全文获取类型
收费全文 | 408篇 |
免费 | 23篇 |
出版年
2018年 | 4篇 |
2016年 | 6篇 |
2015年 | 14篇 |
2014年 | 15篇 |
2013年 | 23篇 |
2012年 | 14篇 |
2011年 | 19篇 |
2010年 | 30篇 |
2009年 | 26篇 |
2008年 | 14篇 |
2007年 | 14篇 |
2006年 | 11篇 |
2005年 | 15篇 |
2004年 | 13篇 |
2003年 | 4篇 |
2002年 | 7篇 |
2001年 | 6篇 |
2000年 | 5篇 |
1999年 | 3篇 |
1998年 | 11篇 |
1997年 | 11篇 |
1996年 | 7篇 |
1995年 | 7篇 |
1994年 | 6篇 |
1993年 | 9篇 |
1992年 | 2篇 |
1991年 | 6篇 |
1989年 | 3篇 |
1988年 | 11篇 |
1987年 | 3篇 |
1985年 | 5篇 |
1984年 | 5篇 |
1983年 | 5篇 |
1982年 | 18篇 |
1981年 | 3篇 |
1979年 | 10篇 |
1978年 | 2篇 |
1977年 | 7篇 |
1976年 | 3篇 |
1975年 | 4篇 |
1972年 | 2篇 |
1959年 | 2篇 |
1958年 | 6篇 |
1956年 | 10篇 |
1955年 | 5篇 |
1954年 | 3篇 |
1952年 | 2篇 |
1950年 | 3篇 |
1949年 | 2篇 |
1948年 | 3篇 |
排序方式: 共有431条查询结果,搜索用时 991 毫秒
1.
1. Few detailed long-term data sets exist for fresh waters with which to examine large-scale temporal changes in community composition. Consequently, insight into community persistence has been restricted to a few, contingent case studies. We collated and analysed data for the aquatic macroinvertebrate community of Broadstone Stream in south-east England, spanning three decades. The pH of this naturally acid stream has risen progressively since the 1970s, and we sought to examine the potential effects of this environmental change upon the community.
2. Persistence within Broadstone was high when compared with other systems that have been analysed using similar methods. The stream was characterised by a `core' community of eight taxa that were always present, and contributed 75–97% of total invertebrate abundance, with a trailing limb of progressively rarer and more acid-sensitive taxa. There was little species turnover, although the time-series exceeded 20 generations for most species.
3. Despite this high persistence, a long-term response to rising pH was detected: species indicating profound acidity (identified a priori from independent studies) have declined since the 1970s, whereas indicators of moderate acidity increased. The structure of the community food web has also changed since the 1970s, with increased predator diversity and abundance, and a lengthening of food chains following the invasion of a new top predator.
4. These changes in the community appeared to be driven by an interaction between pH and climate. The unusually hot, dry summers characteristic of the 1990s may have raised pH during the more sensitive (i.e. early) stages of the life-cycle, and thus provided a window of opportunity for less acid-tolerant taxa to colonise and become established. Changes in pH appeared to set the boundaries of the available local species pool, within which biotic interactions ultimately shaped the community. 相似文献
2. Persistence within Broadstone was high when compared with other systems that have been analysed using similar methods. The stream was characterised by a `core' community of eight taxa that were always present, and contributed 75–97% of total invertebrate abundance, with a trailing limb of progressively rarer and more acid-sensitive taxa. There was little species turnover, although the time-series exceeded 20 generations for most species.
3. Despite this high persistence, a long-term response to rising pH was detected: species indicating profound acidity (identified a priori from independent studies) have declined since the 1970s, whereas indicators of moderate acidity increased. The structure of the community food web has also changed since the 1970s, with increased predator diversity and abundance, and a lengthening of food chains following the invasion of a new top predator.
4. These changes in the community appeared to be driven by an interaction between pH and climate. The unusually hot, dry summers characteristic of the 1990s may have raised pH during the more sensitive (i.e. early) stages of the life-cycle, and thus provided a window of opportunity for less acid-tolerant taxa to colonise and become established. Changes in pH appeared to set the boundaries of the available local species pool, within which biotic interactions ultimately shaped the community. 相似文献
2.
3.
4.
5.
6.
7.
Experimental Investigations on the Environmental Determination of {delta}13C at Different Altitudes 总被引:1,自引:0,他引:1
The influence of environmental variables which are known tochange with altitude on the 13C of leaves of Nardus stricta,have been investigated in controlled environment experiments.Low temperature and an increased incidence of freezing temperaturescaused 13C to increase. In contrast, lowered atmospheric pressurereduced 13C. Increased leaf surface wetness and soil water contentalso caused a decrease. Plants from different altitudes didnot differ consistently or significantly in their responses. Key words: Carbon isotope discrimination, altitude, temperature, atmospheric pressure, freezing 相似文献
8.
The phylogeny of Greya Busck (Lepidoptera: Prodoxidae) was inferred from
nucleotide sequence variation across a 765-bp region in the cytochrome
oxidase I and II genes of the mitochondrial genome. Most parsimonious
relationships of 25 haplotypes from 16 Greya species and two outgroup
genera (Tetragma and Prodoxus) showed substantial congruence with the
species relationships indicated by morphological variation. Differences
between mitochondrial and morphological trees were found primarily in the
positions of two species, G. variabilis and G. pectinifera, and in the
branching order of the three major species groups in the genus. Conflicts
between the data sets were examined by comparing levels of homoplasy in
characters supporting alternative hypotheses. The phylogeny of Greya
species suggests that host-plant association at the family level and larval
feeding mode are conservative characters. Transition/transversion ratios
estimated by reconstruction of nucleotide substitutions on the phylogeny
had a range of 2.0-9.3, when different subsets of the phylogeny were used.
The decline of this ratio with the increase in maximum sequence divergence
among taxa indicates that transitions are masked by transversions along
deeper internodes or long branches of the phylogeny. Among transitions,
substitutions of A-->G and T-->C outnumbered their reciprocal
substitutions by 2-6 times, presumably because of the approximately 4:1
(77%) A+T-bias in nucleotide base composition. Of all transversions,
73%-80% were A<-->T substitutions, 85% of which occurred at third
positions of codons; these estimates did not decrease with an increase in
maximum sequence divergence of taxa included in the analysis. The high
frequency of A<-->T substitutions is either a reflection or an
explanation of the 92% A+T bias at third codon positions.
相似文献
9.
10.
The occurrence of Almeidini (Hemiptera: Anthocoridae) in Australia, with the description of a new genus 总被引:1,自引:0,他引:1
T. E. WOODWARD 《Systematic Entomology》1977,2(3):255-260
Abstract. The tribe Almeidini Carayon, 1972 is recorded from Australia and Australmeida gen.n. described. The brachypterous male and female of the typespecies Lasiochilus derricki Gross, 1954 are redescribed and the macropterous male and female and the submacropterous female are newly described. One possibly conspecific specimen is recorded from Kerevat, New Britain. The structure of the male and female internal reproductive systems reinforces the external characters used by Carayon (1972a) as indicating close affinity of the Almeidini to the Xylocorini. The genus Almeida Distant is recorded from Australia and the genuse Limppomanus Distant from New caledonia. 相似文献