Water voles are making a comeback on the River Meon in the South Downs National Park.
The animal, made famous by The Wind in the Willows, had become a rare sight on Britain's riverbanks.
A recent ecological report has shown that they are now thriving and breeding on the Hampshire river.
The survey comes a decade after a reintroduction programme was initiated to combat the water vole's declining population, which has shrunk by 95 per cent over the past century.
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The main reasons for this decline have been habitat loss and predation by the non-native American mink.
The research found more than 2,500 signs of water vole activity at 27 sites along the river, with 20 of these showing evidence of breeding.
Since 2013, more than 2,800 water voles have been released along the river, and the survey indicates that thousands have made the River Meon their permanent home.
Dr Rowenna Baker, who led the survey, said: "It’s been fantastic to find evidence that water voles are persisting at their release sites and colonising new habitat up and down the river corridor.
"The abundance of field signs shows that water voles are now thriving on the River Meon, and this is incredibly important when nationally their future still remains uncertain."
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The presence of water voles has contributed to the return of the otter to the River Meon, with evidence suggesting there may be three breeding females on the river.
The Meon Valley Partnership, formed in 2008, has played a major role in this success, working with landowners to restore natural features of the river, reintroduce water voles, control American mink populations, and improve water quality.
Elaina Whittaker-Slark, lead ranger for the Hampshire area of the National Park, said: "Unfortunately re-introductions are not always successful, so it’s wonderful to see this latest survey that shows good old ‘Ratty’ really is flourishing.
"It’s a good indicator of the overall health of this very rare chalk river, which has really turned a corner in the past decade or so after water voles were considered locally extinct back in 2003."
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