全文获取类型
收费全文 | 905篇 |
免费 | 15篇 |
专业分类
920篇 |
出版年
2019年 | 2篇 |
2017年 | 3篇 |
2016年 | 4篇 |
2015年 | 9篇 |
2014年 | 16篇 |
2013年 | 23篇 |
2012年 | 27篇 |
2011年 | 52篇 |
2010年 | 67篇 |
2009年 | 70篇 |
2008年 | 70篇 |
2007年 | 74篇 |
2006年 | 68篇 |
2005年 | 53篇 |
2004年 | 28篇 |
2003年 | 20篇 |
2002年 | 5篇 |
2001年 | 11篇 |
2000年 | 14篇 |
1999年 | 10篇 |
1998年 | 33篇 |
1997年 | 22篇 |
1996年 | 23篇 |
1995年 | 13篇 |
1994年 | 13篇 |
1993年 | 9篇 |
1992年 | 12篇 |
1991年 | 14篇 |
1990年 | 22篇 |
1989年 | 14篇 |
1988年 | 16篇 |
1987年 | 15篇 |
1986年 | 10篇 |
1985年 | 6篇 |
1984年 | 8篇 |
1983年 | 7篇 |
1982年 | 6篇 |
1981年 | 9篇 |
1980年 | 6篇 |
1979年 | 5篇 |
1978年 | 3篇 |
1976年 | 2篇 |
1972年 | 4篇 |
1971年 | 5篇 |
1965年 | 2篇 |
1955年 | 1篇 |
1954年 | 1篇 |
1952年 | 3篇 |
1950年 | 1篇 |
1949年 | 1篇 |
排序方式: 共有920条查询结果,搜索用时 15 毫秒
121.
Coping with the cold: minimum temperatures and thermal tolerances dominate the ecology of mountain ants 下载免费PDF全文
TOM R. BISHOP MARK P. ROBERTSON BERNDT J. VAN RENSBURG CATHERINE L. PARR 《Ecological Entomology》2017,42(2):105-114
1. Ants (Hymenoptera: Formicidae) are often cited as highly thermophilic and this has led to a range of studies investigating their thermal tolerances. It is unknown, however, if the geographic distribution of ant thermal tolerance conforms to the two major macropyhsiological rules that have been found in other taxa: Janzen's and Brett's rules. In addition, there is a paucity of data on how the lower thermal tolerances of ants are able to influence behaviour. 2. These two knowledge gaps were addressed here by sampling ants across a 1500 m elevational gradient in southern Africa and estimating the upper (CTmax) and lower (CTmin) thermal tolerances of 31 and 28 species, respectively. Ant abundances and soil temperatures were also recorded across the gradient over 6 years. 3. It was found that the average CTmin of the ants declined with elevation along with environmental temperatures. It was also found that the correlation between abundance and local temperature depended on the ant species' CTmin. The activity of species with a low CTmin was not constrained by temperature, whereas those with a high CTmin were limited by low temperatures. 4. For the first time, evidence is provided here that the thermal tolerances of ants are consistent with two major macrophysiological rules: Brett's rule and Janzen's rule. A mechanistic link between physiology, behaviour and the environment is also shown, which highlights that the ability of ants to deal with the cold may be a key, but often overlooked, factor allowing multiple ant species to succeed within an environment. 相似文献
122.
Six microsatellite loci were identified and characterized in the eastern form of the widespread and commercially important giant freshwater prawn (Macrobrachium rosenbergii). The loci were detected by randomly screening for dinucleotide and trinucleotide repeat units within a partial genomic library developed for the species. In a sample of 29 prawns, number of alleles and heterozygosity per locus ranged from 12 to 18 and from 0.66 to 0.90, respectively. These markers provide powerful tools for the conservation and management of wild stocks, the improvement of cultured stocks of M. rosenbergii, and for investigating evolutionary processes underlying genetic divergence among populations. 相似文献
123.
