One of the common themes running through his films is an ecological bias. The industrial civilization devastated the earth and polluted the atmosphere. Toxic forests comprise mutated vegetation and gigantic insects. These forests belch forth deadly spores so everyone in their proximity must wear air filters. Seas are highly acidic and burn all who touch them.
Ecological responsibility is the bedrock upon which the story is built.
A Totoro is a mythical, magical and cuddly creature. The Totoros are, in a sense, the physical embodiment of the spirit of the forest and trees. Miyazaki is fond of using tree or forest spirits in his movies. Mononoke Hime, as it is also called, is a story about a mystic fight between the Animal Gods of the forest and humans. The humans had to cut the forest down so that they could mine the land for iron, and the Gods tried to protect their land.