Over the summer, Yellowstone has flourished - in late August there are more living things here than at any other time of the year. But winter is around the corner and there are just two months for all Yellowstone's animals to get ready or get out.

As winter turns to spring, temperatures rise and Yellowstone bursts into life. Beavers feast on new green shoots, baby bison take their first faltering steps, and grizzly bear cubs emerge from dens to explore this new green world. But spring is also a perilous time. Wolves are hungrier than usual after slim winter pickings, and rising temperatures melt the vast mountain snow pack, sending a million tons of water flooding into the rivers. Only the toughest will survive.

As the spring melts the winter snow, the herds of elk, pronghorn and bison return from their winter feeding grounds to take advantage of America's richest natural grasslands.