Water voles in parts of East Yorkshire are being targeted by gangs of wild American mink.
Water voles have no escape from the American mink
The mink, which have been released or escaped from fur farms, are threatening the voles' population in Driffield.
The Yorkshire Wildlife Trust said it was monitoring the situation to protect the voles, which are said to be one of the country's most threatened mammals.
"Wherever a water vole goes a mink can follow them and unfortunately catches them and eats them," a spokesman said.
Water voles tend to live in streams, ditches and ponds on agricultural land. Although once a common sight in the countryside, their population has been in decline since the 1900s and studies have predicted they will become extinct in the UK by 2012 if more is not done to protect the species.
Anyone who spots water voles in the rivers of East Yorkshire are asked to report their sightings to the Yorkshire Wildlife Trust.