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   93948篇
  免费   328篇
  国内免费   813篇
  95089篇
  2023年   7篇
  2022年   11篇
  2021年   19篇
  2020年   9篇
  2019年   13篇
  2018年   11849篇
  2017年   10683篇
  2016年   7469篇
  2015年   616篇
  2014年   321篇
  2013年   351篇
  2012年   4251篇
  2011年   12864篇
  2010年   12026篇
  2009年   8249篇
  2008年   9806篇
  2007年   11399篇
  2006年   278篇
  2005年   535篇
  2004年   987篇
  2003年   1041篇
  2002年   806篇
  2001年   274篇
  2000年   178篇
  1999年   38篇
  1998年   10篇
  1997年   26篇
  1996年   15篇
  1994年   12篇
  1993年   34篇
  1992年   32篇
  1991年   52篇
  1990年   16篇
  1989年   10篇
  1988年   24篇
  1987年   20篇
  1986年   8篇
  1985年   15篇
  1984年   16篇
  1983年   23篇
  1981年   10篇
  1980年   11篇
  1975年   9篇
  1972年   253篇
  1971年   281篇
  1970年   7篇
  1965年   13篇
  1962年   25篇
  1944年   12篇
  1940年   10篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
961.
Habitat fragmentation reduces the available habitat area and increases both the distance between fragments and the amount of fragment edges. Therefore, there are more probabilities of plant population size reduction and species extinction. In the same way, biotic and abiotic changes associated with forest fragmentation can dramatically alter plant growth and phenological patterns. We conducted a 3-year study to analyze effects of habitat fragmentation and seasonal variation on host plant quality (quantity of leaves, diameter at breast height, tree height), gall abundance and species richness in a temperate oak forest. Our results show that host plant quality was significantly higher in isolated oaks and small fragments, increasing the abundance and species richness of oak gall wasp species in most fragmented habitats. Oak canopy cover is altered by forest fragmentation, there being higher production of leaves on trees that are more exposed to fragmentation, and can provide important resources for maintaining gall wasp species diversity in a fragmented landscape. We found higher gall wasp richness and abundance in autumn than in the spring, which matches with the higher quantity of leaves in this season.  相似文献   
962.
963.
The year 2016 will see significant improvements for the Journal of Cell Communication and Signaling with the earning of an official Impact Factor and the merger of Springer Science + Business Media and part of Macmillan Sciences and Education. It will also be an important year for the International CCN Society (ICCNS) with the nomination of a new Scientific Board and reinforcement of interactions with other major scientific societies interested in various aspects of Cell Signaling. Starting this year the ICCNS will become an official provider of CCN3 knock out mice to ICCNS members. This first step in opening up access to certified reagents as a service for our CCN scientific community is part of our intention and efforts to attain the highest degree of assistance and enabler of scientific cooperation and communication in Science Communication.  相似文献   
964.
Many empirical food webs contain multiple resources, which can lead to the emergence of sub-communities—partitions—in a food web that are weakly connected with each other. These partitions interact and affect the complete food web. However, the fact that food webs can contain multiple resources is often neglected when describing food web assembly theoretically, by considering only a single resource. We present an allometric, evolutionary food web model and include two resources of different sizes. Simulations show that an additional resource can lead to the emergence of partitions, i.e. groups of species that specialise on different resources. For certain arrangements of these partitions, the interactions between them alter the food web properties. First, these interactions increase the variety of emerging network structures, since hierarchical bodysize relationships are weakened. Therefore, they could play an important role in explaining the variety of food web structures that is observed in empirical data. Second, interacting partitions can destabilise the population dynamics by introducing indirect interactions with a certain strength between predator and prey species, leading to biomass oscillations and evolutionary intermittence.  相似文献   
965.
966.
The coffee berry borer (CBB), Hypothenemus hampei (Ferrari), is the most important insect pest of coffee worldwide. In this study, we used headspace solid-phase microextraction coupled with gas chromatography–mass spectrometry to sample and identify volatile compounds from Robusta coffee berries, Coffea canephora Pierre ex Froehner, infested with CBB and with mechanical damage. Furthermore, we evaluated the behavioral responses of the CBB and two of its parasitoids, Prorops nasuta Waterstone and Phymastichus coffea LaSalle, to three selected coffee volatile compounds in a Y-tube olfactometer. We found in the effluvia of red coffee berry compounds not previously reported for this coffee species. Our results show that Robusta coffee berries release induced volatiles either by insect herbivory or by mechanical damage. Small amount of butyl acetate, unknown compound 2, α-longipinene, longiborneol and longiborneol acetate are produced only in infested coffee berries fruits. Quantitatively, nine compounds account for the difference between healthy berries, infested, or mechanically damaged berries. Trans-ocimene, 4,8-dimethyl-3,7-nonadien-2-ol, α-copaene and kaurene increased amount levels in infested berries, while amount of methyl salicylate and linalool increased in mechanically damaged coffee berries. The olfactometric bioassays showed that CBB females and its two parasitoids were attracted to methyl salicylate. In addition, H. hampei and P. nasuta were attracted to linalool, and P. nasuta and P. coffea were attracted to trans-ocimene.  相似文献   
967.
Potential for oilseed rape resistance in pollen beetle control   总被引:1,自引:0,他引:1  
Breeding for plant resistance to insect pests is a classic strategy in integrated management, but it has never been developed for use against European pests of oilseed rape (Brassica napus) (OSR), especially one of the most damaging ones, the pollen beetle (Meligethes aeneus). In this paper we look at the three strategies that could be employed to improve OSR resistance (based on transgenes, relatives of B. napus or OSR natural variation) and review our current knowledge as to how these strategies could be put into practice. We identify the drawbacks which are specific to the pollen beetle that could impede breeding programs for resistance, and propose an approach to circumvent them. Finally, we detail the steps of the interaction between OSR and the pollen beetle that could be targeted in order to improve plant resistance (host plant location, adult survival, adult feeding, egg production and oviposition, larval development) and discuss their efficiency and durability potential.  相似文献   
968.
969.
970.
On the basis of theoretical predictions, pollination networks seem to be resilient to random node elimination but sensitive to targeted exclusion. However, such predictions have a very weak empirical basis. In order to test the robustness of the pollination network to short-term disturbances, we removed inflorescences of the most connected species occurring in a lowland meadow network using the before–after approach and compared the result with that obtained by network modelling. The manipulated network showed no significant differences for the most commonly used metrics, but was more generalized than control networks, owing to a change in the preferences of pollinators. Furthermore, no secondary extinctions (emigrations) were found, owing to the considerable natural variation found among insect species assemblages. Following elimination of the most linked plant species, a new hub was detected in the experimental meadow, the hub node being a plant species with a similar inflorescence to that removed, and formerly playing the role of a peripheral node. We conclude that exclusion of the main food source forced insects to change their specialized preferences to other plant species that were available. Mostly, these had inflorescences similar to those that were removed.  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