They keep the gravel all messed up with their digging.
Since they climb, this means you also have to protect from that likelihood with either a cover or lining your barrier with flashing or some other material on which they cannot get traction.
Bunny Guinness, another regular panellist and columnist for the Sunday Telegraph, said: "I'm intolerant of squirrels and rabbits and pigeons. Coming from a farming background, I'm intolerant." She recommends using a Kania Trap, which crushes the neck of squirrels.