全文获取类型
收费全文 | 512篇 |
免费 | 46篇 |
专业分类
558篇 |
出版年
2022年 | 3篇 |
2021年 | 4篇 |
2020年 | 2篇 |
2018年 | 4篇 |
2017年 | 4篇 |
2016年 | 5篇 |
2015年 | 20篇 |
2014年 | 14篇 |
2013年 | 21篇 |
2012年 | 40篇 |
2011年 | 40篇 |
2010年 | 26篇 |
2009年 | 17篇 |
2008年 | 48篇 |
2007年 | 34篇 |
2006年 | 31篇 |
2005年 | 33篇 |
2004年 | 33篇 |
2003年 | 35篇 |
2002年 | 26篇 |
2001年 | 4篇 |
2000年 | 5篇 |
1999年 | 7篇 |
1998年 | 10篇 |
1997年 | 3篇 |
1996年 | 4篇 |
1995年 | 3篇 |
1994年 | 5篇 |
1993年 | 3篇 |
1992年 | 5篇 |
1991年 | 6篇 |
1990年 | 8篇 |
1989年 | 3篇 |
1988年 | 7篇 |
1987年 | 3篇 |
1986年 | 3篇 |
1985年 | 5篇 |
1983年 | 4篇 |
1982年 | 4篇 |
1981年 | 2篇 |
1980年 | 4篇 |
1978年 | 3篇 |
1977年 | 2篇 |
1976年 | 3篇 |
1971年 | 2篇 |
1959年 | 1篇 |
1946年 | 1篇 |
1934年 | 1篇 |
1929年 | 1篇 |
1925年 | 1篇 |
排序方式: 共有558条查询结果,搜索用时 15 毫秒
81.
State-dependent ideal free distributions 总被引:1,自引:0,他引:1
Summary The standard ideal free distribution (IFD) states how animals should distribute themselves at a stable competitive equilibrium. The equilibrium is stable because no animal can increase its fitness by changing its location. In applying the IFD to choice between patches of food, fitness has been identified with the net rate of energetic gain. In this paper we assess fitness in terms of survival during a non-reproductive period, where the animal may die as a result of starvation or predation. We find the IFD when there is a large population that can distribute itself between two patches of food. The IFD in this case is state-dependent, so that an animal's choice of patch depends on its energy reserves. Animals switch between patches as their reserves change and so the resulting IFD is a dynamic equilibrium. We look at two cases. In one there is no predation and the patches differ in their variability. In the other, patches differ in their predation risk. In contrast to previous IFDs, it is not necessarily true that anything is equalized over the two patches. 相似文献
82.
The past few years have seen significant advances in our understanding of eukaryotic genomes. In the field of parasitology, this is best exemplified by the application of genome mapping techniques to the study of genome structure and function in the protozoan parasite, Leishmania. Although much is known about the organism and the diseases it causes, molecular genetics has only recently begun to play a major part in elucidating some of the unusual characteristics of this interesting parasite. Mapping of the small (35 Mb) genome and determination of the functional role of genes by the application of in vitro homologous gene targeting techniques are revealing novel avenues for the development of prophylactic measures. 相似文献
83.
84.
85.
Enantiomeric Degradation of 2-(4-Sulfophenyl)Butyrate via 4-Sulfocatechol in Delftia acidovorans SPB1 总被引:2,自引:0,他引:2 下载免费PDF全文
Enrichment cultures with enantiomeric 2-(4-sulfophenyl)butyrate (SPB) as the sole added source(s) of carbon and energy for growth yielded a pure culture of a degradative bacterium, which was identified as Delftia acidovorans SPB1. The organism utilized the enantiomers sequentially. R-SPB was utilized first (specific growth rate [μ] = 0.28 h−1), with transient excretion of an unknown intermediate, which was identified as 4-sulfocatechol (4SC). Utilization of S-SPB was slower (μ = 0.016 h−1) and was initiated only after the first enantiomer was exhausted. Suspensions of cells grown in S-SPB excreted 4SC, so metabolism of the two enantiomers converged at 4SC. The latter was degraded by ortho cleavage via 3-sulfo-cis,cis-muconate. Strain SPB1 grew with 4SC and with 1-(4-sulfophenyl)octane (referred to herein as model LAS) but not with commercial linear alkylbenzenesulfonate (LAS) surfactant, which is subterminally substituted but nontoxic. It would appear that metabolism of the model LAS does not represent metabolism of commercial LAS. 相似文献
86.
