Deer are a great example in fact of the problems we face with damaging "farming". they, along with sheep, help strip the land of any shrubs/bushes and trees, keeping places like the highlands, Dartmoor and many of our so called national parks* relatively barren of life and "unnatural" through the overgrazing of these areas. It's great for the farmers though, they can make a nice profit when they otherwise wouldn't be able to. The same can be said with growse moors.
* There is an international ranking of protected areas. 1, being unspoilt wilderness (National Parks) and 5 being protected zones where people live, our so called national parks are ranked 4, heavily managed and little natural about them.