The biosphere is the blanket of living things that surrounds the substratum of the earth. The biosphere is composed of living organisms as well as the physical environment. The physical environment includes the rocky material of the earth's crust, the water on or near the earth's surface, and the thin blanket of gas surrounding the earth. All life is confined to a five‐mile vertical space around the surface of the earth.

Ecologists study the living components of the biosphere in subunits called biomes. A biome is a group of communities dominated by a particular climax community. Deserts, forests, and prairies are examples of biomes. Other examples are the tundra, taiga (the southern edge of the tundra), and temperate forests. Each biome represents a unique situation where the ecosystem is defined by the environment. The broad diversity of living things that characterizes the earth exists in the biome. Each type of living thing is adapted to its own habitat and niche within the biome. The general composition of a biome remains uniform, but local differences arise as a result of population fluctuations, floods, fire, and other ecological factors.