The environment is the surroundings or conditions in which organisms live and interact. It includes the physical world: air, water, soil, sunlight, and temperature. It also includes the biological world: other organisms, both predators and prey, competitors and partners. The term is broad. A bacterium's environment is a drop of water. A city dweller's environment includes buildings, traffic, and social institutions. Ecologists usually focus on the natural and physical components that shape life.

Environments are not passive backdrops. They shape what can live where. A desert supports different species than a rainforest because of water and temperature. A polluted river supports fewer species than a clean one. Organisms also shape their environments. Beavers build dams that create wetlands. Earthworms aerate soil. Humans have altered environments more than any other species, clearing forests, damming rivers, and changing the atmosphere. Those changes feed back on human life through floods, heat waves, and crop failures.

Components of the environment

• Abiotic: sunlight, water, air, soil, temperature.
• Biotic: plants, animals, fungi, microbes.
• Social: built environments and institutions.
• Global: climate and atmospheric systems.

Environment and organism are not separate. Each defines the other, and the relationship changes over time.