Refer to the early portions of the text referring to the color of the hills. (This is wonderful imagery describing the correlation between the death of the land and the death of the tribe. Hint: They are intertwined.)
The setting is essential to the book, at its heart, the book is about South Africa more then it is about any one character or idea. The land in particular is emphasized, and man's relationship to the land is seen as out of balance.
The breakdown of the tribe mirrors the breakdown of the land, as people exploit the land, build huge cities, destroy the beautiful earth, and render themselves apart. They do not know themselves or the land, and this is the central problem and theme.