全文获取类型
收费全文 | 2446篇 |
免费 | 164篇 |
国内免费 | 40篇 |
专业分类
林业 | 226篇 |
农学 | 63篇 |
基础科学 | 9篇 |
500篇 | |
综合类 | 192篇 |
农作物 | 114篇 |
水产渔业 | 504篇 |
畜牧兽医 | 556篇 |
园艺 | 134篇 |
植物保护 | 352篇 |
出版年
2024年 | 5篇 |
2023年 | 32篇 |
2022年 | 47篇 |
2021年 | 63篇 |
2020年 | 48篇 |
2019年 | 27篇 |
2018年 | 84篇 |
2017年 | 79篇 |
2016年 | 78篇 |
2015年 | 81篇 |
2014年 | 121篇 |
2013年 | 157篇 |
2012年 | 127篇 |
2011年 | 156篇 |
2010年 | 144篇 |
2009年 | 139篇 |
2008年 | 103篇 |
2007年 | 139篇 |
2006年 | 120篇 |
2005年 | 97篇 |
2004年 | 75篇 |
2003年 | 89篇 |
2002年 | 78篇 |
2001年 | 70篇 |
2000年 | 52篇 |
1999年 | 38篇 |
1998年 | 33篇 |
1997年 | 44篇 |
1996年 | 26篇 |
1995年 | 29篇 |
1994年 | 16篇 |
1993年 | 34篇 |
1992年 | 22篇 |
1991年 | 16篇 |
1990年 | 19篇 |
1989年 | 17篇 |
1988年 | 19篇 |
1987年 | 28篇 |
1986年 | 43篇 |
1985年 | 6篇 |
1984年 | 7篇 |
1983年 | 8篇 |
1982年 | 5篇 |
1981年 | 4篇 |
1979年 | 4篇 |
1976年 | 3篇 |
1974年 | 2篇 |
1973年 | 5篇 |
1964年 | 2篇 |
1963年 | 3篇 |
排序方式: 共有2650条查询结果,搜索用时 36 毫秒
211.
Two outbreaks of phocine distemper have severely affected harbour seal (Phoca vitulina) populations in European and UK waters. The first occurred in 1988 when the causative virus was identified as a new member of the genus morbillivirus. The second outbreak in 2002 was first detected on the same Danish Island of Anholt and involved similar populations and geographical locations. However, despite the obvious similarities between the epidemics, differences in viral transmission and case mortality were found. Harbour seals are highly susceptible to infection while sympatric grey seals (Halichoerus grypus) are resistant but could be important asymptomatic carriers of the disease. Arctic phocid seals remain the most likely source of the virus and grey seals could be the link between these primary hosts and the harbour seal populations further south. Future epidemiological models should therefore consider including multiple host species. The future conservation and management of harbour seal populations vulnerable to PDV relies on the ability to accurately predict the long-term impact on population abundance and distribution. Although knowledge about the behaviour and pathogenesis of the virus has increased substantially and data on host movements and contact rates are accumulating, studies into the determinants of the host range have lagged behind. The development of more realistic epidemiological models should be combined with studies into the factors controlling species and individual susceptibility. Assessing the risk of infection to endangered but currently unexposed potential host species (such as the Hawaiian monk seal, Monachus schauinslandi) is essential for guiding potential conservation management options, such as vaccination 相似文献
212.
213.
Reserve corridors in production forestry landscapes are frequently concentrated in riparian areas. This study describes the numerical response of ground-dwelling beetles to increasing distance from streams, with the aim of evaluating the effectiveness of such a bias in reserve allocation. Patterns in abundance and diversity of epigaeic beetles were quantified at four first order streams in wet eucalypt forest in Tasmania, Australia. The depth and pattern of beetles’ riparian response varied between streams. Commonly trapped beetles were less abundant near to three of the four streams, and the numerical response model differed in each case. Species richness of common beetles was also lower near one of the streams. Pooled abundance and richness of rare species did not vary in response to the riparian-upslope transition. No riparian or upslope specialist species were identified among the most commonly collected species. Compared to upslope habitat, the riparian-upslope transition encompassed greater variability in species composition without actually increasing overall richness. The study findings demonstrate the need for ecosystem-specific data to optimize reserve placement, since the results were not predicted from general vegetation patterns, ecological theory, or the widely held assumption that riparian areas support greater abundance and diversity of organisms than adjacent upslope areas. Reserves encompassing more upslope habitat are recommended to complement those located in riparian areas. 相似文献
214.