Foraging theory has typically been concerned with the acquisition of a single resource even though organisms from mammals to protozoa are capable of balancing their requirements for multiple resources. Existing theory concerning multiple nutrients from multiple foods does not predict the sequence of food selection. We constructed an optimisation model of the simplest case of two foods containing differing amounts of two nutrients. We begin with the well-supported assumption that reproductive value declines with the distance from target nutrient intake. We show that nutrient space divides into two distinct areas where the animal should exclusively consume one food or the other. The organism thus initially concentrates on one food type until the border between the areas is reached and then moves as closely as possible along the border to approach the target. This strategy is commonly observed in a range of organisms, suggesting that the assumed fitness function is common. 相似文献
87.
Ryan K. Brook Susan J. Kutz Alasdair M. Veitch Richard A. Popko Brett T. Elkin Glen Guthrie 《EcoHealth》2009,6(2):266-278
Many northern Canadians have continued a subsistence lifestyle of wildlife harvesting and, therefore, value sustainable wildlife
populations. At a regional wildlife workshop in the Sahtu Settlement Area, Northwest Territories in 2002, elders and community
leaders raised concerns regarding wildlife health, food safety, and the effects of climate change on wildlife. They requested
that efforts be put toward training youth in science and increasing involvement of hunters and youth in wildlife research.
In response, we initiated a long-term, integrated approach to foster community-based wildlife health monitoring and research.
Annual trips were made to all schools in the Sahtu from 2003 to 2009 to provide hands-on learning for 250–460 students on
a range of wildlife topics. In addition, interviews were conducted with 31 hunters and elders to document their local ecological
knowledge of wildlife health and local hunters were trained as monitors to collect tissue samples and measurements to assess
body condition and monitor health of harvested caribou (n = 69) and moose (n = 19). In 2007 the program was extended to include participation in the annual caribou hunt held by one community. Each year
since 2005, a graduate student and/or a postdoctoral trainee in the veterinary or biological sciences has participated in
the program. The program has evolved during the last 6 years in response to community and school input, results of empirical
research, hunter feedback, local knowledge, and logistical constraints. The continuity of the program is attributed to the
energetic collaboration among diverse partners and a unified approach that responds to identified needs. 相似文献
88.
We propose operational definitions of reproductive optimism and pessimism; optimism involves behaving in a way that gives too much weight (in terms of producing surviving offspring) to positive events, pessimism gives too much weight to negative events. Natural selection maximizes the long-term growth of a lineage rather than short-term measures such as numbers of offspring. Consequently, optimism or pessimism can be favoured by natural selection, even though such biases appear irrational from a short-term perspective. We investigate the evolution of optimism in a metapopulation. The circumstances of a patch change over time, independently of other patches. With sufficient dispersal between patches, stochasticity affects members of a lineage largely independently and optimism is favoured. With little dispersal, the temporal fluctuations of a patch affect many members similarly; pessimism is then favoured. Our results establish that the spatial and temporal structure of the environment is crucial in determining the direction of evolved biases. 相似文献
89.
Graphical representation of molecular and cellular features is a key form of communication in structural biology in which abstract symbols of an economical and visually appropriate kind, pseudo-color coding, and dynamic animations all play their parts. Accordingly, it should not be surprising that many structural biologists--like traditional biologists before them--are talented artists who also express themselves on "non-scientific" topics. This article illustrates the approaches of and pictures by several practicing scientist-artists-mainly, in this sampling, electron microscopists. 相似文献
90.