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   372篇
  免费   51篇
  国内免费   439篇
  2024年   13篇
  2023年   32篇
  2022年   65篇
  2021年   66篇
  2020年   60篇
  2019年   70篇
  2018年   48篇
  2017年   72篇
  2016年   52篇
  2015年   45篇
  2014年   48篇
  2013年   48篇
  2012年   21篇
  2011年   47篇
  2010年   33篇
  2009年   24篇
  2008年   15篇
  2007年   24篇
  2006年   22篇
  2005年   11篇
  2004年   6篇
  2003年   3篇
  2002年   8篇
  2001年   12篇
  2000年   3篇
  1999年   1篇
  1998年   1篇
  1997年   1篇
  1996年   4篇
  1995年   2篇
  1994年   1篇
  1992年   1篇
  1991年   1篇
  1988年   1篇
  1982年   1篇
排序方式: 共有862条查询结果,搜索用时 281 毫秒
61.
There is increasing interest worldwide regarding managing plantation forests in a manner that maintains or improves timber production, enhances ecosystem services, and promotes long‐term sustainability of forest resources. We selected the Gan River Basin, the largest catchment of Poyang Lake and a region with a typical plantation distribution in South China, as the study region. We evaluated and mapped four important forest ecosystem services, including wood volume, carbon storage, water yield, and soil retention at a 30 × 30 m resolution, then quantified their trade‐offs and synergies at the county and subwatershed scales. We found that the wood volume and carbon storage services, as well as the soil retention and water yield, exhibited synergistic relationships. However, the carbon storage displayed a trade‐off relationship with the water yield. Additionally, we compared the beneficial spatial characteristics among dominant species in the study region. The results showed that the Chinese fir forest and the pine forest exhibited lower overall benefits than natural forests including the broad‐leaved forest and the bamboo forest. To propose a suitable management strategy for the study region, method of spatial cluster analysis was used based on the four eco‐services at the subwatershed scale. The basin was divided into four management groups instead of treating the region as a homogenous management region. Finally, we proposed more specific and diverse management strategies to optimize forest benefits throughout the entire region.  相似文献   
62.
In this paper, I examine the mastery of mothering skills and satisfaction with associated health services in women who had recently given birth in Montreal (n = 33). I compare experience between women of two distinct ethnocultural groups: Anglophone Euro-Canadian and Anglophone Afro-Caribbean. The overall aim is to discern differentials in the mastery of mothering skills and associated satisfaction with maternal and child health services. The study is framed by neo-Weberian social theory suggesting that modernization and bureaucratization increasingly eviscerate everyday skills and knowledge. These processes also lead to changes regarding what is considered credible ‘authoritative knowledge.’ I found that older Anglophone Euro-Canadians expressed the greatest skill deficits. They attempted to redress these deficits through consultation of professionally authored books, medical Web sites and health professionals. Older Anglophone Euro-Canadians saw these resources as sources of ‘authoritative knowledge.’ They also expressed dissatisfaction with related health services. In contrast, Anglophone Afro-Caribbeans and younger lower-income Anglophone Euro-Canadians expressed satisfaction with their skills. This derived from widespread previous experience with children and more extensive and readily available kith and kin networks. These were considered sources of ‘authoritative knowledge’ in this group. This group expressed less dissatisfaction with health services, as they did not need, or expect, these services to redress skill deficits.
Rob WhitleyEmail:
  相似文献   
63.
Apis cerana japonica Radoszkowski, endemic to Japan, is known to be one of the most important pollinators for wild plants and crops, such as buckwheat, in cool to warm temperate Japan. To determine the degree of dependence of A. cerana japonica on forest resources, we analyzed pollen brought back to nests in a typical “Satoyama” landscape with relatively high deciduous forest coverage in northern Japan. We divided the landscape elements of the study area into three types: deciduous forest, conifer plantation and open land according to landcover digital data, and each pollen taxon was assigned to one of these three types of landscape elements. We collected total pollen loads of 15.75 g (total of colonies A and B) in May (spring), 1.57 g (total of colonies A and C) in June (early summer), 19.03 g (total of colonies A, B and C) in July (mid‐summer) and 45.61 g (total of colonies A, B and D) in September (autumn). Deciduous forests are the most important foraging habitats for A. cerana japonica in the “Satoyama” landscape especially from spring to mid‐summer when mass flowering of tall trees and shrubs species provides rich floral resources for developing bee colonies. On the other hand, the bees frequently foraged from herbaceous plant species in autumn when flowering of tree species reduces and herbaceous plant species have flowering peaks. In turn, the bees provide pollination services to a number of wild flowers blooming in various forest layers ranging from the canopy to the understory layer.  相似文献   
64.
