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   1196篇
  免费   125篇
  国内免费   61篇
  1382篇
  2024年   9篇
  2023年   22篇
  2022年   39篇
  2021年   57篇
  2020年   51篇
  2019年   70篇
  2018年   51篇
  2017年   42篇
  2016年   42篇
  2015年   52篇
  2014年   59篇
  2013年   84篇
  2012年   56篇
  2011年   52篇
  2010年   60篇
  2009年   49篇
  2008年   78篇
  2007年   60篇
  2006年   52篇
  2005年   45篇
  2004年   35篇
  2003年   32篇
  2002年   28篇
  2001年   30篇
  2000年   18篇
  1999年   17篇
  1998年   17篇
  1997年   17篇
  1996年   20篇
  1995年   17篇
  1994年   10篇
  1993年   13篇
  1992年   14篇
  1991年   8篇
  1990年   7篇
  1989年   3篇
  1988年   5篇
  1987年   12篇
  1986年   5篇
  1985年   7篇
  1984年   5篇
  1983年   6篇
  1982年   4篇
  1981年   4篇
  1980年   3篇
  1979年   4篇
  1978年   7篇
  1977年   2篇
  1976年   1篇
  1974年   1篇
排序方式: 共有1382条查询结果,搜索用时 15 毫秒
51.
陕西延安北洛河流域蝶类资源调查及区系研究   总被引:3,自引:0,他引:3  
通过对延安北洛河流域的蝶类资源进行调查,发现该地区共有蝴蝶6科54属75种(凤蝶科2属3种、粉蝶科7属10种、眼蝶科10属13种、蛱蝶科15属25种、灰蝶科11属15种、弄蝶科8属9种),其中34种为单属种,陕西省新纪录5种;区系结构分析表明该地区蝴蝶以古北种为主。标本保存于延安大学生命科学学院标本馆。  相似文献   
52.
秦岭宁陕县森林植被碳储量与碳密度特征   总被引:1,自引:0,他引:1  
邓蕾  上官周平 《西北植物学报》2011,31(11):2310-2320
以秦岭南坡中段宁陕县林区2003年二类森林调查资料为基础,采用政府间气候变化委员会(IPCC)推荐使用的森林碳储量估算方法,从森林类型、林种、年龄和林分起源的角度,对该林区森林植被碳储量和碳密度进行估算。结果显示:(1)宁陕县森林植被碳储量为12.31Tg(1Tg=1×1012 g),平均碳密度为66.36Mg/hm2(1Mg=1×106 g),其各乡镇森林植被碳储量和碳密度在空间上的分布不平衡。(2)各森林类型中针叶林总碳储量为0.71Tg,平均碳密度为64.11 Mg/hm2,阔叶林总碳储量为11.61Tg,占宁陕县总碳储量的94.3%,碳密度为67.65Mg/hm2。(3)各林种中防护林碳储量最大(8.13Tg),占宁陕县总碳储量的66%,特种用途林碳密度最大(81.43Mg/hm2)。(4)不同林分起源中,天然林碳储量为12.231Tg,占宁陕县总碳储量的99.3%,人工林碳储量较小。(5)不同年龄森林中未成熟森林(包括幼龄林、中龄林和近熟林)碳储量为12.13Tg,占总碳储量的98.5%,近熟林碳密度最大(80.14Mg/hm2),幼龄林碳密度最小(39.85Mg/hm2)。研究表明,宁陕县森林具有较大的固碳能力和固碳潜力,其森林面积和蓄积是决定森林碳储量大小的重要因子,而森林碳密度的大小与森林类型、年龄组成和林分起源方式密切相关。  相似文献   
53.
Eighty-six species of fungi belonging to sixty-four genera were examined for their ability to metabolize naphthalene. Analysis by thin-layer and high pressure liquid chromatography revealed that naphthalene metabolism occurred in forty-seven species belonging to thirty-four genera from the major fungal taxa. All organisms tested from the order Mucorales oxidized naphthalene with species of Cunninghamella, Syncephalastrum and Mucor showing the greatest activity. Significant metabolism was also observed with Neurospora crassa, Claviceps paspali and four species of Psilocybe. The predominant metabolite formed by most organisms was 1-naphthol. Other products identified were, 4-hydroxy-1-tetralone, trans-1,2-dihydroxy-1,2-dihydronaphthalene, 2-naphthol, 1,2-and 1,4-naphthoquinone.  相似文献   
54.
