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   4543篇
  免费   511篇
  国内免费   512篇
  2024年   16篇
  2023年   81篇
  2022年   91篇
  2021年   120篇
  2020年   148篇
  2019年   211篇
  2018年   184篇
  2017年   177篇
  2016年   192篇
  2015年   163篇
  2014年   177篇
  2013年   236篇
  2012年   149篇
  2011年   175篇
  2010年   153篇
  2009年   230篇
  2008年   256篇
  2007年   293篇
  2006年   252篇
  2005年   222篇
  2004年   201篇
  2003年   153篇
  2002年   171篇
  2001年   120篇
  2000年   102篇
  1999年   116篇
  1998年   118篇
  1997年   106篇
  1996年   87篇
  1995年   82篇
  1994年   88篇
  1993年   91篇
  1992年   59篇
  1991年   85篇
  1990年   74篇
  1989年   45篇
  1988年   62篇
  1987年   41篇
  1986年   34篇
  1985年   26篇
  1984年   42篇
  1983年   19篇
  1982年   30篇
  1981年   25篇
  1980年   8篇
  1979年   9篇
  1978年   10篇
  1977年   12篇
  1976年   6篇
  1973年   8篇
排序方式: 共有5566条查询结果,搜索用时 203 毫秒
121.
Abstract. A vegetation map at scale 1:5 million is presented. * * Attached on the inside of this issue's back cover.
It covers Bangladesh, Burma (Myanmar), India, Cambodia, Laos, Thailand, Vietnam and Sri Lanka and fills a conspicuous gap in the cartography of tropical vegetation, following the publication of vegetation maps of South America, Africa and Malaysia. For conformity, it is presented as one sheet at a scale of 1:5 million. It uses the basic map of the American Geographical Society (1942; bipolar oblique conformal projection) which forms the base for FAO's Soil map of the world. Basic information was obtained from many published maps, unpublished observations and satellite data. The limits of the main vegetation types have been updated with a complete set of Landsat MSS images (369 scenes) with a mosaic of Landsat TM data for 1991 and with recent forest maps from Asia. Nine main vegetation units, which are groups of forest formations, have been identified and mapped, including woodlands, thickets and wooded savannas. Agricultural land has been shown in a uniform pale green colour in order to clearly express the extent of human impacts on woody vegetation. In spite of the necessary oversimplification of the ground data, this map is probably the most explicit expression of the remaining forest stands and of the regression of natural vegetation in the region. It can be considered as a benchmark for future monitoring of tropical vegetation.  相似文献   
122.
Abstract. This study includes 24 transects on slopes of the Velebit, Biokovo and Orjen mountains, the tallest coastal mountains along the Adriatic Sea, with a very high species richness according to European standards and having the largest regional concentrations of endemics. Within each transect the presence or absence of phytosociological alliances, according to the Zürich-Montpellier School, was interpreted from local studies. The alliance composition of the transects was compared with methods used in numerical syntaxonomy. Three types of vegetation complexes (N. Dinaric inland, transitional and S. Dinaric coastal) were derived from a numerical classification of the transects; they were defined on the basis of their characteristic alliances. The maritime impact affecting the mountains through the river canyons reduces the contrast in vegetation between inland and coastal slopes. The existence of a climatic macrogradient is indicated by seriation of the basic alliance/transect data matrix leading to a diagonal structure, by a gradual shift of the alliance composition within the same vegetational type, and by the distribution of the transects and alliances along the first axis of a Correspondence Analysis ordination. Transect scores on the second ordination axis are negatively correlated with transect vegetational diversity (number of alliances per transect); this axis may reflect a decrease in humidity and temperature range. This interpretation is supported by the result of a comparison of the total mountain vegetational diversity (Mann-Whitney test), which showed that Orjen is the most diverse mountain ridge. The expected correlation between plant community diversity and altitude of the transects is blurred, probably due to the impact of the cold Bora wind and subsequent compression of vegetational zones.  相似文献   
123.
