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   38329篇
  免费   3492篇
  国内免费   19篇
  2023年   212篇
  2022年   154篇
  2021年   1028篇
  2020年   610篇
  2019年   772篇
  2018年   917篇
  2017年   770篇
  2016年   1194篇
  2015年   1985篇
  2014年   2210篇
  2013年   2345篇
  2012年   3315篇
  2011年   3022篇
  2010年   1940篇
  2009年   1666篇
  2008年   2397篇
  2007年   2349篇
  2006年   2124篇
  2005年   2006篇
  2004年   1843篇
  2003年   1736篇
  2002年   1614篇
  2001年   339篇
  2000年   237篇
  1999年   341篇
  1998年   413篇
  1997年   243篇
  1996年   209篇
  1995年   209篇
  1994年   214篇
  1993年   219篇
  1992年   215篇
  1991年   203篇
  1990年   189篇
  1989年   165篇
  1988年   173篇
  1987年   161篇
  1986年   128篇
  1985年   128篇
  1984年   164篇
  1983年   122篇
  1982年   153篇
  1981年   145篇
  1980年   132篇
  1979年   103篇
  1978年   107篇
  1977年   98篇
  1976年   82篇
  1974年   71篇
  1973年   82篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
41.
Over-exploitation of wildlife is a significant threat to global biodiversity, but addressing the sustainability of harvests can be difficult when trade is conducted illegally. The wildlife trade is driven chiefly by consumer demand, largely in developed nations (but increasingly in Asia), and more species are traded to meet international demand for pets than for any other purpose. We surveyed traders of amphibians and reptiles in the Indonesian provinces of Maluku, West Papua and Papua between September 2010 and April 2011. We recorded 5,370 individuals representing 52 species collected solely for the pet trade. At least 44?% were either fully protected or had not been allocated a harvest quota, making their harvest and trade illegal. Approximately half were listed within the Convention on International Trade in Endangered Species of Wild Fauna and Flora. Trade operates via a complex chain, with hunters receiving little income compared to middlemen and exporters. Examination of Indonesian harvest quotas for amphibians and reptiles suggests limited knowledge of species distributions, with quotas often set for species in provinces where they do not occur. Illegal trade is due, partly, to an inadequate understanding of the species being traded and is facilitated by poor monitoring and enforcement at key trade hubs. As a first step to combatting illegal trade, and to better understand the effects of harvest on wild populations, we recommend the need for increased monitoring and enforcement, improving the knowledge base of species traded and educating consumers about the effects their demand for pets has on these species.  相似文献   
42.
The economic viability of the wildlife based enterprises (bee-keeping and caterpillar utilization) in Malawi is discussed in relation to conventional agricultural enterprises (maize, beans and ground-nuts). A strong incentive emerges for rural people to adopt wildlife management as an adjunct to subsistence agriculture, and therefore, to promote conservation of natural ecosystems and wildlife habitats in the face of growing human population and demand for land. Dependence on agriculture has depleted the wildlife resource outside protected areas and has been less effective in improving the wealth and living standards of most rural people. This study illustrates that the Malawi Department of National Parks and Wildlife needs to introduce economic incentives that integrate biological conservation with economic development for the rural people. The management programme involves the adoption of a rotation burning policy that promotes vegetation coppicing, eases harvesting and promotes high caterpillar yields.  相似文献   
43.
Vocalizations of South African clawed frogs are produced by contractions of laryngeal muscles innervated by motor neurons of the caudal medulla (within cranial nerve nucleus IX-X). We have traced afferents to laryngeal motor neurons in male and female frogs using retrograde axonal transport of horseradish peroxidase conjugated to wheat germ agglutinin (HRP-WGA). After iontophoretic injection of HRP-WGA into n. IX-X, retrogradely labelled neurons were seen in the contralateral n. IX-X, in rhombencephalic reticular nuclei, and in the pre-trigeminal nucleus of the dorsal tegmental area (DTAM) of both males and females.  相似文献   
44.
45.
Ca2+ is an important intracellular messenger affecting many diverse processes. In eukaryotic cells, Ca2+ storage is achieved within specific intracellular organelles, especially the endoplasmic/sarcoplasmic reticulum, in which Ca2+ is buffered by specific proteins known as Ca2+ buffers. Ca2+ buffers are a diverse group of proteins, varying in their affinities and capacities for Ca2+, but they typically also carry out other functions within the cell. The wide range of organelles containing Ca2+ and the evidence supporting cross-talk between these organelles suggest the existence of a dynamic network of organellar Ca2+ signaling, mediated by a variety of organellar Ca2+ buffers.  相似文献   
46.
47.
48.
49.
50.
1. Host plant phenotypic traits affect the structure of the associated consumer community and mediate species interactions. Intraspecific variation in host traits is well documented, although a functional understanding of variable traits that drive herbivore community response is lacking. We address this gap by modelling the trait-environment relationship using insect traits and host plant traits in a multilevel model. 2. We compare herbivore assemblages from the canopy of the phenotypically variable tree Metrosideros polymorpha on Hawai‘i Island. Multiple distinct varieties of M. polymorpha frequently co-occur, with variation in morphological traits. Using this system, we identify host and insect traits that underlie patterns of herbivore abundance and quantify the strength of host-insect trait interactions. 3. This work examines plant-insect interactions at a community scale, across 36 herbivore species in three orders. We find that co-occurring trees of varying phenotype support distinct communities. Leaf traits, including specific leaf area, trichome presence, and leaf nutrients, explain 46% of variation in insect communities. We find that feeding guild and nymphal life history are correlated with host plant traits, and we show that model predictions are improved by including the host and insect trait interaction. 4. This study demonstrates how insect herbivores traits influence community response to morphologically variable hosts. Environmental heterogeneity indirectly affected herbivore community structure via intraspecific variation in host plants, providing an important source of variation for maintaining diversity in the broader community.  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