全文获取类型
收费全文 | 6126篇 |
免费 | 651篇 |
国内免费 | 482篇 |
出版年
2023年 | 160篇 |
2022年 | 195篇 |
2021年 | 286篇 |
2020年 | 323篇 |
2019年 | 293篇 |
2018年 | 261篇 |
2017年 | 241篇 |
2016年 | 241篇 |
2015年 | 232篇 |
2014年 | 372篇 |
2013年 | 416篇 |
2012年 | 251篇 |
2011年 | 297篇 |
2010年 | 243篇 |
2009年 | 369篇 |
2008年 | 341篇 |
2007年 | 337篇 |
2006年 | 295篇 |
2005年 | 222篇 |
2004年 | 184篇 |
2003年 | 147篇 |
2002年 | 142篇 |
2001年 | 135篇 |
2000年 | 102篇 |
1999年 | 98篇 |
1998年 | 78篇 |
1997年 | 84篇 |
1996年 | 68篇 |
1995年 | 69篇 |
1994年 | 70篇 |
1993年 | 57篇 |
1992年 | 50篇 |
1991年 | 63篇 |
1990年 | 58篇 |
1989年 | 48篇 |
1988年 | 34篇 |
1987年 | 32篇 |
1986年 | 21篇 |
1985年 | 48篇 |
1984年 | 45篇 |
1983年 | 26篇 |
1982年 | 38篇 |
1981年 | 35篇 |
1980年 | 22篇 |
1979年 | 26篇 |
1978年 | 18篇 |
1977年 | 18篇 |
1976年 | 14篇 |
1975年 | 14篇 |
1973年 | 13篇 |
排序方式: 共有7259条查询结果,搜索用时 15 毫秒
1.
Removal of feral cats from island environments is a useful mechanism by which their ecological impact on endangered species can be reduced or ended. Nevertheless, because cats are anthropogenic in their origins, social perceptions of management practices play a large role in their implementation. Four-hundred questionnaires were delivered (386 were returned) with 100 going to each of the following: local residents; environmental workers; tourists; and, hunters. Questions explored respondents’ knowledge about island biodiversity and invasive species as well as attitudes towards cat population management methods. Habitat destruction and introduction of invasive species were considered the main threats for the conservation of island biodiversity. Most respondents considered cats to have a negative impact on biodiversity and sterilization campaigns were considered most appropriate for cat population control. Several free sterilization campaigns have been conducted in La Palma Island Biosphere Reserve in order to reduce free-ranging cats and were well received by local people. This research, which combined concepts of management, ecology and social sciences, provides valuable insights which may to be applicable on several other islands where cats and people are present and in conflict with conservation priorities. 相似文献
2.
Paul M. Severns 《Journal of Insect Conservation》2008,12(6):651-661
Fender’s blue butterfly is an endangered species restricted to fragmented, grassland remnants that are becoming increasingly
dominated by tall, invasive grasses in western Oregon, USA. I performed a removal experiment to assess the impacts of structural
degradation accompanying the invasion of Arrhenatherum elatius, tall oat grass, on butterfly fitness and fitness related behaviors. Clipping of A. elatius to native grass sward height resulted in 2.5–5 times as many eggs laid per leaf of host plant. Both male and female butterflies
basked more frequently in areas removed of A. elatius inflorescences and upon encountering the treatment edge butterflies had a high rate of return into a large area removed of
the grass inflorescences. Although butterfly behavior appeared to be affected by the change in sward height on the treatment
edge, there was no evidence for the edge causing a disproportionate egg load. Invasion and dominance by A. elatius appeared to diminish host plant apparency which may result in overloading of eggs on conspicuous host plants, increased incidence
of emigration, and a decrease in the likelihood of colonization because female butterflies appeared indifferent to larval
resources beneath A. elatius inflorescences. Dominance of natural shortgrass prairies by tall stature grasses like A. elatius may be an insidious form of habitat degradation for grassland Lepidoptera worldwide, but it may go largely unnoticed because
larval and adult resources can persist under the unnaturally tall grass canopy. 相似文献
3.
Andreas König 《European Journal of Wildlife Research》2008,54(1):101-109
Throughout Central Europe, foxes have taken over urban areas as their habitat. In Southern Germany, these foxes are also carriers
of the small fox tapeworm, which causes a serious zoonotic infection in humans. Therefore, a survey was carried out in a suburb
of the city of Munich. A postal questionnaire was used to analyse the attitudes, opinions and fears of these participants
towards their urban foxes and the background to these attitudes. Questionnaires were sent to all households with gardens and
collected in again via the community council. Seven hundred and seventy-nine or 31% of questionnaires were returned. Only
a few people are afraid of the fox itself; however, 55% are afraid of the fox tapeworm. Worming the animals is the preferred
counter-measure, with 81% in favour. The majority of inhabitants are pleased to see a fox in the community and feel the animals
have a right to live. People are afraid of the tapeworm either because they have children in the household or because of increased
knowledge of the subject or because it has increasingly become an issue. On the basis of the results of this study, it is
to be expected that radical solutions such as killing the foxes are unlikely to be accepted among the population. Worming
of the foxes does, however, meet with general approval. 相似文献
4.
