全文获取类型
收费全文 | 438篇 |
免费 | 30篇 |
国内免费 | 35篇 |
出版年
2023年 | 3篇 |
2022年 | 7篇 |
2021年 | 8篇 |
2020年 | 13篇 |
2019年 | 13篇 |
2018年 | 5篇 |
2017年 | 12篇 |
2016年 | 11篇 |
2015年 | 19篇 |
2014年 | 21篇 |
2013年 | 15篇 |
2012年 | 17篇 |
2011年 | 15篇 |
2010年 | 13篇 |
2009年 | 27篇 |
2008年 | 25篇 |
2007年 | 23篇 |
2006年 | 17篇 |
2005年 | 28篇 |
2004年 | 20篇 |
2003年 | 27篇 |
2002年 | 15篇 |
2001年 | 17篇 |
2000年 | 19篇 |
1999年 | 11篇 |
1998年 | 14篇 |
1997年 | 13篇 |
1996年 | 7篇 |
1995年 | 9篇 |
1994年 | 9篇 |
1993年 | 6篇 |
1992年 | 3篇 |
1991年 | 3篇 |
1990年 | 3篇 |
1989年 | 4篇 |
1988年 | 7篇 |
1987年 | 5篇 |
1986年 | 6篇 |
1985年 | 2篇 |
1984年 | 1篇 |
1983年 | 2篇 |
1982年 | 3篇 |
1980年 | 1篇 |
1979年 | 1篇 |
1978年 | 1篇 |
1977年 | 1篇 |
1974年 | 1篇 |
排序方式: 共有503条查询结果,搜索用时 296 毫秒
101.
Perry R. Ian Walters Carl J. Boutillier James A. 《Reviews in Fish Biology and Fisheries》1999,9(2):125-150
A framework is developed for the provision of scientific advice to support the management of new and developing marine invertebrate fisheries. These fisheries often occur on species for which little biological or exploitation information is available. The framework explicitly endorses the precautionary approach to fisheries management and research. Three general management strategies (size/sex limits, regulation by total allowable catch, control of the exploitation rate) and their needs for supporting scientific information are identified. The significance of spatial pattern, and of recognizing the need for different approaches to obtain scientific information and to manage sedentary benthic and mobile pelagic species, is a central theme. Three 'phases' are proposed to obtain the necessary scientific information: (a) Phase 0, 'collecting existing information', consisting of syntheses of available biological and fisheries information on the target (and similar) species, leading to formulation of potential management strategies; (b) Phase 1, 'collecting new information', to obtain the essential information that is lacking or insufficient from the Phase 0 analysis, and to evaluate alternative management strategies and propose regulatory actions; and (c) Phase 2, 'fishing for commerce', to implement the chosen management actions and to monitor fishing operations, so as to increase the information base available to refine the results from previous phases. Phase 1 activities may consist of surveys, site-specific depletion experiments and studies to obtain biological information, and development of experimental management areas to test different exploitation rates. A strategy that includes establishing reserve areas recognises the inherent uncertainties associated with developing fisheries and provides a buffer against mistakes or 'surprises'; it also provides control areas to compare stock productivity in fished and unfished locations. The application of this framework to a developing sea cucumber fishery in British Columbia is presented as an example. Throughout, strong interaction and collaboration among science, management, and stakeholders is crucial to the provision of scientific advice for precautionary management of new invertebrate fisheries. 相似文献
102.
103.
Vigor and salt tolerance in 3 lines of tall wheatgrass 总被引:1,自引:0,他引:1
The F1 progeny of the cross of two salt-tolerant lines of Thinopyrum elongatum [Host] D. R. Dewey grew better than either parent under non-saline and saline growth conditions. Under non-saline conditions, the hybrid produced 1.8 times as much vegetative tissue as one parent and 3.2 times more than the other parent in the same length of time. The relative growth rates of the 2 parental lines decreased equally as media osmotic potentials decreased. The relative growth rate of the hybrid did not decrease as rapidly as that of the parents; therefore, it was concluded that the greater growth of the hybrid was due to increased salt tolerance. Carbohydrate reserves and water-soluble solutes believed to be involved in osmotic adjustment were assayed to determine if there were any differences between the hybrid and its parents in their abilities to accumulate these compounds. The concentrations of these constituents were measured at dawn and at dusk of the same day in plants grown in media at osmotic potentials ranging from –0.1 to –1.2 MPa. There were no differences in pool sizes of the organic compounds in the 3 lines. Starch increased 10–40 fold in leaves from dawn to dusk and sucrose increased 100-fold. However, this pattern was unaffected by salinity. Conversely, betaine concentrations increased with increasing salinity but were the same at dawn and dusk. Na+ and K+ were affected by both light and salinity. Cl was one-half (Na++ K+) on a molar basis under all conditions. Proline accumulated when (Na++ K+) exceeded 200 μmol (g fresh weight)?1. Since this amount of (Na++ K+) existed only in tissues harvested at dusk from severely saline-stressed plants, only leaves from such plants harvested at dusk contained proline. 相似文献
104.
