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   1103篇
  免费   113篇
  国内免费   127篇
  2024年   1篇
  2023年   27篇
  2022年   22篇
  2021年   30篇
  2020年   48篇
  2019年   69篇
  2018年   39篇
  2017年   42篇
  2016年   52篇
  2015年   50篇
  2014年   60篇
  2013年   77篇
  2012年   61篇
  2011年   64篇
  2010年   47篇
  2009年   92篇
  2008年   71篇
  2007年   66篇
  2006年   60篇
  2005年   46篇
  2004年   26篇
  2003年   42篇
  2002年   30篇
  2001年   27篇
  2000年   25篇
  1999年   19篇
  1998年   8篇
  1997年   11篇
  1996年   11篇
  1995年   16篇
  1994年   9篇
  1993年   6篇
  1992年   13篇
  1991年   10篇
  1990年   8篇
  1989年   8篇
  1988年   3篇
  1987年   7篇
  1986年   6篇
  1985年   6篇
  1984年   2篇
  1983年   2篇
  1982年   9篇
  1981年   3篇
  1980年   4篇
  1979年   1篇
  1978年   2篇
  1976年   2篇
  1975年   2篇
  1970年   1篇
排序方式: 共有1343条查询结果,搜索用时 31 毫秒
1.
Lu Chen  Shuqiang Li  Zhe Zhao 《ZooKeys》2015,(541):41-56
One new genus of the spider subfamily Coelotinae, Flexicoelotes gen. n., with five new species is described from southern China: Flexicoelotes huyunensis sp. n. (female), Flexicoelotes jiaohanyanensis sp. n. (male and female), Flexicoelotes jinlongyanensis sp. n. (male and female), Flexicoelotes pingzhaiensis sp. n. (female), Flexicoelotes xingwangensis sp. n. (male and female).  相似文献   
2.
M. Ohsawa 《Plant Ecology》1995,121(1-2):3-10
A new template for mountain vegetation zonation along latitudinal gradients is proposed for examining geographical pattern of various forest attributes in humid monsoon Asia. The contrasting temperature regime in tropical and temperate mountains, i.e., the former is a non-seasonal, temperature-sum controlled environment, and the latter is a seasonal, low temperature limiting environment, leads to different altitudinal patterns of tree height distribution and species richness. In the tropical mountains, both tree height and species richness decrease steeply, and the tree height often stepwise. The decline of tree height and species diversity in the temperate mountains is far less pronounced except near the forest limit. Both trends are explained by their temperature regime.  相似文献   
3.
4.
Aim We examine the genetic diversity within the lizard genus Gekko in the Philippine islands to understand the role of geography and geological history in shaping species diversity in this group. We test multiple biogeographical hypotheses of species relationships, including the recently proposed Palawan Ark Hypothesis. Location Southeast Asia and the Philippines. Methods Samples of all island endemic and widespread Philippine Gekko species were collected and sequenced for one mitochondrial gene (NADH dehydrogenase subunit 2) and one nuclear gene (phosducin). We used maximum likelihood and Bayesian phylogenetic methods to derive the phylogeny. Divergence time analyses were used to estimate the time tree of Philippine Gekko in order to test biogeographical predictions of species relationships. The phylogenetic trees from the posterior distribution of the Bayesian analyses were used for testing biogeographical hypotheses. Haplotype networks were created for the widespread species Gekko mindorensis to explore genetic variation within recently divergent clades. Results Both maximum likelihood and Bayesian phylogenetic analyses indicated that Philippine Gekko species are a diverse clade with a long history in the archipelago. Ancestral range reconstruction and divergence time analyses suggest a Palawan microcontinental origin for this clade, coinciding with Palawan’s separation from Asia beginning 30 Ma, with subsequent diversification in the oceanic Philippine islands. The widespread species G. mindorensis and G. monarchus diversified in the late Miocene/early Pliocene and are potentially complexes of numerous undescribed species. Main conclusions The view of the Philippine islands as a ‘fringing archipelago’ does not explain the pattern of species diversity in the genus Gekko. Philippine Gekko species have diversified within the archipelago over millions of years of isolation, forming a large diverse group of endemic species. Furthermore, the Philippine radiation of gekkonid lizards demonstrates biogeographical patterns most consistent with stochastic colonization followed by in situ diversification. Our results reveal the need to consider deeper time geological processes and their potential role in the evolution of some Philippine terrestrial organisms.  相似文献   
5.
East Asian species of the genera Hybrizon and Ghilaromma are reviewed. Four species of Hybrizon, H. buccatus (Brébisson 1825), H. ghilarovi Tobias, 1988, H. juncoi (Ceballos 1957) and H. flavofacialis Tobias, 1988 and two species of Ghilaromma, G. orientalis Tobias, 1988 and G. ussuriensis Tobias, 1988, were recognized. H. ghilarovi was recorded from Korea, Japan and China, while H. juncoi was recorded from Korea, for the first time. The specimens recorded from Japan as G. fuliginosi (Wilkinson, 1930) by Watanabe (1984) are referred to G. orientalis herein. This species is newly recorded from Korea and Japan. Keys to East Asian species of Hybrizon and the world species of Ghilaromma are also provided.  相似文献   
6.
