A heterotroph is a creature that must ingest biomass to obtain its energy and nutrition. In direct contrast, autotrophs are capable of assimilating diffuse, inorganic energy and materials and using these to synthesize biochemicals. Green plants, for example, use sunlight and simple inorganic molecules to photosynthesize organic matter. All heterotrophs have an absolute dependence on the biological products of autotrophs for their sustenance—they have no other source of nourishment.
All animals are heterotrophs, as are most microorganisms (the major exceptions being microscopic algae and blue-green bacteria). Heterotrophs can be classified according to the sorts of biomass that they eat. Animals that eat living plants are known as herbivores, while those that eat other animals are known as carnivores. Many animals eat both plants and animals, and these are known as omnivores. Animal parasites are a special type of carnivore that are usually much smaller than their prey, and do not usually kill the animals that they feed upon.
Heterotrophic microorganisms mostly feed upon dead plants and animals, and are known as decomposers. Some animals also specialize on feeding on dead organic matter, and are known as scavengers or detritivores. Even a few vascular plants are heterotrophic, parasitizing the roots of other plants and thereby obtaining their own nourishment. These plants, which often lack chlorophyll, are known as saprophytes.
Humans, of course, are heterotrophs. This means that humans can only sustain themselves by eating plants, or by eating animals that have themselves grown by eating plants. All of these foods must be specifically grown for human consumption in agricultural ecosystems, or be gathered from natural ecosystems. If humans and their societies are to be sustained over the long term, it can only be through the wise use of the species and ecosystems that sustain them. This is a fact, and a consequence of the inextricable links of humans with other species and with the products and services of ecosystems. The intimate dependency of humans on other creatures is a biological and ecological relationship that can be difficult for modern people to remember as they purchase their food in stores, and do not directly participate in its growth, harvesting, and processing.