference’ and ‘cultural sameness’ is a construction, resulting in mutual accommodation and intolerant judgments, respectively. That is, when host parents recognised the ‘cultural difference’ of the foreign students’ behaviour, they often sought to understand meanings behind the behaviour, resulting in mutual accommodation. It turned the space into what Michael Taussig calls the ‘space between’, in which it is unclear who is imitating whom. When the host parents recognised ‘cultural sameness’ based on superficial similarity, however, their efforts to understand meanings behind behaviour was halted and, when the behaviour was considered problematic, led to the student being asked to move out. This article details an ironic space of study abroad where alterity inspired tolerance and similarity inspired intolerance.  相似文献   

63.
Tail biting is a welfare and economical concern in modern pig production. One common preventive measure used throughout the world is tail docking, which is generally considered one of the most effective methods for limiting tail biting. However, tail docking is a painful mutilation and systematic tail docking is not allowed in the EU. Therefore, the aim was to compare pig behaviour and the prevalence of tail biting in finishing pigs with intact tails housed in two different pen designs under Danish commercial conditions. PEN1 was a traditional Danish pen and PEN2 was inspired by Swedish finisher pen design and had a larger proportion of solid floor area (PEN1: 1/3 and PEN2: 2/3), reduced group size (PEN1: 15 and PEN2: 12), increased space allowance per head (PEN1: 0.7 m2 and PEN2: 0.89 m2) and straw allocated on the floor (PEN2) whereas straw was provided in a straw rack in PEN1. Tail damage observations were carried out daily by the stockperson and every 2 weeks one trained research technician assessed tail damages according to a tail scoring system. Tail lesions were observed in 51% of PEN1 and in 11% of PEN2 (P < 0.001). PEN1 had higher prevalence of tail damages than PEN2 (23% v. 5%, P < 0.001). Behavioural observations were carried out by the use of video recordings. Pigs in PEN2 tended to spend more time on tail-directed behaviour than pigs in PEN1 (P = 0.07), whereas pigs in PEN1 tended to spend more time on ear-directed behaviour (P = 0.08). Pigs in PEN2 spent more time on straw-directed behaviour compared to pigs in PEN1 (P < 0.001). Pen design did not affect time spent on other penmate-directed behaviour. In addition, the level of welfare between the two pen designs was compared using the Welfare Quality® protocol. PEN2 received an overall score of ‘excellent’ while PEN1 scored ‘enhanced’. PEN2 scored higher on all principles besides ‘good health’, where PEN1 scored better on lameness and wounds. The main measurements accounting for the differences were water supply, huddling, tail biting, social behaviour and fear of humans. In conclusion, the combination of increased space allowance, increased area of solid flooring, straw allocated onto the floor and reduced group size (PEN2) resulted in fewer tail damaged pigs and a better overall welfare assessment, despite a tendency for more tail-directed behaviour.  相似文献   
64.
“三生”空间的优化布局,有赖于对"三生"空间变化所产生的地域功能效应的认识,社会生态服务价值分析为这一效应评价提供了新的途径。以邛海国家湿地公园为例,采用参与式制图与半结构化访谈方法调查当地居民感知的社会生态服务价值变化类型,最终获取有效问卷与填图393份,提取空间数据点1830个。通过质性空间分析等方法研究湿地公园建设以来"三生"空间变化对社会生态服务价值的影响,探讨地域功能的尺度特征及其变化趋势与效应。研究表明:(1)湿地公园建设以来"三生"空间格局变化剧烈,主要表现为生产空间转换为生态空间与生活空间,前者占据主导地位,呈带状分布在邛海湿地周围,后者则呈斑块状分布在各村落周边;(2)利用Nvivo11软件进行质性分析,将归纳出的6种社会生态服务价值与区域性、地方性地域功能相联系,发现在城郊保护地"三生"空间变化中,区域性地域功能增益明显,而地方性地域功能表现出增益与贬损交织的复杂变化特征,不同尺度地域功能间存在协同与冲突的多重关系;(3)研究指出区域"三生"空间配置过程中,既要关注地域功能在地域间的水平综合均衡,也要兼顾不同尺度地域功能间的垂直综合均衡,才能实现区域效益的最优化。城郊保护地在受到外部结构化力量和社区内部适应能力的双重作用下发生地域功能转化,社区参与的质性空间评价则为认识地域功能转化过程、效应及调控策略提供了途径。  相似文献   
65.
