全文获取类型
收费全文 | 6534篇 |
免费 | 803篇 |
国内免费 | 836篇 |
出版年
2024年 | 13篇 |
2023年 | 178篇 |
2022年 | 126篇 |
2021年 | 267篇 |
2020年 | 314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 | 269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 | 161篇 |
1999年 | 152篇 |
1998年 | 120篇 |
1997年 | 74篇 |
1996年 | 75篇 |
1995年 | 80篇 |
1994年 | 52篇 |
1993年 | 48篇 |
1992年 | 47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 | 21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8173条查询结果,搜索用时 15 毫秒
51.
Maxime Eeraerts 《Journal of Applied Entomology》2020,144(8):751-754
Mason bees (Osmia spp.) are receiving increasing attention as they are very effective pollinators of multiple crops. However, different bee-associated kleptoparasites are known which can be detrimental to the development of the bees’ offspring. To date, clear evidence-based guidelines to control these kleptoparasites are lacking for growers. This study investigates whether different types of artificial nesting materials—cardboard tubes and wooden grooved boards—influence the development of the European orchard bee (Osmia cornuta) as well as the infestation rate of kleptoparasitic mites in the bees’ nest. This preliminary study provides insight that cardboard tubes reduce the infestation rate of mites by 81.8% and as such promote successful development of the European orchard bee by 15.5%. Using cardboard tubes over wooden grooved boards should be advised to growers as an effective management strategy to avoid infestation of kleptoparasitic mites and to enhance successful bee development. 相似文献
52.
《Current biology : CB》2020,30(7):1280-1286.e2
53.
A set of eight simple ecological and social principles is proposed that could enhance the understanding of what constitutes fish 'habitat' and, if implemented, could contribute to improved management and conservation strategies. The habitat principles are a small, interrelated sub‐set that may be coupled with additional ones to formulate comprehensive guidelines for management and conservation strategies. It is proposed that: 1) habitat can be created by keystone species and interactions among species; 2) the productivity of aquatic and riparian habitat is interlinked by reciprocal exchanges of material; 3) the riparian zone is fish habitat; 4) fishless headwater streams are inseparable from fish‐bearing rivers downstream; 5) habitats can be coupled – in rivers, lakes, estuaries and oceans, and in time; 6) habitats change over hours to centuries; 7) fish production is dynamic due to biocomplexity, in species and in habitats; 8) management and conservation strategies must evolve in response to present conditions, but especially to the anticipated future. It is contended that the long‐term resilience of native fish communities in catchments shared by humans depends on incorporating these principles into management and conservation strategies. Further, traditional strategies poorly reflect the dynamic nature of habitat, the true extent of habitat, or the intrinsic complexity in societal perspectives. Forward‐thinking fish management and conservation plans view habitat as more than water. They are multilayered, ranging from pools to catchments to ecoregions, and from hours to seasons to centuries. They embrace, as a fundamental premise, that habitat evolves through both natural and anthropogenic processes, and that patterns of change may be as important as other habitat attributes. 相似文献
54.
55.
STEPHEN M. OGLE F. JAY BREIDT MARK EASTER STEVE WILLIAMS KENDRICK KILLIAN KEITH PAUSTIAN 《Global Change Biology》2010,16(2):810-822
Process‐based model analyses are often used to estimate changes in soil organic carbon (SOC), particularly at regional to continental scales. However, uncertainties are rarely evaluated, and so it is difficult to determine how much confidence can be placed in the results. Our objective was to quantify uncertainties across multiple scales in a process‐based model analysis, and provide 95% confidence intervals for the estimates. Specifically, we used the Century ecosystem model to estimate changes in SOC stocks for US croplands during the 1990s, addressing uncertainties in model inputs, structure and scaling of results from point locations to regions and the entire country. Overall, SOC stocks increased in US croplands by 14.6 Tg C yr?1 from 1990 to 1995 and 17.5 Tg C yr?1 during 1995 to 2000, and uncertainties were ±22% and ±16% for the two time periods, respectively. Uncertainties were inversely related to spatial scale, with median uncertainties at the regional scale estimated at ±118% and ±114% during the early and latter part of 1990s, and even higher at the site scale with estimates at ±739% and ±674% for the time periods, respectively. This relationship appeared to be driven by the amount of the SOC stock change; changes in stocks that exceeded 200 Gg C yr?1 represented a threshold where uncertainties were always lower than ±100%. Consequently, the amount of uncertainty in estimates derived from process‐based models will partly depend on the level of SOC accumulation or loss. In general, the majority of uncertainty was associated with model structure in this application, and so attaining higher levels of precision in the estimates will largely depend on improving the model algorithms and parameterization, as well as increasing the number of measurement sites used to evaluate the structural uncertainty. 相似文献
56.
57.
Resource availability or distribution may affect interindividual competition in species such as primates, which forage in social groups, and several field studies suggest that dominance status predicts access to restricted food sources. Increased competition due to restricted resources may result in the intensification of aggressive behaviors. The study reported here examines the impact of manipulation of the distribution of food resources in a laboratory-housed group of bonnet macaques to assess the impact of distribution on aggressive behaviors. Food was restricted to a space which allowed access to only one or two individuals at a time or was dispersed throughout the living enclosure to allow universal access in alternating periods. During periods of clumped distribution, the monkeys obtained access to the food in order of their dominance status in the group. In addition, overall levels of aggressive behaviors increased, submissive behavior increased, and play decreased during periods of clumped distribution of food. These results confirm the findings from field studies indicating increases in agonism with restriction of food resources. These findings also have implications for primate colony management, in terms of developing strategies for minimizing intragroup aggression in captive groups. 相似文献
58.
59.
The mechanism of mass collaboration in risk management was studied during the Sichuan earthquake under a Web-based “PeopleFinder” project, where information is contributed and shared among mass contributors. The case study is provided by a great earthquake that happened in Wenchuan County, Sichuan Province, of southwestern China at 2:28 p.m. on May 12, 2008. We witnessed and experienced the rescue and relief efforts for the great earthquake. In this article, two fundamental frameworks are developed to study the mechanism of mass collaboration. Mass collaboration is proven to be effective in a big public crisis such as the Sichuan earthquake. 相似文献
60.
Keith W.T.Goulding 《中国科学:生命科学英文版》2005,48(Z2)
The nitrogen (N) cycle is essentially 'leaky'. The losses of small amounts of nitrate to waters and of ammonia and nitrous oxide to the atmosphere are a part of the global biogeo-chemical N cycle. However, intensive agricultural production, industry and vehicle use have more than doubled the amount of 'reactive' N in the environment, resulting in eutrophication, ecosystem change and health concerns. Research has identified agricultural practices that cause large losses of N and, in some cases, developed solutions. This paper discusses the problems of maintaining productivity while reducing N losses, compares conventional with low input (integrated) and organic farming systems, and discusses wider options. It also looks at the need to integrate studies on N with other environmental impacts, set in the context of the whole farm system, to provide truly sustainable agricultural systems. 相似文献