全文获取类型
收费全文 | 277篇 |
免费 | 7篇 |
专业分类
284篇 |
出版年
2016年 | 4篇 |
2015年 | 6篇 |
2013年 | 8篇 |
2012年 | 4篇 |
2011年 | 9篇 |
2010年 | 13篇 |
2009年 | 13篇 |
2008年 | 9篇 |
2007年 | 12篇 |
2006年 | 12篇 |
2005年 | 9篇 |
2004年 | 5篇 |
2003年 | 6篇 |
2001年 | 6篇 |
2000年 | 2篇 |
1999年 | 4篇 |
1998年 | 3篇 |
1997年 | 6篇 |
1996年 | 3篇 |
1995年 | 10篇 |
1994年 | 3篇 |
1993年 | 4篇 |
1992年 | 7篇 |
1991年 | 8篇 |
1990年 | 3篇 |
1989年 | 4篇 |
1988年 | 8篇 |
1987年 | 6篇 |
1986年 | 3篇 |
1985年 | 3篇 |
1984年 | 4篇 |
1983年 | 4篇 |
1982年 | 5篇 |
1981年 | 10篇 |
1980年 | 10篇 |
1979年 | 5篇 |
1978年 | 9篇 |
1977年 | 4篇 |
1976年 | 5篇 |
1975年 | 2篇 |
1974年 | 3篇 |
1973年 | 2篇 |
1972年 | 2篇 |
1971年 | 2篇 |
1956年 | 2篇 |
1954年 | 2篇 |
1952年 | 2篇 |
1951年 | 2篇 |
1950年 | 3篇 |
1934年 | 1篇 |
排序方式: 共有284条查询结果,搜索用时 0 毫秒
111.
Abstract: Gray wolf (Canis lupus) populations are recovering in many parts of the world and managers from various jurisdictions will be faced with difficult decisions as wolf populations continue to increase. Wolf management in Alberta, Canada, is achieved mostly through trapping on a registered trapline system. Wolf harvest increased over the last 22 years relative to population increases. Most wolf harvests occurred in the Rocky Mountains and surrounding foothills area and this pattern was consistent over time. On average, trappers only harvested an estimated 9.8% of the provincial population annually despite the lack of bag limits or quotas. Harvests are spatially autocorrelated with peak autocorrelation coinciding with average home-range size for wolves in Alberta. When wolf control actions are deemed necessary, trappers are unlikely to remove a sufficient number of wolves over a large enough area to limit subpopulations under the registered trapline system. 相似文献
112.
HÉLÈNE VERHEYDEN PHILIPPE BALLON† VALERIE BERNARD CHRISTINE SAINT-ANDRIEUX‡ 《Mammal Review》2006,36(3):217-234
- 1 The literature on bark‐stripping by red deer Cervus elaphus in Europe is reviewed to reveal quantitative variation in this behaviour and relate it to deer density and local characteristics such as dominant tree species, occurrence of artificial feeding, altitude, region and size of the study site. We also review the importance of bark in red deer diets over the seasons and discuss the causes of bark‐stripping, focusing on the significance of bark as food.
- 2 Over the 36 sites examined, the rate of bark‐stripping was highly variable (from 0 to 84% of susceptible trees debarked), with less damage in Scotland than in other European sites for which bark‐stripping rates were higher at high red deer density. Altitude, the size of the study site, the number of dominant tree species and the occurrence of artificial feeding do not significantly relate to the rate of bark‐stripping.
- 3 Bark sometimes made up a large proportion of red deer diet (> 10%), especially in areas with severe winters (high levels of snow), whereas in study sites with mild winters, bark was practically not eaten at all.
- 4 These results suggest that severe bark‐stripping could be related to a reduction in food resource availability. This food availability hypothesis needs to be better documented, dealing particularly with the possible interaction between food availability and red deer density.
113.
