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   7210篇
  免费   931篇
  国内免费   482篇
  2024年   10篇
  2023年   213篇
  2022年   162篇
  2021年   241篇
  2020年   377篇
  2019年   427篇
  2018年   352篇
  2017年   354篇
  2016年   306篇
  2015年   320篇
  2014年   340篇
  2013年   486篇
  2012年   308篇
  2011年   279篇
  2010年   266篇
  2009年   412篇
  2008年   427篇
  2007年   420篇
  2006年   367篇
  2005年   341篇
  2004年   285篇
  2003年   246篇
  2002年   226篇
  2001年   243篇
  2000年   197篇
  1999年   191篇
  1998年   152篇
  1997年   126篇
  1996年   85篇
  1995年   82篇
  1994年   99篇
  1993年   50篇
  1992年   42篇
  1991年   36篇
  1990年   52篇
  1989年   8篇
  1988年   12篇
  1987年   16篇
  1986年   10篇
  1985年   10篇
  1984年   12篇
  1983年   13篇
  1982年   10篇
  1981年   3篇
  1980年   4篇
  1979年   1篇
  1978年   1篇
  1977年   2篇
  1974年   1篇
排序方式: 共有8623条查询结果,搜索用时 156 毫秒
31.
Tissue culture techniques for the propagation and conservation of endemic or threatened plants can be used to complement the methods usually applied in ex situ conservation. Thus, Minuartia valentina (Caryophyllaceae), an endangered plant species endemic to the Valencia Community (Eastern Spain), was successfully regenerated through shoot proliferation from wild plants growing in their natural area. Nodal segments, 10~mm long, were cut from rametes of adult material, sterilised and established in vitro. Equally successful shoot multiplication was achieved on Murashige and Skoog (MS) medium with 80 mg l-1 phloroglucinol in combination with either 1 mg l-1 6-benzylaminopurine or 1 mg l-1 kinetin. Excised shoots were rooted in MS medium supplemented with an auxin (indole acetic acid, indole-3-butyric acid, or napththalene acetic acid). Shoots rooted well (96–100%) within three weeks in all auxin treatments. However, the use of napththalene acetic acid was discarded because this auxin delayed root differentiation, and induced adventitious root malformation. Rooted plantlets were transferred to pots and 85% of them acclimatized successfully four weeks after transfer to greenhouse conditions, where they exhibited normal morphology and growth.  相似文献   
32.
Using the land‐bound vertebrates on the marine islands as model organisms, two metrics are presented that permit quantitative and succinct synopses of the ‘evolutionary maturity’ of the hosted faunal assemblages. In turn, these reflect the geo‐physical settings and geological developments of the substrates. The assemblage lineage‐taxonomy spectrum (ALTS) is based on the constituent lineages’ taxonomic distinctiveness and diversity. Individual lineages within assemblages can in most cases be assigned to one of six categories, LT1?LT6: LT1 is a non‐endemic taxon, whereas LT6 comprises multiple endemic genera from a family that arose elsewhere. If required, the scheme can be expanded: LT9 is an endemic order. The data can then be combined to provide an assemblage spectrum, for example, 00:08:38:30:08:15[ 13 ]. Here, the first six values denote the number of lineages assigned to each category expressed as percentages of the overall total, with the sum of the processed lineages listed as the seventh (in brackets and bold). The ALTS metric highlights efficiently the key features of a marine island's biological assemblage. Notably, the contrast between spectra for suites on geologically and geo‐physically varied island types can be striking, for instance the squamate suite on the young, proximate orogenic margin island of Taiwan is coded 78:16:05:00:00:00[ 37 ] whereas the one on the distantly located, Late Eocene composite terrane island of New Caledonia is 00:11:00:11:33:44[ 9 ]. To overcome the subjectivity that is inherent in assigning supraspecific ranks, an alternative assemblage lineage‐age spectrum (ALAS) is also introduced that makes use of the binary logarithm values of the colonization times of the island lineages (0–2, 2–4, … , 32–64, >64 Ma). It is represented using a seven‐plus‐two‐number code, for instance Madagascar's squamates are 00:06:00:00:19:62:12[ 19 ( 16 )]; most colonizations took place in the Palaeogene (66–23 Ma); there are 19 lineages, but only 16 are presently age‐dated. In addition to marine‐island biogeography studies, the ALTS–ALAS spectrum approach is potentially useful for encapsulating biotas in other sorts of insular setting (e.g. lakes, mountain tops), and for evaluating palaeogeographical models. Furthermore, it may help emphasize the conservation value of an island's faunal assemblage.  相似文献   
33.