The horse mussel Atrina zelandica (Gray) is a large, suspension feeding pinnid bivalve, common in coastal and estuarine areas of northern New Zealand. As a suspension feeder, Atrina is likely to be influenced by suspended sediment loads. We conducted a laboratory experiment to determine the effect of short-term elevations in turbidity levels, such as those commonly recorded during storms, on the physiological condition and clearance rates of Atrina. We also conducted a field survey and a 3-month transplant experiment at multiple sites along a gradient of increasing suspended sediment load in a New Zealand estuary. Laboratory clearance rates of Atrina declined above a threshold suspended sediment concentration, and Atrina physiological condition at the end of this experiment was lower in high cf. low turbidity treatments. Decreases in Atrina condition were detected after exposure to elevated levels for only 3 days. The field survey and transplant experiment provided empirical evidence of a strong, negative effect of increasing suspended sediment flux on the physiological condition of Atrina. We suggest that relationships between the physiological condition of suspension feeders and sediment settling flux could provide a link between sediment inputs, which commonly occur as a result of catchment runoff during rainfall events, and the ecological health of estuarine and shallow coastal areas. Our study also demonstrated that Atrina have a natural distribution limit controlled by suspended sediment load. Thus, there is potential for larger-scale functional and structural effects on benthic communities in estuarine and coastal areas with high rates of sedimentation.  相似文献   
55.
The effective management and conservation of animal populations relies on statistically-sound and replicable surveys to obtain estimates of abundance and assess trends. Surveys of cetaceans, such as humpback whales Megaptera novaeangliae, are difficult to conduct and are particularly affected by bias in detection probability. For example, the probability of detection of whales from land decreases substantially with increased distance from the platform. This distance effect is also true for aerial surveys, combined with the problem that animals are unavailable for detection (underwater) whilst in the field of view. We present a novel approach that combines corrected double-platform land surveys with corrected aerial surveys to obtain a robust estimate of g(0), the probability of detection on the survey line, for aerial surveys of migrating humpback whales. Several sources of heterogeneity in detection probabilities were identified within the land and aerial surveys (including group composition, bearing of first sighting, number of groups being tracked simultaneously and cloud cover). After including these into our estimate of ĝ(0), we found that only 29% of available whales are being detected on the survey line (ĝ(0) = 0.288), which is a considerably smaller estimate than many available for humpback whales using other methods. Incorporating heterogeneity into the population surveys shows that we are likely to be underestimating the population size of whales on the east coast of Australia. The implications of this result for their conservation and management in light of increased whale-human conflict is discussed.  相似文献   
56.
Sparsely distributed species attract conservation concern, but insufficient information on population trends challenges conservation and funding prioritization. Occupancy‐based monitoring is attractive for these species, but appropriate sampling design and inference depend on particulars of the study system. We employed spatially explicit simulations to identify minimum levels of sampling effort for a regional occupancy monitoring study design, using white‐headed woodpeckers (Picoides albolvartus), a sparsely distributed, territorial species threatened by habitat decline and degradation, as a case study. We compared the original design with commonly proposed alternatives with varying targets of inference (i.e., species range, space use, or abundance) and spatial extent of sampling. Sampling effort needed to achieve adequate power to observe a long‐term population trend (≥80% chance to observe a 2% yearly decline over 20 years) with the previously used study design consisted of annually monitoring ≥120 transects using a single‐survey approach or ≥90 transects surveyed twice per year using a repeat‐survey approach. Designs that shifted inference toward finer‐resolution trends in abundance and extended the spatial extent of sampling by shortening transects, employing a single‐survey approach to monitoring, and incorporating a panel design (33% of units surveyed per year) improved power and reduced error in estimating abundance trends. In contrast, efforts to monitor coarse‐scale trends in species range or space use with repeat surveys provided extremely limited statistical power. Synthesis and applications. Sampling resolutions that approximate home range size, spatially extensive sampling, and designs that target inference of abundance trends rather than range dynamics are probably best suited and most feasible for broad‐scale occupancy‐based monitoring of sparsely distributed territorial animal species.  相似文献   
57.
