全文获取类型
收费全文 | 147篇 |
免费 | 4篇 |
国内免费 | 10篇 |
专业分类
林业 | 30篇 |
农学 | 1篇 |
36篇 | |
综合类 | 57篇 |
农作物 | 3篇 |
水产渔业 | 7篇 |
畜牧兽医 | 24篇 |
园艺 | 1篇 |
植物保护 | 2篇 |
出版年
2023年 | 3篇 |
2022年 | 7篇 |
2021年 | 2篇 |
2019年 | 4篇 |
2018年 | 3篇 |
2017年 | 3篇 |
2016年 | 3篇 |
2015年 | 4篇 |
2014年 | 4篇 |
2013年 | 2篇 |
2012年 | 5篇 |
2011年 | 19篇 |
2010年 | 6篇 |
2009年 | 19篇 |
2008年 | 15篇 |
2007年 | 11篇 |
2006年 | 13篇 |
2005年 | 5篇 |
2004年 | 3篇 |
2003年 | 3篇 |
2002年 | 3篇 |
2001年 | 2篇 |
2000年 | 3篇 |
1999年 | 6篇 |
1998年 | 4篇 |
1996年 | 2篇 |
1995年 | 3篇 |
1994年 | 1篇 |
1993年 | 1篇 |
1991年 | 1篇 |
1990年 | 1篇 |
排序方式: 共有161条查询结果,搜索用时 31 毫秒
71.
Jonathan S. Walker 《Biological conservation》2006,130(1):60-69
Relatively few studies have focussed on the relationship between resource use and rarity. Rare species could differ from common species in two ways: rare species may utilise resources that are themselves rarer or rare species may utilise a narrower range of resources than common species (i.e., they are more specialised). I investigated the relationship between local abundance and patterns of food resource use for an assemblage of 19 frugivorous birds in the tropical lowland rain forest of Sulawesi, Indonesia. Over a 10-month period I collected data on two measures of rarity, the local abundance and monthly variability in local abundance of each bird species, together with data on their feeding ecology including dietary specialisation, resource availability, resource overlap and habitat use. These measures produced 18 ecological variables, 17 of which I correlated with the two measures of rarity using univariate correlations and minimum adequate multiple regression models. These were carried out using the original data and independent contrasts that controlled for potential phylogenetic bias. Both measures of rarity were most strongly correlated with measures of resource availability. In both the original data and independent contrasts regression models of the temporal variability in bird abundance, only temporal fruit availability was included as a significant predictor. In the regression models of bird abundance, the strongest significant predictor was the amount of rare fruit in the diet (a negative relationship). Both models also included measures of resource specialisation, however, these were not significant in the absence of the measure of resource availability. On Sulawesi, rare frugivorous birds utilise rare resources. Specialisation, however, might be an important factor in species persistence once they become rare. The generality of the findings need to be tested. 相似文献
72.
针茅属植物是内蒙古草原的主要优势种或建群种,具有重要的经济和生态价值。本研究以内蒙古地区7种针茅属植物为研究对象,分析其在群落中的相对重要值、生态位宽度及其在群落中的生态位重叠。结果表明:短花针茅(Stipa breviflora)、戈壁针茅(S. tianschanica)、小针茅(S. klemenzii)、沙生针茅(S. glareosa)和大针茅(S. grandis)在群落中的相对重要值大于30%,而克氏针茅(S. krylovii.)和贝加尔针茅(S. baicalensis)的相对重要值较小。生态位宽度分析发现:短花针茅、戈壁针茅、小针茅、沙生针茅和大针茅的生态位宽度较高,在群落中具有优势地位,且对生长环境适应能力较强。而克氏针茅和贝加尔针茅生态位宽度显著低于上述5种针茅属植物。贝加尔针茅、克氏针茅、沙生针茅及大针茅的平均生态位重叠指数较大;而戈壁针茅、短花针茅及小针茅的平均生态位重叠指数较小。 相似文献
73.
74.
