Autotrophs make their own food, most through photosynthesis. Photosynthesis uses the energy of the sun to make sugars from carbon dioxide and water. This process sustains plants and some other organisms, such as algae and phytoplankton.

Photosynthetic organisms are known as the "primary producers" of the food chain. They are the foundation on which all other organisms depend. In general, the food chain moves from plants and other autotrophs to herbivores, and then to omnivores and carnivores, which eat the herbivores.

In contrast to autotrophs, heterotrophs survive through respiration, using oxygen and an energy source (carbohydrates, fats or protein) to produce ATP, which powers cells. They depend on other organisms for food and oxygen. Photosynthesis benefits heterotrophs in several different ways. First, photosynthesis consumes carbon dioxide (a waste product of respiration) and produces oxygen (necessary for respiration). Heterotrophs therefore depend on photosynthesis as a source of oxygen. In addition, photosynthesis sustains the organisms that heterotrophs consume in order to stay alive. Even if a heterotroph is strictly carnivorous and does not eat plants, it must eat animals that eat plants to survive.

The complex interactions between different types of organisms in a given environment make up an ecosystem, where all the species are dependent on each other. Although the energy flows in a given ecosystem may change over time or differ greatly in comparison to others, a stable ecosystem exists in a careful balance. Loss of a single species, pollution, or destruction of habitat all can throw off this balance and make the ecosystem less functional and more prone to collapse.