From the introduction: Ecology is concerned with how the living part (or biosphere) of the planet works. The biosphere is driven by the energy of the sun, and this energy is passed on from species to species and then finally radiated as excess heat back into space. Green plants start this cycle, and exist anywhere under the sun where the earth is moist, warm, or nutrient-rich enough to let them live. The plants use the solar energy to drive a chemical synthesis of carbohydrate fuels from water and carbon dioxide. This carbohydrate fuel is what all the rest of the planet's creatures live off of. Without plants there would be no food for the rest of the creatures.