The grey squirrel was introduced from  America in 1876 and continued until approximately 1930. It was at this point it was recognised the effects and damage the grey squirrel was having on our native flora & fauna.
As the grey squirrel can establish and live in higher population density it's food demands are much greater than our native red squirrel.
It can eat a greater variety of plant foods at an earlier stage of maturity. Acorns are a prime example of this, red squirrels find the tannin in acorns unpalatable and grey squirrels will utilise this food source in advance of reds, along with most other food sources.
This results in the red squirrels being starved of vital food sources or being displaced from prime habitat.