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   298篇
  免费   26篇
  国内免费   1篇
  325篇
  2024年   1篇
  2023年   4篇
  2022年   4篇
  2021年   3篇
  2020年   11篇
  2019年   19篇
  2018年   17篇
  2017年   14篇
  2016年   13篇
  2015年   11篇
  2014年   12篇
  2013年   16篇
  2012年   12篇
  2011年   19篇
  2010年   16篇
  2009年   12篇
  2008年   15篇
  2007年   25篇
  2006年   12篇
  2005年   4篇
  2004年   15篇
  2003年   7篇
  2002年   12篇
  2001年   9篇
  2000年   4篇
  1999年   7篇
  1998年   5篇
  1997年   2篇
  1996年   3篇
  1994年   2篇
  1993年   4篇
  1992年   2篇
  1991年   3篇
  1989年   1篇
  1987年   1篇
  1985年   2篇
  1982年   1篇
  1981年   1篇
  1978年   1篇
  1977年   1篇
  1975年   1篇
  1973年   1篇
排序方式: 共有325条查询结果,搜索用时 10 毫秒
101.
Passion fruit seed extract (PFSE), a product rich in stilbenes such as piceatannol and scirpusin B, has various physiological effects. It is unclear whether PFSE and its stilbene derivatives inhibit cancer cell proliferation via human glyoxalase I (GLO I), the rate-limiting enzyme for detoxification of methylglyoxal. We examined the anticancer effects of PFSE in two types of human cancer cell lines with different GLO I expression levels, NCI–H522 cells (highly-expressed GLO I) and HCT116 cells (lowly-expressed GLO I). PFSE and its stilbenes inhibited GLO I activity. In addition, PFSE and its stilbenes supressed the cancer cell proliferation of NCI–H522 cells more than HCT116 cells. These observations suggest that PFSE can provide a novel anticancer strategy for prevention and treatment.  相似文献   
102.
We contrast two methods for estimating the trends of bowhead whales (Balaena mysticetus) in West Greenland: (1) double platform visual aerial survey, corrected for missed sightings and the time the whales are available at the surface; and (2) a genetic capture‐recapture approach based on a 14‐yr‐long biopsy sampling program in Disko Bay. The aerial survey covered 39,000 km2 and resulted in 58 sightings, yielding an abundance estimate of 744 whales (CV = 0.34, 95% CI: 357–1,461). The genetic method relied on determining sex, mitochondrial haplotypes and genotypes of nine microsatellite markers. Based on samples from a total of 427 individuals, with 11 recaptures from previous years in 2013, this resulted in an estimate of 1,538 whales (CV = 0.24, 95% CI: 827–2,249). While the aerial survey is considered a snapshot of the local spring aggregation in Disko Bay, the genetic approach estimates the abundance of the source of this aggregation. As the whales in Disko Bay primarily are adult females that do not visit the bay annually, the genetic method would presumably yield higher estimates. The studies indicate that an increase in abundance observed between 1998 and 2006 has leveled off.  相似文献   
103.
We used direct observation via snorkeling surveys to quantify microhabitat use by native brook (Salvelinus fontinalis) and non‐native brown (Salmo trutta) and rainbow (Onchorynchus mykiss) trout occupying natural and restored pool habitats within a large, high‐elevation Appalachian river, United States. Permutational multivariate analysis of variance (PERMANOVA) and subsequent two‐way analysis of variance (ANOVA) indicated a significant difference in microhabitat use by brook and non‐native trout within restored pools. We also detected a significant difference in microhabitat use by brook trout occupying pools in allopatry versus those occupying pools in sympatry with non‐native trout—a pattern that appears to be modulated by size. Smaller brook trout often occupied pools in the absence of non‐native species, where they used shallower and faster focal habitats. Larger brook trout occupied pools with, and utilized similar focal habitats (i.e. deeper, slower velocity) as, non‐native trout. Non‐native trout consistently occupied more thermally suitable microhabitats closer to cover as compared to brook trout, including the use of thermal refugia (i.e. ambient–focal temperature >2°C). These results suggest that non‐native trout influence brook trout use of restored habitats by: (1) displacing smaller brook trout from restored pools, and (2) displacing small and large brook trout from optimal microhabitats (cooler, deeper, and lower velocity). Consequently, benefits of habitat restoration in large rivers may only be fully realized by brook trout in the absence of non‐native species. Future research within this and other large river systems should characterize brook trout response to stream restoration following removal of non‐native species.  相似文献   
104.
Angola’s intermittent 27‐year civil war displaced over four million people and decimated wildlife populations. During the 1980s, African elephants (Loxodonta africana Blumenbach) in Angola drew international alarm with reports of 100,000 elephants killed. Luiana Partial Reserve (PR), a conservation area in south‐east Angola, was the military operations centre for UNITA (National Union for the Total Independence of Angola), which used elephant ivory to pay for arms and meat. However, the full impact of the civil war on elephants is uncertain because there are no reliable estimates of Angolan elephant populations. Following the end of the civil war in 2002, our three aerial surveys of Luiana PR indicated that elephant numbers are increasing rapidly, from 366 in January 2004 to 1827 in November 2005, and expanding their range in the Reserve. Concurrently, elephants tagged with satellite collars in northern Botswana and the Caprivi Strip, Namibia, moved into Luiana PR. To facilitate re‐colonization and conservation of elephants and other wildlife in Luiana PR, we recommend: (i) realignment of the veterinary fence on the Botswana–Namibia border; (ii) development of effective land use management and anti‐poaching programmes; (iii) clearing of landmines; (iv) designation of the Reserve a national park; and (v) development of ecotourism and community conservation programmes.  相似文献   
105.
