The Serengeti National Park is a large national park in northern Tanzania that stretches over 14,763 km2 . It is located in eastern Mara Region and northeastern Simiyu Region and contains over 1,500,000 hectares of virgin savanna. The Serengeti is well known for the largest annual animal migration in the world of over 1.5 million blue wildebeest and 250,000 zebra along with smaller herds of Thomson’s gazelle and eland. The national park is also home to the largest lion population in Africa. It is under threat from deforestation, population growth and ranching. Serengeti National Park forms a Lion Conservation Unit since 2005 together with Maasai Mara National Reserve.[10] More than 3,000 lions live in this ecosystem.[11] The population density of the African leopard is estimated at 5.41 individuals per 100 km2 (39 sq mi) in the dry season. Other mammal carnivores include the Cheetah,[17] about 3,500 spotted hyena, Black-backed jackal, African golden wolf, honey badger, striped hyena, caracal, serval, banded mongoose, and two species of otters.[citation needed] The African wild dog was reintroduced to the area in 2012 after disappearing in 1991.[18] Other mammals include hippopotamus, common warthog, aardvark, aardwolf, African wildcat, African civet, common genet, zorilla, african striped weasel, bat-eared fox, ground pangolin, crested porcupine, three species of hyraxes and cape hare.[19] Primates such as yellow and olive baboons, patas monkeys, and vervet monkey, mantled guereza are also seen in the gallery forests of the Grumeti River. More than 500 bird species can be seen such as Masai ostrich, secretarybird, kori bustards, helmeted guineafowls, Grey-breasted spurfowl, blacksmith lapwing, african collared dove, red-billed buffalo weaver, southern ground hornbill, crowned cranes, sacred ibis, cattle egrets, black herons, knob-billed ducks, saddle-billed storks, white stork, goliath herons, marabou storks, yellow-billed stork, spotted thick-knees, lesser flamingo, shoebills, abdim’s stork, hamerkops, hadada ibis, african fish eagles, pink-backed pelicans, tanzanian red-billed hornbill, martial eagles, egyptian geese, lovebirds, spur-winged geese, oxpeckers, and many species of vultures.It is also famous for the great Widbeast Migration.