全文获取类型
收费全文 | 6637篇 |
免费 | 765篇 |
国内免费 | 870篇 |
专业分类
8272篇 |
出版年
2024年 | 37篇 |
2023年 | 185篇 |
2022年 | 162篇 |
2021年 | 278篇 |
2020年 | 317篇 |
2019年 | 411篇 |
2018年 | 309篇 |
2017年 | 383篇 |
2016年 | 350篇 |
2015年 | 373篇 |
2014年 | 389篇 |
2013年 | 473篇 |
2012年 | 270篇 |
2011年 | 306篇 |
2010年 | 280篇 |
2009年 | 390篇 |
2008年 | 406篇 |
2007年 | 419篇 |
2006年 | 306篇 |
2005年 | 322篇 |
2004年 | 250篇 |
2003年 | 181篇 |
2002年 | 206篇 |
2001年 | 193篇 |
2000年 | 166篇 |
1999年 | 153篇 |
1998年 | 121篇 |
1997年 | 78篇 |
1996年 | 79篇 |
1995年 | 80篇 |
1994年 | 52篇 |
1993年 | 48篇 |
1992年 | 48篇 |
1991年 | 37篇 |
1990年 | 42篇 |
1989年 | 26篇 |
1988年 | 21篇 |
1987年 | 15篇 |
1986年 | 20篇 |
1985年 | 15篇 |
1984年 | 22篇 |
1983年 | 11篇 |
1982年 | 13篇 |
1981年 | 6篇 |
1980年 | 6篇 |
1979年 | 4篇 |
1978年 | 5篇 |
1977年 | 3篇 |
1975年 | 2篇 |
1950年 | 1篇 |
排序方式: 共有8272条查询结果,搜索用时 15 毫秒
1.
2.
3.
4.
5.
6.
Management of a harem breeding colony of rhesus monkeys to reduce trauma-related morbidity and mortality 总被引:1,自引:0,他引:1
M J Kessler W T London R G Rawlins J Gonzalez H S Martinez J Sanchez 《Journal of medical primatology》1985,14(2):91-98
A management procedure was developed for a harem breeding colony of rhesus monkeys to reduce trauma-related injuries and deaths resulting from the periodic removal of pregnant monkeys for research and their subsequent return to the population. Lower morbidity and mortality rates, a reduced mean conception interval, and a higher mean conception rate occurred when monkeys were maintained in permanent harems to which returning females were reintroduced compared to new social groups formed from aggregates of unfamiliar animals. 相似文献
7.
In the ecotone research region of the Danube in Austria (Man-and-the-Biosphere (MaB)- project 5/21, Austrian Academy of Science) the macrophytes are one of the most important groups of organisms investigated. The species composition and the plant mass in hydrologically dynamic, and in predominantly stagnant sections of a system of lateral branches and sloughs were studied. This study showed that areas protected from frequent disturbance by floods had a greater number of species and higher biomass of aquatic macrophytes. Some species were shown to be evenly distributed throughout the branch system, whereas other species tended to form rare, but large singular patches. A set of new data elaboration techniques enabled us to describe the distribution pattern of the aquatic vegetation in this large branch system of the River Danube. 相似文献
8.
M. A. Burgess 《Biodiversity and Conservation》1994,3(2):126-136
Understanding the relationships between indigenous people and their threatened economic plants can aid the conservation effort on many levels. Understanding ethnic perceptions of the taxon is critical toin situ andex situ conservation projects and enhances the accompanying educational effort. Examples are discussed from the experience of a grassroots conservation group in southwestern United States, Native Seed/SEARCH. Four levels of economic plant vulnerability are examined among 1) wild-harvested plants, 2) husbanded wild plants, 3) domesticates, and 4) wild relatives of domesticates. Legal interpretations of endangered husbanded and domestic plants are discussed, and further documentation encouraged. Genetic dynamism of threatened indigenous crops is examined and the concept of Systems Conservation (i.e. the plant/human interactive systems) is introduced. Guidelines are offered for incorporating better cultural responsibility intoex situ conservation strategies. The concept of Biocultural Restoration is introduced with an example from an O'odham community. Examples are given of ways indigenous peoples and their knowledge can assist in the conservation effort. 相似文献
9.
Simone Baumann-Pickering Jennifer S. Trickey Alba Solsona-Berga Ally Rice Erin M. Oleson John A. Hildebrand Kaitlin E. Frasier 《Diversity & distributions》2023,29(4):478-491
Aim
Understanding cetacean species' distributions and population structure over space and time is necessary for effective conservation and management. Geographic differences in acoustic signals may provide a line of evidence for population-level discrimination in some cetacean species. We use acoustic recordings collected over broad spatial and temporal scales to investigate whether global variability in echolocation click peak frequency could elucidate population structure in Blainville's beaked whale (Mesoplodon densirostris), a cryptic species well-studied acoustically.Location
North Pacific, Western North Atlantic and Gulf of Mexico.Time period
2004–2021.Major taxa studied
Blainville's beaked whale.Methods
Passive acoustic data were collected at 76 sites and 150 cumulative years of data were analysed to extract beaked whale echolocation clicks. Using an automated detector and subsequent weighted network clustering on spectral content and interclick interval of clicks, we determined the properties of a primary cluster of clicks with similar characteristics per site. These were compared within regions and across ocean basins and evaluated for suitability as population-level indicators.Results
Spectral averages obtained from primary clusters of echolocation clicks identified at each site were similar in overall shape but varied in peak frequency by up to 8 kHz. We identified a latitudinal cline, with higher peak frequencies occurring in lower latitudes.Main conclusions
It may be possible to acoustically delineate populations of Blainville's beaked whales. The documented negative correlation between signal peak frequency and latitude could relate to body size. Body size has been shown to influence signal frequency, with lower frequencies produced by larger animals, which are subsequently more common in higher latitudes for some species, although data are lacking to adequately investigate this for beaked whales. Prey size and depth may shape frequency content of echolocation signals, and larger prey items may occur in higher latitudes, resulting in lower signal frequencies of their predators. 相似文献10.
J. R. Vallentyne M. Munawar 《Journal of Aquatic Ecosystem Stress and Recovery (Formerly Journal of Aquatic Ecosystem Health)》1993,2(4):231-235
The development of an ecosystem (social, economic, environmental) approach to water management is traced from its origins in the Great Lakes of North America. The focus on health and integrity of ecosystems is an outgrowth of the Lamarckian concept of The Biosphere as a global system of matter, life, and mind. The driving forces behind the development of an ecosystem approach have been negative feedback from excessive demotechnic growth and faith that we can maintain a healthy relationship with Mother Earth. 相似文献