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21.
为探讨近三十年来我国皱纹盘鲍养殖模式对群体遗传结构产生的影响,利用线粒体细胞色素C氧化酶亚基 (
22.
Naugle David E. Higgins Kenneth F. Nusser Sarah M. Johnson W. Carter 《Landscape Ecology》1999,14(3):267-276
We evaluated the influence of scale on habitat use for three wetland-obligate bird species with divergent life history characteristics and possible scale-dependent criteria for nesting and foraging in South Dakota, USA. A stratified, two-stage cluster sample was used to randomly select survey wetlands within strata defined by region, wetland density, and wetland surface area. We used 18-m (0.1 ha) fixed radius circular-plots to survey birds in 412 semipermanent wetlands during the summers of 1995 and 1996. Variation in habitat use by pied-billed grebes (Podilymbus podiceps) and yellow-headed blackbirds (Xanthocephalus xanthocephalus), two sedentary species that rarely exploit resources outside the vicinity of nest wetlands, was explained solely by within-patch variation. Yellow-headed blackbirds were a cosmopolitan species that commonly nested in small wetlands, whereas pied-billed grebes were an area-sensitive species that used larger wetlands regardless of landscape pattern. Area requirements for black terns (Chlidonias niger), a vagile species that typically forages up to 4 km away from the nest wetland, fluctuated in response to landscape structure. Black tern area requirements were small (6.5 ha) in heterogeneous landscapes compared to those in homogeneous landscapes (15.4–32.6 ha). Low wetland density landscapes composed of small wetlands, where few nesting wetlands occurred and potential food sources were spread over large distances, were not widely used by black terns. Landscape-level measurements related to black tern occurrence extended past relationships between wetlands into the surrounding matrix. Black terns were more likely to occur in landscapes where grasslands had not been tilled for agricultural production. Our findings represent empirical evidence that characteristics of entire landscapes, rather than individual patches, must be quantified to assess habitat suitability for wide-ranging species that use resources over large areas. 相似文献
23.
对石河子市土地利用结构进行了评价。结果表明,石河子市正处于经济发展的加速阶段,城市化进程还在加快,土地利用结构也处于不断变化之中。同时分析了石河子市土地利用结构存在的问题。 相似文献
24.
F. Scasso N. Mazzeo J. Gorga C. Kruk G. Lacerot J. Clemente D. Fabin S. Bonilla 《水产资源保护:海洋与淡水生态系统》2001,11(1):31-44
1. Lake Rodó is a turbid system, a condition attributed to algal biomass. The proximal source of the eutrophication was stormwater discharges from an ill‐defined urban area. This paper describes an attempt to restore the water quality of Lake Rodó, the first time this has been done in Uruguay. In spring 1996 it was drained, sediments were removed and stream inputs were diverted. Groundwater was used to re‐fill the lake. Due to its high nutrient concentration a re‐circulation system was designed, pumping water from associated pools covered with free‐floating plants. 2. After the lake was refilled, the system was characterized by oxygen saturation or over‐saturation, neutral to basic pH, and high phosphorus, nitrogen and silicate concentrations. Ratios of total nitrogen (TN):total phosphorus (TP) and chlorophyll a (Chl a):TP indicated that phosphorus was the primary limiting nutrient during the period of groundwater supply. Once groundwater pumping had ceased, there was a decrease in TN:TP and Chl a:TP ratios, suggesting N‐limiting conditions prevailed in some periods. 3. Before restoration, the phytoplankton community was dominated year‐round by Planktothrix agardhii; since restoration the community has been more diverse. This change has favoured grazing by mesozooplankton, and the onset of clear‐water phases in spring. 4. Abundant populations of small omnivorous fish maintained a high predation pressure on zooplankton, restricting the abundance of large‐bodied herbivores, which, in turn, allowed an increase in phytoplanton biomass and a decrease in water transparency. Based on this observation, together with the phosphorus concentration and the low abundance of filamentous cyanobacteria compared with previous studies, we suggest that top‐down control has played a key role in increasing transparency in Lake Rodó. 5. A nutrient reduction programme, by the mechanical harvest of floating plants, and a removal of small omnivorous fishes and stocking strictly with piscivores, could be key factors in the achievement of a stable clear‐water phase. However, if blooms of Microcystis or other similar genera occur in summer, additional measures (e.g. reduction of the hydraulic residence time) will be needed to improve water transparency. Copyright © 2001 John Wiley & Sons, Ltd. 相似文献
25.
西双版纳望天树林的群落组成和结构特征研究 总被引:1,自引:0,他引:1
利用1992~1993年在西双版纳自然保护区的勐腊县补蚌片区建立的5块望天树林固定样地的观测资料,从物种组成、科属区系特征、水平布局和垂直布局方面对该群落的组成和结构特征进行了研究,采用α多样性指数及联合熵H(X,Y)分析了望天树林群落结构的多样性;采用β多样性指数比较了样地间的多样性。结果表明:西双版纳望天树林物种相当丰富,在5块50 m×50 m的固定样地内计有种子植物68科176属380种。该群落中热带科有57科,占89.1%,热带属有163属,占92.6%,反映出其强烈的热带性质。根据树高的分布情况,将该群落的地上垂直结构分为5层,即计有乔木3层,灌木层和草本层。而对群落内物种的水平分布格局所进行的检测结果显示,所选择的3个主要树种(望天树、木奶果、假海桐)在5块固定样地内均为典型的集群分布。联合熵分析表明,其群落结构的多样性指数为15.800 8~21.005 3。 相似文献
26.
典型选样调查测定,分析马尾松、大头茶常绿针阔混交林的森林特征、种群结构、林分结构、生物量结构及生态功能。结果表明:1.该混交林是亚热带常绿阔叶林前一阶段的过渡性森林类型,在干扰状态下保持其相对的稳定性。2.复层、异龄现象是该混交林的主要特征。建群种充分利用了林地空间营养面积,发挥了林地的生产潜力。叶面积指数达6.86~8.83,林分木材蓄积量达160~222m~3/ha,林分总生物量达114~163t/ha。其中,乔木层生物量达105~154t/ha。光照充足的林下,草本层生物量可达2.5t/ha,对庇护地表起到了最直接、最明显的作用,而调节林冠密度,可控制或促进草本层生物量的增长。3.此种混交林改善了森林立地的蓄水功能和贮水方式。林地土壤最大贮水量达2645~4100t/ha,降水贮存量达565~1256t/ha,林地枯枝落叶层饱和持水量达4589~8702kg/ha。该类型达到了生态与经济效益兼顾,可在我国南方亚热带山地各类工程造林中推广应用。 相似文献
27.
28.
家蚕质型多角体病毒研究的若干新进展 总被引:2,自引:0,他引:2
家蚕质型多角体病毒(BmCPV)分为9个株系,可使20种昆虫感染发病。本文阐述了近年来BmCPV在交叉感染、结构、基因组、结合蛋白与功能等方面的一些研究新进展。据报道BmCPV-1的两个毒株(H株与I株)的基因组全序列已被测定公布,由10个节段的双链RNA构成;众多研究认为BmCPV具有5种结构蛋白(VP1-5),推测VP1为依赖于RNA的RNA聚合酶,VP2、VP5分别为核衣壳蛋白、外壳蛋白,VP3、VP4各为三磷酸核苷酸结合蛋白、非特异性的核酸结合的关键蛋白,5种结构蛋白在BmCPV病毒复制包装过程中发挥着不同的功能。 相似文献
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30.