Course: BIS 105 Environmental Biology

Unit 2 Learning Module

Consumers

Consumers

Consumers are organisms that depend on producers or other types of organisms for food. They are also called heterotrophs, which literally means “other nutrition.” Heterotrophs are unable to make organic compounds from inorganic molecules and energy. Instead, they take in organic molecules by consuming other organisms. All animals and fungi and many bacteria are heterotrophs. A few insect-eating plants are also heterotrophic. Heterotrophs can be classified on the basis of the types of organisms they consume. They include herbivores, omnivores, and carnivores.

Herbivores

Herbivores are organisms that consume only producers such as plants or algae. In most ecosystems, herbivores form a necessary link between producers and other consumers. Herbivores transform the energy stored in producers to compounds that can be used by other organisms. Another name for herbivores is primary consumers, since they are the first organisms to use the energy stored by producers.

In terrestrial ecosystems, many animals and fungi and some bacteria are herbivores. Herbivorous animals include deer, rabbits, and mice. Herbivores may specialize in particular types of plants, such as grasses, or specific plant parts, such as leaves, nectar, or roots. Examples of herbivores are shown in Figure below.

Various types of herbivores. Source: CK12 Foundation

In aquatic ecosystems, the main herbivores are the heterotrophic organisms that make up zooplankton. Zooplankton refers to all the small organisms that feed on phytoplankton. These organisms include both single-celled organisms such as protozoa and multicellular organisms such as jellyfish. Phytoplankton and zooplankton together make up large communities of producers and herbivores called plankton.

Carnivores

Carnivores are organisms that eat a diet consisting mainly of herbivores or other carnivores. Carnivores include lions, wolves, polar bears, hawks, frogs, fish, and spiders. Animals that eat only meat are called obligate carnivores. They generally have a relatively short digestive system that cannot break down the tough cellulose found in plants. Other carnivores, including dogs, can digest plant foods but do not commonly eat them. Certain carnivores, called scavengers, mainly eat the carcasses of dead animals. Scavengers include vultures, raccoons, and blowflies.

A tiny minority of plants—including Venus flytraps and pitcher plants—are also carnivorous. These plants trap and digest insects. Some fungi are carnivorous as well. Carnivorous fungi capture and digest microscopic protozoan organisms such as amoebas.

Omnivores

Omnivores are organisms that eat both plants and animals as primary food sources. Humans are an example of an omnivorous species. Although some humans eat foods derived only from plants or only from animals, the majority of humans eat foods from both sources. Other examples of omnivorous animals are pigs, brown bears, gulls, and crows. Aquatic omnivores include some species of fish, such as piranhas.