Impact of migratory snow geese on nitrogen and phosphorus dynamics in a freshwater reservoir 总被引:3,自引:1,他引:2
MARK H. OLSON MELISSA M. HAGE MARK D. BINKLEY JAMES R. BINDER 《Freshwater Biology》2005,50(5):882-890
1. We examined impacts of nutrient loading, particularly of nitrogen and phosphorus, from greater snow geese (Chen caerulescens atlantica) on a reservoir in south‐eastern Pennsylvania, U.S.A. Approximately 100 000 geese use the reservoir for 2–6 weeks prior to their spring migration northward. 2. We estimated the magnitude of nutrient loading by geese during their presence and compared that to surface input and output rates. We also conducted nutrient limitation bioassay experiments to examine patterns of algal nutrient limitation upstream and downstream of the reservoir. 3. During their presence from 1 February to 27 March 2001, snow geese contributed 85–93% of the phosphorus and 33–44% of the nitrogen loaded to the reservoir. Both nutrients were exported from the reservoir slowly rather than as a quick pulse. Consequently, phosphorus concentrations in the outflow were higher than in the inflow from February to the end of July. However, nitrogen concentrations were consistently lower in the outflow than the inflow. 4. Nutrient limitation bioassays conducted in June and July indicated that primary production in the outflow was limited by nitrogen whereas the inflow was co‐limited by nitrogen and phosphorus. Further downstream from the reservoir, primary production was consistently phosphorus limited. Therefore, nitrogen limitation persisted long after the geese had left, but was relatively localised. 相似文献
124.
125.
126.
Scaling of respiration from the leaf to the canopy level currently depends on identification of physiological parameters that are tightly linked to respiration and that can readily be determined. Several recent studies have helped provide guides to predicting whole canopy respiration on the basis of foliar nitrogen (N). This approach is potentially powerful owing to the well‐described patterns of allocation of N that follow interception of radiation. In the present study, we investigated the sensitivity of the N–respiration correlation to environmental and developmental factors, in order to evaluate its usage for attempts to scale respiration to the organism and ecosystem level. We studied fully expanded, 1 and 2‐year‐old, and current‐year needles from canopies of Pinus radiata that had been treated (unthinned, thinned and thinned+fertilized treatments) in ways likely to induce a wide range of growth and respiratory responses. We examined respiration in detail during the growth period in spring and again at the end of summer, using calorespirometric methods (combined measurements of CO2 and heat rates) to determine the respiration rates , instantaneous enthalpic growth rates (RSGΔHB, a measure of the conservation of electrons in anabolic products) and the enthalpy conversion efficiency (ηH) of needles differing in age. A general linear model revealed that was positively correlated with needle N, but this correlation was strongly dependent on the season and the needle age – indicating an important physiological difference between expanding young needles and fully expanded old needles. Furthermore, the strength of the correlation between needle N and respiration was comparatively weak for the current year, expanding foliage, indicating that factors other than foliage N significantly influenced the respiration of young needles. The analysis of instantaneous growth rates revealed two general processes. Older, nonexpanding foliage showed considerable rates of enthalpic growth (increases in enthalpy) that was mainly caused by the increment of lignin during secondary growth. Secondly, canopy development appeared dynamic and to be optimized according to environmental drivers and constraints – such as light and water availability. In late spring, needle extension slowed in the upper, but not the lower canopy, because the upper canopy appeared to be affected first by the onset of drought stress in late spring. Growth rates were reduced in the upper canopy despite greater rates of respiration, indicating higher demand of ATP for the maintenance of protein and for export of sugars. Consequently, the enthalpy conversion efficiency and enthalpic N productivity (enthalpic growth per unit N) were comparatively poor indicating advanced development of needles in the upper canopy. We suggest that the growth and maintenance paradigm of respiration is, at best, only moderately useful when applied to whole trees, and is not valid at the cellular level or that of the plant organ. A different concept, namely that of respiratory efficiency, seems a more suitable way to represent respiration in carbon (C) balance models and should help provide a better mechanistic understanding of how respiration affects the C conversion efficiency of plants, and ultimately the net primary productivity of ecosystems. 相似文献
127.
