全文获取类型
收费全文 | 657篇 |
免费 | 2篇 |
国内免费 | 6篇 |
专业分类
系统科学 | 9篇 |
教育与普及 | 2篇 |
理论与方法论 | 20篇 |
现状及发展 | 66篇 |
研究方法 | 100篇 |
综合类 | 381篇 |
自然研究 | 87篇 |
出版年
2022年 | 1篇 |
2021年 | 2篇 |
2020年 | 1篇 |
2019年 | 2篇 |
2018年 | 2篇 |
2017年 | 4篇 |
2016年 | 9篇 |
2015年 | 4篇 |
2014年 | 3篇 |
2013年 | 10篇 |
2012年 | 43篇 |
2011年 | 146篇 |
2010年 | 28篇 |
2009年 | 4篇 |
2008年 | 50篇 |
2007年 | 49篇 |
2006年 | 44篇 |
2005年 | 56篇 |
2004年 | 59篇 |
2003年 | 53篇 |
2002年 | 45篇 |
2001年 | 1篇 |
2000年 | 5篇 |
1999年 | 5篇 |
1996年 | 2篇 |
1995年 | 2篇 |
1994年 | 2篇 |
1993年 | 1篇 |
1992年 | 1篇 |
1991年 | 1篇 |
1990年 | 4篇 |
1988年 | 3篇 |
1986年 | 2篇 |
1985年 | 6篇 |
1984年 | 6篇 |
1982年 | 2篇 |
1980年 | 3篇 |
1978年 | 1篇 |
1977年 | 1篇 |
1976年 | 1篇 |
1972年 | 1篇 |
排序方式: 共有665条查询结果,搜索用时 796 毫秒
141.
Is there a common model inherent in macroeconomic data? Macroeconomic theory suggests that market economies of various nations should share many similar dynamic patterns; as a result, individual country empirical models, for a wide variety of countries, often include the same variables. Yet, empirical studies often find important roles for idiosyncratic shocks in the differing macroeconomic performance of countries. We use forecasting criteria to examine the macrodynamic behaviour of 15 OECD countries in terms of a small set of familiar, widely used core economic variables, omitting country‐specific shocks. We find this small set of variables and a simple VAR ‘common model’ strongly support the hypothesis that many industrialized nations have similar macroeconomic dynamics. Copyright © 2005 John Wiley & Sons, Ltd. 相似文献
142.
Leech species of the mid-Snake River of Idaho and Oregon are described, and the distribution of the extant leech Mooreobdella microstoma Moore in the Snake River paleodrainage is delineated. Samples were collected from aquatic surveys in the Snake River using suction dredging by the Idaho Power Company and U.S. Bureau of Reclamation between 1995 and 2006. Supplementing these surveys, opportunities were provided for leech identification in water-quality analyses in Arizona and Wyoming and in other surveys in California, Oregon, Washington, and Idaho. Eight species of leeches were found in the Snake River surveys. Erpobdella parva Moore was the most widely distributed species, occurring both above and below Shoshone Falls. Mooreobdella microstoma was widely distributed below Shoshone Falls. Other leech species were rare, although Helobdella stagnalis Linnaeus was very common above and less common below Shoshone Falls, a natural barrier to the anadromous fish. Mooreobdella microstoma is an extant species that links the Snake River to the lower Colorado River by various paleodrainages. It probably colonized the Snake River by upstream movement, whereas Erpobdella parva likely colonized this river by downstream movement. 相似文献
143.
Due to an error in printing quality of an earlier article, the distribution maps for the night snake ( Hypsiglena torquata ) and the tiger salamander ( Ambystoma tigrinum ) in Idaho are reprinted. 相似文献
144.
The fluctuating characteristics (numbers, biomass, condition, and young-adult ratios) of the Lahontan (Humboldt) cutthroat trout population in Chimney Creek, Nevada, are discussed in relationship to the unpredictable and unstable habitat in which the population occurs. One possible means of adapting to environmental capriciousness, staggered spawning, occurred during 1982, and clues as to the cause of this unusual event are sought by examining the runoff hydrographs of a nearby watershed for 1981 through 1984. The management values of the environmental tolerance of these native trout with respect to restoring viable trout fisheries in degraded Great Basin streams are also considered. 相似文献
145.
A general botanical inventory of a part of northwestern Moffat County, Colorado, resulted in the location of “ remnants ” of the presettlement vegetation spectrum that are largely unaltered by grazing, logging, or other recent human - related land uses. The 69 samples taken from these remnants were classified into 22 plant associations. Composition, structure, environmental location, geographical range, and response to disturbance are discussed for each association, and a photograph of each is presented. Seven of the 22 associations are apparently restricted to the study area. Restricted associations occur in the more extreme environments of the study area, such as on calcareous substrata or very xeric sites. More mesic sites along ephemeral creeks, on north - facing slopes, or on sandstones support plant associations that have much wider ranges, many of them extending across the northern Great Basin. 相似文献
146.
