An ecosystem includes all living things (biotic factors) and nonliving things (abiotic factors) in an area, and the interactions between them. Biotic factors include plants, animals, and microorganisms. Abiotic factors include sunlight, water, temperature, and soil.

Energy flows through ecosystems in food chains and food webs. Producers (plants) capture energy from the Sun. Primary consumers (herbivores) eat producers. Secondary consumers (carnivores) eat primary consumers. Decomposers break down dead organisms and recycle nutrients back into the soil.

Only about 10% of energy transfers from one level to the next. This is why there are fewer top predators than herbivores in any ecosystem.

Biomes are large-scale ecosystems defined by climate. Major biomes include tropical rainforest, desert, grassland, temperate forest, taiga (boreal forest), and tundra. Each biome supports different types of organisms adapted to its conditions.