全文获取类型
收费全文 | 3333篇 |
免费 | 282篇 |
国内免费 | 2篇 |
出版年
2023年 | 17篇 |
2022年 | 13篇 |
2021年 | 49篇 |
2020年 | 39篇 |
2019年 | 42篇 |
2018年 | 51篇 |
2017年 | 50篇 |
2016年 | 87篇 |
2015年 | 152篇 |
2014年 | 163篇 |
2013年 | 221篇 |
2012年 | 254篇 |
2011年 | 265篇 |
2010年 | 185篇 |
2009年 | 182篇 |
2008年 | 245篇 |
2007年 | 239篇 |
2006年 | 203篇 |
2005年 | 209篇 |
2004年 | 207篇 |
2003年 | 175篇 |
2002年 | 187篇 |
2001年 | 56篇 |
2000年 | 33篇 |
1999年 | 35篇 |
1998年 | 32篇 |
1997年 | 26篇 |
1996年 | 18篇 |
1995年 | 25篇 |
1994年 | 12篇 |
1993年 | 14篇 |
1992年 | 8篇 |
1991年 | 11篇 |
1990年 | 10篇 |
1989年 | 6篇 |
1988年 | 9篇 |
1987年 | 5篇 |
1986年 | 5篇 |
1985年 | 10篇 |
1984年 | 6篇 |
1983年 | 11篇 |
1982年 | 12篇 |
1981年 | 10篇 |
1980年 | 2篇 |
1979年 | 3篇 |
1978年 | 6篇 |
1977年 | 5篇 |
1976年 | 5篇 |
1973年 | 2篇 |
1968年 | 1篇 |
排序方式: 共有3617条查询结果,搜索用时 78 毫秒
1.
2.
Sam M. Ferreira Cathy Greaver Grant A. Knight Mike H. Knight Izak P. J. Smit Danie Pienaar 《PloS one》2015,10(6)
The onslaught on the World’s rhinoceroses continues despite numerous initiatives aimed at curbing it. When losses due to poaching exceed birth rates, declining rhino populations result. We used previously published estimates and growth rates for black rhinos (2008) and white rhinos (2010) together with known poaching trends at the time to predict population sizes and poaching rates in Kruger National Park, South Africa for 2013. Kruger is a stronghold for the south-eastern black rhino and southern white rhino. Counting rhinos on 878 blocks 3x3 km in size using helicopters, estimating availability bias and collating observer and detectability biases allowed estimates using the Jolly’s estimator. The exponential escalation in number of rhinos poached per day appears to have slowed. The black rhino estimate of 414 individuals (95% confidence interval: 343-487) was lower than the predicted 835 individuals (95% CI: 754-956). The white rhino estimate of 8,968 individuals (95% CI: 8,394-9,564) overlapped with the predicted 9,417 individuals (95% CI: 7,698-11,183). Density- and rainfall-dependent responses in birth- and death rates of white rhinos provide opportunities to offset anticipated poaching effects through removals of rhinos from high density areas to increase birth and survival rates. Biological management of rhinos, however, need complimentary management of the poaching threat as present poaching trends predict detectable declines in white rhino abundances by 2018. Strategic responses such as anti-poaching that protect supply from illegal harvesting, reducing demand, and increasing supply commonly require crime network disruption as a first step complimented by providing options for alternative economies in areas abutting protected areas. 相似文献
3.
4.
5.
6.
7.
Correlation of surface sediment diatoms with the present lake water pH in 28 Algoma lakes,Ontario, Canada 总被引:1,自引:1,他引:0
The surface sediment diatom analysis of 28 Algoma lakes (pH 4.40–8.13) indicates that even though each lake has a widely different aquatic environment and characteristic diatom assemblage, a definite relationship exists between the lake water pH and their diatom assemblages. In the acidic lakes acidobiontic and acidophilous diatom species predominate whereas in circumneutral and alkaline lakes circumneutral and alkaliphilous diatoms were most common. Cluster analysis of the pH indicator diatom assemblages grouped the study lakes into three distinct cluster groups. These groups also closely corresponded to lake water pH. On the basis of published ecological information as well as their presence in our study lakes, the pH indicator status of a number of diatom taxa have been discussed. A detailed listing of the diatom taxa identified and their pH indicator status is provided in order to facilitate their use in future diatom-inferred pH studies. 相似文献
8.
Jesús Olivero John E. Fa Miguel A. Farfán Jerome Lewis Barry Hewlett Thomas Breuer Giuseppe M. Carpaneto María Fernández Francesco Germi Shiho Hattori Josephine Head Mitsuo Ichikawa Koichi Kitanaishi Jessica Knights Naoki Matsuura Andrea Migliano Barbara Nese Andrew Noss Dieudonné Ongbwa Ekoumou Pascale Paulin Raimundo Real Mike Riddell Edward G. J. Stevenson Mikako Toda J. Mario Vargas Hirokazu Yasuoka Robert Nasi 《PloS one》2016,11(1)
Pygmy populations occupy a vast territory extending west-to-east along the central African belt from the Congo Basin to Lake Victoria. However, their numbers and actual distribution is not known precisely. Here, we undertake this task by using locational data and population sizes for an unprecedented number of known Pygmy camps and settlements (n = 654) in five of the nine countries where currently distributed. With these data we develop spatial distribution models based on the favourability function, which distinguish areas with favourable environmental conditions from those less suitable for Pygmy presence. Highly favourable areas were significantly explained by presence of tropical forests, and by lower human pressure variables. For documented Pygmy settlements, we use the relationship between observed population sizes and predicted favourability values to estimate the total Pygmy population throughout Central Africa. We estimate that around 920,000 Pygmies (over 60% in DRC) is possible within favourable forest areas in Central Africa. We argue that fragmentation of the existing Pygmy populations, alongside pressure from extractive industries and sometimes conflict with conservation areas, endanger their future. There is an urgent need to inform policies that can mitigate against future external threats to these indigenous peoples’ culture and lifestyles. 相似文献
9.
10.