Organisms and populations of organisms are dependent on their environmental interactions both with other living things and with non-living factors. In any ecosystem, organisms and populations with similar requirements for food, water, oxygen. Or other resources may compete with each other for limited resources.

Food webs are models that demonstrate how matter and energy is transferred between producers (generally plants and other organisms that engage in photosynthesis), consumers, and decomposers as the three groups interact—primarily for food—within an ecosystem. Transfers of matter into and out of the physical environment occur at every level—for example, when molecules from food react with oxygen captured from the environment, the carbon dioxide and water thus produced are transferred back to the environment, and ultimately so are waste products, such as fecal material.