Quantitative steps in the evolution of metabolic organisation as specified by the Dynamic Energy Budget theory 总被引:1,自引:0,他引:1
The Dynamic Energy Budget (DEB) theory quantifies the metabolic organisation of organisms on the basis of mechanistically inspired assumptions. We here sketch a scenario for how its various modules, such as maintenance, storage dynamics, development, differentiation and life stages could have evolved since the beginning of life. We argue that the combination of homeostasis and maintenance induced the development of reserves and that subsequent increases in the maintenance costs came with increases of the reserve capacity. Life evolved from a multiple reserves - single structure system (prokaryotes, many protoctists) to systems with multiple reserves and two structures (plants) or single reserve and single structure (animals). This had profound consequences for the possible effects of temperature on rates. We present an alternative explanation for what became known as the down-regulation of maintenance at high growth rates in microorganisms; the density of the limiting reserve increases with the growth rate, and reserves do not require maintenance while structure-specific maintenance costs are independent of the growth rate. This is also the mechanism behind the variation of the respiration rate with body size among species. The DEB theory specifies reserve dynamics on the basis of the requirements of weak homeostasis and partitionability. We here present a new and simple mechanism for this dynamics which accounts for the rejection of mobilised reserve by busy maintenance/growth machinery. This module, like quite a few other modules of DEB theory, uses the theory of Synthesising Units; we review recent progress in this field. The plasticity of membranes that evolved in early eukaryotes is a major step forward in metabolic evolution; we discuss quantitative aspects of the efficiency of phagocytosis relative to the excretion of digestive enzymes to illustrate its importance. Some processes of adaptation and gene expression can be understood in terms of allocation linked to the relative workload of metabolic modules in (unicellular) prokaryotes and organs in (multicellular) eukaryotes. We argue that the evolution of demand systems can only be understood in the light of that of supply systems. We illustrate some important points with data from the literature. 相似文献
105.
106.
Marcelo H. O. Pinheiro Luiz C. Almeida De Neto Reinaldo Monteiro 《Biodiversity and Conservation》2006,15(8):2747-2764
The International Union for Conservation of Nature and Natural Resources (IUCN) and the World Wide Fund for Nature (WWF) advocates
an increase of the number of botanical gardens throughout the world as one of the measures that can help to preserve the world's
biodiversity. To implement this strategy, the present work brings forward a suggestion particularly suited to tropical regions:
establishing municipal botanical gardens. It refers to the experience of a newly opened municipal botanical garden in Brazil,
comparing its attractive power on visitors to that of other botanical gardens included in the Brazilian network of Botanical
Gardens. It also presents considerations on in situ conservation in small remnants and on the importance of urban reserves to preserve the regional biodiversity and spread the
conservationist philosophy. The present proposal promotes the participation of local communities making the public opinion
more aware and active, besides being able to counterbalance proposals that support protecting the world biodiversity through
interventionist actions. It assumes that, through actions planned and coordinated by regional and national botanical garden
networks, the measure proposed can mitigate the anthropic actions exerted on important natural reserves all over the world. 相似文献
107.
Yael Pazy Stephanie A. Thomas Edward J. Collins Robert B. Bourret 《Journal of molecular biology》2009,392(5):1205-1220
In two-component regulatory systems, covalent phosphorylation typically activates the response regulator signaling protein, and hydrolysis of the phosphoryl group reestablishes the inactive state. Despite highly conserved three-dimensional structures and active-site features, the rates of catalytic autodephosphorylation for different response regulators vary by a factor of almost 106. Previous studies identified two variable active-site residues, corresponding to Escherichia coli CheY residues 59 and 89, that modulate response regulator autodephosphorylation rates about 100-fold. Here, a set of five CheY mutants, which match other “model” response regulators (ArcA, CusR, DctD, FixJ, PhoB, or Spo0F) at variable active-site positions corresponding to CheY residues 14, 59, and 89, were characterized functionally and structurally in an attempt to identify mechanisms that modulate autodephosphorylation rate. As expected, the autodephosphorylation rates of the CheY mutants were reduced 6- to 40-fold relative to wild-type CheY, but all still autodephosphorylated 12- to 80-fold faster than their respective model response regulators. Comparison of X-ray crystal structures of the five CheY mutants (complexed with the phosphoryl group analogue BeF3−) to wild-type CheY or corresponding model response regulator structures gave strong evidence for steric obstruction of the phosphoryl group from the attacking water molecule as one mechanism to enhance phosphoryl group stability. Structural data also suggested that impeding the change of a response regulator from the active to the inactive conformation might retard the autodephosphorylation reaction if the two processes are coupled, and that the residue at position ‘58’ may contribute to rate modulation. A given combination of amino acids at positions ‘14’, ‘59’, and ‘89’ adopted similar conformations regardless of protein context (CheY or model response regulator), suggesting that knowledge of residue identity may be sufficient to predict autodephosphorylation rate, and hence the kinetics of the signaling response, in the response regulator family of proteins. 相似文献
108.