Habitat loss is one of the main threats to wildlife. Therefore, knowledge of habitat use and preference is essential for the design of conservation strategies and identification of priority sites for the protection of endangered species. The yellow‐tailed woolly monkey (Lagothrix flavicauda Humboldt, 1812), categorized as Critically Endangered on the IUCN Red List, is endemic to montane forests in northern Peru where its habitat is greatly threatened. We assessed how habitat use and preference in L. flavicauda are linked to forest structure and composition. The study took place near La Esperanza, in the Amazonas region, Peru. Our objective was to identify characteristics of habitat most utilized by L. flavicauda to provide information that will be useful for the selection of priority sites for conservation measures. Using presence records collected from May 2013 to February 2014 for one group of L. flavicauda, we classified the study site into three different use zones: low‐use, medium‐use, and high‐use. We assessed forest structure and composition for all use zones using 0.1 ha Gentry vegetation transects. Results show high levels of variation in plant species composition across the three use zones. Plants used as food resources had considerably greater density, dominance, and ecological importance in high‐use zones. High‐use zones presented similar structure to medium‐ and low‐use zones; thus it remains difficult to assess the influence of forest structure on habitat preference. We recommend focusing conservation efforts on areas with a similar floristic composition to the high‐use zones recorded in this study and suggest utilizing key alimentation species for reforestation efforts.  相似文献   
34.
35.
36.
37.
38.
Intra‐cohort cannibalism is an example of a size‐mediated priority effect. If early life stages cannibalize slightly smaller individuals, then parents face a trade‐off between breeding at the best time for larval growth or development and predation risk from offspring born earlier. This game‐theoretic situation among parents may drive adaptive reproductive phenology toward earlier breeding. However, it is not straightforward to quantify how cannibalism affects seasonal egg fitness or to distinguish emergent breeding phenology from alternative adaptive drivers. Here, we devise an age‐structured game‐theoretic mathematical model to find evolutionary stable breeding phenologies. We predict how size‐dependent cannibalism acting on eggs, larvae, or both changes emergent breeding phenology and find that breeding under inter‐cohort cannibalism occurs earlier than the optimal match to environmental conditions. We show that emergent breeding phenology patterns at the level of the population are sensitive to the ontogeny of cannibalism, that is, which life stage is subject to cannibalism. This suggests that the nature of cannibalism among early life stages is a potential driver of the diversity of reproductive phenologies seen across taxa and may be a contributing factor in situations where breeding occurs earlier than expected from environmental conditions.  相似文献   
39.
40.
During the last few decades, human-driven activities have led to indiscriminate habitat destruction and exploitation of many plant species in Kashmir Himalaya. As a result, many species have become threatened and are struggling for survival. Of particular concern are the endemic and critically endangered species which have the highest risk of the extinction, hence warranting immediate conservation actions. Therefore the current study was carried out to understand the distribution, ecology and conservation implications of the four critically endangered endemic plants of Kashmir Himalaya. Habitat distribution modelling showed that the suitable potential areas for the species occurred from subalpine to alpine meadowlands with an elevational range of 1500–4600 m asl. The output of the MaxEnt model and field surveys have revealed that their highest potential distribution is in Panchari, Khrew, Ramnagar, Pahalgam, Gurez, Sonamarg, Gulmarg and Kishtwar forest ranges. Based on the field explorations and herbarium records, Saussurea costus (Falc.) Lipsch have 27 distribution areas, Gentiana kurroo Royle 18, Lilium polyphyllum D. Don 12 and Aconitum chasmanthum Stapf have 15. Precipitation of the driest month and annual mean temperature played an important role in the distribution of the studied species. The species started their lifecycle with the onset of the spring season, flowered in summer, fruit in autumn and senesce in the winter season. Under natural conditions, the number of days required for germination ranged from 180 to 210 where cold stratification played a pivotal role. Since last few decades, the populations of these species have been shrinking in their natural habitats due to over-exploitation for medicinal purposes and habitat destruction through amplified humanoid interferences like the expansion of agricultural land, road building, grazing and urbanization. Thus there is an urgent need to come up with positive strategies to save whatever is left and plan long term rescue measures not only to protect these species from extinction but also to reintroduce them along with framing the plans to supply sustained raw materials for medicine.  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