The first episode PRIME MOVERS shows how wild creatures help to build the foundations upon which environments depend. In North America herds of bison maintain the prairies that used to spread across the continent, while in Africa giraffes work as tree pruners and help maintain the savannah grasslands. In Africa the great migrations play a crucial role in the health of the grasslands and the final act of clearance by vultures and dung beetles is at the very heart of the cycle of life and death.