全文获取类型
收费全文 | 5227篇 |
免费 | 486篇 |
国内免费 | 2篇 |
专业分类
5715篇 |
出版年
2024年 | 14篇 |
2023年 | 63篇 |
2022年 | 144篇 |
2021年 | 290篇 |
2020年 | 136篇 |
2019年 | 168篇 |
2018年 | 202篇 |
2017年 | 168篇 |
2016年 | 249篇 |
2015年 | 401篇 |
2014年 | 383篇 |
2013年 | 403篇 |
2012年 | 546篇 |
2011年 | 468篇 |
2010年 | 265篇 |
2009年 | 203篇 |
2008年 | 313篇 |
2007年 | 271篇 |
2006年 | 223篇 |
2005年 | 214篇 |
2004年 | 172篇 |
2003年 | 131篇 |
2002年 | 100篇 |
2001年 | 31篇 |
2000年 | 19篇 |
1999年 | 19篇 |
1998年 | 17篇 |
1997年 | 11篇 |
1996年 | 6篇 |
1995年 | 8篇 |
1994年 | 3篇 |
1993年 | 2篇 |
1992年 | 5篇 |
1991年 | 2篇 |
1990年 | 3篇 |
1989年 | 8篇 |
1988年 | 2篇 |
1987年 | 5篇 |
1986年 | 5篇 |
1985年 | 2篇 |
1984年 | 5篇 |
1983年 | 2篇 |
1982年 | 6篇 |
1980年 | 5篇 |
1978年 | 2篇 |
1976年 | 2篇 |
1973年 | 2篇 |
1926年 | 1篇 |
1912年 | 1篇 |
1907年 | 1篇 |
排序方式: 共有5715条查询结果,搜索用时 0 毫秒
11.
Steinhour E Sherwani SI Mazerik JN Ciapala V O'Connor Butler E Cruff JP Magalang U Parthasarathy S Sen CK Marsh CB Kuppusamy P Parinandi NL 《Molecular and cellular biochemistry》2008,315(1-2):97-112
We have earlier reported that the redox-active antioxidant, vitamin C (ascorbic acid), activates the lipid signaling enzyme, phospholipase D (PLD), at pharmacological doses (mM) in the bovine lung microvascular endothelial cells (BLMVECs). However, the activation of phospholipase A(2) (PLA(2)), another signaling phospholipase, and the modulation of PLD activation by PLA(2) in the ECs treated with vitamin C at pharmacological doses have not been reported to date. Therefore, this study aimed at the regulation of PLD activation by PLA(2) in the cultured BLMVECs exposed to vitamin C at pharmacological concentrations. The results revealed that vitamin C (3-10 mM) significantly activated PLA(2) starting at 30 min; however, the activation of PLD resulted only at 120 min of treatment of cells under identical conditions. Further studies were conducted utilizing specific pharmacological agents to understand the mechanism(s) of activation of PLA(2) and PLD in BLMVECs treated with vitamin C (5 mM) for 120 min. Antioxidants, calcium chelators, iron chelators, and PLA(2) inhibitors offered attenuation of the vitamin C-induced activation of both PLA(2) and PLD in the cells. Vitamin C was also observed to significantly induce the formation and release of the cyclooxygenase (COX)- and lipoxygenase (LOX)-catalyzed arachidonic acid (AA) metabolites and to activate the AA LOX in BLMVECs. The inhibitors of PLA(2), COX, and LOX were observed to effectively and significantly attenuate the vitamin C-induced PLD activation in BLMVECs. For the first time, the results of the present study revealed that the vitamin C-induced activation of PLD in vascular ECs was regulated by the upstream activation of PLA(2), COX, and LOX through the formation of AA metabolites involving oxidative stress, calcium, and iron. 相似文献
12.
13.
Replicate harvest simulations were conducted in a large natural population of Panax quinquefolius L.␣(Araliaceae) to determine the selective effects of harvest. We investigated how minimum size requirements and the influence
of size on apparency to human harvesters could result in preferential removal of large plants. To determine which plants were
encountered in the large population, harvesters were tracked using GPS as they searched for every legally harvestable, adult
plant they could find. Plants were assigned stage-specific fitness measures based on their contributions to population growth
rate (λ) under three demographically based harvest regimes: no harvest, harvest and harvest removing seeds. Plant size was
codified into a size-index equal to the product of total leaf area and stem height. Heterogeneity of slopes was tested to
determine if the selection gradients (β) describing the relationship between fitness and size varied among the three harvest
regimes. Harvest differentially reduced the fitness of larger plants in one of four individual harvest simulations. The combined
harvest simulation significantly altered the selection regime for size in the population of juvenile and adult (harvestable)
plants. Seed removal by harvesters intensified fitness declines for larger plants. Because larger plants contribute most to
population growth, the selective effects of harvest could result in a shift in the evolutionary dynamics of this species with
significant conservation implications. 相似文献
14.
