Populations of organisms can be categorized by the function they serve in an ecosystem. Plants and some micro-organisms are producers. They make their own food. 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. Energy entering ecosystems as sunlight is converted by producers into stored chemical energy through photosynthesis, It then 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 the soil composition. Limitations of resources and other factors such as predation and climate limit the growth of populations in specific niches in the ecosystem. Ecosystems are dependent on interrelationships within the environment.