Investigations concerned with ground beetle (Carabidae) dynamics in the agroecosystem have generally been limited to relatively short time periods in one crop, commonly wheat, whilst other studies have concentrated on the influence of landscape structure and non-crop habitat on beetle activity, again usually in one crop. Results of a seven-year survey at Nafferton in northern England, with recording from all crops in an organic rotation and from three types of field boundary, indicated that the lack of field boundary management had the greatest influence on ground beetle activity/density in this particular agroecosystem. However, within the crop rotation, grass/clover limited activity/density compared with that in cereal, potato and bean crops and there were also differences within cereal crops. Data from a 17-year survey at Drayton Environmental Change Network (ECN) site in midland England indicate that previous understanding of ground beetle activity and distribution in intensively managed landscapes may have been limited by temporal constraints. At Drayton, changes in surrounding crop cover, with the introduction of willow coppice and reduced area of arable land and agricultural inputs, considerably influenced ground beetle activity/density and assemblages in non-crop habitat. Small, active species dominating the assemblage in the first few years of the Drayton survey were gradually replaced by larger, non-flying species, especially after all the willow coppice had been planted. In both surveys, activity/density of ground beetles was related to disturbance, affecting vegetation cover and structure, but at the field scale at Nafferton and at the farm scale at Drayton. Longer surveys in a variety of landscapes such as those by the United Kingdom Environmental Change Network are likely to improve understanding of activity, diversity and distribution of invertebrates, which are fundamental requirements if predators such as ground beetles are required for ecosystem service provision.  相似文献   
106.
An evaluation methodology is outlined for roadside surveys of blood alcohol content in drivers, where correlative attributes are absent or at most weakly related to the alcohol levels. The methodology is based on concepts of statistical information theory, and may be extended into a continuous model of the distribution systems arising from such or similar surveys with voluntary participation.  相似文献   
107.
108.
East Asian nations have become increasingly diverse in their ethnic composition since the 1990s. A large proportion of recent immigrants consists of women from China, Vietnam, Indonesia, Thailand and the Philippines, who migrated following their marriage to male citizens from South Korea and Taiwan. To study this new group of foreign residents, the governments of both Taiwan and South Korea have conducted national surveys since 2000. This paper conducts a content analysis of the questions of these surveys to elaborate a criticism of the ethnic statistics produced from them. These surveys provide relevant information on this group of new residents, but they also have the potential to reinforce existing negative stereotypes due to prior assumptions that shaped the content of questionnaires. The analysis reveals that the production of statistics on migrants is influenced by three ideologies: assimilation, patriarchy and nationalism.  相似文献   
109.
Understanding species distributions, habitat requirements, and population trends is helpful for implementing effective conservation. But expense often prevents such evaluations. Here, we present a preliminary assessment for a low‐cost method – road‐side observations – to see how effective it can be. The bamboo zone of Bwindi Impenetrable National Park is restricted, covering 0.3% of park area yet is poorly known. Our study evaluates the use of the bamboo zone and surrounding forest by species of large mammals and large ground birds. Data were collected while driving a road section running through this zone. During 416 trips, seven species of mammal and one bird were recorded. Distributions for most species were significantly nonrandom in both space (P < 0.02) and time (Hour of day: P < 0.040; Month: P < 0.0001) for all species but black‐fronted duikers and blue monkeys for which 0.085 > P > 0.050 for month. Of the species observed, francolins were more frequently seen in the zone of visible bamboo than the nonbamboo zone (P = 0.002). Black‐and‐white colobus, L’Hoest’s monkey, and jackals were more common in nonbamboo forest (P ≤ 0.05). Road sightings have potential for low‐cost habitat use assessments and monitoring.  相似文献   
110.
We conducted shore-based sighting surveys to estimate the number of northbound migrating gray whale calves passing Piedras Blancas, California, for seven consecutive years (1994–2000). In addition, we conducted aerial surveys to determine offshore distribution of the migration in 1994 and 1995, measured day/night migration rates with thermal sensors in 1994–1996, and maintained concurrent replicate watches near the peak of each migration to estimate the proportion of the cow/calf pairs missed by the standard watch team. During good weather, we counted 325, 194, 407, 501, 440, 141, and 96 calves during 1994–2000, respectively. Correcting these counts for periods not on watch and for calves missed, produced final estimates of 945 calves (SE = 68.21) for 1994, 619 calves (SE = 67.19) for 1995, 1,146 calves (SE = 70.67) for 1996, 1,431 calves (SE = 82.02) for 1997, 1,388 calves (SE = 91.84) for 1998, 427 calves (SE = 41.10) for 1999, and 279 calves (SE = 34.79) for 2000. Calf production indices (calf estimate/total population estimate) are 4.2%, 2.7%, 4.8%, 5.8%, 5.5%, 1.7%, and 1.1% for the years 1994–2000, respectively. Fluctuations in calf production over this time period were positively correlated with the length of time that primary feeding habitat was free of seasonal ice during the previous year.  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