Risks and benefit evaluation for controlled human infection studies, where healthy volunteers are deliberately exposed to infectious agents to evaluate vaccine efficacy, should be explicit, systematic, thorough, and non-arbitrary. Decision analysis promotes these qualities using four steps: (1) determining explicit criteria and measures for evaluation, (2) identifying alternatives to the study, (3) defining the models used to estimate the measures for each alternative, and (4) running the models to produce the estimates and compare the alternatives. In this paper, we describe how decision analysis might be applied by funders and regulators, as well as by others contemplating the use of novel controlled human infection studies for vaccine development and evaluation. 相似文献
5.
《Bioorganic & medicinal chemistry letters》2020,30(9):127071
New therapies for treating drug-resistant pneumococcal infections are urgently needed. The novel scaffold 6-hydroxy-4-oxo-1,2-dihydro-4H-quinoline was shown to have similar efficacies against all three different serotypes of S. pneumoniae, ATCC 49617™ (19F), ATCC BAA-1663™ (15B), and ATCC 700904™ (19A), in a resazurin-based high-throughput screen using the Korea Chemical Bank library. Further studies to identify a new lead with this scaffold, including tricyclic pyrrolo[3,2,1-ij]quinolone and pyrido[3,2,1-ij]quinolone derivatives, led to the identification of 6d, 7d and 12a. Compound 6d (IC50 = 0.92, 0.75, and 0.77 µM), 7d (IC50 = 0.57, 0.66, and 0.38 µM) and 12a (IC50 = 0.27, 1.03, and 0.62 µM) showed submicromolar IC50 values against 19F, 15B, and 19A, respectively, and thus serve as a starting point for further optimization. While some of compounds in this series exhibited acceptable pharmacokinetic profiles in preliminary in vivo rat experiments, the most active compound 12a showed poor solubility and high plasma protein binding. Our current research efforts are focused on optimizing compounds to improve physicochemical properties as well as potency. 相似文献
6.
A 3-year microplot study was conducted to characterize the interaction between Meloidogyne arenaria race 1 (MA1) and M. hapla (MH), as affected by the five peanut genotypes: Florigiant, NC 7, NC 6, NC Ac 18416, and NC Ac 18016. The interactive effects on infection (total parasitic forms per root unit) and reproduction potentials of each nematode species and crop damage were determined. As a single population, MA1 had greater infection capacity and caused more crop damage than did MH, but both species had similar reproduction potentials. In mixed infestations, MA1 was more competitive than MH, as reflected by incidence of infection. Infection and reproduction potentials, and crop-damage capabilities of the mixed populations were similar to those of MA1 alone. All peanut genotypes were susceptible to infection by both nematodes. NC 6 was less susceptible to damage by MA1 and the mixed populations than other genotypes. A nematode treatment x genotype interaction was detected for root infection and crop damage, but not for population density or reproduction. With high preplant nematode levels (Pi), the populations reached their peak by midseason, whereas those with low Pi peaked after midseason. Crop damage in the second and third years was correlated with Pi level. 相似文献
7.
Uninfected adultAphis gossypii(Homoptera: Aphididae) apterae (sentinel aphids) on cotton leaves were exposed for 8 h to the air over a commercial cotton field in Louisiana during the night of 1–2 July 1995. At 0015 h there were 90,437 primary conidia/m3air of the fungusNeozygites fresenii(Entomophthorales: Neozygitaceae) at the midfield position as determined from Rotorod samples. Forty-eight percent (n = 106) of the sentinel aphids exposed for 8 h at midfield were infected by aerial conidia ofN. fresenii.Exposure of sentinel aphids outside the cotton field, at 10 and 100 m downwind and 10 m upwind, resulted in 34.8% (n = 131), 24.0% (n = 129), and 17.4% (n = 146) infected aphids, respectively. These data demonstrate that wind-dispersed aerial conidia ofN. freseniiare infective and rapidly and efficiently disperse the pathogen throughout aphid populations within and between fields. 相似文献
8.
9.
In Central Europe as in most other temperate regions of the world, Buddleja davidii has become a very successful invader. A thorough observation, documentation and analysis of the spread of invasive species is the precondition for the understanding of invasion processes. Therefore, I documented the occurrence of the species along a west–east transect as well as an altitudinal transect, and I tried to reconstruct the spread of the species in the course of the last decades along railroad areas, which have proved to be the most favorized habitats for colonization of Buddleja. Additionally, a literature review is given on its general spread and distribution in Germany. Based on the investigation of 52 stations, the results show that the species, in Germany, has its optimum in the Rhein-Ruhr- and the Rhein-Main-area, that its abundance significantly decreases from west to east and with increasing altitude. A literature review combined with own investigations shows, that it was very successful in Germany on ruins of World War II but decreased and sometimes totally disappeared in cities of East Germany and of the altitudinal higher regions of Germany, i.e. also in many towns of South Germany. In West Germany, the recent spread started about three decades ago and is still in process. As cold winters seem to be the limiting factor for the spread of Buddleja, even an accelerated spread of this species and perhaps a loss of its ruderal character can be expected, considering the progress of climate change. 相似文献
10.