BRENDAN C. EBNER JASON D. THIEM DEAN M. GILLIGAN MARK LINTERMANS IAN J. WOODEN SIMON LINKE 《Austral ecology》2008,33(7):891-901
Abstract Biodiversity estimates are typically a function of sampling effort and in this regard it is important to develop an understanding of taxon‐specific sampling requirements. Northern hemisphere studies have shown that estimates of riverine fish diversity are related to sampling effort, but such studies are lacking in the southern hemisphere. We used a dataset obtained from boat electro‐fishing the fish community along an essentially continuous 13‐km reach of the Murrumbidgee River, Australia, to investigate sampling effort effects on fish diversity estimates. This represents the first attempt to investigate relationships between sampling effort and the detection of fish species in a large lowland river in Australia. Seven species were recorded. Species‐specific patterns in catch per unit effort were evident and are discussed in terms of solitary and gregarious species, recreational fishing and the monitoring of rare and threatened species. There was a requirement to sample substantial lengths of river to describe total species richness of the fish community in this river reach. To this end, randomly allocated sampling effort and use of species richness estimators produced accurate estimates of species richness without the requirement for excessive levels of effort. Twenty operations were required to estimate species richness at this site, highlighting the need for comparable studies of river fish communities in lowland rivers elsewhere in Australia and the southern hemisphere. 相似文献
128.
MARK BOLTON ANDREA L. SMITH ELENA GÓMEZ‐DÍAZ VICKI L. FRIESEN RENATA MEDEIROS JOËL BRIED JOSE L. ROSCALES ROBERT W. FURNESS 《Ibis》2008,150(4):717-727
The existence of two seasonally distinct breeding populations of Oceanodroma storm‐petrels in the Azores islands was first documented in 1996. The discovery of morphological differences between the populations led to the suggestion that they may represent cryptic sibling species. Recent mtDNA and microsatellite analysis from storm‐petrel populations has considerably advanced our understanding of their taxonomic relationships. Here we present new information on the timing of breeding and moult of the two Azores populations, the extent of exchange of individuals between seasons, and diet from feather isotopes. We conclude that the hot‐season Azores population should be considered a new species for which we propose the name Oceanodroma monteiroi, Monteiro's Storm‐petrel. The species is both genetically distinct and genetically isolated from the sympatric cool‐season population of Madeiran Storm‐petrel Oceanodroma castro, and from all other populations of Oceanodroma castro in the Atlantic and Pacific Oceans examined to date. Differences in the vocalizations permit species recognition, and the extent of primary feather wear and stage of moult aids separation of the two species in the Azores, which is especially valuable during August when both attend the breeding colonies in large numbers. Feather carbon and nitrogen isotopes reveal that the diet of Monteiro's Storm‐petrel differs from that of the sympatric Madeiran Storm‐petrel during both breeding and non‐breeding seasons, and unlike the Madeiran Storm‐petrel, Monteiro's Storm‐petrel appears to maintain the same foraging environment during the summer and winter months, though it shows a dietary shift to higher trophic levels during the non‐breeding season. Monteiro's Storm‐petrel is thought to be confined to the Azores archipelago, where it is currently known to nest on just two small neighbouring islets. The total population size was estimated at 250–300 pairs in 1999. 相似文献
129.
130.
DAVID T. TINGEY ROBERT B. MCKANE DAVID M. OLSZYK MARK G. JOHNSON PAUL T. RYGIEWICZ E. HENRY LEE 《Global Change Biology》2003,9(7):1038-1050
The effects of elevated CO2 and temperature on principal carbon constituents (PCC) and C and N allocation between needle, woody (stem and branches) and root tissue of Pseudotsuga menziesii Mirb. Franco seedlings were determined. The seedlings were grown in sun‐lit controlled‐environment chambers that contained a native soil. Chambers were controlled to reproduce ambient or ambient +180 ppm CO2 and either ambient temperature or ambient +3.5 °C for 4 years. There were no significant CO2 × temperature interactions; consequently the data are presented for the CO2 and temperature effects. At the final harvest, elevated CO2 decreased the nonpolar fraction of the PCC and increased the polar fraction and amount of sugars in the needles. In contrast, elevated temperature increased the nonpolar fraction of the PCC and decreased sugars in needles. There were no CO2 or temperature effects on the PCC fractions in the woody tissue or root tissue. Elevated CO2 and temperature had no significant effects on the C content of any of the plant tissues or fractions. In contrast, the foliar N content declined under elevated CO2 and increased under elevated temperature; there were no significant effects in other tissues. The changes in the foliar N concentrations were in the cellulose and lignin fractions, the fractions, which contain protein, and are the consequences of changes in N allocation under the treatments. These results indicate reallocation of N among plant organs to optimize C assimilation, which is mediated via changes in the selectivity of Rubisco and carbohydrate modulation of gene expression. 相似文献