The African landscape so often depicted in movies and imagined by travelers is a park-like vista of grassland dotted with flat-topped trees. This is the savannah, a mosaic of woodland and grassland. The ration of trees to grass and the dominant species of trees is determined by rainfall and the type of soil. Savannah is the dominant habitat in most of the large wildlife reserves in southern Africa, with thorny acacia trees being the most prominent. Seasonal grass fires are an important mechanism in maintaining the savannah ecosystem, as they encourage grass growth and limit the spread of woody plants. Large herbivores including giraffe, zebra, buffalo and wildebeest favor the savannah which also supports the highest density of lions and other large predators, as well as bird diversity with eagles, vultures, bustards and rollers.