I. Esteves B. Peteira S. Powers N. Magan B.R. Kerry 《Biocontrol Science and Technology》2009,19(2):185-199
For the first time, the effects of varying osmotic and matric potential on fungal radial growth and accumulation of polyols were studied in three isolates of Pochonia chlamydosporia. Fungal radial growth was measured on potato dextrose agar modified osmotically using potassium chloride or glycerol. PEG 8000 was used to modify matric potential. When plotted, the radii of the colonies were found to grow linearly with time, and regression was applied to estimate the radial growth rate (mm day?1). Samples of fresh mycelia from 25-day-old cultures were collected and the quantity (mg g?1 fresh biomass) of four polyols (glycerol, erythritol, arabitol and mannitol) and one sugar (glucose) was determined using HPLC. Results revealed that fungal radial growth rates decreased with increased osmotic or matric stress. Statistically significant differences in radial growth were found between isolates in response to matric stress (P<0.006) but not in response to osmotic stress (P=0.759). Similarly, differences in the total amounts of polyols accumulated by the fungus were found between isolates in response to matric stress (P<0.001), but not in response to osmotic stress (P=0.952). Under water stress, the fungus accumulated a combination of different polyols important in osmoregulation, which depended on the solute used to generate the stress. Arabitol and glycerol were the main polyols accumulated in osmotically modified media, whereas erythritol was the main polyol that was accumulated in media amended with PEG. The results found that Pochonia chlamydosporia may use different osmoregulation mechanisms to overcome osmotic and matric stresses. 相似文献
109.
Natalie C. Ban 《Biodiversity and Conservation》2009,18(7):1829-1845
Marine protected areas (MPAs) can be an effective tool for marine biodiversity conservation, yet decision-makers usually have
limited and biased datasets with which to make decisions about where to locate MPAs. Using commonly available abiotic and
biotic datasets, I asked how many datasets are necessary to achieve robust patterns of conservation importance. I applied
a decision support tool for marine protected area design in two regions of British Columbia, Canada, and sequentially excluded
the datasets with the most limited geographic distribution. I found that the reserve selection method was robust to some missing
datasets. The removal of up to 15 of the most geographically limited datasets did not significantly change the geographic
patterns of the importance of areas for conservation. Indeed, including abiotic datasets plus at least 12 biotic datasets
resulted in a spatial pattern similar to including all available biotic datasets. It was best to combine abiotic and biotic
datasets in order to ensure habitats and species were represented. Patterns of clustering differed according to whether I
used one set alone or both combined. Biotic datasets served as better surrogates for abiotic datasets than vice versa, and
both represented more biodiversity features than randomly selected reserves. These results should provide encouragement to
decision-makers engaged in MPA planning with limited spatial data. 相似文献
110.
The energy density ( E D ) of anchovy Engraulis encrasicolus in the Bay of Biscay was determined by direct calorimetry and its evolution with size, age and season was investigated. The water content and energy density varied seasonally following opposite trends. The E D g−1 of wet mass ( M W ) was highest at the end of the feeding season (autumn: c . 8 kJ g−1 M W ) and lowest in late winter ( c . 6 kJ g−1 M W ). In winter, the fish lost mass, which was partially replaced by water, and the energy density decreased. These variations in water content and organic matter content may have implications on the buoyancy of the fish. The water content was the major driver of the energy density variations for a M W basis. A significant linear relationship was established between E D g−1 ( y ) and the per cent dry mass ( M D ; x ): y =−4·937 + 0·411 x . In the light of the current literature, this relationship seemed to be not only species specific but also ecosystem specific. Calibration and validation of fish bioenergetics models require energy content measurements on fish samples collected at sea. The present study provides a first reference for the energetics of E. encrasicolus in the Bay of Biscay. 相似文献