全文获取类型
收费全文 | 141篇 |
免费 | 4篇 |
出版年
2010年 | 4篇 |
2009年 | 3篇 |
2006年 | 5篇 |
2005年 | 3篇 |
2001年 | 3篇 |
2000年 | 2篇 |
1998年 | 2篇 |
1997年 | 4篇 |
1996年 | 8篇 |
1995年 | 4篇 |
1994年 | 4篇 |
1993年 | 3篇 |
1992年 | 3篇 |
1991年 | 3篇 |
1990年 | 5篇 |
1989年 | 5篇 |
1984年 | 2篇 |
1980年 | 2篇 |
1979年 | 2篇 |
1978年 | 3篇 |
1977年 | 2篇 |
1976年 | 2篇 |
1975年 | 3篇 |
1973年 | 2篇 |
1972年 | 4篇 |
1971年 | 3篇 |
1966年 | 4篇 |
1964年 | 2篇 |
1962年 | 2篇 |
1960年 | 1篇 |
1959年 | 2篇 |
1958年 | 3篇 |
1957年 | 2篇 |
1956年 | 5篇 |
1955年 | 4篇 |
1954年 | 5篇 |
1953年 | 1篇 |
1951年 | 2篇 |
1950年 | 2篇 |
1949年 | 1篇 |
1946年 | 1篇 |
1935年 | 1篇 |
1933年 | 1篇 |
1932年 | 1篇 |
1931年 | 1篇 |
1929年 | 1篇 |
1927年 | 1篇 |
1911年 | 1篇 |
1910年 | 1篇 |
1906年 | 1篇 |
排序方式: 共有145条查询结果,搜索用时 15 毫秒
31.
MARJAN JONGEN MIKE B. JONES THOMAS HEBEISEN HERBERT BLUM GEORGE HENDREY 《Global Change Biology》1995,1(5):361-371
Lolium perenne and Trifolium repens were grown in a Free Air CO2 Enrichment (FACE) system at elevated (600 μimol mol-1) and ambient (340 μmol mol-1) carbon dioxide concentrations during a whole growing season. Using a root ingrowth bag technique the extent to which CO2 enrichment influenced the growth of L, perenne and T. repens roots under two contrasting nutrient regimes was examined. Root ingrowth bags were inserted for a fixed time into the soil in order to trap roots. It was also possible to follow the mortality of roots in bags inserted for different time intervals. Root ingrowth of both L. perenne and T. repens increased under elevated CO2 conditions. In L. perenne, root ingrowth decreased with increasing nutrient fertilizer level, but for T. repens the root ingrowth was not affected by the nutrient application rate. Besides biomass measurements, root length estimates were made for T, repens. These showed an increase under elevated CO2 concentrations. Root decomposition appeared to decrease under elevated CO2 concentrations. A possible explanation for this effect is the observed changes in tissue composition, such as the increase in the carbon: nitrogen ratio in roots of L. perenne at elevated CO2 concentrations. 相似文献
32.
Strategies of passerine migration across the Mediterranean Sea and the Sahara Desert: a radar study 总被引:2,自引:0,他引:2
HERBERT BIEBACH IRIS BIEBACH WOLFGANG FRIEDRICH GEORG HEINE JESKO PARTECKE DIETER SCHMIDL 《Ibis》2000,142(4):623-634
Radar observations of the diurnal timing of bird migration in the Sahara Desert are presented for autumn migration. Study sites were on a transect along the north-south migratory direction. Three groups of birds migrating either during day, evening or night in the northern part of the Western desert in Egypt were identified. The maximum of day and night groups occurred later the further south the study sites were. Based on the distance between sites and the timing of peak migration, birds were flying at an estimated ground speed of about 20 m/s. The maximum of the evening group was at about 21:00 h at all sites. The three groups were classified according to three different strategies of migration across the Mediterranean Sea and the Sahara Desert: (1) the day group of birds performed a non-stop flight across the sea and at least the northern part of the desert; [2] the night group performed an intermittent migratory strategy with stopover at the coast of Egypt to continue migration the next evening; (3) the evening group birds were also intermittent migratory fliers, but they stopped somewhere in the desert after a continuous flight across the sea and part of the desert. About 20% of all migrants are involved in non-stop migration and 80% in intermittent migration with stopover at the coast (70%) or with stopover in the desert (10%). It is argued that any species of small passerine has the option to use any of the three strategies. 相似文献
33.
The effects of stimulus flow rate on perceived taste intensitywere studied in man using five concentrations of each of fourstimulus compounds (sodium chloride, hydrochloric acid, quininehydrochloride, and sucrose) and six different flow rates from1.2 to 12.0 cc/sec. AH stimuli were presented through a tonguechamber. Perceived taste intensity increased monotonically withflow rate for every stimulus condition tested. Psychophysicalfunctions calculated across flow rates (rather than across physicalconcentrations) support the view that flow rates mimic physicalconcentration. 相似文献
34.
