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

Ecologists study the living components of the biosphere in subunits called biomes. A biome is a group of communities with stable ecosystems 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 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.