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   32209篇
  免费   308篇
  国内免费   289篇
测绘学   811篇
大气科学   2883篇
地球物理   6645篇
地质学   11102篇
海洋学   2487篇
天文学   6890篇
综合类   82篇
自然地理   1906篇
  2020年   229篇
  2019年   242篇
  2018年   518篇
  2017年   516篇
  2016年   739篇
  2015年   481篇
  2014年   723篇
  2013年   1453篇
  2012年   780篇
  2011年   1080篇
  2010年   923篇
  2009年   1296篇
  2008年   1114篇
  2007年   975篇
  2006年   1077篇
  2005年   912篇
  2004年   878篇
  2003年   887篇
  2002年   892篇
  2001年   760篇
  2000年   797篇
  1999年   676篇
  1998年   648篇
  1997年   676篇
  1996年   583篇
  1995年   544篇
  1994年   488篇
  1993年   436篇
  1992年   420篇
  1991年   419篇
  1990年   426篇
  1989年   399篇
  1988年   382篇
  1987年   474篇
  1986年   436篇
  1985年   469篇
  1984年   565篇
  1983年   562篇
  1982年   505篇
  1981年   491篇
  1980年   449篇
  1979年   433篇
  1978年   448篇
  1977年   394篇
  1976年   355篇
  1975年   355篇
  1974年   407篇
  1973年   390篇
  1972年   245篇
  1971年   225篇
排序方式: 共有10000条查询结果,搜索用时 27 毫秒
941.
Abstract

Increasing wolf populations are a concern for wildlife managers in the Midwestern U.S. Understanding the psychological mechanisms that contribute to public perceptions of risk will enable development of strategies that seek to mitigate these risks, and suggest where outreach efforts may facilitate acceptance of wolves. We examined the psychological factors that influence Illinois residents’ perceived risks from wolves. We hypothesized that individuals’ perceived risks from wolves were a function of their attitudes toward wolves, negative affect toward wolves, and basic beliefs about wildlife. Data were obtained from a survey of the Illinois public (n?=?784). Negative affect and attitudes toward wolves were direct predictors of perceived risks. Basic beliefs predicted attitudes and negative affect toward wolves. Negative affect predicted attitudes. Basic beliefs had direct and indirect effects on perceived risks.  相似文献   
942.
943.
944.
945.
"Using urban places of 15,000 or more inhabitants as its point of reference, this paper identifies and investigates the most rapidly growing towns and cities of the USSR during the recent intercensal periods of 1970-79 and 1979-89. Rapidly growing towns are defined as towns that grew by at least 50 percent overall for the intercensal period and at a rate of equal to or exceeding 4.1 percent annually. In addition, a category of 'doubling towns' is investigated, defined as towns that increased in population by at least 100 percent (or 6.3 percent or more annually). Special attention is devoted to the geographical aspects, economic functions, and size characteristics of the towns involved. Comparisons with 1959-70, an overall examination from 1959-89, and an update for contemporary Russia during 1989-93 also are undertaken."  相似文献   
946.
Paleolimnological techniques were used to assess human impacts onLake Blanca, a small (0.6 km2), coastal fresh waterbodyin southern Uruguay, which is the drinking water source for 100,000 localresidents. We retrieved a core that extends to about 1100 14Cyr BP. 210Pb ages, organic mater, CO3, totalcarbon, nutrients, fossil pigments and diatoms allowed us to establishlimnological conditions before and after cultural impacts. Soil removal(1880–1960) and intensive cattle and sheep grazing (1943–1966) ledto gully formation in the catchment. This watershed erosion resulted inincreased sedimentation rates. The aquatic system appeared to be mesotrophicwith dominance of epibenthic diatoms until 1966, at which timeeutrophication intensified with forestry activities. Increases in nutrients, aswell as blooms of planktonic diatoms, were observed. During the last decade,tourist/urban development as well as high drinking water demand caused areduction in lake area. Subsequent marked increases in rainfall led to furtherphytoplankton blooms and macrophyte proliferation.  相似文献   
947.
Frozen ground phenomena in the Northern Foothills, Northern Victoria Land, Antarctica, include large–scale polygons, 15–20 m in diameter, and small frost mounds, 1–5 m high. The polygons are most widespread on terrain formed upon Younger Drift and are usually surrounded by interpolygon furrows or troughs, 10–30 cm deep and 10–100 cm wide. The troughs contain shallow wedges of sandy gravel (sand wedges) near the surface but excavations into underlying permafrost indicate that small ice wedges or ice veins are locally present. Field and anecdotal evidence suggest that thermal contraction cracking is active under today's climate. Frost mounds occur in association with a number of perennially frozen lakes in the region. In most cases they appear related to frost and icing blister activity caused by the episodic injection of free water from below. The debris–covered nature of the centre of Enigma Lake is best explained in terms of basal ice accretion beneath the lake–ice cover.  相似文献   
948.
949.
950.
A morphometric analysis of the gills ofOreochromis alcalicus grahami has been carried out on specimens from ecologically distinct lagoons and a water-holding tank of Lake Magadi, a highly alkaline salt lake situated in a volcanically active region of the southern part of the Great Rift Valley in Kenya. The data were compared with those fromOreochromis niloticus, a close relative that lives in fresh water and with data from other fresh water and marine fish. Our primary goal was to identify the possible adaptive features which enable the fish to survive in an environment characterized by severely fluctuating levels of oxygen, a condition exacerbated by factors such as high temperature, alkalinity and osmolarity. The specimens ofO. a. grahami from the south-western lagoons of the lake had gills better adapted for gas exchange with a body mass specific diffusing capacity for oxygen which was about 2 times greater than that of the gills of the specimens from the fish spring lagoons and 2.5 times that of those from the water-holding tanks. Some parameters of the gills ofO. a. grahami, e.g. the gill filament length and number of gill filaments are significantly greater than those ofO. niloticus but the number of secondary lamellae, area of secondary lamellae and the diffusing capacity of the gills are similar in the two species. Compared with most other fish, the gills ofO. a. grahami appear to be particularly well adapted for gas exchange especially by having a thin water-blood barrier. Perhaps in no other extant fish have the gills had to be so exquisitely designed to meet environmental extremes and regulate complex and at times conflicting functions such as gas exchange, iono-regulation, acid-base balance and nitrogenous waste excretion as inO. a. grahami  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