Sheep and goats are especially bad as grazers go, because they pull up plants by the roots. Cattle and horses nip off the grass, but don't pull it up, so it can grow back. Greece was once completely forested. They cut all the oak trees down to build ships. Then they grew wheat. Eventually the soil deteriorated and they couldn't grow grain anymore. Then they grazed cattle. The forage became too poor for cattle. Finally they grazed goats. The goats left the land as the rocky semi-desert you see today. Same thing happened in the Middle East and in North Africa, for example, Carthage.