The annual return from the Pacific Ocean of millions of salmon to the streams where they were born in North America in order to spawn and die. Grizzly bears depend for their survival on this event, too.

How one lion family - the Ntudu pride - struggles to survive until the return of the great migration of wildebeest and zebra to the Serengeti in Africa.

In Africa, a great flood in the Okavango turns four thousand square miles of arid plains into a wetland. The lush grass attracts elephants, deer, zebra, buffalo and hippos come for the water.