全文获取类型
收费全文 | 405篇 |
免费 | 7篇 |
专业分类
412篇 |
出版年
2017年 | 4篇 |
2016年 | 2篇 |
2014年 | 8篇 |
2013年 | 12篇 |
2012年 | 18篇 |
2011年 | 16篇 |
2010年 | 19篇 |
2009年 | 35篇 |
2008年 | 30篇 |
2007年 | 41篇 |
2006年 | 25篇 |
2005年 | 22篇 |
2004年 | 14篇 |
2003年 | 6篇 |
2001年 | 4篇 |
2000年 | 2篇 |
1999年 | 3篇 |
1998年 | 3篇 |
1997年 | 10篇 |
1996年 | 10篇 |
1995年 | 7篇 |
1994年 | 4篇 |
1993年 | 8篇 |
1992年 | 4篇 |
1991年 | 7篇 |
1990年 | 4篇 |
1989年 | 2篇 |
1988年 | 6篇 |
1987年 | 5篇 |
1986年 | 4篇 |
1985年 | 5篇 |
1984年 | 8篇 |
1983年 | 6篇 |
1982年 | 4篇 |
1981年 | 2篇 |
1980年 | 3篇 |
1979年 | 7篇 |
1978年 | 5篇 |
1975年 | 3篇 |
1974年 | 4篇 |
1973年 | 3篇 |
1972年 | 7篇 |
1971年 | 6篇 |
1965年 | 1篇 |
1963年 | 1篇 |
1959年 | 1篇 |
1958年 | 1篇 |
1950年 | 1篇 |
1949年 | 1篇 |
1948年 | 1篇 |
排序方式: 共有412条查询结果,搜索用时 15 毫秒
71.
Daphnia community analysis in shallow Kenyan lakes and ponds using dormant eggs in surface sediments
JOACHIM MERGEAY STEVEN DECLERCK DIRK VERSCHUREN LUC DE MEESTER 《Freshwater Biology》2006,51(3):399-411
1. Water fleas of the genus Daphnia are considered rare in tropical regions, and information on species distribution and community ecology is scarce and anecdotal. This study presents the results of a survey of Daphnia species distribution and community composition in 40 standing waterbodies in southern Kenya. The study sites cover a wide range of tropical standing aquatic habitats, from small ephemeral pools to large permanent lakes between approximately 700 and 2800 m a.s.l. Our analysis combines data on Daphnia distribution and abundance from zooplankton samples and dormant eggs in surface sediments. 2. Nearly 70% (27 of 40) of the sampled waterbodies were inhabited by Daphnia. Although their abundance in the active community was often very low, this high incidence shows that Daphnia can be equally widespread in tropical regions as in temperate regions. 3. Analysis of local species assemblages from dormant eggs in surface sediments was more productive than snapshot sampling of zooplankton communities. Surface‐sediment samples yielded eight Daphnia species in total, and allowed the detection of Daphnia in 25 waterbodies; zooplankton samples revealed the presence of only four Daphnia species in 16 waterbodies. 4. Daphnia barbata, D. laevis, and D. pulex were the most frequently recorded and most abundant Daphnia species. Canonical correspondence analysis of species–environment relationships indicates that variation in the Daphnia community composition of Kenyan waters was best explained by fish presence, temperature, macrophyte cover and altitude. Daphnia barbata and D. pulex tended to co‐occur with each other and with fish. Both species tended to occur in relatively large (>10 ha) and deeper (>2 m) alkaline waters (pH 8.5). Daphnia laevis mainly occurred in cool and clear, macrophyte‐dominated lakes at high altitudes. 相似文献
72.
Differences between individuals collected from sun and shade populations could result from either somatic or genetic differences, particularly in populations of perennial plants. Our objective in this study was to separate somatic from genetic differences. We collected Anthoxanthum odoratum and Plantago lanceolata from sun and shade populations and made measurements on both vegetatively propagated clones and seed progeny from each clone. The parental populations differed in a wide range of physiological and morphological traits. However, only photosynthetic capacity was significantly different between both the original sun and shade populations and their seed progeny. In both species, plants from the sun population had higher photosynthetic capacities than those from the shade population when grown in a common environment. This demonstrates that there was genetic differentiation between the sun and shade populations. Photosynthetic capacity of parents and offspring also differed, suggesting a somatic effect. Since many of the original clones were virus-infected, but all but one of the offspring were virus-free, this might have been a result of virus infection. However, in spite of the fact that the parents and offspring clones were propagated vegetatively so that the plants were at the same developmental stage at the time of measurement, we cannot rule out the possibility that differences in age of cell lines could also have been a factor. 相似文献
73.
A number of studies in recent years have attempted to elucidate the effects of learning and environmental change on brain biochemistry, especially protein synthesis1. There has also been much interest in the circadian rhythms exhibited by animals, at both the behavioural and biochemical levels (for a review see ref. 2). It therefore seemed of interest to determine whether or not such cyclical responses are reflected in intrinsic rates of protein synthesis. We describe here a diurnal rhythmicity in the incorporation of tritiated lysine into various brain regions and the liver. 相似文献
74.
Interest in bay scallop conservation has resulted in organized stock enhancement efforts and increased attention to fisheries management issues. Genetic markers can facilitate the monitoring of enhancement efforts, characterization of wild populations, and optimize hatchery practices. We have identified eight polymorphic simple sequence repeat markers including one dinucleotide, six trinucleotide and one compound dinucleotide repeats, in expressed sequence tags generated from multiple bay scallop cDNA libraries. The numbers of alleles range from two to five. The expected and observed heterozygosities range from 0.093 to 0.720 and 0.095 to 0.600, respectively. 相似文献
75.
