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Flow of Energy

Introduces producers, consumers, and decomposers.

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Flow of Energy

What is happening inside each leaf and blade of grass?

Photosynthesis. Maybe the most important biochemical reaction of Earth. As sunlight shines down on this forest, the sunlight is being absorbed, and the energy from that sunlight is being transformed into chemical energy. That chemical energy is then distributed to all other living organisms in the ecosystem.

Flow of Energy

Ecosystems need energy. Many ecosystems get their energy in the form of sunlight, which enters the ecosystem through photosynthesis. This energy then flows through the ecosystem, passed from producers to consumers. Plants are producers in many ecosystems. Energy flows from plants to the herbivores that eat the plants, and then to the carnivores that eat the herbivores. The flow of energy depicts interactions of organisms within an ecosystem.

Matter is also recycled in ecosystems. Biogeochemical cycles recycle nutrients, like carbon and nitrogen, so they are always available. These nutrients are used over and over again by organisms. Water is also continuously recycled. The flow of energy and the recycling of nutrients and water are examples of the interactions between organisms and the interactions between the biotic and abiotic factors of an ecosystem.


Producers are organisms that produce food for themselves and other organisms. They use energy and simple inorganic molecules to make organic compounds. The stability of producers is vital to ecosystems because all organisms need organic molecules. Producers are also called autotrophs. There are two basic types of autotrophs: photoautotrophs and chemoautotrophs.

  1. Photoautotrophs use energy from sunlight to make food by photosynthesis. They include plants, algae, and certain bacteria (see Figure below).
  2. Chemoautotrophs use energy from chemical compounds to make food by chemosynthesis. They include some bacteria and also archaea. Archaea are microorganisms that resemble bacteria.

Different types of photoautotrophs are important in different ecosystems.


Consumers are organisms that depend on other organisms for food. They take in organic molecules by essentially “eating” other living things. They include all animals and fungi. (Fungi don't really “eat”; they absorb nutrients from other organisms.) They also include many bacteria and even a few plants, such as the pitcher plant shown in Figure below. Consumers are also called heterotrophs. Heterotrophs are classified by what they eat:

  • Herbivores consume producers such as plants or algae. They are a necessary link between producers and other consumers. Examples include deer, rabbits, and mice.
  • Carnivores consume animals. Examples include lions, polar bears, hawks, frogs, salmon, and spiders. Carnivores that are unable to digest plants and must eat only animals are called obligate carnivores. Other carnivores can digest plants but do not commonly eat them.
  • Omnivores consume both plants and animals. They include humans, pigs, brown bears, gulls, crows, and some species of fish.

Pitcher Plant. Virtually all plants are producers. This pitcher plant is an exception. It consumes insects. It traps them in a sticky substance in its “pitcher.” Then it secretes enzymes that break down the insects and release nutrients. Which type of consumer is a pitcher plant?


When organisms die, they leave behind energy and matter in their remains. Decomposers break down the remains and other wastes and release simple inorganic molecules back to the environment. Producers can then use the molecules to make new organic compounds. The stability of decomposers is essential to every ecosystem. Decomposers are classified by the type of organic matter they break down:

  • Scavengers consume the soft tissues of dead animals. Examples of scavengers include vultures, raccoons, and blowflies.
  • Detritivores consume detritus—the dead leaves, animal feces, and other organic debris that collects on the soil or at the bottom of a body of water. On land, detritivores include earthworms, millipedes, and dung beetles (see Figure below). In water, detritivores include “bottom feeders” such as sea cucumbers and catfish.
  • Saprotrophs are the final step in decomposition. They feed on any remaining organic matter that is left after other decomposers do their work. Saprotrophs include fungi and single-celled protozoa. Fungi are the only organisms that can decompose wood.

Dung Beetle. This dung beetle is rolling a ball of feces to its nest to feed its young.

KQED: Banana Slugs: The Ultimate Recyclers

One of the most beloved and iconic native species within the old growth redwood forests of California is the Pacific Banana Slug. These slimy friends of the forest are the ultimate recyclers. Feeding on fallen leaves, mushrooms or even dead animals, they play a pivotal role in replenishing the soil. QUEST goes to Henry Cowell Redwoods State Park near Santa Cruz, California on a hunt to find Ariolomax dolichophallus, a bright yellow slug with a very big personality. See http://ww2.kqed.org/quest/2011/04/13/science-on-the-spot-banana-slugs-unpeeled/ for more information.


  • Ecosystems require constant inputs of energy from sunlight or chemicals.
  • Producers use energy and inorganic molecules to make food.
  • Consumers take in food by eating producers or other living things.
  • Decomposers break down dead organisms and other organic wastes and release inorganic molecules back to the environment.


1. Explain the difference between a producer and consumer.
2. Give an example of a producer and a consumer.
3. What are the two basic types of autotrophs?

4. Identify three different types of consumers. Name an example of each type.

5. What can you infer about an ecosystem that depends on chemoautotrophs for food?

6. What is a decomposer?
7. List the three types of decomposers and give an example of each.

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