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   2124篇
  免费   87篇
  国内免费   194篇
  2024年   1篇
  2023年   35篇
  2022年   64篇
  2021年   38篇
  2020年   34篇
  2019年   56篇
  2018年   56篇
  2017年   76篇
  2016年   105篇
  2015年   81篇
  2014年   121篇
  2013年   118篇
  2012年   65篇
  2011年   82篇
  2010年   71篇
  2009年   202篇
  2008年   224篇
  2007年   204篇
  2006年   151篇
  2005年   126篇
  2004年   82篇
  2003年   79篇
  2002年   52篇
  2001年   31篇
  2000年   32篇
  1999年   25篇
  1998年   26篇
  1997年   23篇
  1996年   17篇
  1995年   30篇
  1994年   16篇
  1993年   14篇
  1992年   10篇
  1991年   14篇
  1990年   11篇
  1989年   8篇
  1988年   7篇
  1987年   6篇
  1986年   7篇
  1985年   1篇
  1984年   3篇
  1982年   1篇
排序方式: 共有2405条查询结果,搜索用时 31 毫秒
91.
There is an urgent need for a new conservation approach as mere designation of protected areas, the primary approach to conserving biodiversity, revealed its low conservation efficiency and inability to cope with numerous challenges faced by nature in the Anthropocene. The paper discusses the new concept, which proposes that ecological restoration becomes an integral part of conservation planning and implementation, and is done using threatened plant species that are introduced not only into locations where they currently grow or grew in the recent past, but also into suitable locations within their potential distribution range. This new concept is called conservation-oriented restoration to distinguish it from the traditional restoration. Although the number of restoration projects focusing on recreation of once existing natural habitats is instantly growing, the majority of ecological restoration projects, in contrast to conservation-oriented restoration, have predominantly utilitarian goals, e.g.improvement or air quality, erosion control or soil replenishment. Conservation-oriented restoration should not be seen as an alternative either to the latter, or to the conservation dealing with particular threatened species(species-targeted conservation). These three conservation approaches, traditional ecological restoration, species-targeted conservation, and conservation-oriented restoration differ not only in broadly defined goals and attributes of their targets, but also in the types of ecosystems they are applicable to, and complement each other in combating global deterioration of the environment and biodiversity loss.  相似文献   
92.
Sustainable use of wildlife: The view from archaeozoology   总被引:1,自引:0,他引:1  
The concept of “sustainable use” is a polemic, especially when referring to wildlife and other “natural resources.” Claims vary from persistent overuse to traditional practices of sustaining the resource base. Arguments for either side routinely draw from contemporary situations, relying on relatively short time periods. But, if the notion “sustainable” is to be consequential, the period under consideration must be meaningful – claims that trends over a few years or even decades are representative of long-term phenomena are questionable, for they depend on numerous untested, if not untestable, assumptions. Research in anthropology, biology, biological conservation, ecology, policy, restoration ecology, social development, and wildlife management is routinely constrained to relatively short time periods, and such results are of limited value for evaluating sustainable use. Archaeozoology provides unique tools for investigating trends in human-resource relations over periods of centuries and millennia; it is one of the few disciplines explicitly dedicated to the study of material evidence over significant time periods that bears directly on the question of sustainable use. Hence, objective, scientific evaluation of sustainable use must contemplate archaeozoological evidence.Yet, archaeozoology often does not provide definitive answers: it is wrought with fundamental problems in sampling, data gathering, analysis, and interpretation (e.g., varying screen sizes; meta-sampling procedures; comparisons across diverse spatial, temporal, and environmental continua; etc.). Typically data must be evaluated through inference, rather than through direct observations and experiments. Moreover, environmental variation (e.g., climate change, sea level change, tectonic movement, etc.), as well as changes in societies, confounds the interpretation of long-term human–environmental trends. But, those challenges are not unique to archaeozoology, for biological and ecological phenomena are no less subject to multiple sources of environmental and social variation. The difference is that archaeologists are patently cognizant of these sources of variability, and they openly debate them when interpreting data, while in other disciplines these sources of change are routinely ignored. Nonetheless, despite the unique relevance of their work, few archaeozoologists have ventured outside their discipline to engage natural scientists and policy makers in issues of biological conservation and wildlife biology, and particularly the root questions of sustainable use. In addition to considering data and hypotheses regarding long-term prehistoric human–wildlife interactions, it is essential to view this information within broader perspectives, linked to other disciplines as well as to contemporary social and policy issues. Because sustainable use is mandated in international instruments and national policies, the notion has relevance in arenas far beyond academia, where archaeozoologists must be integrated into interdisciplinary teams.  相似文献   
93.
The Hardangervidda in southern Norway, the largest mountain plateau in Europe, has thousands of lakes and streams, mainly between 1000 and 1300 m above sea level, where brown trout is the only fish species. To describe the current genetic diversity of brown trout in this area, a total of 863 fish from 20 lakes were genotyped with eleven microsatellites. Most diversity is within lake populations, but diversity among geographical groups and populations within groups was significant, too. Neighbor-joining, principle coordinate analysis and Bayesian clustering show three major geographic groups in accordance with the river systems. Bias was caused by recent stocking in two lakes. Low/no genetic differentiation among some populations indicates that intermixing is common when lakes are well-connected, as was also shown by assignment test. We recommend preserving the genetic diversity of brown trout in this unique area by managing stocking in lake systems according to genetic structure.  相似文献   
94.
