全文获取类型
收费全文 | 603篇 |
免费 | 88篇 |
出版年
2021年 | 9篇 |
2019年 | 6篇 |
2018年 | 8篇 |
2017年 | 5篇 |
2016年 | 9篇 |
2015年 | 16篇 |
2014年 | 24篇 |
2013年 | 30篇 |
2012年 | 36篇 |
2011年 | 21篇 |
2010年 | 15篇 |
2009年 | 19篇 |
2008年 | 27篇 |
2007年 | 25篇 |
2006年 | 14篇 |
2005年 | 19篇 |
2004年 | 9篇 |
2003年 | 17篇 |
2002年 | 22篇 |
2001年 | 17篇 |
2000年 | 13篇 |
1999年 | 23篇 |
1998年 | 6篇 |
1997年 | 10篇 |
1996年 | 7篇 |
1995年 | 8篇 |
1994年 | 13篇 |
1993年 | 5篇 |
1992年 | 5篇 |
1991年 | 9篇 |
1990年 | 10篇 |
1989年 | 14篇 |
1988年 | 14篇 |
1987年 | 15篇 |
1986年 | 15篇 |
1985年 | 10篇 |
1984年 | 11篇 |
1982年 | 6篇 |
1981年 | 9篇 |
1980年 | 7篇 |
1979年 | 11篇 |
1978年 | 6篇 |
1976年 | 5篇 |
1974年 | 10篇 |
1973年 | 12篇 |
1971年 | 6篇 |
1970年 | 8篇 |
1969年 | 9篇 |
1968年 | 12篇 |
1965年 | 4篇 |
排序方式: 共有691条查询结果,搜索用时 150 毫秒
611.
612.
613.
Tuanyok A Stone JK Mayo M Kaestli M Gruendike J Georgia S Warrington S Mullins T Allender CJ Wagner DM Chantratita N Peacock SJ Currie BJ Keim P 《PLoS neglected tropical diseases》2012,6(1):e1453
Lipopolysaccharide (LPS) is one of the most important virulence and antigenic components of Burkholderia pseudomallei, the causative agent of melioidosis. LPS diversity in B. pseudomallei has been described as typical, atypical or rough, based upon banding patterns on SDS-PAGE. Here, we studied the genetic and molecular basis of these phenotypic differences. Bioinformatics was used to determine the diversity of genes known or predicted to be involved in biosynthesis of the O-antigenic moiety of LPS in B. pseudomallei and its near-relative species. Multiplex-PCR assays were developed to target diversity of the O-antigen biosynthesis gene patterns or LPS genotypes in B. pseudomallei populations. We found that the typical LPS genotype (LPS genotype A) was highly prevalent in strains from Thailand and other countries in Southeast Asia, whereas the atypical LPS genotype (LPS genotype B) was most often detected in Australian strains (~13.8%). In addition, we report a novel LPS ladder pattern, a derivative of the atypical LPS phenotype, associated with an uncommon O-antigen biosynthesis gene cluster that is found in only a small B. pseudomallei sub-population. This new LPS group was designated as genotype B2. We also report natural mutations in the O-antigen biosynthesis genes that potentially cause the rough LPS phenotype. We postulate that the diversity of LPS may correlate with differential immunopathogenicity and virulence among B. pseudomallei strains. 相似文献
614.
Global amphibian declines suggest a major shift in the amount and quality of habitat for these sensitive taxa. Many species that were once widespread are now experiencing declines either in part of or across their historic range. The northern leopard frog (Rana [Lithobates] pipiens] has undergone significant declines particularly in the western United States and Canada. Leopard frog population losses in Nevada are largely due to habitat fragmentation and the introduction of nonnative fish, amphibian, and plant species. Only two populations remain in the Truckee and Carson River watersheds of western Nevada which represents the western boundary of this species range. We used sequence data for an 812 base pair fragment of the mitochondrial NADH dehydrogenase 1 (ND1) gene to support a native origin for western Nevada populations. All frogs had a single haplotype (W07) from the distinct western North America ND1 haplotype clade. Data from seven polymorphic microsatellite loci show that Truckee and Carson River populations are highly differentiated from each other and from leopard frogs collected from eastern Nevada sites. Lack of gene flow among and distinct color morphs among the western Nevada populations likely predates the current geographical isolation. Comparisons with other peripheral L. pipiens populations show western Nevada populations have similar levels of gene diversity despite their contemporary isolation (H(E) 0.411, 0.482). Restoration of leopard frog populations in these watersheds will be challenging given well-entrenched nonnative bullfrog populations and major changes to the riparian zone over the past century. Declines of once common amphibian species has become a major conservation concern. Contemporary isolation of populations on a species range periphery such as the leopard frog populations in the Truckee and Carson rivers further exacerbate extirpation risk as these populations are likely to have fewer genetic resources to adaptively respond to rapidly changing biotic and abiotic environments. 相似文献
615.
