共查询到20条相似文献，搜索用时 281 毫秒
1.
The current situation in mexican immigration 总被引：1，自引：0，他引：1
By 1988, the Mexican-origin population of the United States had grown to 12.1 million, largely from recent, sharp increases in immigration. The policy concerns raised by this phenomenon have been influenced by some perceptions that available research contradicts. Today most Mexican immigrants come to stay, about half are female, and they have increasingly less schooling compared to the native-born population and other immigrants. Nationally, they do not cause adverse economic effects for native-born workers and, across generations, their language and political assimilation is proceeding well. They put greater demands on education than on other public services. However, the Mexican-origin population affects the economy and public services more and differently in the areas where it is concentrated, primarily in the western United States and large urban areas. Further, the recent legalization of 2.3 million Mexican immigrants can be expected to increase the demand on public services, especially in those areas. 相似文献
2.
Experiments were conducted to measure the photosynthetic response of three crop and four tree species to realistic concentrations of ozone and (for tree species only) simulated acidic rain. The ozone concentrations were representative of those found in clean ambient air, in mildly to moderately polluted air such as occurs in much of the United States during the summer, and in more heavily polluted air. However, the highest concentrations of ozone used were lower than those found regularly in the Los Angeles area. The mean pH of the simulated acid rain treatments ranged from more alkaline to much more acidic than the mean pH of precipitation in the United States. Exposure to any increase in ozone reduced net photosynthesis in all species tested. In contrast, acidic rain had no negative effect on photosynthesis in tree species, and no interaction between ozone and acidic rain was observed. Ozone-induced reductions in photosynthesis were related to declines in growth or yield. Species with higher stomatal conductances and thus higher potential for pollutant uptake exhibited greater negative responses to similar ozone treatments. Since exposure to ozone concentrations typical of levels of the pollutant observed in the eastern half of the United States reduced the rates of net photosynthesis of all species tested, reductions in net photosynthesis may be occurring over much of the eastern United States. 相似文献
3.
Dibner MD 《Science (New York, N.Y.)》1986,232(4756):1367-1372
The countries of the European Economic Community have recently mounted considerable efforts to commercialize biotechnology. Together, these efforts approach the same number of companies and level of government spending as those in the United States. In Europe there is more government emphasis on support for industry-university collaborations and industrial projects than in the United States, where basic research is emphasized. European efforts are often not easily delineated from those in the United States; many European companies have extensive U.S. operations and many U.S. companies have involvement in Europe. Strategies and efforts in European biotechnology are examined and compared to those in the United States. 相似文献
4.
R B McCall 《Science (New York, N.Y.)》1970,170(958):644-648
Comparisons between sibling and parent-child pairs with unrelated control pairs matched for year of birth and parental education were made to determine the relative heritability of the general level of intelligence quotient as opposed to that of the sequential pattern of IQ change over age (3 to 12 years). There was greater similarity among related siblings relative to matched controls for general level than for pattern of IQ over age. Relationships between the IQ's of children and that of their parents as children were not consistent across age. 相似文献
5.
Fatal sibling aggression, precocial development, and androgens in neonatal spotted hyenas 总被引：5，自引：0，他引：5
Fatal neonatal sibling aggression is common in predatory birds but has not been previously reported in wild mammals. Spotted hyena females are strongly masculinized, both anatomically and behaviorally, apparently by high levels of androgens during ontogeny. Neonates display elevated androgen levels, precocial motor development, and fully erupted front teeth. Litters are usually twins, and siblings fight violently at birth, apparently leading to the death of one sibling in same-sex litters, whereas in mixed-sex litters both siblings usually survive. 相似文献
6.
Moon RM 《Science (New York, N.Y.)》1985,230(4723):274-280
Neutron scattering is a versatile technique that has been successfully applied to condensed-matter physics, biology, polymer science, chemistry, and materials science. The United States lost its leadership role in this field to Western Europe about 10 years ago. Recently, a modest investment in the United States in new facilities and a positive attitude on the part of the national laboratories toward outside users have resulted in a dramatic increase in the number of U.S. scientists involved in neutron scattering research. Plans are being made for investments in new and improved facilities that could return the leadership role to the United States. 相似文献
7.