Swans, geese and some ducks (Anatidae) are obligate herbivores, many are important quarry species and all contribute to a variety of ecosystem services. Population growth and shifting ranges have led to increasing proximity to man and thus increasing conflicts. We review and synthesize the role of these birds as herbivores on agricultural land (cropland, rotational grassland and pasture) and other terrestrial habitats where conflict with human interests may occur. A bibliographic analysis of peer‐reviewed papers (N = 359) shows that publication activity peaked in 1991–2000 in North America and 2000–2010 in Europe, and has decreased since. Taxonomic and geographical biases are obvious in research to date: Snow Goose Chen caerulescens was the most studied species (N = 98), and Canada Branta canadensis, Barnacle B. leucopsis and Brent geese B. bernicla all featured in more than 40 studies; most studies originated in northwest Europe or North America, very few have been carried out in Asia and European Russia. On the basis of nutrient/energy budgets of herbivorous waterfowl, it is evident that dense single‐species crops (such as rotational grassland, early‐growth cereals and root crops) and spilled grain in agricultural landscapes offer elevated energetic and nutritional intake rates of food of higher quality compared to natural or semi‐natural vegetation. Hence, although affected by seasonal nutritional demands, proximity to roost, field size, disturbance levels, access to water, food depletion and snow cover, agricultural landscapes tend to offer superior foraging opportunities over natural habitats, creating potential conflict with agriculture. Herbivorous waterfowl select for high protein, soluble carbohydrate and water content, high digestibility as well as low fibre and phenolic compounds, but intake rates from grazing varied with goose body and bill morphology, creating species‐specific loci for conflict. Crop damage by trampling and puddling has not been demonstrated convincingly, nor do waterfowl faeces deter grazing stock, but where consumption of crops evidently reduces yields this causes conflict with farmers. Studies show that it is difficult and expensive to assess the precise impacts of waterfowl feeding on yield loss because of other sources of variation. However, less damage has been documented from winter grazing compared to spring grazing and yield loss after spring grazing on grassland appears more pronounced than losses on cereal fields. Although yield losses at national scales are trivial, individual farmers in areas of greatest waterfowl feeding concentrations suffer disproportionately, necessitating improved solutions to conflict. Accordingly, we review the efficacy of population management, disturbance, provision of alternative feeding areas, compensation and large‐scale stakeholder involvement and co‐management as options for resolving conflict based on the existing literature and present a framework of management advice for the future. We conclude with an assessment of the research needs for the immediate future to inform policy development, improve management of waterfowl populations and reduce conflict with agriculture.  相似文献   
65.
