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   617篇
  免费   49篇
林业   62篇
农学   11篇
基础科学   2篇
  179篇
综合类   43篇
农作物   42篇
水产渔业   59篇
畜牧兽医   185篇
园艺   20篇
植物保护   63篇
  2024年   3篇
  2023年   7篇
  2022年   10篇
  2021年   33篇
  2020年   38篇
  2019年   26篇
  2018年   29篇
  2017年   28篇
  2016年   36篇
  2015年   26篇
  2014年   31篇
  2013年   44篇
  2012年   40篇
  2011年   51篇
  2010年   24篇
  2009年   19篇
  2008年   27篇
  2007年   26篇
  2006年   29篇
  2005年   27篇
  2004年   36篇
  2003年   21篇
  2002年   22篇
  2001年   3篇
  2000年   4篇
  1999年   3篇
  1997年   2篇
  1996年   1篇
  1995年   1篇
  1994年   2篇
  1992年   1篇
  1989年   1篇
  1986年   3篇
  1983年   1篇
  1982年   2篇
  1981年   1篇
  1980年   3篇
  1977年   1篇
  1975年   3篇
  1964年   1篇
排序方式: 共有666条查询结果,搜索用时 468 毫秒
661.
In February 2015, an unexpected windstorm downed five hectares of a European black pine Pinus nigra subsp. laricio forest formation located close to Vallombrosa, Florence (Central Italy). In the following spring, an extensive survey was conducted in the area. Felled trees, stumps and all the suitable plant material were screened for the presence of the pinewood nematode (PWN), Bursaphelenchus xylophilus, by sampling wood and bark. Bark beetles were then collected from the gallery systems on the inner side of bark samples and observed in the laboratory. The following bark beetles were morphologically identified: Ips sexdentatus, Orthotomicus erosus, O. laricis and Pityogenes bidentatusa. The dissection of Ips sexdentatus allowed the extraction of numerous nematodes that were morphologically and molecularly identified as Ektaphelenchoides pini. Conversely, only few nematode specimens were isolated from either pine bark or wood. These individuals could be only molecularly identified and belonged to an undescribed nematode taxon. Even though no PWN was recorded in the investigated sites, our survey allowed the detection of a new association between E. pini and I. sexdentatus on P. nigra.  相似文献   
662.
  • 1. Amphibians are declining worldwide in response to local and global pressures. Pond‐breeding species are particularly vulnerable to environmental change because they rely on two components of the landscape: aquatic and terrestrial habitats. Agricultural practices are changing rapidly at world and local scales. As a consequence, farm ponds and their surrounding terrestrial landscapes will probably be affected.
  • 2. This study investigated the main habitat determinants for the occurrence of four species of newts (genus Triturus) inhabiting the Pays de Herve, a rural area in Belgium. Newt occurrence was determined for 258 ponds and the effect of habitat on distribution determinants was evaluated using generalized linear models.
  • 3. Newts were found in 42% of the ponds. Contrary to expectations, the distribution of newts was not positively associated with a high density of ponds. However, a low occurrence of newts and the low water depth of many ponds suggest a low quality of habitats. Proximity to forest, deep water, and an absence of fish in ponds are factors that significantly favour newts, but terrestrial habitat requirements vary among species.
  • 4. These results indicate the necessity of maintaining both forest and meadow patches in close proximity to ponds inhabited by newts, and of controlling fish introduction in those ponds. These findings stress the importance of conservation and management of terrestrial and aquatic habitats for maintaining amphibian diversity.
Copyright © 2007 John Wiley & Sons, Ltd.  相似文献   
663.
The aim of this research was to evaluate the efficiency of diets with Spirulina and plant oils (POs) inclusion for white sturgeon weaning and their effects on the fatty acid (FA) composition of fish flesh. Three isoproteic (45%) and isoenergetic (21 MJ kg?1 DM) diets were formulated: one fish meal‐based diet integrated with fish oil (FMO) and two 40% Spirulina meal‐based diets integrated with corn (SPC) or soybean (SPS) oils respectively. One hundred and thirty‐five white sturgeon fingerlings (mean weight 17.5 g) were stocked randomly in nine fibreglass tanks. At the end of the trial, which lasted 71 days, the growth performance traits and somatic indexes were determined. The chemical composition, gross energy and FA profile were determined on the fish fillets. No significant effects were observed for the growth performances or fillet chemical composition. The FA profile of the fillets reflected those of the diets. In particular, the fillets of the fish fed with the SPC and SPS diets were lower in n‐3 FA, due to the substitution of fish oil (FO) with POs. It is possible to replace FO and meal in sturgeon; therefore, Spirulina meal integrated with POs could be a good alternative to sturgeon diet.  相似文献   
664.
665.
In this paper we investigate the interleukin-10 (IL-10) expression and 3D modelling of the European sea bass (Dicentrarchus labrax L.). IL-10 is a regulatory cytokine that has been intensively studied in mammals and has been found mainly involved in the suppression or deactivation of activated immune responses. The full-length sea bass IL-10 cDNA consists of 1015 bp and is translated in a predicted molecule of 187 amino acids. A multiple alignment of this peptide with other known sequences showed the conservation of fundamental IL-10 family characteristics. Expression analysis by real-time PCR in control fish showed a high basal expression in the head kidney (HK), followed by gut and brain. In vitro treatment of HK leucocytes with LPS showed an intense increase of IL-10 expression at 4 h and a significant decrease at 24 h, with PHA-L no differences were evidenced. A 3D model for sea bass IL-10 was obtained by accurate homology procedures and putative interaction residues with the IL-10 receptor described. The results suggested that sea bass IL-10 could be involved in anti-inflammatory responses, but further experiments are needed to assess its importance in response to pathogenic agents, vaccinations and immunostimulants.  相似文献   
666.
  1. Based on optimal foraging theory, animals are expected to maximize foraging benefits whilst minimizing risks. Despite risking being subjected to anthropogenic impacts such as water contamination, marine traffic, and underwater noise, estuaries have been identified as the preferred habitat of the Indo‐Pacific humpback dolphin (Sousa chinensis, IPHD). However, it remains unclear why this vulnerable species favours such risky habitats.
  2. Here, an exploratory case study in Zhanjiang estuary, China, was conducted to test the hypothesis that IPHDs select estuarine habitats as a trade‐off that maximizes foraging opportunities whilst minimizing the risk of mortality.
  3. The results showed that IPHDs accept greater mortality risks for higher food rewards but select habitats with lower risks when food rewards are similar between two locations.
  4. Although this type of information is important for underpinning models for individual dolphins, its principal role is to show environmental protection agencies why IPHDs favour estuaries despite the increased mortality risks.
  5. Habitat conservation plans should carefully consider prey stocks, possibly through the presence of marine protected areas near estuaries, as local overfishing may lead vulnerable cetacean populations to take greater risks.
  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