Conservation of rare species is frequently complicated by their narrow niches and poor competitive abilities which limit their distribution to a few small populations. We studied how population size and structure of an endemic cycad of northwestern Mexico, Dioon sonorense, known to be threatened by widespread land conversion and plant extraction by humans, varied with 21 environmental variables. We also used matrix population models to show how population structure may affect its long-term persistence. Slope and soil exchangeable potassium explained a moderate proportion of the variance in the abundance of adult individuals, while abundance of adults and slope explained a large proportion of the variance in the abundance of seedlings. The strong non-linear relationships of seedling abundance with slope and adult abundance suggest threshold effects that may limit D. sonorense regeneration in less suitable environments. Modeling suggests that most of the study populations will not be viable in the long-term, except for the largest population. Modeling scenarios with large increments in fecundity and seedling survival produced modest gains in population growth. The long-term persistence of D. sonorense is shown in our study to be further threatened by: (a) small size of adult populations, (b) low-quality habitat in which most populations occur, and potentially, (c) the combined effect of low fecundity and recruitment. 相似文献
75.
In this review we summarize our knowledge of using stable isotopes (15N/14N, 13C/12C) to better understand the trophic ecology of oribatid mites. Our aims are (a) to recapitulate the history of stable isotope research in soil animals with a focus on oribatid mites, (b) to present new stable isotope data for oribatid mites and overview the current state of knowledge of oribatid mite trophic niche differentiation, (c) to compile problems and limitations of stable isotope based analyses of trophic relationships and (d) to suggest future challenges, questions and problems that may be solved using stable isotope analyses and other novel techniques for improving our understanding on the trophic ecology of soil invertebrates. We conclude that (1) in addition to 15N/14N ratios, 13C/12C ratios contribute to our understanding of the trophic ecology of oribatid mites, allowing, e.g. separation of lichen- and moss-feeding species, (2) there likely are many lichen but few moss feeding oribatid mite species, (3) oribatid mite species that are endophagous as juveniles are separated by their stable isotope signatures from all other oribatid mite species, (4) fungivorous oribatid mite species cannot be separated further, e.g. the fungal taxa they feed on cannot be delineated. A particular problem in using stable isotope data is the difficulty in determining signatures for basal food resources, since decomposing material, fungi and lichens comprise various components differing in stable isotope signatures; 13C/12C ratios and potentially other isotopes may help in identifying the role of these resources for decomposer animal nutrition. 相似文献
76.
红花指管蚜及其天敌的时间生态位宽度较小而空间生态位宽度较大,说明这些昆虫种群的发生有高峰期明显和全株分布的特点。红花指管蚜的时动空间生态位宽度与种群密度相关显著;天敌中七星瓢虫和草间小黑蛛的时间×空间生态位宽度较大,说明它们发生期较长、活动范围较广;大草蛉、龟纹瓢虫、蚜茧蜂、食蚜蝇、草间小黑蛛和七星瓢虫与红花指管蚜时间生态位重叠度指数和相似性指数均较大。蚜茧蜂、大草蛉、龟纹瓢虫、食蚜蝇和七星瓢虫空间生态位重叠度指数和相似性指数均较大,与红花指管蚜的时间×空间生态位重叠度,大草蛉、龟纹瓢虫、蚜茧蜂、食蚜蝇、七星瓢虫和草间小黑蛛显著较大。红花指管蚜数量与天敌数量的关联度:龟纹瓢虫>草间小黑蛛>七星瓢虫>大草蛉>蚜茧蜂>食蚜蝇>小花蝽>八斑球腹蛛>黑襟毛瓢虫>三突花蛛>异色瓢虫。根据天敌与红花指管蚜时间生态位重叠度、空间生态位重叠度、时间×空间生态位重叠度、关联度总积分排序结果为:龟纹瓢虫=大草蛉>蚜茧蜂>七星瓢虫>食蚜蝇>草间小黑蛛>黑襟毛瓢虫>八斑球腹蛛=三突花蛛>小花蝽>异色瓢虫。 相似文献
77.
泛化种和特化种对碳及养分等资源利用存在很大差异,在土壤能量和养分循环中发挥着独特功能。目前农田微生物研究主要针对整个细菌、真菌、古菌或其他功能群落,对农田生态系统细菌泛化种和特化种的认识还很缺乏。因此,为探究典型水稻土细菌泛化种和特化种的群落结构、驱动机制及在氮循环中的功能,根据第二次土壤普查数据,从我国东部(江苏、安徽、上海)和西南部(贵州、云南)采集16个土壤表层样品(0~20 cm),进行理化性质测定和高通量测序,并对获得的数据进行相应的生态学分析。结果表明,在所有的OTUs中,3.28%被归为泛化种,9.07%被归为特化种。泛化种和特化种在门水平的分布模式不同,特化种在绿弯菌门(Chloroflexi)、放线菌门(Actinobacteria)、硝化螺旋菌门(Nitrospirae)、厚壁菌门(Firmicutes)和浮霉菌门(Planctomycetes)中所占的比例高于泛化种。基于β多样性零模型的群落构建分析表明,泛化种与特化种均由确定性过程主导,与特化种相比,泛化种受随机性过程的影响更大。驱动泛化种和特化种群落结构变异的环境因子不同,pH、年均降雨量、黏粒含量、全氮是驱动... 相似文献
78.
