Upon arrival, early settlers practiced slash-and-burn agriculture to clear the coastal rainforests for cultivation. The first settlers encountered Madagascar's abundance of megafauna, including giant lemurs, elephant birds, giant fossa and the Malagasy hippopotamus, which have since become extinct because of hunting and habitat destruction. By 600 AD, groups of these early settlers had begun clearing the forests of the central highlands. Arab traders first reached the island between the 7th and 9th centuries. A wave of Bantu-speaking migrants from southeastern Africa arrived around 1000 AD. South Indian Tamil merchants arrived around 11th century. They introduced the zebu, a type of long-horned humped cattle, which they kept in large herds. Irrigated paddy fields were developed in the central highland Betsileo Kingdom and were extended with terraced paddies throughout the neighboring Kingdom of Imerina a century later. The rising intensity of land cultivation and the ever-increasing demand for zebu pasturage had largely transformed the central highlands from a forest ecosystem to grassland by the 17th century. The oral histories of the Merina people, who may have arrived in the central highlands between 600 and 1,000 years ago, describe encountering an established population they called the Vazimba. Probably the descendants of an earlier and less technologically advanced Austronesian settlement wave, the Vazimba were assimilated or expelled from the highlands by the Merina kings Andriamanelo, Ralambo and Andrianjaka in the 16th and early 17th centuries. Today, the spirits of the Vazimba are revered as "tompontany" (ancestral masters of the land) by many traditional Malagasy communities.