Wyttenbach  A.  Schleppi  P.  Tobler  L.  Bajo  S.  Bucher  J. 《Plant and Soil》1995,(1):305-312
The endogenous concentrations of the essential elements Ca, Cl, Cu, Fe, K, Mg, Mn, N, P and Zn, and of the nonessential elements Al, As, Ba, Br, Co, Cr, Cs, Hg, I, La, Na, Rb, Sb, Sc, Si and Sr were determined in 5 successive needle age classes. 40 mature spruce trees from 6 different sites were investigated individually. A given element usually shows smooth changes with the needle age class t. Trees on a given site usually have a similar dynamic behaviour. The same holds for the different site means. The concentrations can be approximated by functions c=f(t). Three different types of functions are required to describe the dynamic behaviour of 3 groups of elements that increase with t, and one for the elements that decrease with t. A given element usually can be described by the same type of function at all sites, even if its concentration differs widely. Exceptions are Mn, Co and Zn, which change from a decreasing function at low concentrations to an increasing function at high concentrations. Further irregulatities are found at some sites with Ca, Sr and Ba. These findings are corroborated by a multivariate statistical analysis.  相似文献   
124.
The age, growth and reproduction of Leuciscus pyrenaicus (Günther, 1868), an endemic cyprinid from the Iberian Peninsula, was studied from November 1987 to September 1989 in a small seasonal tributary of the Guadalquivir river basin. Maximum fork lengths observed were a 160 mm male with six scale annuli and a 171 mm female aged 7 +. Maximum ages observed were 7 + in males and 8 + in females. There were no significant differences in the annual growth increments between sexes. Seasonal growth period started in March and continued for 5 to 6 months. Mean lengths of 1 + specimens onwards diminished during summer and/or autumn. Males and females matured in their third and fourth year of life respectively. The overall sex ratio (272 males: 310 females) differed significantly from equality. Spawning began in May and ended in July. L. pyrenaicus is a multiple spawner that releases a minimum of two batches of eggs per female each year. Eggs in each batch were similar in both size (egg diameter) and number released. The relationship between fecundity (Fee) and fork length (mm) was represented by the formula: Fec=1.96 10−3 L2.50.  相似文献   
125.
Populations of bluegill sunfish Lepomis macrochirus , experiencing heavy juvenile predation, showed increased growth rates and increased age and size at maturity relative to populations experiencing decreased predation on juveniles but increased predation on adults. This study examined bluegills experimentally from both types of populations and a cross between them in a common environment to determine if variation in growth and age at maturity is genetically or environmentally induced. Two factorial experiments, varying strain of bluegills and resource availability, were used to evaluate differences in growth rate. One experiment, varying strain of bluegills, was used to assess differences in age at maturity. Growth was strongly influenced by resource level, but growth rate did not vary among populations. Nearly all bluegills in each population matured at 1 year of age in a common environment. Thus, variation observed in source populations must be mostly attributable to differences in the environment between populations. At least three factors could potentially cause differences in growth and age at maturity: (1) variation in resource availability; (2) variation in demographic structure; and (3) variation in size-specific mortality rates caused by differences in predator abundance between populations. Observed patterns of variation between populations are best explained by effects of differences in predator populations.  相似文献   
126.
The age, growth and reproduction ofRutilus lemmingii (Steindachner, 1866), an endemic cyprinid from the Iberian Peninsula, was studied for over a period of two years in a small seasonal tributary of the Guadalquivir river basin. Approximately 65 % of the total growth in length occured in the first year of life. Males reached a maximum age of 3+ yr (Fork Length, F.L. = 114 mm) and females 4+ yr (F.L. = 144 mm). Both sexes matured during their second year of life (1 +). The overall sex ratio (334 males to 389 females) differed significantly from unity. Somatic condition decreased markedly during the reproductive period of March to May.R. lemmingii is a multiple spawner and releases two batches of eggs per female each year. Mean egg diameter of the first batch was larger than the second one. The regression between fecundity and Fork Length (mm) was: Fec = 0.014 F.L.Z.2.858 Compared with available information, thisR. lemmingii population, located at a lower latitude, is characterized by fast growth, early maturity, high level of reproductive effort, and a short life-span. These life-history characteristices are typical of species in unstable environments, where adult mortality is high, variable or unpredictable.  相似文献   
127.