JOSÉ MARTÍN GALLARDO José Carlos Escudero García Encarnación García Ceballos-Zúñiga Juan José Pérez-Regadera Pérez 《Aquaculture International》2006,14(1-2):163-170
Two physical factors light or substrate (fibre glass, artificial vegetation, gravel and mud) were determined in a 24 m long
artificial channel to have importance on the distribution of 33 tench 2+, mean size 11.8±0.9 cm S.L. For this purpose in five
trials tench could select between the half of the channel occupied by their preferred light intensity (inferior to 10 lux)
combined with the less preferred substrate (fibre glass) or the half with the combination of one of the substrates considered
and higher light conditions (40 lux). Fish preferred, with a 100% frequency, the optimal light-less preferred substrate when
compared with the combinations fibre glass, gravel and mud. When artificial vegetation was the substrate considered, tench
selected the combination preferred substrate – non-optimal light intensity with a 98.26±2.68% frequency. This result changed
when increasing light intensity up to 150 lux, avoiding the shading effect. We demonstrated that light has a bigger influence
than substrate in juvenile tench habitat selection under controlled conditions. 相似文献
79.
This paper focuses on the environmental and ethical attributes of food products and their production processes. These two aspects have been recently recognized and are becoming increasingly important in terms of signaling and of consumer perception. There are two relevant thematic domains: environmental and social. Within each domain there are two movements. Hence the paper first presents the four movements that have brought to the fore new aspects of food product quality, to wit: (1) aspects of environmental ethics (organic agriculture and integrated agriculture), and (2) social ethics (fair trade and ethical trade). Next, it describes how the actors in the movements (producers, retailers, NGOs, and governments) are organized and how consumers perceive each of the movements. From the perspective of the actors in the movements themselves, the movements are grouped into two “actors’ philosophies.” The first is a “radical” philosophy (the organic production and fair trade movements that arose in radical opposition to conventional agriculture or unfair trade relations), and the second is a “reformist” philosophy (the integrated agriculture and ethical trade movements that arose as efforts to modify but not radically change conventional agriculture). From the point of view of consumers, the classification of the movements is based on perceptions of the “domain” of the movements. That is, consumers tend to perceive the organic production movement and the integrated agricultural movement as a single group because they both deal with the environment. By contrast, consumers tend to group the fair trade movement and the ethical trade movement together because they both deal essentially with social ethics. Recently, key players such as large retailers and agribusinesses have adopted as part of their overall quality assurance programs both environmental and ethical attributes. Their involvement in and adoption of the goals of the movements have, however, generated tensions and conflicts. This is particularly true within the radical movements, because of concerns of cooptation. Finally, the paper identifies challenges faced by those promoting food products with environmental and social/ethical attributes as they attempt to communicate coherent signals to consumers at this crucial moment in the emergence of a mass market for these products.
Jean
-Marie Codron
is a Senior Researcher at INRA and co-director of MOISA, a public joint research laboratory involved in the social sciences. His research interests focus on three main lines of research: economics of contracts, economics of the firm, and economics of market institutions, with applications to “complex” food sectors, where product quality is difficult to measure and/or to signal to the consumer.
Lucie Sirieix
is Professor of Marketing and Consumer Behaviour at SupAgro Montpellier, France, a national higher education establishment under the Ministry of Agriculture, Fisheries and Food. Within the MOISA research unit, her main research topics are variety seeking, risk and trust, environmental and ethical consumer concerns, and sustainable consumption. Her specific research areas include organic products, fair trade, and regional products.
Thomas Reardon
is Professor of Agricultural Economics at Michigan State University. His work focuses on globalization, consolidation in the retail and processing sectors, and their effects on agrifood systems and trade as well as on the economics of private quality and safety standards. 相似文献
80.