A few weeks ago I read a book called Feral: Searching for Enchantment on the Frontiers of Rewilding by George Monbiot... it was very good, with a great deal of information. He went into the history of how the landscape changed drastically over the centuries in Britain because of agriculture and farming, (I had no idea sheep had been such a major factor in all this.)
I'm not sure how well some of his ideas would work, (I can't see many people being keen on the reintrodution of wolves!) but he explains how everything can be affected when the predator at the top of a food chain is eliminated. When wolves were allowed back into Yellowstone Park in America, it had a great impact not only on the herd animals, but on the environment and even the shape of the landscape... for the better.
It was a fascinating book, from cover to cover... he's a very readable author. Highly recommended.