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   206篇
  免费   4篇
  210篇
  2013年   14篇
  2012年   4篇
  2011年   7篇
  2010年   18篇
  2009年   7篇
  2008年   7篇
  2007年   17篇
  2006年   3篇
  2005年   5篇
  2004年   2篇
  2002年   2篇
  1999年   4篇
  1998年   6篇
  1997年   6篇
  1996年   3篇
  1994年   5篇
  1992年   4篇
  1991年   4篇
  1990年   7篇
  1989年   3篇
  1988年   4篇
  1987年   3篇
  1986年   3篇
  1985年   3篇
  1984年   3篇
  1983年   3篇
  1982年   2篇
  1981年   2篇
  1980年   5篇
  1978年   4篇
  1977年   3篇
  1976年   2篇
  1972年   1篇
  1971年   5篇
  1970年   1篇
  1969年   1篇
  1968年   1篇
  1960年   1篇
  1958年   3篇
  1957年   6篇
  1956年   3篇
  1955年   1篇
  1954年   1篇
  1953年   4篇
  1952年   2篇
  1951年   4篇
  1950年   1篇
  1949年   1篇
  1928年   1篇
  1925年   1篇
排序方式: 共有210条查询结果,搜索用时 15 毫秒
11.
12.
We isolated 45 new Kentish plover (Charadrius alexandrinus) microsatellite loci. These were tested for polymorphism in 42 Kentish plovers breeding in the Çukurova Delta, Turkey. Thirty‐six of the 45 loci were polymorphic with observed heterozygosity varying between 0.22 and 0.93. Genotypes of individuals of known sex indicated that two loci were sex‐linked (Calex‐26 is located on the Z chromosome and Calex‐31 on the W chromosome). Additionally, we tested all loci for amplification in four other species of Charadridae (Kittlitz's plover, Madagascar plover, three‐banded plover and white‐fronted plover). On average 34 loci amplified per species (range 29–36).  相似文献   
13.
The impact of the crayfish Orconectes virilis on aquatic macrophytes   总被引:1,自引:0,他引:1  
SUMMARY. 1. The impact of crayfish on the biomass, density and shoot morphology of four submersed plant species was examined under semi-natural conditions. Male or female crayfish ( Orconectes virilis ) were held for 5 weeks at biomasses of 0, 5, 10 or 18 g m−2 (live weight) in twelve plastic pools (4.67 m2, surface area) containing Potamogeton richardsonii, Myriophyllum exalbescens, Nuphar variegatum and Sparganium eurycarpum .
2. Crayfish significantly affected biomass, density and/or shoot morphology of all four macrophyte species. Differences in the effect of crayfish on macrophyte growth were related to plant species, crayfish sex and activity, and the abundance of alternative foods.
3. The effect of female crayfish on macrophyte growth was generally stimulatory. Myriophyllum and Potamogeton biomass, Potamogeton density and Myriophyllum length increased in the presence of female crayfish, possibly due to the reduction in herbivorous snails as a result of crayfish predation. In contrast, plant growth decreased in the presence of male crayfish: Myriophyllum, Nuphar and Potamogeton biomass, Myriophyllum and Sparganium density, and Sparganium and Poiamogeton length were reduced at male crayfish biomasses between 5 and 18 g m−2.
4. These results indicate that even relatively low densities of crayfish can greatly affect the growth of submersed aquatic plants. Because of their ability to modify aquatic macrophyte, macroinvertebrate and, ultimately, fish communities, the introduction of crayfish into lakes where they do not occur could have a major effect on the structure and composition of the littoral zone.  相似文献   
14.
Forecasting how species will respond to climatic change requires knowledge of past community dynamics. Here we use time‐series data from the small‐mammal fossil records of two caves in the Great Basin of the American West to evaluate how contrasting and variable local paleoclimates have shaped small‐mammal abundance dynamics over the last ~7500 years of climatic change. We then predict how species and communities will respond to future scenarios of increased warming and aridity coupled with continued spread of an invasive annual grass (Bromus tectorum). We find that most community‐level responses to climatic change occur in the mammalian abundance structure at both sites; the dominance of the community by individuals from species with a southern geographic affinity increases with climatic warming. This suggests that responses occurred in situ rather than by the immigration of new taxa over this time interval. Despite predictability at the community‐scale, species‐level relationships between abundance and climate are variable and are not necessarily explained by a species' geographic affinity. Species present at both sites, however, exhibit remarkably similar responses to climate at each site, indicating that species autecology (specifically dietary functional group) is important in determining response to climatic warming. Regression‐tree analyses show remarkable concordance between the two cave faunas and highlight the importance of a granivorous dietary strategy in this desert ecosystem. Under projections of increased temperature and decreased precipitation over the next 50 years, our results indicate that granivores should thrive as communities become more dominated by individuals with a southern geographic affinity. Granivores, however, are negatively impacted by the invasion of cheatgrass. The last century of anthropogenic impacts has thus placed granivores at a greater risk of extinction than predicted under climate‐only scenarios.  相似文献   
15.