Herbage allowance (HA) management during gestation–lactation cycle of cows grazing native grasslands improves pregnancy rates and calves’ weaning weight records. Those improvements were associated with greater herbage mass, and better body condition score (BCS) and metabolic status of the cows, which could affect grazing and maternal behaviour, particularly when temporary suckling restriction (TSR) and flushing (FL) is applied. The objective of this study was to evaluate the effect of HA during the gestation–lactation period on cows’ and calves’ liveweight (LW), BCS, milk yield, insulin, IGF-1 and leptin concentrations, as well as the proportion of diurnal grazing activities (grazing, ruminating, walking and idling), and maternal behaviour from −10 to 70 days relative to TSR (day 0 = initiation of TSR at 86 ± 10 days relative to calving). Thirty-three primiparous Hereford cows were allocated to HA treatments during gestation and lactation, which annually averaged 2.5 (low = LHA) and 4 kg DM/kg LW (high = HHA). The LW and BCS of cows did not differ during −10 to 50 days but were greater in HHA than LHA at the end of the study (P < 0.05). Concentrations of IGF-1 were greater in HHA compared to LHA, while insulin and leptin did not differ. Grazing was lower in HHA than LHA, and the opposite occurred with ruminating and idling (P < 0.05). Cow–calf physical distance was greater in LHA cows compared to HHA (P < 0.05) and increased greatly in the former group after FL, while this increase was lower and later in HHA cows. Milk yield was greater in HHA (P < 0.05), and calves’ weight did not differ between treatments from day −10 to 35, but was greater in HHA compared to LHA from 45 days until the end of the study. Thus, the HHA in a low herbage height and mass condition resulted in greater IGF-1 concentrations and milk yield, and induced changes in grazing and maternal behaviour that were associated with increased cows’ LW, BCS and calves’ weight at the end of the study.  相似文献   
66.
结肠清洗后肠道微生态平衡的重建   总被引:6,自引:0,他引:6  
目的观察结肠清洗对人肠道菌群的影响和结肠清洗后肠道微生态平衡再建的过程。方法10名健康男性志愿者随机平均分为2组,一组为结肠清洗后食用普通膳食,另一组为结肠清洗后,先食用航天食品,后食用普通膳食,在选定的时相点收集粪便,选择性培养基分别对肠球菌、大肠埃希菌、类杆菌、乳杆菌及双歧杆菌定量测定。结果结肠清洗后,肠道所有的微生物数量均显著减少,7d后肠道菌群恢复正常。结论结肠清洗引起肠道微生态平衡紊乱,航天食品不利于肠道微生态重建。  相似文献   
67.
Bellon  S.  Guérin  G. 《Plant Ecology》1992,99(1):307-316
In the south of France, holm oak (Quercus ilex L.) woodlands, particularly coppices, are currently used by livestock farmers. For this vegetation, a grazing allowance is available, based on the relationship between various layers. As far as livestock is concerned, such woodlands can be utilized for many purposes, e.g. stock feeding during critical periods and to provide flexibility to the pasture grazing sequence. Furthermore, the related grazing intake patterns determine the level of holm oak utilization. Practices integrating both wood and livestock production provide a new perspective to the management of these woodlands in that the presence of livestock promotes specific operations which can lead to more diversified forestry. The renewal of both wood and pastoral resources depends on: (i) the initial level of integration of these two activities and (ii) subsequent technical operations. In the present paper a midterm management scenario is developed which describes the likely development of woodlands with multiple use and periodic thinning.  相似文献   
68.
Mutations in FBXL4 (F-Box and Leucine rich repeat protein 4), a nuclear-encoded mitochondrial protein with an unknown function, cause mitochondrial DNA depletion syndrome. We report two siblings, from consanguineous parents, harbouring a previously uncharacterized homozygous variant in FBXL4 (c.1750 T > C; p.Cys584Arg). Both patients presented with encephalomyopathy, lactic acidosis and cardiac hypertrophy, which are reported features of FBXL4 impairment. Remarkably, dichloroacetate (DCA) administration to the younger sibling improved metabolic acidosis and reversed cardiac hypertrophy. Characterization of FBXL4 patient fibroblasts revealed severe bioenergetic defects, mtDNA depletion, fragmentation of mitochondrial networks, and abnormalities in mtDNA nucleoids. These phenotypes, observed with other pathogenic FBXL4 variants, confirm the pathogenicity of the p.Cys584Arg variant. Although treating FBXL4 fibroblasts with DCA improved extracellular acidification, in line with reduced lactate levels in patients, DCA treatment did not improve any of the other mitochondrial functions. Nonetheless, we highlight DCA as a potentially effective drug for the management of elevated lactate and cardiomyopathy in patients with pathogenic FBXL4 variants. Finally, as the exact mechanism through which FBXL4 mutations lead to mtDNA depletion was unknown, we tested the hypothesis that FBXL4 promotes mitochondrial fusion. Using a photo-activatable GFP fusion assay, we found reduced mitochondrial fusion rates in cells harbouring a pathogenic FBXL4 variant. Meanwhile, overexpression of wildtype FBXL4, but not the p.Cys584Arg variant, promoted mitochondrial hyperfusion. Thus, we have uncovered a novel function for FBXL4 in promoting mitochondrial fusion, providing important mechanistic insights into the pathogenic mechanism underlying FBXL4 dysfunction.  相似文献   
69.
摘要:为了明确大豆航天搭载早期世代农艺性状选择的依据,提高航天搭载育种早期世代选择效果。对6个大豆品种经过航天搭载后的SP2代变异率和SP4代选择效果进行了分析,结果表明遗传力高的SP2代的单株荚数和单株粒数2个农艺性状的变异率与SP4代入选单株数相关达到极显著水平;SP2代遗传力较高底荚高度和节间长度的变异率与SP4代入选单株相关达到显著水平,SP2代遗传力不高的植株高度和主茎节数变异率与SP4代入选单株相关不显著;SP2代对单株荚数、单株粒数、底荚高度、节间长度进行选择有效,而对植株高度和主茎节数选择效果不明显。  相似文献   
70.
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