收费全文 | 1811篇 |
免费 | 178篇 |
国内免费 | 1篇 |
2023年 | 11篇 |
2022年 | 35篇 |
2021年 | 57篇 |
2020年 | 25篇 |
2019年 | 35篇 |
2018年 | 46篇 |
2017年 | 39篇 |
2016年 | 61篇 |
2015年 | 102篇 |
2014年 | 124篇 |
2013年 | 144篇 |
2012年 | 188篇 |
2011年 | 156篇 |
2010年 | 104篇 |
2009年 | 94篇 |
2008年 | 113篇 |
2007年 | 91篇 |
2006年 | 90篇 |
2005年 | 91篇 |
2004年 | 67篇 |
2003年 | 58篇 |
2002年 | 55篇 |
2001年 | 7篇 |
2000年 | 7篇 |
1999年 | 14篇 |
1998年 | 12篇 |
1997年 | 7篇 |
1996年 | 5篇 |
1994年 | 11篇 |
1993年 | 5篇 |
1991年 | 6篇 |
1989年 | 4篇 |
1984年 | 6篇 |
1982年 | 5篇 |
1981年 | 4篇 |
1980年 | 5篇 |
1977年 | 4篇 |
1974年 | 6篇 |
1973年 | 4篇 |
1971年 | 3篇 |
1970年 | 4篇 |
1969年 | 11篇 |
1968年 | 4篇 |
1967年 | 3篇 |
1964年 | 3篇 |
1961年 | 3篇 |
1960年 | 5篇 |
1959年 | 3篇 |
1958年 | 4篇 |
1957年 | 3篇 |
Interbasin water transfers are becoming an increasingly common tool to satisfy municipal and agricultural water demand, but their impacts on movement and gene flow of aquatic organisms are poorly understood. The Grand Ditch is an interbasin water transfer that diverts water from tributaries of the upper Colorado River on the west side of the Continental Divide to the upper Cache la Poudre River on the east side of the Continental Divide. We used single nucleotide polymorphisms to characterize population genetic structure in cutthroat trout (Oncorhynchus clarkii) and determine if fish utilize the Grand Ditch as a movement corridor. Samples were collected from two sites on the west side and three sites on the east side of the Continental Divide. We identified two or three genetic clusters, and relative migration rates and spatial distributions of admixed individuals indicated that the Grand Ditch facilitated bidirectional fish movement across the Continental Divide, a major biogeographic barrier. Previous studies have demonstrated ecological impacts of interbasin water transfers, but our study is one of the first to use genetics to understand how interbasin water transfers affect connectivity between previously isolated watersheds. We also discuss implications on native trout management and balancing water demand and biodiversity conservation.
相似文献Aquatic systems have been extensively altered by human structures (e.g., construction of dams/canals) and these have major impacts on the connectivity of wildlife populations through the loss and isolation of suitable habitats. Habitat loss and isolation affect gene flow and influence the persistence of populations in time and space by restricting movements. Isolation can result in higher inbreeding, lower genetic diversity, and greater genetic structure, which may render populations more vulnerable to environmental changes, and thus to extinction. Given the ubiquity and the persistence of dams and canals in space and time, it is crucial to understand their effects on the population genetics of aquatic species. Here, we documented the genetic diversity and structure of painted turtle (Chrysemys picta) populations in the Rideau Canal, Ontario, Canada. More specifically, we used 13 microsatellites to evaluate the influence of locks on genetic variation in 822 painted turtles from 22 sites evenly distributed along the 202-km canal. Overall, we found low, but significant, genetic differentiation suggesting that some dispersal is occurring throughout the canal. In addition, we showed that locks contribute to the genetic differentiation observed in the system. Clustering analysis revealed two distinct genetic groups whose boundary is associated with a series of six locks. Our results illustrate how artificial waterways, such as canal systems, can influence population genetic structure. We highlight the importance of adopting management plans that can mitigate the impacts of human infrastructure and preserve gene flow across the landscape to maintain viable populations.
相似文献