农业生态学是生态学的一个分支学科,是生态学原理在农业上的具体应用。美国是世界上最大的农业强国。美国农业的发展,得益于农业生态学的理论指导和农业生态学原理的实践运用。在实地调查与研读有关文献资料的基础上,对美国农业生态学的发展进行了考察与分析,认为:(1)美国农业生态学产生于1928年,至今已有85年的发展历史。美国农业生态学可划分为4个发展阶段,即:起始阶段(1928-1961年)、扩展阶段(1962-1979年)、巩固阶段(1980-1999年)和新发展阶段(2000年-)。(2)美国农业生态学的内涵由起初的一门"学科(科学)",逐步向着包括有"过程"、"理念"、"方法"、"行动"、"体系"、"运动"、"实践活动"、"强有力的工具"等多重涵义的方向发展,农业生态学内涵的深度和广度均得到不断丰富和完善。(3)美国农业生态学的基本内容包括背景(农业生态学国际背景)、理论(农业生态学理论基础)、设计(农业生态系统的优化设计)和管理(农业生态学理论的实践应用)等4部分,美国农业生态学具有4个明显特征——视野的全球性、内容的多样性、体系的灵活性和技术的实用性。(4)未来美国农业生态学将朝着"高"(高起点、高标准、高效率、高水平)、"新"(新手段、新方法、新技术,新成果、新成效、新进展)、"长"(长期观察分析、长期资料积累、长期定位试验)、"多"(农业生态学研究与分析采用多点、多地、多样本、多模式、多途径、多指标、多方法)、"实"(联系实际、注重实践、讲求实效、展现实绩)的方向发展。 相似文献
8.
中国大豆进口量持续保持高位，其价格波动一直是关注焦点。为研究不同大豆市场价格传导的差异，选取中国、巴西、阿根廷和美国大豆市场2014年1月—2021年5月的日度价格数据，采用小波模型与Copula函数对中国与各国大豆市场价格关联进行实证分析。结果表明：中国大豆市场价格与各国大豆市场价格之间均存在显著正向关联，关联程度随时间尺度的扩大而增强，其中与巴西关联性最为显著；尾部价格联动效应存在差异，价格波动超过1个月，与阿根廷、美国分别对价格下降、上涨更敏感；中国与巴西、美国大豆市场价格都存在明显的非对称价格传导，不同国家正/负向传导效应在时间尺度上存在差异。基于研究结果，提出了提高国内油料生产，丰富进口来源，加强市场监测预警等政策建议。 相似文献
9.
D Armstrong S Dimmitt D H Boehme S C Leonberg W Vogel 《Science (New York, N.Y.)》1974,186(4159):155-156
Use of a spectrophotomtetric assay of peroxidase with p-phenylenediamine as cosubstrate demonstrated deficient enzymne activity in leukocytes from two patients with a dominantly inherited form of ceroid lipofuscinosis (Kuf's disease) and a clinically hlealthy unaffected sibling. When the reaction was performned in the absence of added hydrogen peroxide, oxidation of the p-phenylenediamnine cosubstrate (indicating the presence of endogenous peroxide) occurred only with enzyme samnples from the three siblings but not with those from a large number of unrelated, unaffected controls. This demonstrates that the deficiency of peroxide) found previously in the recessively inherited infantile and juvenile formns of ceroid lipofuscinosis (Batten-Spielmeyer-Vogt disease) is also present in an adult form with dominant inheritance. 相似文献
10.
为了比较中美两国物联网研究状况的差异,客观认识中国在物联网领域的研究实力、研究水平和研究状况,为促进中国物联网产业快速发展提供有价值的信息,文章以SCI-E(Science Citation Index Expanded)收录的2009年—2018年中美物联网领域的学术论文为数据源,应用文献计量和词云分析的方法,从论文产出、学科、关键词、高被引论文等几个角度揭示中美物联网领域研究现状和发展态势异同。研究结果表明:尽管中国相关论文数量高于美国,但是论文影响力与美国存在差距,今后中国应进一步提升自主创新力,拓宽物联网研究的广度与深度,特别是加强研发具有自主知识产权的关键技术和核心技术,提升国际影响力和竞争力。 相似文献
11.
The tidal rhythmites in the Proterozoic Big Cottonwood Formation (Utah, United States), the Neoproterozoic Elatina Formation of the Flinders Range (southern Australia), and the Lower Pennsylvanian Pottsville Formation (Alabama, United States) and Mansfield Formation (Indiana, United States) indicate that the rate of retreat of the lunar orbit is dxi/dt approximately k2 sin(2delta) (where xi is the Earth-moon radius vector, k2 is the tidal Love number, and delta is the tidal lag angle) and that this rate has been approximately constant since the late Precambrian. When the contribution to tidal friction from the sun is taken into account, these data imply that the length of the terrestrial day 900 million years ago was approximately18 hours. 相似文献
12.
