Donkeys and other animals are usually found in this kind of empty land where they come for grass during the dry season. Although the land is dry I think these animals are still able to find something to eat.
According to Wikipedia, "Serrekunda (or Serekunda) is the largest city in The Gambia, lying southwest of Banjul. Its population as of 2006, was 335,733 people. Although Banjul is The Gambia's capital, it is on an island and cannot grow further, making much of the growth in Serrekunda."