Woody biomass production during the second rotation of a bio-energy Populus plantation increases in a future high CO2 world 总被引:2,自引:0,他引:2
MARION LIBERLOO CARLO CALFAPIETRA† MARTIN LUKAC‡ DOUGLAS GODBOLD‡ ZHI-BIN LUO§ REA POLLE§ MARCEL R. HOOSBEEK¶ OLEVI KULL MICHAL MAREK CHRISTINE RAINES†† MAURO RUBINO‡‡ GAIL TAYLOR§§ GIUSEPPE SCARASCIA-MUGNOZZA† REINHART CEULEMANS 《Global Change Biology》2006,12(6):1094-1106
The quickly rising atmospheric carbon dioxide (CO2)‐levels, justify the need to explore all carbon (C) sequestration possibilities that might mitigate the current CO2 increase. Here, we report the likely impact of future increases in atmospheric CO2 on woody biomass production of three poplar species (Populus alba L. clone 2AS‐11, Populus nigra L. clone Jean Pourtet and Populus×euramericana clone I‐214). Trees were growing in a high‐density coppice plantation during the second rotation (i.e., regrowth after coppice; 2002–2004; POPFACE/EUROFACE). Six plots were studied, half of which were continuously fumigated with CO2 (FACE; free air carbon dioxide enrichment of 550 ppm). Half of each plot was fertilized to study the interaction between CO2 and nutrient fertilization. At the end of the second rotation, selective above‐ and belowground harvests were performed to estimate the productivity of this bio‐energy plantation. Fertilization did not affect growth of the poplar trees, which was likely because of the high rates of fertilization during the previous agricultural land use. In contrast, elevated CO2 enhanced biomass production by up to 29%, and this stimulation did not differ between above‐ and belowground parts. The increased initial stump size resulting from elevated CO2 during the first rotation (1999–2001) could not solely explain the observed final biomass increase. The larger leaf area index after canopy closure and the absence of any major photosynthetic acclimation after 6 years of fumigation caused the sustained CO2‐induced biomass increase after coppice. These results suggest that, under future CO2 concentrations, managed poplar coppice systems may exhibit higher potential for C sequestration and, thus, help mitigate climate change when used as a source of C‐neutral energy. 相似文献
114.
CHRISTINE OUINSAVI NESTOR SOKPON JEAN BOUSQUET CRAIG H. NEWTON DAMASE P. KHASA 《Molecular ecology resources》2006,6(2):480-483
Eleven microsatellite primer pairs were developed for the tropical African tree Milicia excelsa. Genomic DNA was enriched for dinucleotide (TCn and TGn) and tretranucleotide (GATAn), and 188 random clones were sequenced from both orientations. We designed and tested 44 oligonucleotide primer pairs, which were evaluated using genomic DNA from 30 M. excelsa mature trees collected from a natural population in Benin. Eleven of the 44 markers showed good amplification and were polymorphic. The number of putative alleles for polymorphic primer pairs varied from three to seven, with expected and observed heterozygosities ranging from 0.10 to 0.64 and from 0.10 to 0.80, respectively. All 11 loci amplified the related species Milicia regia, indicating that these primers will be useful for population and ecology genetic studies in other species of the genus Milicia. 相似文献
115.
BROU BASILE KOFFI ANGE MARIE RISTERUCCI DOMINIQUE JOULIA PATRICK DURAND NICOLAS BARR THIERRY DE MEEÛS CHRISTINE CHEVILLON 《Molecular ecology resources》2006,6(2):502-504
Nine microsatellite loci from the cattle tick Boophilus microplus were isolated and characterized within New Caledonia. This Pacific island was invaded by the cattle tick from a few immigration events dating from mid‐20th century. A population survey involving 94 adult ticks indicated monomorphism at one locus, presence of null alleles at another, and high genetic diversity (0.61–0.72) at seven loci apparently suitable for population genetics studies. This opens the opportunity to dissect the populational mechanisms involved in the spectacular capacity of B. microplus to cope with local environmental heterogeneity during its recurrent invasions of tropical areas. 相似文献
116.
PETER KORSTEN JAKOB C. MUELLER CHRISTINE HERMANNSTÄDTER KAREN M. BOUWMAN NIELS J. DINGEMANSE PIET J. DRENT MIRIAM LIEDVOGEL ERIK MATTHYSEN KEES
Van OERS THIJS
Van OVERVELD SAMANTHA C. PATRICK JOHN L. QUINN BEN C. SHELDON JOOST M. TINBERGEN BART KEMPENAERS 《Molecular ecology》2010,19(4):832-843
Polymorphisms in the dopamine receptor D4 gene (DRD4) have been related to individual variation in novelty‐seeking or exploratory behaviour in a variety of animals, including humans. Recently, the human DRD4 orthologue was sequenced in a wild bird, the great tit (Parus major) and a single nucleotide polymorphism in exon 3 of this gene (SNP830) was shown to be associated with variation in exploratory behaviour of lab‐raised individuals originating from a single wild population. Here we test the generality of this finding in a large sample of free‐living individuals from four European great tit populations, including the originally sampled population. We demonstrate that the association between SNP830 genotype and exploratory behaviour also exists in free‐living birds from the original population. However, in the other three populations we found only limited evidence for an association: in two populations the association appeared absent; while in one there was a nonsignificant tendency. We could not confirm a previously demonstrated interaction with another DRD4 polymorphism, a 15 bp indel in the promoter region (ID15). As yet unknown differences in genetic or environmental background could explain why the same genetic polymorphism (SNP830) has a substantial effect on exploratory behaviour in one population, explaining 4.5–5.8% of the total variance—a large effect for a single gene influencing a complex behavioural trait—but not in three others. The confirmation of an association between SNP830 genotype and personality‐related behaviour in a wild bird population warrants further research into potential fitness effects of the polymorphism, while also the population differences in the strength of the association deserve further investigation. Another important future challenge is the identification of additional loci influencing avian personality traits in the wild. 相似文献
117.
