全文获取类型
收费全文 | 10551篇 |
免费 | 856篇 |
国内免费 | 8篇 |
出版年
2023年 | 46篇 |
2022年 | 37篇 |
2021年 | 192篇 |
2020年 | 150篇 |
2019年 | 150篇 |
2018年 | 249篇 |
2017年 | 206篇 |
2016年 | 292篇 |
2015年 | 505篇 |
2014年 | 493篇 |
2013年 | 636篇 |
2012年 | 824篇 |
2011年 | 725篇 |
2010年 | 485篇 |
2009年 | 384篇 |
2008年 | 565篇 |
2007年 | 525篇 |
2006年 | 497篇 |
2005年 | 493篇 |
2004年 | 497篇 |
2003年 | 453篇 |
2002年 | 388篇 |
2001年 | 171篇 |
2000年 | 144篇 |
1999年 | 125篇 |
1998年 | 111篇 |
1997年 | 67篇 |
1996年 | 79篇 |
1995年 | 71篇 |
1994年 | 74篇 |
1993年 | 69篇 |
1992年 | 117篇 |
1991年 | 98篇 |
1990年 | 82篇 |
1989年 | 83篇 |
1988年 | 73篇 |
1987年 | 87篇 |
1986年 | 74篇 |
1985年 | 97篇 |
1984年 | 71篇 |
1983年 | 54篇 |
1982年 | 50篇 |
1981年 | 48篇 |
1980年 | 51篇 |
1979年 | 63篇 |
1978年 | 54篇 |
1977年 | 46篇 |
1976年 | 47篇 |
1975年 | 53篇 |
1973年 | 44篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
Tom P. Moorhouse Cecilia A. L. Dahlsj? Sandra E. Baker Neil C. D'Cruze David W. Macdonald 《PloS one》2015,10(10)
Tourism accounts for 9% of global GDP and comprises 1.1 billion tourist arrivals per annum. Visits to wildlife tourist attractions (WTAs) may account for 20–40% of global tourism, but no studies have audited the diversity of WTAs and their impacts on the conservation status and welfare of subject animals. We scored these impacts for 24 types of WTA, visited by 3.6–6 million tourists per year, and compared our scores to tourists’ feedback on TripAdvisor. Six WTA types (impacting 1,500–13,000 individual animals) had net positive conservation/welfare impacts, but 14 (120,000–340,000 individuals) had negative conservation impacts and 18 (230,000–550,000 individuals) had negative welfare impacts. Despite these figures only 7.8% of all tourist feedback on these WTAs was negative due to conservation/welfare concerns. We demonstrate that WTAs have substantial negative effects that are unrecognised by the majority of tourists, suggesting an urgent need for tourist education and regulation of WTAs worldwide. 相似文献
2.
Aida Verdes Wooyoung Cho Marouf Hossain Patricia L. R. Brennan Daniel Hanley Tomá? Grim Mark E. Hauber Mand? Holford 《PloS one》2015,10(12)
Pigment-based coloration is a common trait found in a variety of organisms across the tree of life. For example, calcareous avian eggs are natural structures that vary greatly in color, yet just a handful of tetrapyrrole pigment compounds are responsible for generating this myriad of colors. To fully understand the diversity and constraints shaping nature’s palette, it is imperative to characterize the similarities and differences in the types of compounds involved in color production across diverse lineages. Pigment composition was investigated in eggshells of eleven paleognath bird taxa, covering several extinct and extant lineages, and shells of four extant species of mollusks. Birds and mollusks are two distantly related, calcareous shell-building groups, thus characterization of pigments in their calcareous structures would provide insights to whether similar compounds are found in different phyla (Chordata and Mollusca). An ethylenediaminetetraacetic acid (EDTA) extraction protocol was used to analyze the presence and concentration of biliverdin and protoporphyrin, two known and ubiquitous tetrapyrrole avian eggshell pigments, in all avian and molluscan samples. Biliverdin was solely detected in birds, including the colorful eggshells of four tinamou species. In contrast, protoporphyrin was detected in both the eggshells of several avian species and in the shells of all mollusks. These findings support previous hypotheses about the ubiquitous deposition of tetrapyrroles in the eggshells of various bird lineages and provide evidence for its presence also across distantly related animal taxa. 相似文献
3.
4.
Summary Cellular fatty acids inLactobacillus büchneri were transmethylated with H2SO4 catalyst in methanol at elevated temperature. By optimising the reaction time and the amounts of catalyst and methanol used at a fixed temperature it was possible to maximise the lactobacillic acid yield. The yield of lactobacillic acid with this method was better than with the traditional method using base-catalysed saponification followed by HCL-catalysed methylation. 相似文献
5.
6.
7.
Condon Tom Hakim Abbas Moran Antonio B. Zenteno Blumstein Daniel T. 《Journal of Ethology》2021,39(1):89-96
Journal of Ethology - Animals emit predator-elicited calls in response to potential predation threats. These vocalizations induce a variety of anti-predator behaviors in conspecific receivers... 相似文献
8.
9.
10.