The expansion of large-scale plantations has a major impact on landscapes in the Tropics and Subtropics. Crops like soy bean, oil palm and rubber have led to drastic changes in land cover over the past decades, thereby altering ecosystem functions and services (ESS). Associated shifts in ESS such as climate regulation, erosion and water cycles, biodiversity as well as soil fertility or the provisioning of raw materials have been assessed through several models and software solutions (InVEST, ARIES, MIMES). However, suitable methods for the integration of a range of biodiversity assessments in agricultural landscapes are scarce.With this study, we introduce a methodology for incorporating multiple levels of species diversity into models to allow an integrated evaluation of ESS. We collected data sets from both published and unpublished sources on the distribution of vascular plants, selected pollinator groups, ground beetles, ungulates as well as amphibians, mammals, reptiles and birds in rubber-dominated landscapes, with a focus on our study sites in Southwest China and Thailand. Based on this information, we developed a common classification scheme that enables the integration of different facets of biodiversity (species diversity and functional diversity) to complement an interdisciplinary ESS assessment.Species diversity data were normalized against the most divers habitats reported (using habitat scores) to assess the impact of rubber cultivation on multiple levels of biodiversity. This resulted in a comparable matrix of different land use types and their suitability as habitat for the respective species groups allowing the aggregation of very diverse indicators. The findings were applied to two alternative land use scenarios in southern China to highlight the potential effects of land use and management decisions on species and functional diversity. Our results highlighted that the conservation oriented scenario did score higher for habitat suitability in both total species (+5%) as well as IUCN Red List species (+6%) assessments compared to the current state or business as usual scenarios (-2% and −3% compared to current state).The process presented here allows for an application within established ESS software programs, in our case InVEST, using aggregated indices while additionally providing enhanced opportunities for comparable, spatially explicit assessments of the expected impact of the analyzed scenarios on specific species groups.  相似文献   
66.
Assessing cultural ecosystem services provided by biodiversity requires a combination of ecological and social approaches. In this study, we investigated the capacity of large African mammal species to provide the cultural ecosystem service of wildlife tourism by using a supply and demand framework. First, we tested the relationship between supply and demand for large mammal species in wildlife tourism. Second, we tested whether the trophic level and body size of mammals influenced the mismatch between supply and demand, and whether the patterns of mismatches were consistent among four protected areas (PAs) in three Southern African countries. To quantify supply of species, we counted large mammals along 196 five km road transects within the four PAs; to estimate demand, we gathered 651 face-to-face questionnaires of wildlife tourists and distinguished between their expectation and hope to see specific species. Results show that a higher supply of large mammal species increased the expectation to see a species (linear regression slope β = 0.28, p < 0.01), whereas supply did not affect the hopes to see a specific species (β = −0.04, p = 0.63). Analyses of mismatches revealed that predator species were more demanded in relation to their supply than ungulates. Finally, we found that the demands of wildlife tourists for mammal species in relation to their supply were consistent across the four PAs. Supply-demand analyses reveal that species’ traits, in particular trophic level, shape the hopes of wildlife tourists to see specific mammal species. We propose that the quantification of supply-demand mismatches can be used to identify charismatic species and relevant species’ traits, and can be applied for wildlife tourism assessments within as well as across regions. Supply-demand analyses provide a useful framework and deliver indicators for better assessing cultural ecosystem services involving wildlife and nature-based tourism, and can be used for conservation management.  相似文献   
67.
互花米草群落功率最大化倾向   总被引:2,自引:0,他引:2  
植物新种进入新区域后,通过扰动或搅局,改变生态系统的结构,形成有特色的自组 织生态系统,趋向新条件下的功率最大化.本文应用能值分析的方法,分别对苏北互花米草 生态系统和光滩生态系统进行能值分析和系统评估.结果表明:互花米草生态系统每年能值 产出比光滩生态系统高1.52E+18 sej.能值密度是光滩的4.72倍,基础能值产出率约为光滩生态系统的5倍.互花米草生态系统能更有效地利用能量,增加系统内部的 能值贮存,实现系统内部的功率最大化.互花米草入侵某区域滩涂后,通过自组织促使整个互花米草生态系统趋于功率最大化,充分发挥生态服务功能,但由于其繁殖扩散过快,导致种群爆发,产生了一系列负面效应,如抑制本土物种、侵占航道、危害贝类养殖等.  相似文献   
68.