Liang Y Van Nostrand JD N'guessan LA Peacock AD Deng Y Long PE Resch CT Wu L He Z Li G Hazen TC Lovley DR Zhou J 《Applied and environmental microbiology》2012,78(8):2966-2972
To better understand the microbial functional diversity changes with subsurface redox conditions during in situ uranium bioremediation, key functional genes were studied with GeoChip, a comprehensive functional gene microarray, in field experiments at a uranium mill tailings remedial action (UMTRA) site (Rifle, CO). The results indicated that functional microbial communities altered with a shift in the dominant metabolic process, as documented by hierarchical cluster and ordination analyses of all detected functional genes. The abundance of dsrAB genes (dissimilatory sulfite reductase genes) and methane generation-related mcr genes (methyl coenzyme M reductase coding genes) increased when redox conditions shifted from Fe-reducing to sulfate-reducing conditions. The cytochrome genes detected were primarily from Geobacter sp. and decreased with lower subsurface redox conditions. Statistical analysis of environmental parameters and functional genes indicated that acetate, U(VI), and redox potential (E(h)) were the most significant geochemical variables linked to microbial functional gene structures, and changes in microbial functional diversity were strongly related to the dominant terminal electron-accepting process following acetate addition. The study indicates that the microbial functional genes clearly reflect the in situ redox conditions and the dominant microbial processes, which in turn influence uranium bioreduction. Microbial functional genes thus could be very useful for tracking microbial community structure and dynamics during bioremediation. 相似文献
616.
Michael P. Moulton Wendell P. Cropper Jr. Linda E. Moulton Michael L. Avery David Peacock 《Biodiversity and Conservation》2012,21(1):155-174
Introduced species are widely believed to represent a significant threat to conservation of biological diversity. A better
understanding of the ecological factors associated with successful species establishment should lead to improved management
and mitigation of these introductions. The “propagule pressure hypothesis”, implying a greater chance of successful introduction
with greater numbers introduced, has been widely accepted as a principal ecological factor in explaining establishment of
exotic species. The historical record of bird introductions in a few locations, including the state of Victoria in Australia,
has been advanced as the principal quantitative support for the hypothesis. We compiled lists of bird species introductions
into Australia from several sources, and discovered inconsistencies in the records of introductions. In a series of comparisons,
we found that the historical record of passerine introductions to Australia does not support the propagule pressure hypothesis
unless superfluous introductions of already successful species are included. An additional problem with previous analyses
is the inclusion of unsuccessful haphazard cage escapes. 相似文献
617.
Handley KM Wrighton KC Piceno YM Andersen GL DeSantis TZ Williams KH Wilkins MJ N'Guessan AL Peacock A Bargar J Long PE Banfield JF 《FEMS microbiology ecology》2012,81(1):188-204
There is increasing interest in harnessing the functional capacities of indigenous microbial communities to transform and remediate a wide range of environmental contaminants. Information about which community members respond to stimulation can guide the interpretation and development of remediation approaches. To comprehensively determine community membership and abundance patterns among a suite of samples associated with uranium bioremediation experiments, we employed a high-density microarray (PhyloChip). Samples were unstimulated, naturally reducing, or collected during Fe(III) (early) and sulfate reduction (late biostimulation) from an acetate re-amended/amended aquifer in Rifle, Colorado, and from laboratory experiments using field-collected materials. Deep community sampling with PhyloChip identified hundreds-to-thousands of operational taxonomic units (OTUs) present during amendment, and revealed close similarity among highly enriched taxa from drill core and groundwater well-deployed column sediment. Overall, phylogenetic data suggested that stimulated community membership was most affected by a carryover effect between annual stimulation events. Nevertheless, OTUs within the Fe(III)- and sulfate-reducing lineages, Desulfuromonadales and Desulfobacterales, were repeatedly stimulated. Less consistent, co-enriched taxa represented additional lineages associated with Fe(III) and sulfate reduction (e.g. Desulfovibrionales; Syntrophobacterales; Peptococcaceae) and autotrophic sulfur oxidation (Sulfurovum; Campylobacterales). Data implies complex membership among highly stimulated taxa and, by inference, biogeochemical responses to acetate, a nonfermentable substrate. 相似文献
618.
