全文获取类型
收费全文 | 3817篇 |
免费 | 653篇 |
国内免费 | 514篇 |
出版年
2024年 | 13篇 |
2023年 | 198篇 |
2022年 | 152篇 |
2021年 | 227篇 |
2020年 | 274篇 |
2019年 | 301篇 |
2018年 | 258篇 |
2017年 | 247篇 |
2016年 | 243篇 |
2015年 | 200篇 |
2014年 | 220篇 |
2013年 | 284篇 |
2012年 | 213篇 |
2011年 | 187篇 |
2010年 | 166篇 |
2009年 | 190篇 |
2008年 | 181篇 |
2007年 | 194篇 |
2006年 | 175篇 |
2005年 | 114篇 |
2004年 | 110篇 |
2003年 | 110篇 |
2002年 | 130篇 |
2001年 | 131篇 |
2000年 | 92篇 |
1999年 | 92篇 |
1998年 | 86篇 |
1997年 | 61篇 |
1996年 | 53篇 |
1995年 | 34篇 |
1994年 | 28篇 |
1993年 | 8篇 |
1992年 | 7篇 |
1982年 | 2篇 |
1958年 | 3篇 |
排序方式: 共有4984条查询结果,搜索用时 31 毫秒
991.
Population dynamics and functional roles of Enchytraeidae (Oligochaeta) in hardwood forest and agricultural ecosystems 总被引:1,自引:0,他引:1
Population dynamics of enchytraeids are described for 2 montane forested watersheds in southwestern North Carolina (Coweeta) and an agricultural site under conventional (CT) and no-tillage (NT) management in the northeastern Georgia piedmont (Horseshoe Bend, HSB). Given that much of the taxonomy, ecology and community structure of enchytraeids is poorly known, our objective was to identify key indicators of enchytraeid community structure which could be used, in this case, to better understand their role in soil structure formation. Although population densities of enchytraeids were higher in the forested (Coweeta) than in the arable soils (HSB), the average ash free dry weight per enchytraeid at HSB was nearly double that found at Coweeta. Based on these measurements and an estimate of their gut transit time, we calculated that the enchytraeids at HSB transported 2180 g of soil per m2 per year compared to 443 and 393 g m-2 yr-1 for watershed 18 and 27, respectively at Coweeta. We therefore hypothesize that enchytraeids have a larger influence on soil structure in agricultural fields than in forested areas, in spite of lower population densities. The ash free dry weight and ash wt. per enchytraeid may qualify as key indicator parameters of enchytraeid community structure which helps to understand their functional role in ecosystems, though more studies are called for. 相似文献
992.
Nikolai A. Sobolev Evgeny A. Shvarts Mikhail L. Kreindlin Vadim O. Mokievsky Victor A. Zubakin 《Biodiversity and Conservation》1995,4(9):964-983
Natural habitat preservation, i.e. the creation and management of Protected Natural Areas (PNAs), is one of the most important forms of biodiversity conservation. The most widespread types of PNAs in Russia are Zakazniks (State Nature Refuges) and Natural Monuments, but unlike Zapovedniks (State Nature Reserves) these types of Russian PNAs are little-known to foreign ecologists. Thus the main attention of this article is given to the problems of Zakazniks and Natural Monuments while other types of Russian PNAs are mentioned briefly. In many regions of Russia, Zakazniks and Natural Monuments are considered to be the core components for the regional protection of biodiversity. Non-Governmental Organizations play an important role in the creation and management of PNAs. The recent sudden change of circumstances in Russia have given rise to many problems which threaten the existence of Zakazniks and Natural Monuments. Possible means of saving these PNAs include: (i) promoting the interest of local people in protecting biodiversity; and (ii) supporting local authorities, and public initiatives and regional programmes in the creation of local PNA networks. 相似文献
993.
Paul J. A. Timmermans Johan D. Vochteloo Jo M. H. Vossen 《Primates; journal of primatology》1997,38(2):139-148
In search of the relevant difference between mother rearing and surrogate rearing with respect to the emergence of neophobic
behavior in macaques, we tested the hypothesis that mobility of surrogate mothers prevents the development of neophobic behavior
in cynomolgus macaques. The experimental group consisted of ten surrogate-reared subjects which during their first year got
rides on surrogate mothers moving through the cage. The control group consisted of nine subjects reared with immobile surrogate
mothers. Between the age of 7 and 27 months the reactions to two different novel objects were tested. First exposure to one
object occurred at 6 months of age and to the other object at 20 months. The kind of object and the age of first exposure
made no difference. Contrary to our expectations mobility of surrogates did not affect the incidence of neophobia: most subjects
in both groups persistently avoided the objects. This finding seems to support Hofer's hypothesis that the mother acts as
a regulator of the infant's behavior and physiology. 相似文献
994.
995.
建立生物多样性保护信息系统的意义和途径 总被引:6,自引:0,他引:6
王献溥 《植物资源与环境学报》1996,(1)
生物多样性保护信息系统可提供必要的数据和信息,帮助决策者对生物多样性的保护与持续利用作出科学的规划,因而受到国内外的普遍关注。本文就该信息系统的内容、功能以及建立途径等方面作一简略介绍。 相似文献
996.