Abstract.
- 1 In natural populations, colonies of bumble bees vary in many important life history traits, such as colony size and age at maturity, or the number and sex of reproductives produced. We investigated how the presence of parasites in field populations of the bumble bee Bombus lucorum L. relates to variation in life history traits and reproductive performance. A total of thirty-six colonies was placed in accessible nest sites in the field and monitored at regular intervals throughout a season.
- 2 Among the life history correlates, early nest foundation was strongly associated with large maximum colony size, old age and large size at maturity, and this in turn with successful production of males and queens, as well as with the number of sexuals produced. Overall, reproductive success was highly skewed with only five colonies producing all the queens. Sixteen colonies failed to reproduce altogether.
- 3 The social parasite Psithyrus was abundant early in the Bombus colony cycle and preferentially invaded host nests with many first brood workers and thus disproportionately large size, i.e. those colonies that would otherwise be more likely to reproduce or produce (daughter) queens rather than males. To prevent nest loss, Psithyrus had to be removed soon after invasion. Therefore, the effects reported here can only be crude estimates.
- 4 Parasitoid conopid flies are likely to cause heavy worker mortality when sexuals are reared by the colonies. Their inferred effect was a reduction in biomass that could be invested in sexuals as well as a shift in the sex ratio at the population level resulting from failure to produce queens. We suggest to group the inferred correlates into ‘early events’ surrounding colony initiation and social parasitism, and ‘late events’ surrounding attained colony size in summer and parasitism by conopid flies. Our evidence thus provides a heuristic approach to understand the factors that affect reproductive success of Bombus colonies.
118.
Abstract 1. Variation in plant chemistry does not only mediate interactions between plants and herbivores but also those between herbivores and their natural enemies, and plants and natural enemies. 2. Endophytic fungi complete their whole life cycle within the host plant’s tissue and are associated with a large diversity of plant species. Endophytes of the genus Neotyphodium alter the chemistry of the host plant by producing herbivore toxic alkaloids. 3. Here we asked whether the endophyte‐tolerant aphid species Metopolophium festucae could be defended against its parasitoid Aphidius ervi when feeding on endophyte‐infected plants. In a laboratory experiment, we compared life‐history traits of A. ervi when exposed to hosts on endophyte‐infected or endophyte‐free Lolium perenne. 4. The presence of endophytes significantly increased larval and pupal development times, but did not affect the mortality of immature parasitoids or the longevity of the adults. Although the number of parasitoid mummies tended to be reduced on endophyte‐infected plants, the number of emerging parasitoids did not differ significantly between the two treatments. 5. This shows that the metabolism of individual aphids feeding on infected plants may be changed and help in the defence against parasitoids. An increase in parasitoid development time should ultimately reduce the population growth of A. ervi. Therefore, endophyte presence may represent an advantage for endophyte‐tolerant aphid species through extended parasitoid development and its effect on parasitoid population dynamics. 相似文献
119.
120.
Colloidal‐size synthetic‐polymer microspheres of three sizes (0.15, 0.31 and 0.70‐µm diameter) were each suspended in liquids of three different colors (clear, tan and dark brown) at a range of concentrations. The samples were presented to panelists in a viewing box. Visual‐threshold determinations were made for each of the nine sample sets using the ascending method of limits (AMLs) and bright illumination (1,192 lux) with each of the three viewing backgrounds: black velvet, white cotton and white taffeta. The thresholds were much more similarly expressed as turbidimeter observations than as either weight or number concentration. With the black‐velvet background, the thresholds ranged from 0.206 to 2.190 nephelometric turbidity units (NTU). With white cotton and white taffeta, they were considerably higher (P < 0.001), ranging from 1.97 to 41.00 and 2.97 to 34.40 NTU, respectively. This large difference appears to be a matter of contrast – scattered white light is much easier to perceive against a dark background. 相似文献