A population consists of all individuals of a species that occur together in a given place and time. All populations living together and the physical factors with which they interact compose an ecosystem. Populations of organisms can be categorized by the function they serve in an ecosystem. All animals, including humans, are consumers, which obtain food by eating other organisms. Decomposers, primarily bacteria and fungi, are consumers that use waste materials and dead organisms for food.
Food webs identify the relationships among producers, consumers, and decomposers in an ecosystem.
For ecosystems, the major source of energy is sunlight, which passes from organism to organism in food webs. The number of organisms an ecosystem can support depends on the resources available and abiotic factors, such as quantity of light and water, range of temperatures, and soil composition. Soil consists of weathered rocks and decomposed organic material from dead plants, animals, and bacteria. Water is a solvent. As it passes through the water cycle, it dissolves minerals and gases and carries them to the oceans.