Grey squirrels have survived repeated Government-sanctioned attempts at extermination by shooting, trapping and poisoning. Greys do considerable damage to trees when present in large numbers. They attack trees in early summer gnawing at the main stem, seeking the sweet, sappy layers immediately beneath the bark. Sometimes the tree is completely ringed and as a consequence will die. If it doesn't die then it will usually be checked or spoilt. It is now illegal to import a grey squirrel or keep one as a pet. They will adapt to any area covered by trees, whether broad leaved or conifer, with an average density of about 5 per acre. Greys will live in suburban parks and gardens, becoming tame enough to feed from the hand.
Nuisance concerns: Squirrels have easily adapted to humans. They frequently use buildings as nesting areas. Squirrels love to break into a house and stay in an attic or soffit. They often find a small opening and will chew a wider hole to gain access to the building. They bring nesting material into the home, and make quite a bit of noise scurrying around and caching nuts. They often fall down the chimney flue and make a lot of noise or enter the fireplace. Sometimes they fall down a wall from the attic and get stuck. They often chew their way into commercial buildings or apartments. They often enter the attic through the gable vent. Squirrels can cause a fire hazard in homes by bringing in nesting material, and by chewing on power lines. Squirrels can leave behind a lot of droppings and urine in the attic. The droppings not only smell bad, but they pose a biohazard, and the smell attracts new squirrels. I highly recommend attic decontamination services if you've had squirrels living in your attic.