When Leopold wrote: "I now suspect that just as a deer herd lives in mortal fear of its wolves, so does a mountain live in mortal fear of its deer.” he was referring to how the deer were a predator to the mountain just as the wolves were predator to the deer. Once humans had killed most of the wolves around the mountain there wasn’t a good control on the deer population. The deer population was eating the majority of edible plants and trees. The balance of nature keeps most populations in check when a species is not singled out and destroyed. All animals have a predator that preys on them and they themselves are in someway a predator as well. Populations of each species are kept in check not only be being killed and eaten by other animals