Between the 1940s and 1970s Southern Hemisphere populations of humpback whales (including eastern Australia and Oceania populations) were hunted to near extinction by extensive commercial whaling activities in Antarctica, with fewer whales taken in shore whaling operations in New Zealand, Australia (including Norfolk Island) and Tonga. Variable rates of recovery of these populations have been documented, ranging from recovery to prewhaling numbers in eastern Australian humpbacks to very little sign of recovery in many Oceania populations. Here we analyze recent sighting data collected over 12 yr, from annual surveys in Cook Strait, New Zealand. The data show an increase in sightings, at an estimated rate of 13% (95% CI of 4.9% and 21.7%) in 2015, of humpback whales migrating through Cook Strait. The wide confidence intervals preclude substantive conclusions about the rate of increase but suggest humpback whales are returning to this region in increasing numbers, indicating an influx of immigrants from neighboring areas, namely eastern Australia.  相似文献   
58.
  1. Freshwater conservation is vital to the maintenance of global biodiversity. Ponds are a critical, yet often under‐recognized, part of this, contributing to overall ecosystem functioning and diversity. They provide habitats for a range of aquatic, terrestrial, and amphibious life, often including rare and declining species.
  2. Effective, rapid, and accessible survey methods are needed to enable evidence‐based conservation action, but freshwater taxa are often viewed as “difficult”—and few specialist surveyors are available. Datasets on ponds are therefore limited in their spatiotemporal coverage.
  3. With the advent of new recording technologies, acoustic survey methods are becoming increasingly available to researchers, citizen scientists, and conservation practitioners. They can be an effective and noninvasive approach for gathering data on target species, assemblages, and environmental variables. However, freshwater applications are lagging behind those in terrestrial and marine spheres, and as an emergent method, research studies have employed a multitude of different sampling protocols.
  4. We propose the Pond Acoustic Sampling Scheme (PASS), a simple protocol to allow a standardized minimal sample to be collected rapidly from small waterbodies, alongside environmental and methodological metadata. This sampling scheme can be incorporated into a variety of survey designs and is intended to allow access to a wide range of participants, without requiring complicated or prohibitively expensive equipment.
  5. Adoption of this sampling protocol would enable consistent sound recordings to be gathered by researchers and conservation organizations, and allow the development of landscape‐scale surveys, data sharing, and collaboration within an expanding freshwater ecoacoustic community—rather than individual approaches that produce incompatible datasets. The compilation of standardized data would improve the prospects for effective research into the soundscapes of small waterbodies and aid freshwater conservation efforts.
  相似文献   
59.
60.
Exposure to workplace hazards, such as dust, solvents, and fumes, has the potential to adversely affect the health of people. However, the effects of workplace hazards on health may differ when exposure occurs at different times in the circadian cycle, and among people who work longer hours or who do not obtain adequate sleep. The aim of the present study was to document exposures to workplace hazards across a national sample of New Zealanders, comparing people who work a standard 08:00 ?17:00 h Monday-to-Friday working week (Std hours) and those who do not (N-Std hours). New Zealanders (n = 10 000) aged 20–64 yrs were randomly selected from the Electoral Roll to take part in a nationwide survey of workplace exposures. Telephone interviews were conducted between 2004 and 2006, using a six-part questionnaire addressing demographics, detailed information on the current or most recent job (including exposures to a range of workplace hazards), sleep, sleepiness, and health status. N-Std hours were categorised on the basis of: being required to start work prior to 07:00 h or finish work after 21:00 h and/or; having a regular on-call commitment (at least once per week) and/or; working rotating shifts and/or; working night shift(s) in the last month. The response rate was 37% (n = 3003), with 22.2% of participants (n = 656) categorised as working N-Std hours. Industry sectors with the highest numbers of participants working N-Std hours were manufacturing, health and community services, and agriculture, fishing, and forestry. Response rate was 37% (n = 3003) with 22.2% (n = 656) categorised as working N-Std hours. Participants working N-Std hours were more likely to be exposed to all identified hazards, including multiple hazards (OR = 2.45, 95% CI = 2.01–3.0) compared to those working Std hours. Participants working N-Std hours were also more likely to report ‘never/rarely’ getting enough sleep (OR = 1.38, 95% CI = 1.15–1.65), ‘never/rarely’ waking refreshed (OR = 1.23, 95% CI = 1.04–1.47), and excessive sleepiness (OR = 1.77, 95% CI = 1.29–2.42). New Zealanders working N-Std hours are more likely to be exposed to hazards in the workplace, to be exposed to multiple hazards, and to report inadequate sleep and excessive sleepiness than their colleagues working a standard 08:00?17:00 h Monday-to-Friday working week. More research is needed on the effects of exposure to hazardous substances outside the usual waking day, on the effects of exposure to multiple hazards, and on the combination of hazard exposure and sleep restriction as a result of shift work.  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