In the present paper 29 species ofCaulerpa from Indonesia, the Philippines and Papua New Guinea are listed, with reference to their distribution and occurrence of ecological phenotypes (ecads). Other names ofCaulerpa taxa recorded for Malesia in literature are dealt with in the paragraph on excluded and changed names. The variability and nomenclature of mainly the species recorded from Indonesia are discussed, especially regarding the implications of the recognition of ecads in several species. The ecads in the sectionSedoideae are discussed as an example of use of the designation ecad for a number of growth forms. Biogeography of the genusCaulerpa is discussed with focus on areas of high biodiversity, both inside and outside Malesia, followed by a discussion on biodiversity assessment. It is suggested that the genusCaulerpa is a good indicator for the quantitative assessment of biodiversity. The need for a modern world-monograph of this genus is stressed. Such a monograph can be a stimulus for ecosystem studies ofCaulerpa stands. 相似文献
997.
The Cape Peninsula, South Africa: physiographical, biological and historical background to an extraordinary hot-spot of biodiversity 总被引:1,自引:0,他引:1
The Cape Peninsula, a 470 km2 area of rugged scenery and varied climate, is located at the southwestern tip of the Cape Floristic Region, South Africa. The Peninsula is home to 2285 plant species and is a globally important hot-spot of biodiversity for higher plants and invertebrates. This paper provides a broad overview of the physiography, biological attributes and history of human occupation of the Peninsula. The Peninsula is characterized physiographically by extremely high topographical heterogeneity, very long and steep gradients in annual rainfall, and a great diversity of nutrient-poor soils. Thus, the Peninsula supports a high number of habitats and ecological communities. The predominant vegetation is fynbos, a fire-prone shrubland, and 12 broadly characterized fynbos types have been described on the Peninsula. Animal community structure, especially with regard to invertebrates, is poorly known. Vertebrate community structure is probably strongly influenced by nutrient poverty and recurrent fire. Generally, most vertebrates are small and typically occur in low numbers. Some invertebrates play keystone roles in facilitating ecological processes. Human occupation of the Peninsula was limited, until relatively recently, by nutrient poverty. After Dutch colonization in 1652, direct and indirect impacts on the natural ecosystems of the Peninsula escalated dramatically, and by 1994, some 65% of original natural habitat was either transformed by urbanization and agriculture, or invaded by alien plants. Nonetheless, there is still excellent potential to conserve the Cape Peninsula's remaining biodiversity. 相似文献
998.
Claude Combes 《Biodiversity and Conservation》1996,5(8):953-962
The influence of parasites in ecosystems, especially on biodiversity, is discussed. Various examples illustrate the role that parasites play in the outcome of interspecific competition, in the success of invading species, and in the separation of emerging species. Parasites can be stabilizers or destabilizers, depending on factors such as susceptibility of hosts and size of the ecosystem. Parasites play a major role each time something disturbs living beings at the populational and/or specific level, as they do at the individual level. 相似文献
999.
Angelika Brandt 《Biodiversity and Conservation》1997,6(11):1533-1556
During three expeditions with the RVs Meteor and Polarstern more than sixty thousand peracarids were sampled from the deep Arctic Ocean (northern North Atlantic) by means of an epibenthic sledge. Sampling areas were the Kolbeinsey Ridge north of Iceland (800–1100 m), the Northeast Water Polynya, off Greenland (45–517m), and 75°N east of Greenland (197–2681m). Until now 288 species of Peracarida have been identified to species level. These 288 species comprise 152 genera and 59 families of Amphipoda, Cumacea, Isopoda, Mysidacea and Tanaidacea. Thirty-eight genera were very frequent and were sampled at each expedition (these were 22 species of Isopoda, seven species of Cumacea, three species of Amphipoda and Mysidacea, each, and two species of Tanaidacea). Sixty genera are eurybathic, occurring at least over a depth range of 1000m, some even from the shelf up to 2681m depth. Only 10 genera are stenobathic, occurring only in the deep sea. No significant decrease in species number with depth or latitude could be observed. The influencing factors probably causing different community structures are diverse, ranging from food availability over substrate or hydrographical qualities to interspecific competition. 相似文献
1000.
Jan Decher 《Biodiversity and Conservation》1997,6(7):1007-1026
This article considers the conservation of relict natural habitat in West Africa, especially habitat preserved in traditional
sacred groves. Government-sanctioned conservation is contrasted with local grassroots efforts of conservation. Evidence for
the ecological value of sacred groves is based on results of a field study of small mammal communities conducted on the Accra
Plains of Ghana and on published sources on the conservation and use of sacred groves from various countries. The study employed
standard mark-and-recapture techniques for the sampling for terrestrial small mammals, and mist netting for the sampling of
bat communities. Pragmatic approaches combining conservation and sustainable use are considered, as are traditional values
that have preserved the sacred groves in the past for up to several hundred years in some cases. In part because these groves
shelter unique small mammal and plant communities, traditional values and protection mechanisms should be integrated into
the newly emerging cultural and religious contexts. The issues encountered during this study reveal that effective conservation
involving local peoples requires a concerted interdisciplinary effort.
This revised version was published online in November 2006 with corrections to the Cover Date. 相似文献