Ethiopia is an ecologically diverse country, ranging from the deserts along the eastern border to the tropical forests in the south to extensive Afromontane in the northern and southwestern parts. Lake Tana in the north is the source of the Blue Nile. It also has many endemic species, notably the gelada, the walia ibex and the Ethiopian wolf ("Simien fox"). The wide range of altitude has given the country a variety of ecologically distinct areas, and this has helped to encourage the evolution of endemic species in ecological isolation.