MARY E. A. WHITEHOUSE SCOTT HARDWICK BRAD C. G. SCHOLZ AMANDA J. ANNELLS ANDREW WARD PAUL R. GRUNDY STEVEN HARDEN 《Austral ecology》2009,34(1):10-23
The most common explanation for species diversity increasing towards the tropics is the corresponding increase in habitats (spatial heterogeneity). Consequently, a monoculture (like cotton in Australia) which is grown along a latitudinal gradient, should have the same degree of species diversity throughout its range. We tested to see if diversity in a dominant cotton community (spiders) changed with latitude, and if the community was structurally identical in different parts of Australia. We sampled seven sites extending over 20° of latitude. At each site we sampled 1–3 fields 3–5 times during the cotton growing season using pitfall traps and beatsheets, recording all the spiders collected to family. We found that spider communities in cotton are diverse, including a large range of foraging guilds, making them suitable for a conservation biological control programme. We also found that spider diversity increased from high to low latitudes, and the communities were different, even though the spiders were in the same monocultural habitat. Spider beatsheet communities around Australia were dominated by different families, and responded differently to seasonal changes, indicating that different pest groups would be targeted at different locations. These results show that diversity can increase from high to low latitudes, even if spatial heterogeneity is held constant, and that other factors external to the cotton crop are influencing spider species composition. Other models which may account for the latitudinal gradient, such as non‐equilibrium regional processes, are discussed. 相似文献
76.
77.
JONATHAN N. PAULI MATTHEW B. HAMILTON EDWARD B. CRAIN STEVEN W. BUSKIRK 《The Journal of wildlife management》2008,72(7):1650-1652
Abstract Although techniques to analyze and quantify DNA-based data have progressed, methods to noninvasively collect samples lag behind. Samples are generally collected from devices that permit coincident sampling of multiple individuals. Because of cross-contamination, substantive genotyping errors can arise. We developed a cost-effective (US$4.60/trap) single-capture hair trap for American martens (Martes americana). In the field, traps effectively targeted martens; >75% of all hair samples were identified as marten. Eighty percent of marten hair (n = 180) contained sufficient quality for DNA-based analyses. This effective and affordable trap can be used by managers to monitor mesocarnivore populations noninvasively. 相似文献
78.
AUDRONE R. BIKNEVICIUS STEVEN R. LEIGH 《Zoological Journal of the Linnean Society》1997,120(2):139-161
Differences in jaw morphology among adult carnivorans are well established, but the ontogenetic mechanisms by which these differences arise are largely unexplored. Mandibular ontogeny in Crocuta crocuta and Puma concolor is analysed biomechanically using principles of beam theory. In each species, the development of cross-sectional properties of the mandibular corpus associated with rigidity under loading follows a biphasic pattern of growth. In early postnatal growth, deposition of cortical bone appears to be constrained by the overall weaker tissue with which juvenile skeletons are constructed and by the need to volumelrically accommodate the developing teeth within their bony crypts. Thus, this stage of growth is characterized by a net periosteal deposition of bone and a swelling of the medullary cavity. In late postnatal growth, the constraints on endosteal deposition of bone are relieved as the permanent teeth erupt; thus, cortical thicknesses increase sharply by periosteal expansion as well as medullary contraction. Finally, it is noted that basic differences in jaw construction between Crocuta and Puma appear to develop prenatally as they are largely in place at birth. Hence, postnatal development enhances, but does not soley contribute to, the biomechanical differences in the jaws of these species. 相似文献
79.
Reconciling landscape and local views of aquatic communities: lessons from Michigan trout streams 总被引:10,自引:0,他引:10
1. Rapidly advancing geographical information systems (GIS) technologies are forcing a careful evaluation of the roles and biases of landscape and traditional site-based perspectives on assessments of aquatic communities. Viewing the world at very different scales can lead to seeming contradictions about the nature of specific ecological systems. In the case of Michigan trout streams, landscape analyses suggest a predictable community shaped by large-scale patterns in hydrology and geology. Most site-based studies, however, suggest these communities are highly variable in structure over time, and are strongly shaped by site-specific physical and biological dynamics. As the real world is comprised of processes operating both at local and landscape scales, an analytical framework for integrating these paradigms is desirable.
2. Decomposition of variances by factorial ANOVA into time, space and time–space interaction terms can provide a conceptual and analytical model for integrating processes operating at landscape and local scales. Using this approach, long-term data sets were examined for three insects and two fishes common in Michigan trout streams. Each taxon had a unique variance structure, and the observed variance structure was highly dependent upon sample size.
3. Both spatially extensive designs with little sampling over time (typical of many GIS studies) and temporally extensive designs with little or no spatial sampling (typical of population and community studies), are biased in terms of their view of the relative importance of local and landscape factors. The necessary, but in many cases costly, solution is to develop and analyse data sets that are both spatially and temporally extensive. 相似文献
2. Decomposition of variances by factorial ANOVA into time, space and time–space interaction terms can provide a conceptual and analytical model for integrating processes operating at landscape and local scales. Using this approach, long-term data sets were examined for three insects and two fishes common in Michigan trout streams. Each taxon had a unique variance structure, and the observed variance structure was highly dependent upon sample size.
3. Both spatially extensive designs with little sampling over time (typical of many GIS studies) and temporally extensive designs with little or no spatial sampling (typical of population and community studies), are biased in terms of their view of the relative importance of local and landscape factors. The necessary, but in many cases costly, solution is to develop and analyse data sets that are both spatially and temporally extensive. 相似文献
80.