As Predator
In December 2007, the European pine marten was credited with reducing the population of the invasive grey squirrel in the UK. Where the range of the expanding European pine marten population meets that of the grey squirrel, the population of the squirrels quickly retreats. It is theorised that because the grey squirrel spends more time on the ground than the red squirrel, they are far more likely to come in contact with this predator.
Read more about this topic: European Pine Marten