Energy and the u.s. Economy: a biophysical perspective 总被引：1，自引：0，他引：1
A series of hypotheses is presented about the relation of national energy use to national economic activity (both time series and cross-sectional) which offer a different perspective from standard economics for the assessment of historical and current economic events. The analysis incorporates nearly 100 years of time series data and 3 years of cross-sectional data on 87 sectors of the United States economy. Gross national product, labor productivity, and price levels are all correlated closely with various aspects of energy use, and these correlations are improved when corrections are made for energy quality. A large portion of the apparent increase in U.S. energy efficiency has been due to our ability to expand the relative use of high-quality fuels such as petroleum and electricity, and also to relative shifts in fuel use between sectors of the economy. The concept of energy return on investment is introduced as a major driving force in our economy, and data are provided which show a marked decline in energy return on investment for all our principal fuels in recent decades. Future economic growth will depend largely on the net energy yield of alternative fuel sources, and some standard economic models may need to be modified to account for the biophysical constraints on human economic activity. 相似文献
13.
Global Positioning System (GPS) measurements across the New Madrid seismic zone (NMSZ) in the central United States show little, if any, motion. These data are consistent with platewide continuous GPS data away from the NMSZ, which show no motion within uncertainties. Both these data and the frequency-magnitude relation for seismicity imply that had the largest shocks in the series of earthquakes that occurred in 1811 and 1812 been magnitude 8, their recurrence interval should well exceed 2500 years, longer than has been assumed. Alternatively, the largest 1811 and 1812 earthquakes and those in the paleoseismic record may have been much smaller than typically assumed. Hence, the hazard posed by great earthquakes in the NMSZ appears to be overestimated. 相似文献
14.
The association between climatic anomalies in the tropical Pacific Ocean, often called El Ni?o events, and annual corn production in the United States was investigated. Temperature and atmospheric pressure in parts of the United States have been correlated with El Ni?o events. This research suggests that in years in which an El Ni?o event causes surface temperatures in the tropical Pacific to become warmer than normal, there is a higher probability of an above-average corn crop in the United States. For years when sea surface temperatures are average or cool, no significant association is observed. 相似文献
15.
【目的】通过对中国食用菌出口美国市场影响因素的分析,为中国食用菌出口美国市场提供建议,以应对中美贸易摩擦给中国食用菌出口美国市场带来的挑战。【方法】基于联合国粮农组织数据库、Uncomtrade数据库、国际清算银行官网、中国产业信息网中国食用菌出口美国的年度面板数据,运用扩展的引力模型对中国食用菌出口美国的影响因素进行实证分析,并将食用菌分为干食用菌和食用菌罐头产品分别测算不同类别食用菌贸易差异性,探究我国与美国食用菌贸易的发展措施。【结果】自加入WTO以来,中国食用菌产量逐年增加,由2010年的1837.00万t增至2018年的3842.04万t,其中2015年后增幅趋于稳定。中国食用菌目前以内需为主,出口潜力巨大,2011年之后由于美国设置的关税制度、反倾销政策及技术壁垒等贸易障碍减少了中国食用菌对美国的出口量,从2011年的79 t下降至2012年的3 t;2015—2016年,中国食用菌行业进行一系列质量管控措施,制定相应的农产品安全标准及物流运输建设,促进了食用菌出口,2016年出口美国总量达435 t;在2017年随着中美贸易摩擦的升级,出口美国总量回落至216 t。2010—2017年中国食用菌出口美国被FDA通报食用菌不洁物、农药残留超标、标签信息错误、厂商信息错误共830起。实证分结果表明,中国食用菌产量、美国国民收入对中国食用菌罐头和干食用菌出口美国具有显著正向影响;中国食用菌出口价格、美国实施食用菌贸易摩擦手段、中美两国之间距离对中国出口美国食用菌罐头和干食用菌贸易产生显著负向影响。但对干食用菌和食用菌罐头出口美国贸易的影响程度体现出明显差异,中国食用菌产量对干食用菌出口美国贸易比食用菌罐头出口美国贸易正向影响程度更大,影响系数分别为0.901和0.137。中美货币汇率对干食用菌罐头出口美国贸易呈显著正向影响,但对干食用菌出口美国贸易呈显著负向影响,影响系数分别为0.291和-0.341。【建议】优化食用菌产品结构,通过深加工提高食用菌产品附加值,增强我国食用菌出口竞争力;通过干湿搭配,轻重搭配,提高配载效率,在保证食用菌产品质量及交付时间的前提下,最大程度地降低物流成本;构建食用菌贸易摩擦预警机制,通过规模化种植、精细化管理提高食用菌产品质量,完善食用菌质量标准体系。 相似文献
16.