We report the first records of the western gray squirrel ( Sciurus griseus ) from Modoc County and eastern Lassen County in northeastern California. Specimens from Modoc County represent the first records of the species from the Great Basin geographic province. We conclude that these records represent a range expansion based upon previous fieldwork that did not report the species in the region. 相似文献
147.
William H. Behle 《西北部美国博物学家》2011,44(2)
Seville Flowers was the foremost authority of his time on cryptogamic botany in the intermountain region, having published monographs on the mosses, lichens, and ferns of Utah. He also had strong interests in algae, grasses, composites, and the history of botany. In addition to his systematic research, his expertise extended to field ecology. Early in his career he made a classic study of the vegetation of the Great Salt Lake region that led in later years to his participation in the ecological studies at Dugway Proving Grounds in western Utah. Still later he had charge of the predam vegetative surveys of several reservoir sites along the Upper Colorado River, including Glen Canyon, which was later submerged under Lake Powell. He served as professor of botany at the University of Utah from 1936 to 1968. His professional career started in 1929. A bibliography of his writings is included in this account of his life and professional career. 相似文献
148.
A survey of terrestrial vertebrates was conducted at the Scotts Bluff National Monument (SBNM), Nebraska, to determine species composition, relative abundance, and distribution by habitat of the existing amphibians, reptiles, birds, and mammals. Various sampling methods were used to detect animal species richness and to estimate population densities. The seven major habitat types at SBNM contained 4 species of amphibians, 8 reptiles, 96 birds, and 28 mammals. The only endangered or threatened species observed was a dead bald eagle ( Haliaeetus leucocephalus ). The racer ( Coluber constrictor ) and prairie rattlesnake ( Crotalus viridis ) were the two most abundant snakes. The prairie falcon ( Falco mexicanus ) was a common raptor that nested in Scotts Bluff. Sympatric populations of mule deer ( Odocoileus hemionus ) and white-tailed deer ( Odocoileus virginianus ) appeared to be hybridizing. The riverine woodland contained the greatest number of fauna species with only 4% surface area of SBNM. Sharp-tailed grouse ( Tympanuchus phasianellus ) and pronghorn antelope ( Antilocapra americana ), absent from the Monument, are two potential native species for reintroduction. 相似文献
149.
Using sweep samples, we surveyed leafhoppers (Homoptera: Cicadellidae) on grassland sites in the Gallatin Valley of Montana during 1988 and 1991. We sampled 12 sites representing 2 habitat types defined by their dominant plant species in an undisturbed state ( Stipa comata / Bouteloua gracilis and Festuca idahoensis / Agropyron spicatum ). At half of the sites the native plant communities were present, whereas the remainder had been reseeded with either Agropyron spicatum (to replace the S. comata / B. gracilis assemblage ) or Bromus inermis (to replace the F. idahoensis / A. spicatum assemblage). We found at least 66 species of leafhoppers among 44,428 adults collected. Seven taxa comprised 83% of all individuals collected: Doratura stylata (26%), Ceratagallia spp. (18%), Endria inimica (17%), Orocastus perpusillus (7%), Sorhoanus spp. (6%), Athysanella spp. (5%), and Psammotettix lividellus (4%). Sites with similar vegetation had broadly similar leafhopper assemblages, and assemblages differed most between the relatively xeric Stipa comata / Bouteloua gracilis sites and the more mesic sites dominated by Bromus inermis . The composition of a leafhopper assemblage at a site tended to be more similar to those on noncontiguous sites with the same overall vegetation than to those on contiguous sites with different vegetation. These patterns are likely related to the fact that many Cicadellidae are host specialists. In fact, variation in abundance of some of the most common leafhopper taxa on our sites was correlated with the percent cover of their known host plants. Our analyses of the leafhopper assemblages generally support the contention that terrestrial plant associations are among the more useful indicators of insect community composition. 相似文献
150.
We report the 1st record of the crayfish Orconectes virilis (Hagen 1870) from Idaho and the Pacific Northwest, USA. We also provide an updated checklist of recent Idaho crayfishes, which now includes 2 families (Astacidae and Cambaridae), 3 genera ( Pacifasticus, Orconectes , and Procambarus ), and 7 species and subspecies ( Pacifasticus connectus (Faxon 1914), P. gambelii (Girard 1852), P. leniusculus leniusculus (Dana 1852), P. leniusculus klamathensis (Stimpson 1857), P. leniusculus trowbridgii (Stimpson 1857), O. virilis , and Procambarus clarkii (Girard 1852). Native crayfish were not found at the O. virilis collection sites. The localities where O. virilis was found were characterized by low-gradient streams impacted by fine sediment. Aquatic invertebrates found in association with O. virilis tended to be the more pollution-tolerant taxa. 相似文献