全文获取类型
收费全文 | 24799篇 |
免费 | 2392篇 |
国内免费 | 18篇 |
出版年
2023年 | 118篇 |
2021年 | 682篇 |
2020年 | 318篇 |
2019年 | 401篇 |
2018年 | 500篇 |
2017年 | 425篇 |
2016年 | 763篇 |
2015年 | 1191篇 |
2014年 | 1229篇 |
2013年 | 1490篇 |
2012年 | 1877篇 |
2011年 | 1858篇 |
2010年 | 1155篇 |
2009年 | 962篇 |
2008年 | 1350篇 |
2007年 | 1360篇 |
2006年 | 1335篇 |
2005年 | 1180篇 |
2004年 | 1131篇 |
2003年 | 1007篇 |
2002年 | 1008篇 |
2001年 | 210篇 |
2000年 | 149篇 |
1999年 | 234篇 |
1998年 | 255篇 |
1997年 | 164篇 |
1996年 | 193篇 |
1995年 | 143篇 |
1994年 | 202篇 |
1993年 | 170篇 |
1992年 | 153篇 |
1991年 | 135篇 |
1990年 | 139篇 |
1989年 | 135篇 |
1988年 | 105篇 |
1987年 | 114篇 |
1986年 | 104篇 |
1985年 | 145篇 |
1984年 | 158篇 |
1983年 | 129篇 |
1982年 | 182篇 |
1981年 | 202篇 |
1980年 | 191篇 |
1979年 | 121篇 |
1978年 | 160篇 |
1977年 | 145篇 |
1976年 | 116篇 |
1975年 | 102篇 |
1974年 | 133篇 |
1973年 | 134篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
11.
The probability that protected areas will deliver their potential for maintaining or enhancing biodiversity is likely to be
maximised if they are appropriately and effectively managed. As a result, governments and conservation agencies are devoting
much attention to the management of protected areas. In the U.K., the demand for performance accountability has resulted in
Public Service Agreements (PSA) that set out targets for government departments to deliver results in return for investments
being made. One such target for England is to ensure that all nationally important wildlife sites are in favourable condition
by 2010. Here, we tested the hypothesis, of potential strategic importance, that the ecological condition of these sites is
predictable from relationships with a range of physical, environmental and demographic variables. We used binary logistic
regression to investigate these relationships, using the results of English Nature’s 1997–2003 condition assessment exercise.
Generally, sites in unfavourable condition tend to be larger in area, located at higher elevations, but with higher human
population density and are more spatially isolated from units of the same habitat. However, despite the range of different
parameters included in our models, the extent to which the condition of any given site could be predicted was low. Our results
have implications for the delivery of PSA targets, funding allocation, and the location of new protected areas. 相似文献
12.
13.
14.
15.
Hermógenes Fernández-Marín David R. Nash Sarah Higginbotham Catalina Estrada Jelle S. van Zweden Patrizia d'Ettorre William T. Wcislo Jacobus J. Boomsma 《Proceedings. Biological sciences / The Royal Society》2015,282(1807)
Fungus-farming ant colonies vary four to five orders of magnitude in size. They employ compounds from actinomycete bacteria and exocrine glands as antimicrobial agents. Atta colonies have millions of ants and are particularly relevant for understanding hygienic strategies as they have abandoned their ancestors'' prime dependence on antibiotic-based biological control in favour of using metapleural gland (MG) chemical secretions. Atta MGs are unique in synthesizing large quantities of phenylacetic acid (PAA), a known but little investigated antimicrobial agent. We show that particularly the smallest workers greatly reduce germination rates of Escovopsis and Metarhizium spores after actively applying PAA to experimental infection targets in garden fragments and transferring the spores to the ants'' infrabuccal cavities. In vitro assays further indicated that Escovopsis strains isolated from evolutionarily derived leaf-cutting ants are less sensitive to PAA than strains from phylogenetically more basal fungus-farming ants, consistent with the dynamics of an evolutionary arms race between virulence and control for Escovopsis, but not Metarhizium. Atta ants form larger colonies with more extreme caste differentiation relative to other attines, in societies characterized by an almost complete absence of reproductive conflicts. We hypothesize that these changes are associated with unique evolutionary innovations in chemical pest management that appear robust against selection pressure for resistance by specialized mycopathogens. 相似文献
16.
17.
Harbor seal ( Phoca vitulina ) haul-out site use may be affected by natural or anthropogenic factors. Here, we use an 11-yr (1997–2007) study of a seal colony located near a mariculture operation in Drakes Estero, California, to test for natural (El Niño-Southern Oscillation (ENSO), density-dependence, long-term trends) and anthropogenic (disturbance or displacement related to oyster production activities) factors that may influence the use of haul-out subsites. Annual mariculture related seal disturbance rates increased significantly with increases in oyster harvest ( r s = 0.55). Using generalized linear models (GLMs) ranked by best fit and Akaike's Information Criteria, ENSO and oyster production (as a proxy for disturbance/displacement) best explained the patterns of seal use at all three subsites near the mariculture operations, with effects being stronger at the two subsites closest to operations. Conversely, density-dependence and linear trend effects poorly explained the counts at these subsites. We conclude that a combination of ENSO and mariculture activities best explain the patterns of seal haul-out use during the breeding/pupping season at the seal haul-out sites closest to oyster activities. 相似文献
18.
19.
20.