When monsoon mists lift from the Dhofar mountains of Oman, camel herds begin their autumn migration from the plateau into cloud forests. But the beasts that once connected the ancient frankincense capital of southern Arabia to trade routes through Petra and Gaza have multiplied, threatening the trees whose fragrant resin brought the area its wealth. The migration, called the khateel, has changed little over the centuries but the land can no longer support traditional pastoralism. Large herds have had devastating consequences for rare vegetation found nowhere else on Earth.