The biosphere refers to the parts of Earth where life exists or where known life has existed in the past. The biosphere is comprised of the atmosphere, geosphere, and hydrosphere because life exists in each of those three spheres, from birds in the sky to fish in the water to worms in the dirt. Food chains represent interconnected life cycles in the biosphere. Energy is transferred from one organism to the next and, as apex predators die, nutrients are returned to the soil. Readers will learn how people affect the biosphere and how life and energy are maintained in the biosphere.