How did the grey squirrels get rid of the red squirrels?

The Red Squirrel was originally from Scotland and the Grey Squirrel from America. They both thrived in their separate lands until someone brought over the Greys to the UK. They reproduced rapidly and stole the Red Squirrels food and attacked Red Squirrel populaces. Now the Red Squirrel is endangered. But NOT exctinct. The Grey Squirrels have not got rid of the Red Squirrels.