Recent work has dealt with the local management of aquatic resources as an alternative to Hardin's (1968) tragedy of the commons. In communities with no formal management of resources, informal ownership of fishing spots or conflicts with outside competitors may determine the basis for future local management. In this study, I analyze the use of aquatic resources by five fishing communities on the Atlantic Forest coast of southeast Brazil: Búzios Island, Puruba, and Picinguaba in São Paulo State, and Jaguanum and Itacuruçá Islands at Sepetiba Bay in Rio de Janeiro State. Informal ownership of fishing spots, used for set gillnet fishing, is regulated by kin ties at Búzios Island. The artisanal fishers of Sepetiba Bay, especially those from Jaguanum Island, have a conflict with Bay intruders, such as the shrimp and herring trawlers. Two coastal communities, Puruba and Picinguaba, have conflicts with fishing regulations from a State Park (Parque Estadual da Serra do Mar),created in 1977. The transformation of populated areas of the Atlantic Forest to Extractive Reserves might be a way to avoid conflicts with intruders and with governmental agencies, and to involve local populations in management. Kinship rules at Búzios Island and the territorial behavior of fishers at Sepetiba Bay may form a basis for local organization.  相似文献   
128.
There is a need for a consistent forest restoration strategy for the `Black Triangle', Central Europe. In the past 50 years, forests in this area have been heavily affected by industrial pollution. Recently, the amount of pollutants has decreased. This means that effective forest restoration programmes can be started. Forest decline must be seen as an ecological disturbance which cannot be solved by applying technical measures only. An ecosystem approach to forest restoration must be introduced into restoration policy and management. Basic principles and working methods of such an approach are briefly described and proposals are made for forest restoration policy, management and research in the area.  相似文献   
129.
Understanding of factors influencing settlement(attachment and metamorphosis) of marine invertebratelarvae is of great importance in aquaculture andcontrol of biofouling. The influence of two factors onsettlement of larvae was assessed from two separateinvestigations: 1, the influence of age (endogenousfactor) on cyprids of the barnacle Balanusamphitrite; and 2, the influence of a microbial film(exogenous factor) on pediveligers of the mussel Mytilus galloprovincialis.The settlement response of cypris larvae of B.amphitrite was found to be age-dependent. Oldercyprids responded more readily to settlement factorsthan newly molted ones. In M.galloprovincialis, competent pediveligers settled inresponse to a microbial filmed surface but not toan unfilmed surface. Moreover, a factor with MW of lessthan 5000 dalton, derived from culture medium of abacterial strain C1.1 (Pseudomonas-Alteromonasgroup), induced the settlement of M. galloprovincialis larvae.Thus, marine invertebrate larvae may require a periodof competence acquisition, during which they arepoorly responsive to settlement inducers. Uponacquisition of competence, larvae readily respondto external cues (e.g. microbial film, bacterialextracellular products).  相似文献   
130.
This paper provides an analysis of infant body weights obtained from a sample of 38 rhesus monkey infants (Macaco mulatta) aged 29–165 days, i.e., animals still nutritionally dependent on their mothers. We examine the data on neonatal weights in relation to a number of factors, most notably, the sex of the infants, and the age and adiposity of their mothers. The infant body weights represent cross-sectional rather than longitudinal data; because they were mostly free-ranging animals, the infants were weighed just once each. Nevertheless, the results of our analysis strongly suggest that early postnatal growth in free-ranging rhesus is dependent on both maternal fatness and age. They also suggest that, although male infants are generally heavier than like-aged female infants, they do not grow any faster during the early postnatal period. Here, we speculate that the associations between infant size and both maternal age and adiposity are the result of between-mother differences in lactational output. © 1995 Wiley-Liss, Inc.  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