农业信息化的理论研究与实践总结是学界的研究热点.利用文献计量学方法,以CiteSpace为手段,基于Web of Science数据库和CNKI数据库,分析了近年来国内外农业信息化领域研究的发展态势,为该领域科研工作者与决策者提供参考.结果显示:国外方面,美国、英国、澳大利亚、荷兰等国家的研究处于国际领先地位;国家科研机构和大学为主要发文机构,其中美国研究机构发文最多;来自荷兰和美国的作者发文最多,但也比较分散;高影响力论文大多来自美国、欧洲等发达国家和地区;来自美国和荷兰等国的期刊较多,且语种多为English;更加关注现代信息技术在具体农业管理和农业生产中的应用研究.国内方面,中国虽与美国、澳大利亚、加拿大、英国等发达国家开展了比较频繁的国际合作,但发文质量存在较大差距;中国农业科学院是相对优势比较突出的机构;研究比较宽泛,且发表刊物总体水平不高,中文核心期刊较少. 相似文献
17.
本文基于1924—2019年美国大豆生产分布县域数据,采用变异系数、空间分析等方法,分析了美国大豆产量、种植面积和单产的时空分布特征和重心迁移轨迹,为中国了解美国大豆产业发展现状,应对大豆国际市场变化,制定大豆生产、贸易进出口政策等提供参考。结果表明:①近百年美国大豆生产经历了引种扎根、迅速发展、波动性发展、稳定成熟四个发展阶段。1924—2019年,美国大豆的产量、种植面积和单产总体呈现波动上升趋势,年均增长率分别为7.17%、4.17%、1.55%。②中西部地区一直是美国大豆的主产区,研究时段内大平原地区大豆生产地位上升,南部地区地位下降。伊利诺伊州、印第安纳州和艾奥瓦州的大豆产量多年位于全美前列,大豆高产的州多位于大豆主产区。各州之间大豆产量、面积差距逐渐缩小,单产差距不大。③美国大豆产量在空间上呈现显著的集聚特征,且集聚性逐渐增强。高高集聚区和低高集聚区分布较广,此两类集聚区均呈现扩张趋势。④1930—2010年,美国大豆生产重心总体呈倒“几”字形向西北方向移动,2010年后又向东南方向移动,总体移动距离为63.54千米。基于美国的经验,未来中国大豆生产应注重安排轮作模式,制定大豆品种种植规划;改革创新大豆农业保险制度;加快大豆科技创新;推进大豆规模化经营、标准化种植;培养壮大中国大豆行业协会,争夺中国大豆行业话语权。 相似文献
18.
Thrips: Pests of Concern to China and the United States 总被引：5，自引：0，他引：5
Thrips are among the most important agricultural pests globally because of the damage inflicted by their oviposition, feeding, and ability to transmit plant viruses. Because of their invasiveness, a number of pest species are common to both China and the United States and present significant challenges to growers of a wide range of crops in both countries. Among the pest thfips common to both countries are four of the major global thrips pests, Frankliniella occidentalis (Pergande), Scirtothrips dorsalis Hood, Thrips palmi Karny, and Thrips tabaci Lindeman. This review addresses characteristics that enable thrips to be such damaging pests and how biological attributes of thrips create challenges for their management. Despite these challenges, a number of successful management tactics have been developed for various cropping systems. We discuss some of these tactics that have been developed, including the use of cultural controls, biological controls, and judicious use of insecticides that do not disrupt overall pest management programs. The exchange of this type of information will help to facilitate management of pest thrips, especially in regions where species have recently invaded. A prime example is F. occidentalis, the western flower thrips, which is native to the United States, but has recently invaded China. Therefore, management tactics developed in the United States can be adapted to China. Because further success in management of thrips requires a thorough understanding of thrips ecology, we discuss areas of future research and emphasize the importance of collaboration among different countries to enhance our overall understanding of the biology and ecology of thrips and to improve management programs for these widespread pests. 相似文献
19.
O. V. Krupnova 《Russian Agricultural Sciences》2010,36(5):321-323
The relation between 1000 grain weight and falling number in soft spring wheat varieties and lines is studied in years with
a different temperature regime, precipitation amount, and time of its fall. Differently directed correlations (from weak negative
to weak positive) between 1000 grain weight and falling number are established. 相似文献
20.
闽台入境旅游发展存在明显的时空差异。虽然福建省的土地面积和人口数均大于台湾地区,但历年接待的入境旅游人数却明显少于台湾,只是差距正在不断缩小。目前,闽台国际入境旅游客源国分别以美国和日本居首位。虽然历年入境旅游收入福建省远低于台湾地区,但其占GDP的比重二者渐趋一致。闽台入境旅游发展的时空差异是多因素综合作用的结果。其中,台湾入境旅游发展较早,在市场竞争中占据先机具有重要影响;两地旅游发展政策和经济发展轨迹不同等方面也有一定的影响。 相似文献