Fallen coarse woody debris (CWD) is critical to forest biodiversity and function. Few studies model factors that influence CWD availability, although such investigations are critically needed to inform sustainable forest management. We assess benchmark levels of CWD in unharvested native forests and those harvested for timber, across a range of forests in north‐east New South Wales, Australia. We found timber‐harvesting was the dominant driver of CWD, with almost double the count (pieces ha?1) and volume (m3 ha?1) of total CWD in selectively harvested than unharvested sites. This pattern was consistent across wet and dry forest types. Harvested sites had greater counts of hollow‐bearing logs, and greater volumes of small and medium‐sized CWD (15–50 cm diameter) than unharvested sites. There was no effect of harvesting on the volume of large CWD (>51 cm diameter). Total volumes of CWD (>15 cm diameter) varied from 114 to 166 m3 ha?1. We found few differences in CWD counts and volumes between forest types, with grassy woodlands and forests containing less CWD than other dry and shrubby forest types, reflecting lower potential input rates. The CWD levels recorded here are similar to those recorded in dry and wet sclerophyll forests elsewhere in Australia and are typical of global estimates for ‘old growth’ forests. Using general linear models we captured up to 57% of the variation in CWD across sites, and found that timber harvesting, topography and the numbers of standing hollow‐bearing and dead trees were significant predictors of CWD. Values for unharvested forest provide a benchmark that could be used to inform retention guidelines for CWD in managed forests in this region. Further assessment of the effect of repeat timber harvesting is needed to fully understand its impact on CWD dynamics, especially if forest residues resulting from timber harvesting are removed from native forests for bioenergy production. 相似文献
619.
Julia Cooke Yoseph Araya Karen L. Bacon Joanna M. Bagniewska Lesley C. Batty Tom R. Bishop Moya Burns Magda Charalambous David R. Daversa Liam R. Dougherty Miranda Dyson Adam M. Fisher Dan Forman Cristina Garcia Ewan Harney Thomas Hesselberg Elizabeth A. John Robert J. Knell Kadmiel Maseyk Alice L. Mauchline Julie Peacock Angelo P. Pernetta Jeremy Pritchard William J. Sutherland Rebecca L. Thomas Barbara Tigar Philip Wheeler Rachel L. White Nicholas T. Worsfold Zenobia Lewis 《Oikos》2021,130(1):15-28
We currently face significant, anthropogenic, global environmental challenges and the role of ecologists in mitigating these challenges is arguably more important than ever. Consequently there is an urgent need to recruit and train future generations of ecologists, both those whose main area is ecology, but also those involved in the geological, biological and environmental sciences. Here we present the results of a horizon scanning exercise that identified current and future challenges facing the teaching of ecology, through surveys of teachers, students and employers of ecologists. Key challenges identified were grouped in terms of the perspectives of three groups: students, for example the increasing disconnect between people and nature; teachers, for example the challenges associated with teaching the quantitative skills that are inherent to the study of ecology; and society, for example poor societal perceptions of the field of ecology. In addition to the challenges identified, we propose a number of solutions developed at a workshop by a team of ecology teaching experts, with supporting evidence of their potential to address many of the problems raised. These proposed solutions include developing living labs, teaching students to be ecological entrepreneurs and influencers, embedding skills-based learning and coding in the curriculum, an increased role for learned societies in teaching and learning, and using new technology to enhance fieldwork studies including virtual reality, artificial intelligence and real-time spoken language translation. Our findings are focused towards UK higher education, but they should be informative for students and teachers of a wide range of educational levels, policy makers and professional ecologists worldwide. 相似文献
620.
S Vamsee Raju Jody H Tate Sandra KG Peacock Ping Fang Robert A Oster Mark T Dransfield Steven M Rowe 《Respiratory research》2014,15(1):18