35.
ADOLFO AMZQUITA ALBERTINA P. LIMA ROBERT JEHLE LINA CASTELLANOS
SCAR RAMOS ANDREW J. CRAWFORD HERBERT GASSER WALTER H
DL 《Biological journal of the Linnean Society. Linnean Society of London》2009,98(4):826-838
Evolutionary divergence in behavioural traits related to mating may represent the initial stage of speciation. Direct selective forces are usually invoked to explain divergence in mate‐recognition traits, often neglecting a role for neutral processes or concomitant differentiation in ecological traits. We adopted a multi‐trait approach to obtain a deeper understanding of the mechanisms behind allopatric divergence in the Amazonian frog, Allobates femoralis. We tested the null hypothesis that geographic distance between populations correlates with genetic and phenotypic divergence, and compared divergence between mate‐recognition (acoustic) and ecological (coloration, body‐shape) traits. We quantified geographic variation in 39 phenotypic traits and a mitochondrial DNA marker among 125 individuals representing eight populations. Geographic variation in acoustic traits was pronounced and tracked the spatial genetic variation, which appeared to be neutral. Thus, the evolution of acoustic traits tracked the shared history of the populations, which is unexpected for pan‐Amazonian taxa or for mate‐recognition traits. Divergence in coloration appeared uncorrelated with genetic distance, and might be partly attributed to local selective pressures, and perhaps to Batesian mimicry. Divergence in body‐shape traits was low. The results obtained depict a complex evolutionary scenario and emphasize the importance of considering multiple traits when disentangling the forces behind allopatric divergence. ©2009 The Linnean Society of London, Biological Journal of the Linnean Society, 2009, 98 , 826–838. 相似文献
36.
Invertebrate diversity is seldom included in conservation assessments, primarily because information is lacking. Broad surveys may be too costly, difficult or ineffective. Here we assess a ‘shopping basket’ approach, targeting 17 taxa using a range of methods. We sampled 43 one‐hectare sites stratified within 560 km2 of heterogenous African savanna. We achieved up to 80% sampling completeness for epigaeic fauna, but generally much lower completeness (around 50%) for plant‐dwelling and flying taxa. For the former we identified duplication of methods, and for the latter, addition of methods and increased temporal variation rather than effort would improve completeness. Within a taxon, sampling 75% of species present required, on average, about 784 individuals. When considering the local richness, 75% completeness required about 27 individuals per species, but these figures require validation in other areas. About 58 sites were required to achieve 75% sampling completeness, translating to about one site per 10 km2. The percentage of species sampled only in a particular month ranged between 4% and 46%, with greater temporal effects recorded for flying taxa than for epigaeic ones. The trend was similar for species unique to a particular year, with the most extreme case being 67% of the butterfly species sampled one year not previously recorded. We demonstrated and evaluated the feasibility of a simultaneous multi‐taxon survey approach to produce data useful for conservation planning and monitoring. We strongly recommend a quantified approach for surveys and inventories, with details such as specific methods decided based on the biome sampled, and taxonomic expertise available for identification. 相似文献
37.
MICHAEL DRESCHER IGNAS M. A. HEITK
NIG PAUL J. VAN DEN BRINK HERBERT H. T. PRINS 《Austral ecology》2006,31(1):76-87
Abstract: The grass layer of savannas is characterized by strong temporal and spatial heterogeneity in the quantity and quality of forage. Besides this, there is strong variation in other aspects of sward structure, here defined as sward chemistry, morphology, architecture and species composition. The forage maturation hypothesis (FMH) is based on the temporal dynamics of forage quantity and quality of grasslands. Recently this hypothesis has been used to explain the foraging behaviour of large herbivores as an optimal solution for balancing forage ingestion and forage digestion, leading to a maximization of daily rates of forage intake in patches of intermediate forage quantity. However, so far studies using this hypothesis have been constrained to mono‐specific forage resources or have ignored inter‐patch variation in sward structure. We studied the foraging behaviour of cattle in a South African savanna. We explicitly addressed forage quantity and quality, and the structure of seven sward types in order to investigate (i) their effects on foraging behaviour; and (ii) the assumptions and predictions of the FMH for foraging behaviour in grasslands with pronounced variation in sward structure. The results indicated that the assumptions of the FMH were met and that forage quantity affected forage intake the most, with sward structure having little or no effect. The predicted maximum rate of daily forage intake agreed with the results of previous studies. However, the forage quantity at which this maximum intake rate was found, was larger than expected based on the results of some studies. It is likely that this discrepancy resulted from the use of artificial leaf‐only grass swards in these studies. The results suggest that the FMH can be used to explain the foraging behaviour of herbivores over a wide range of sward structures. 相似文献
38.
39.
40.