Lowland rainforests on Borneo are being degraded and lost at an alarming rate. Studies on mammals report species responding in various ways to habitat changes that occur in commercial forestry concessions. Here we draw together information on the relationship between the ecological, evolutionary, and biogeographic characteristics of selected Bornean non-volant mammals, and their response to timber harvesting and related impacts. Only a minority of species show markedly reduced densities after timber harvesting. Nonetheless there are many grounds for concern as various processes can, and often do, reduce the viability of wildlife populations. Our review of what we know, and of current understanding, helps predict mammalian dynamics and subsequent mammal-induced ecosystem changes in logged forests. We identify groups of mammal species that, although largely unstudied, are unlikely to tolerate the impacts associated with timber harvesting. On a positive note we find and suggest many relatively simple and low-cost ways in which concession management practices might be modified so as to improve the value of managed forests for wildlife conservation. Improving forest management can play a vital role in maintaining the rich biodiversity of Borneo’s tropical rain forests.  相似文献   
7.
Aim The main aim of the present study is to infer the post‐glacial history of Abies species from north‐east Asia and to test the hypotheses that coastal Abies populations suffered less from climatic fluctuations during Pleistocene glacial periods than their more continental counterparts, and that Sakhalin was a major area of introgression. Location Natural ranges of the fir species Abies nephrolepis, Abies sachalinensis and Abies holophylla in the Russian Far East, and of Abies gracilis, which is endemic to the Kamchatka Peninsula. Methods Nineteen populations were sampled for allozyme analysis. Seventeen of these populations were also screened for variation at two paternally inherited chloroplast DNA microsatellite loci (cpSSR) and variation at one maternally inherited mitochondrial marker (nad4‐3/4). Finally a subset of 11 populations was analysed with amplified fragment length polymorphism (AFLP). Comparisons were made with already available Abies sibirica data. For all sets of markers, we estimated genetic diversity and differentiation using an analysis of molecular variance (AMOVA). Population clustering was assessed with a Bayesian approach implemented in structure v.2.3. Results Among the three major species, A. sibirica, A. nephrolepis and A. sachalinensis, A. sachalinensis demonstrated the highest cytoplasmic and nuclear diversity and the most continental species, A. sibirica, the lowest. Both nuclear and mitochondrial DNA markers revealed the presence of a transitional zone on Sakhalin Island between A. nephrolepis and A. sachalinensis of south Sakhalin. The structure analysis delivered very clear results confirming the admixed origin of A. sachalinensis, with a genetic contribution from A. nephrolepis. No variation in cytoplasmic markers was found in A. gracilis, suggesting the occurrence of a recent bottleneck. Main conclusions There is a clear reduction of genetic diversity in Abies species from the Pacific coast into the continent. The higher diversity in A. sachalinensis could have two causes: a larger effective population size in the islands due to relatively stable climatic conditions and consequently less pronounced demographic fluctuations in population size and/or hybridization with continental and Japanese populations. Sakhalin Island is a major transitional zone for conifer species. Finally, the fir from Kamchatka, A. gracilis, should be regarded as a separate species closely related to the A. nephrolepisA. sachalinensis complex.  相似文献   
8.
The domestication of cattle fuelled the development of agricultural society in the history of human being.The evolution and genetic relationship of cattle can be elucidated by investigating the variation of mitochondrial DNA(mtDNA) D-loop sequence.In this study,we built a cattle phylogeny with a pool of 856 individual D-loop sequences,of which 264 Chinese cattle D-loop sequences were obtained in this study(141 ones were first analyzed,and 123 were first submitted) and the rest sequences of cattle from six A...  相似文献   
9.
Aeschynanthus Jack, an epiphytic genus with c.160 species, is widespread in SE Asia. We selected 50 species for ITS nrDNA sequencing, to include all biogeographic areas and all infrageneric groupings, which are currently based on seed morphology. Some species were sequenced directly from PCR product; others cloned because of ITS length polymorphisms. The clone sequences were analysed individually and combined in an elision matrix. Results extend earlier findings that Aeschynanthus is divided into two clades, one occurring primarily in mainland SE Asia and the other in Malesia. This pattern is interpreted as indicating an ancient vicariance event followed by dispersal and plate fusion. Clade I has straight or clockwise spiral orientation of the testa cells and clade II anticlockwise spiral orientation. In clade I some species of section Microtrichium form a basal group with other sections being polyphyletic or paraphyletic. In clade II the monophyletic section Aeschynanthus is nested within the paraphyletic basal Microtrichium. Received February 8, 2001 Accepted June 8, 2001  相似文献   
10.
Ten diurnal raptor communities (Falconiformes) were studied in continental and peninsular situations, and on landbridge and oceanic islands of various sizes, from Southern India to Southern Vietnam and from Sri Lanka to Java. An index of abundance was derived from 1-km2 sample plots. A consistent decrease of species richness occurred from continent to peninsulas and to large landbridge islands, then more abruptly to oceanic islands. The impoverishment process was much faster for open habitat raptors than for forest species, and for rarest and most specialized raptors than for common and more generalist species. Large taxa survived on islands as well as smaller species. Specific habitat requirements, historical factors and forest fragmentation were probably more important determinants of community composition than land area itself. An insular syndrome was documented in forest species on islands, including significant examples of habitat niche expansion, interspecific segregation and density compensation. Some cases suggested that interspecific competition was involved. Such relaxation of habitat and density constraints may enhance the survival probability of these species on islands.  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