E.?A.?CatchpoleEmail author Y.?Fan B.?J.?T.?Morgan T.?H.?Clutton-Brock T.?Coulson 《Journal of Agricultural, Biological & Environmental Statistics》2004,9(1):1-26
A detailed and extensive mark-recapture-recovery study of red deer on the island of Rum forms the basis of the modeling of
this article. We analyze male and female deer separately, and report results for both in this article, but use the female
data to demonstrate our modeling approach. We provide a model-selection procedure that allows us to describe the survival
by a combination of age-classes, with common survival within each class, and senility, which is modeled continuously as a
parametric function of age. Dispersal out of the study area is modeled separately. Survival and dispersal probabilities are
examined for the possible influence of both environmental and individual covariates, including a range of alternative measures
of population density. The resulting model is succinct and biologically realistic. We compare and contrast survival rates
of male and female deer of different ages and compare the factors that affect their survival. We demonstrate large differences
in the rate of senescence between males and females even though their senescence begins at the same age. The differences between
the sexes suggest that, in population modeling of sexually size-dimorphic species, it is important to identify sex-specific
survival functions. 相似文献
215.
Ruth?KingEmail author Stephen?P.?Brooks 《Journal of Agricultural, Biological & Environmental Statistics》2003,8(4):486-513
This article is motivated by a series of data on a population of mouflons (Ovisgmelini) in the Caroux-Espinouse massif and focuses upon discriminating between competing biological hypotheses corresponding to the dependence of any or all of the population parameters upon either sex, location, or age. We show how we can analyze the data using a Bayesian approach, where we are able to take into account prior information obtained via a previous radio-tagging study. We consider the Arnason-Schwarz model together with its submodels to describe the data. Efficiently exploring model space using reversible jump Markov chain Monte Carlo methodology, we are able to calculate model-averaged estimates of parameters of interest, which incorporate both parameter and model uncertainty. In addition, we quantitatively compare different biological hypotheses by calculating their corresponding posterior probabilities. In particular, we show that survival rates tend to remain constant with some evidence to suggest a slight senescent decline. We also provide evidence to suggest that movement around the habitat is largely the same for both sexes up until age 4, when the males appear to extend their migration range, venturing further from the main flock in search of better grazing. 相似文献
216.
217.
218.
We explored the main factors affecting the global distribution of tree cavities - a habitat component of mostly biotic origin that is crucial for many animal species. We considered the influence of eight environmental variables (ranging from the single-tree to the biogeographic-region scale) on cavity density in a meta-analysis of 103 published studies. The global median density of cavities was 16 ha−1, with densities highest in Australasia and lowest in the Palaearctic region. Two major factors influencing density were identified: cavity density was positively related to the amount of precipitation, and was higher in natural than in managed forests. These effects suggest that the distribution of tree cavities largely reflects the incidence of fungal heart-rot in trees, and that forest management, by affecting wood decay processes, can have a broad-scale impact on tree microhabitat availability. Although air temperature, forest composition and wood hardness had suggestive univariate effects, neither these variables nor biogeographic region explained any additional variation in multifactor models. In regions where woodpeckers are present there was an upper limit to the density of woodpecker-excavated cavities (approximately 10-20 cavities ha−1) that was considerably lower than the highest total cavity densities encountered (up to 140 ha−1). This indicates that primary cavity-nesters are particularly important keystone species in cavity-poor forests where wood decay processes are suppressed either climatically or by forest management. 相似文献
219.
220.
对黄土高原日光温室黄瓜与其他作物建立的不同种植制度研究表明,冬春茬黄瓜收获后采用不同轮作体系可显著减少土壤微生物数量;休闲降低土壤细菌和真菌数量效果最佳,其次为种植豇豆或与其他豆科作物轮作。翻青玉米、翻青黑豆和种植豇豆处理黄瓜产量较高,其次为夏季休闲,而种植番茄和小白菜处理黄瓜产量较低。并提出黄瓜与翻青玉米、豇豆、翻青黑豆轮作是有效预防和克服土壤连作障碍的较佳种植制度。 相似文献