The objective was to determine the effects of reproductive tract score (RTS) on reproductive performance in beef heifers bred by timed artificial insemination followed by natural service (AI-NS) or by natural service only (NSO). Angus cross beef heifers (n = 2660) in the AI-NS group were artificially inseminated at a fixed time (5- or 7-day CO-Synch + controlled internal drug release protocol) once, then exposed to bulls 2 weeks later (bull-to-heifer ratio = 1:40–1:50) for the reminder of the 85-day breeding season. Angus cross beef heifers (n = 1381) in NSO group were submitted to bulls (bull-to-heifer ratio = 1:20–1:25) for the entire 85-day breeding season. Heifers were reproductive tract scored from 1 (prepubertal) to 5 (cyclic) 4 weeks before, and were body condition scored (BCS) from 1 (emaciated) to 9 (obese) at the beginning of breeding season. Pregnancy diagnosis was performed 70 days after AI for AI-NS group and 2 months after the end of breeding season for both groups. Heifers in both groups were well managed and of similar age (14.9 ± 0.4 [AI-NS] and 14.7 ± 0.8 [NSO] months). Pregnancy rates (PRs) and number of days to become pregnant were calculated using PROC GLIMMIX and PROC LIFETEST procedures of SAS. Adjusting for BCS (P = 0.07), expressed estrus (P < 0.05), year (P < 0.05), and BCS by year interaction (P < 0.05), the AI-PR was greater for heifers in AI-NS group with higher RTS (P < 0.0001; 40.7%, 48.3%, 57.6%, and 64.6% for RTS of 2 or less, 3, 4, and 5, respectively). Controlling for BCS (P < 0.05), year (P < 0.05) and the breeding season pregnancy rates (BS-PRs) were greater for heifers in the AI-NS group with higher RTS (P < 0.01; 81.2%, 86.5%, 90.4%, and 95.2% for RTS of 2 or less, 3, 4, and 5, respectively). Similarly, adjusting for BCS, year (P < 0.05), the BS-PR was greater for heifers in NSO group with higher RTS (P < 0.01; 79.7%, 84.3%, 88.4%, and 90.2% for RTS of 2 or less, 3, 4, and 5, respectively). Heifers with higher RTS in both groups became pregnant earlier in the breeding season compared with heifers with lower RTS (log-rank statistics: P < 0.0001). Heifers in the AI-NS group become pregnant at a faster rate compared with those in the NSO group (P < 0.01). The BS-PR for heifers with RTS 5 was different between AI-NS and NSO groups (P < 0.0001). In conclusion, the RTS influenced both the number of beef heifers that became pregnant during the breeding season and the time at which they become pregnant. Furthermore, irrespective of RTS, heifers bred by NSO required more time to become pregnant than their counterparts in herds that used timed AI. The application of RTS system is reliant on the use of synchronization protocol. The application of RTS for selection may plausibly remove precocious females with lower RTS. On the contrary, application of RTS would help select heifers that will become pregnant earlier in breeding season.  相似文献   
69.
目的 比较济宁市各级各类医疗机构的医疗服务、质量、效率、费用等关键要素变化,为完善“先看病后付费”诊疗服务模式提供参考依据。方法 选取济宁市266家各级各类医疗机构为调查对象,采用自行设计的“济宁市‘先看病后付费’工作推广效果评估调查表”进行问卷调查,同时分层抽样15家医院进行现场访谈。结果 济宁市享受“先看病后付费”住院患者惠及率达到59.45%,年增加37.29%;全市各级各类医疗机构全面实施了此项制度,二级及以下医疗机构惠及率高于三级医疗机构,分别达到77.18%、63.71%。结论 “先看病后付费”服务模式改革,满足了群众医疗需求,医保(新农合)住院患者总费用得到合理控制,患者自付就医经济负担进一步减轻。为规避风险,提出解决垫付资金和恶意欠费问题解决方案。  相似文献   
70.
目的 通过历史流量数据及流量监测系统,对医院门诊流量情况进行分析,为门诊人力资源合理分配提供依据。 方法 应用时间序列数据季节指数分析法对医院历史门诊流量数据按月份、周和每日不同时间段流量进行分析,并对流量监测系统的历史数据进行统计分析。 结果 门诊流量3、5、7、11、12月份较大,1、2月份较少;一周中周一至周三流量较大,周四、周五流量较少;一天中早上8:00~10:00流量较大,呈明显就诊高峰,下午14:00~16:00为就诊低峰。 结论 医院门诊流量在一年的不同月份、每周的不同天次、每日的不同时间段都有不同的变化规律。  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