The return of the otter to many British waterways is helping to reverse the decline of another species - the water vole - a study suggests.
Otters began to decline around 50 years ago due to pesticide use
The research has been done by conservationists at Oxford University, who are monitoring the government's commitment to promote biodiversity.
Otters have been taking on the water voles' enemy - American mink - which thrived after escaping from fur farms.
The study is part of a review of the government's biodiversity commitments.
Professor David Macdonald, director of the wildlife conservation unit at Oxford, said the combating of American mink by otters may create "conditions for the recovery of the water vole".
However, he warned it was "too late" to leave the species alone to recover.
"We're now looking at reintroducing water voles as part of a recovery programme," he said.