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Red squirrels, rabbits and hedgehogs: Why their numbers are falling
The number of red squirrels, rabbits and hedgehogs in Britain is shrinking.
Experts think that almost one in five mammal species could be at high risk of extinction - that means they wouldn't exist any more.
Twelve species have been placed on the country's first ever "Red List" of threatened mammals, including the red squirrel, wildcat and greater mouse-eared bat.
In fact there is just one known greater mouse-eared bat in the whole of Britain!
The study's been carried out by The Mammal Society and Natural England.
It also found that the populations of animals such as the hedgehog and water vole have gone down by two thirds.
There are lots of reasons they say the numbers are dropping - for example, climate change and a loss of habitat. This means there are fewer places which are ideal for these creatures to live and find food.
There is good news though.
The number of animals such as otters, beavers and wild boar have gone up.
According to the report, otters have been helped by changes in the law to better protect them.