Abstract: State wildlife agencies often use input obtained through public meetings to develop management policies. Because public meetings can be dominated by single stakeholder groups, these policies may not reflect the attitudes of new wildlife stakeholders. In 2000 the Utah Wildlife Board, after a series of public meetings, adopted a statewide policy for winter-feeding mule deer (Odocoileus hemionus). The policy was implemented by the Utah Division of Wildlife Resources from 2001 to 2007 in Cache County of northern Utah, USA. In 2007, we surveyed Utah households representing metropolitan, nonmetropolitan, and Cache County residents (n = 1,800) to evaluate whether the winter-feeding policy reflected the attitudes of all wildlife stakeholders. Survey respondents, regardless of residence strata, believed winterfeeding programs were essential for managing mule deer in Utah (χ26 = 7.02, P = 0.32). However, most respondents were reluctant to support feeding programs at the expense of habitat restoration projects (χ26 = 11.64, P = 0.07). Our results suggest that the winter-feeding policy represented the attitudes of the Utah residents surveyed, though few had participated in its development. Respondents' strong utilitarian attitudes toward wildlife (e.g., strong support for hunting and feeding) influenced those respondents' perceptions of the policy. Given the effects of increased urbanization on utilitarian attitudes toward wildlife in many parts of the United States, coupled with decreasing numbers of traditional wildlife stakeholders, state wildlife agencies should continually reevaluate their public involvement processes to ensure new wildlife stakeholders' attitudes and concerns are represented.  相似文献   
16.
Abstract: African elephants (Loxodonta africana) are intensively managed in southern Africa and are routinely translocated between reserves. Domesticated elephants are used for elephant-back safaris and interactions with guests. Understanding how elephants respond to such activities is critical because of welfare issues associated with both humans and elephants. We investigated the stress response (i.e., fecal glucocorticoid metabolite secretion [FGM]) of working elephants in Letsatsing Game Reserve, South Africa, over 1 year to evaluate their response to transportation and ecotourism activities. We used free-ranging elephants in adjacent Pilanesburg National Park as controls. Fecal glucocorticoid metabolites were greatest prior to and during translocation and declined over the year. Within 1–2 months of transportation, FGM levels in working elephants became indistinguishable from those in wild elephants. Fecal glucocorticoid metabolite levels were higher during human interaction days than days without interaction. The highest observed FGM levels were associated with transportation and episodic loud noises. Transportation is a stressful activity for elephants, and ≥3 months should be provided to translocated elephants to acclimate to their new surroundings. Although stress levels of elephants increased slightly when interacting with humans in the contexts we studied, evaluating interactions under a wider range of contexts is necessary to minimize danger to elephants and humans.  相似文献   
17.
The ontogeny of the Weberian apparatus was examined in the zebrafish, Danio rerio , using both cleared and stained specimens and histology. Over 300 individuals from four independent zebrafish lineages, ranging in size from 3 to 28 mm TL, were examined for this study. Results provide a basic understanding of the development of the Weberian apparatus in the wild-type zebrafish. Our results, in conjunction with those already published, point to substantial variation in the development of the Weberian apparatus among otophysans and new interpretations of the homology of certain ossicles (e.g. tripus and claustrum). Hypotheses of homology among various Weberian ossicles are considered and represent an important step in understanding the evolution of sound transmission in ostariophysan fishes.  © 2004 The Linnean Society of London, Zoological Journal of the Linnean Society , 2004, 140 , 241–254.  相似文献   
18.
By using-with the permission of the subject-a simple test entailing injection of Nalline(R) (N-allylnormorphine) and noting the reaction in the pupils of the eyes of the subject, it is possible to determine whether a patient is addicted to a narcotic, is an occasional user or is a nonuser.  相似文献   
19.
20.
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