全文获取类型
收费全文 | 8046篇 |
免费 | 960篇 |
国内免费 | 892篇 |
出版年
2024年 | 14篇 |
2023年 | 212篇 |
2022年 | 156篇 |
2021年 | 323篇 |
2020年 | 356篇 |
2019年 | 445篇 |
2018年 | 365篇 |
2017年 | 399篇 |
2016年 | 397篇 |
2015年 | 418篇 |
2014年 | 447篇 |
2013年 | 561篇 |
2012年 | 309篇 |
2011年 | 363篇 |
2010年 | 340篇 |
2009年 | 459篇 |
2008年 | 462篇 |
2007年 | 495篇 |
2006年 | 399篇 |
2005年 | 395篇 |
2004年 | 328篇 |
2003年 | 236篇 |
2002年 | 263篇 |
2001年 | 225篇 |
2000年 | 194篇 |
1999年 | 184篇 |
1998年 | 158篇 |
1997年 | 91篇 |
1996年 | 110篇 |
1995年 | 105篇 |
1994年 | 90篇 |
1993年 | 82篇 |
1992年 | 74篇 |
1991年 | 58篇 |
1990年 | 58篇 |
1989年 | 53篇 |
1988年 | 30篇 |
1987年 | 32篇 |
1986年 | 26篇 |
1985年 | 30篇 |
1984年 | 32篇 |
1983年 | 19篇 |
1982年 | 19篇 |
1981年 | 17篇 |
1980年 | 19篇 |
1979年 | 12篇 |
1978年 | 12篇 |
1977年 | 13篇 |
1976年 | 4篇 |
1975年 | 3篇 |
排序方式: 共有9898条查询结果,搜索用时 78 毫秒
81.
82.
83.
Early developmental and demographic events in many marsupials occur in the mother's pouch out of animal managers' sight. Because pouch examination is perceived as being stressful both to animal and handler, the pouch has proven to be a formidable physical and psychological barrier to the study of early life history events in marsupials in zoos. Demographic and developmental data such as litter size at birth, secondary sex ratios, infant mortality rates, and early rates of growth and development, normally considered essential information for the development of breeding programs, are therefore essentially unavailable for most zoo populations of marsupials. Here we describe a technique for the safe and non-stressful examination of the pouches of small (< 2 kg) marsupials that enables the capture of such data. The technique, involving the use of transparent plastic tubes of slightly different diameters for restraining the animals and an otoscope for examining the pouch and its contents, has been very successfully applied to life history studies of four species at the National Zoological Park: Pseudocheirus peregrinus, Petaurus breviceps, Philander opossum, and Chironectes minimus. 相似文献
84.
85.
Mitchell's water monitors (Varanus mitchelli) have been maintained on display at Perth Zoo since 1997. They are generally a timid species but have been maintained and bred in a mixed species water feature exhibit. In this article we describe their captive management and behavior with an insight into their reproductive biology. Between 2002 and 2005, 11 clutches were laid ranging from 13 to 27 (X? = 20) eggs from one female. Egg size ranged between 3.00 and 6.08 g (X? = 4.77 g) in weight, 22.8 and 31.9 mm (X? = 28.3 mm) in length, and 11.1 and 19.3 mm (X? = 17.1 mm) in width. Oviposition included double and triple clutches ranging between 41 and 60 days apart (X? = 48 days), events n = 6. Four clutches were incubated at three different temperatures and hatchlings emerged after 157–289 days. The weight of the hatchlings ranged between 2.60 and 4.52 g (X? = 4.34 g). Total length ranged between 140.1 and 178.0 mm (X? = 165.9 mm) and snout–vent length ranged from 53.8 to 70.0 (X? = 64.4 mm). Juvenile growth and development information is presented from hatching through to approximately 3 years of age. Zoo Biol 29:615–625, 2010. © 2009 Wiley‐Liss, Inc. 相似文献
86.
Keith W.T.Goulding 《中国科学:生命科学英文版》2005,48(Z2)
The nitrogen (N) cycle is essentially 'leaky'. The losses of small amounts of nitrate to waters and of ammonia and nitrous oxide to the atmosphere are a part of the global biogeo-chemical N cycle. However, intensive agricultural production, industry and vehicle use have more than doubled the amount of 'reactive' N in the environment, resulting in eutrophication, ecosystem change and health concerns. Research has identified agricultural practices that cause large losses of N and, in some cases, developed solutions. This paper discusses the problems of maintaining productivity while reducing N losses, compares conventional with low input (integrated) and organic farming systems, and discusses wider options. It also looks at the need to integrate studies on N with other environmental impacts, set in the context of the whole farm system, to provide truly sustainable agricultural systems. 相似文献
87.
88.
Demographic properties of an outlier population of Orchis militaris L. (Orchidaceae) in England 总被引:3,自引:0,他引:3
M. J. HUTCHINGS A. MENDOZA W. HAVERS 《Botanical journal of the Linnean Society. Linnean Society of London》1998,126(1-2):95-107
Orchis militaris L. underwent a catastrophic decline in range and plant numbers in the British Isles around the end of the nineteenth century, and was long thought to be nationally extinct. A small number of colonies have been discovered since then, including one in Buckinghamshire, which has been recorded annually almost every year since its discovery in 1947. This paper presents an analysis of the demography and behaviour of this population using census data which have been collected annually from 1977 to 1995. Yew trees which shaded the site occupied by the orchids were removed in 1984 and 1989, and the growth of the herb layer has been strongly constrained by active management since 1989. Since the start of this period of intensive management, the annual gains and losses of plants in the population have become more pronounced, but in most years there has been a net gain in the number of plants. The number and proportion of the emergent plants which flower each year has increased considerably since 1986, and the age structure, which was dominated by older plants prior to 1984, has since become dominated by younger plants, reflecting the increased rate of recruitment and lack of a corresponding increase in mortality. Analyses are presented to show changes in behaviour in consecutive years between the pre-1984 and post-1986 management eras. 相似文献
89.
This article outlines the subsector of the information and communication technology (ICT) industry concerned with reducing the economy's environmental impact, dubbed ICT‐enabled low carbon technologies (ICTeLCTs). The article is based on a study funded by United Kingdom (UK) Trade and Investment, a division of the UK Department for Business, Innovation and Skills. ICTeLCTs can be segmented into specialist and generalist operators. Specialists focus on one or two ICT applications to monitor or reduce environmental issues, while generalists supply products and services enabling a firm or a private household to reduce the environmental impact of its activities. The subsector can be further segmented into green ICT, energy management, building management, carbon accounting, waste management, intelligent transport systems (ITSs), and water management. The main factors driving ICTeLCTs include legislation, voluntary environmental standards, corporate social responsibility (CSR) activities, customer demand, and competitive market factors. Policy makers should continue to drive the growth of ICTeLCTs with the introduction and refinement of environmental legislation regulating energy use and markets. 相似文献
90.