Genomics meets applied ecology: Characterizing habitat quality for sloths in a tropical agroecosystem 下载免费PDF全文
Emily D. Fountain Jung koo Kang Douglas J. Tempel M. Zachariah Peery 《Molecular ecology》2018,27(1):41-53
Understanding how habitat quality in heterogeneous landscapes governs the distribution and fitness of individuals is a fundamental aspect of ecology. While mean individual fitness is generally considered a key to assessing habitat quality, a comprehensive understanding of habitat quality in heterogeneous landscapes requires estimates of dispersal rates among habitat types. The increasing accessibility of genomic approaches, combined with field‐based demographic methods, provides novel opportunities for incorporating dispersal estimation into assessments of habitat quality. In this study, we integrated genomic kinship approaches with field‐based estimates of fitness components and approximate Bayesian computation (ABC) procedures to estimate habitat‐specific dispersal rates and characterize habitat quality in two‐toed sloths (Choloepus hoffmanni) occurring in a Costa Rican agricultural ecosystem. Field‐based observations indicated that birth and survival rates were similar in a sparsely shaded cacao farm and adjacent cattle pasture–forest mosaic. Sloth density was threefold higher in pasture compared with cacao, whereas home range size and overlap were greater in cacao compared with pasture. Dispersal rates were similar between the two habitats, as estimated using ABC procedures applied to the spatial distribution of pairs of related individuals identified using 3,431 single nucleotide polymorphism and 11 microsatellite locus genotypes. Our results indicate that crops produced under a sparse overstorey can, in some cases, constitute lower‐quality habitat than pasture–forest mosaics for sloths, perhaps because of differences in food resources or predator communities. Finally, our study demonstrates that integrating field‐based demographic approaches with genomic methods can provide a powerful means for characterizing habitat quality for animal populations occurring in heterogeneous landscapes. 相似文献
15.
Inga species present brush‐type flower morphology allowing them to be visited by distinct groups of pollinators. Nectar features in relation to the main pollinators have seldom been studied in this genus. To test the hypothesis of floral adaptation to both diurnal and nocturnal pollinators, we studied the pollination ecology of Inga sessilis, with emphasis on the nectar secretion patterns, effects of sequential removals on nectar production, sugar composition and the role of diurnal and nocturnal pollinators in its reproductive success. Inga sessilis is self‐incompatible and pollinated by hummingbirds, hawkmoths and bats. Fruit set under natural conditions is very low despite the fact that most stigmas receive polyads with sufficient pollen to fertilise all ovules in a flower. Nectar secretion starts in the bud stage and flowers continually secreting nectar for a period of 8 h. Flowers actively reabsorbed the nectar a few hours before senescence. Sugar production increased after nectar removal, especially when flowers were drained during the night. Nectar sugar composition changed over flower life span, from sucrose‐dominant (just after flower opening, when hummingbirds were the main visitors) to hexose‐rich (throughout the night, when bats and hawkmoths were the main visitors). Diurnal pollinators contributed less than nocturnal ones to fruit production, but the former were more constant and reliable visitors through time. Our results indicate I. sessilis has floral adaptations, beyond the morphology, that encompass both diurnal and nocturnal pollinator requirements, suggesting a complementary and mixed pollination system. 相似文献
16.
17.
River Restoration in the Twenty-First Century: Data and Experiential Knowledge to Inform Future Efforts 总被引:1,自引:0,他引:1
Margaret Palmer J. David Allan Judy Meyer Emily S. Bernhardt 《Restoration Ecology》2007,15(3):472-481
Despite some highly visible projects that have resulted in environmental benefits, recent efforts to quantify the number and distribution of river restoration projects revealed a paucity of written records documenting restoration outcomes. Improving restoration designs and setting watershed priorities rely on collecting and making accessible this critical information. Information within the unpublished notes of restoration project managers is useful but rarely documents ecological improvements. This special section of Restoration Ecology is devoted to the current state of knowledge on river restoration. We provide an overview of the section’s articles, reflecting on lessons learned, which have implications for the implementation, legal, and financing frameworks for restoration. Our reflections are informed by two databases developed under the auspices of the National River Restoration Science Synthesis project and by extensive interactions with those who fund, implement, and permit restoration. Requiring measurable ecological success criteria, comprehensive watershed plans, and tracking of when and where restoration projects are implemented are critical to improving the health of U.S. waters. Documenting that a project was put in the ground and stayed intact cannot be equated with ecological improvements. However, because significant ecological improvements can come with well‐designed and ‐implemented stream and river restorations, a small investment in documenting the factors contributing to success will lead to very large returns in the health of our nation’s waterways. Even projects that may appear to be failures initially can be turned into success stories by applying the knowledge gained from monitoring the project in an adaptive restoration approach. 相似文献
18.
Isabelle M. C?té Emily S. Darling Christopher J. Brown 《Proceedings. Biological sciences / The Royal Society》2016,283(1824)
Interactions between multiple ecosystem stressors are expected to jeopardize biological processes, functions and biodiversity. The scientific community has declared stressor interactions—notably synergies—a key issue for conservation and management. Here, we review ecological literature over the past four decades to evaluate trends in the reporting of ecological interactions (synergies, antagonisms and additive effects) and highlight the implications and importance to conservation. Despite increasing popularity, and ever-finer terminologies, we find that synergies are (still) not the most prevalent type of interaction, and that conservation practitioners need to appreciate and manage for all interaction outcomes, including antagonistic and additive effects. However, it will not be possible to identify the effect of every interaction on every organism''s physiology and every ecosystem function because the number of stressors, and their potential interactions, are growing rapidly. Predicting the type of interactions may be possible in the near-future, using meta-analyses, conservation-oriented experiments and adaptive monitoring. Pending a general framework for predicting interactions, conservation management should enact interventions that are robust to uncertainty in interaction type and that continue to bolster biological resilience in a stressful world. 相似文献
19.
20.