Rare minnow (Gobiocypris rarus) is an endangered small fish endemic to upper reach of the Yangtze River. From a (GT)n enriched genomic library, 32 microsatellites were isolated and characterized. Nineteen of these loci were polymorphic in a test population with alleles ranging from 2–7, and observed and expected heterozygosities from zero to 0.8438, and 0.2679 to 0.8264, respectively. In the cross-species amplifications, 13 out of 19 polymorphic loci were found to be also polymorphic in at least one of the 7 closely related species of the subfamily Gobioninae. These polymorphic microsatellite loci should provide sufficient level of genetic diversity to evaluate the fine-scale population structure in rare minnow and its closely related species for the conservation purpose.  相似文献   
95.
Panax ginseng C.A. Meyer, commonly known as Korean or Asian ginseng, is a perennial herb native to Korea and China. Its roots are highly prized for several medicinal properties. The present study describes development and characterization of twenty-two polymorphic microsatellite markers for this species. A total of 99 alleles were detected with an average of 4.5 alleles per locus across 20 accessions. Values for observed (H O ) and expected (H E ) heterozygosities ranged from 0.05 to 1.00 and from 0.18 to 0.73, respectively. Eleven loci deviated from Hardy–Weinberg equilibrium (P < 0.001). Significant (P < 0.05) heterozygote deficiency was observed at 13 loci. Exact test for linkage disequilibrium showed significant values (P < 0.05) between 12 pairs of loci. These microsatellite markers provide powerful tools for understanding population and conservation genetics of this species and also for genetic differentiation and authentication of different Panax species being used in commercial ginseng products.  相似文献   
96.
The hexaploid herbaceous species Primula glaucescens Moretti and Primula spectabilis Tratt. (2n = 6x = 66) are two endemics of the Italian Pre-Alps protected by national and international laws. In order to plan conservation strategies for natural populations and to study the influence of the latest glaciation on them we developed a set of microsatellites markers for the endangered Primula species: ten primer pairs allowed analysis of polymorphic disomic loci in P. glaucescens samples and seven of them also amplified polymorphic disomic markers in P. spectabilis. Kirsten Wolff and Giorgio Binelli contributed equally to the paper  相似文献   
97.
Mitochondrial DNA cytochrome oxidase subunit I and nuclear ITS2 sequences were surveyed from Canary Islands threatened species of the genera Purpuraria and Acrostira (Orthoptera: Pamphagidae). Phylogenetic and population analyses show that the two previously recognized Purpuraria erna subspecies are not valid as conservation units, and that there is a new unrecognized species of Purpuraria, coincident with recently discovered morphological variation within the genus. In addition, mitochondrial introgression seems to occur between the two Purpuraria species in southwest Lanzarote. Species-delimitation based on the morphological taxonomy of Acrostira, which recognizes four single-island endemics, is only partially supported by the genetic data. It shows that currently admitted species from the central and western islands of Tenerife, La Gomera and La Palma are closely related, with evidence of recent gene flow between the Tenerife and La Gomera populations. MtDNA variation also showed that A. euphorbiae, currently considered as the most critically endangered grasshopper species in the Canaries, has lower population diversity than its close relatives.  相似文献   
98.
Over the last 50 years, Spanish Atlantic salmon (Salmo salar) populations have been in decline. In order to bolster these populations, rivers were stocked with fish of northern European origin during the period 1974–1996, probably also introducing the furunculosis-inducing pathogen, Aeromonas salmonicida. Here we assess the relative importance of processes influencing mitochondrial (mt)DNA variability in these populations from 1948 to 2002. Genetic material collected over this period from four rivers in northern Spain (Cantabria) was used to detect variability at the mtDNA ND1 gene. Before stocking, a single haplotype was found at high frequency (0.980). Following stocking, haplotype diversity (h) increased in all rivers (mean h before stocking was 0.041, and 0.245 afterwards). These increases were due principally to the dramatic increase in frequency of a previously very low frequency haplotype, reported at higher frequencies in northern European populations proximate to those used to stock Cantabrian rivers. Genetic structuring increased after stocking: among-river differentiation was low before stocking (1950s/1960s Φ ST = –0.00296–0.00284), increasing considerably at the height of stocking (1980s Φ ST = 0.18932) and decreasing post-stocking (1990s/2002 Φ ST = 0.04934–0.03852). Gene flow from stocked fish therefore seems to have had a substantial role in increasing mtDNA variability. Additionally, we found significant differentiation between individuals that had probably died from infectious disease and apparently healthy, angled fish, suggesting a possible role for pathogen-driven selection of mtDNA variation. Our results suggest that stocking with non-native fish may increase genetic diversity in the short term, but may not reverse population declines.  相似文献   
99.
100.
Santalum album L. is an economically important tropical tree species. Owing to extensive logging, changes in land-use patterns and poor natural regeneration, the natural sandal populations are rapidly dwindling. It is feared that such threats could easily undermine the genetic diversity of sandal populations. Effective measures to prevent such loss are prerequisite. We have developed baseline data on the levels of genetic diversity and its distribution on a geographic scale for sandal in India. Based on Geographic Information System, distribution map of sandal was developed and found that the populations were geographically more concentrated in the Deccan plateau. It was found that, for past 53 years, there was a monotonic decrease in the extraction of sandal in the state of Karnataka, India. Using allozyme markers, the genetic diversity of 19 sandal populations in peninsular India were determined and over all observed heterzygosity (Ho) was found to be 31%. The percent Ho was positively correlated with the density of the sandal populations (r = 0.44) and with increasing longitude it was found to be negatively correlated (r = −0.51). The dendrogram analysis indicated a clear clustering of sandal populations based on their geographic occurrence. The Deccan plateau populations were found to be genetically the most diverse and seemed to represent the ‘hot-spot’ of sandal genetic resources in peninsular India. Our results have important implications for the conservation strategies for sandal populations in peninsular India and can be applied for the conservation of other taxa as well.  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