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Trophic levels review

Key terms

Autotroph (producer)An organism that produces its own food using sunlight or chemical energy
Heterotroph (consumer)An organism that consumes another organism for food
Food chainA series of organisms in which energy is transferred to another
Food webA network of feeding interactions, usually consisting of multiple food chains
Trophic levelEach step in a food chain or food web
BiomassThe total amount of living tissue within a trophic level

Autotrophs vs. heterotrophs

Organisms get their food in one of two ways.
Autotrophs (or producers) make their own food using light or chemical energy. Examples of autotrophs include plants, algae, and some bacteria.
Heterotrophs (or consumers) get organic molecules by eating other organisms or their by-products. Animals, fungi, and many bacteria are heterotrophs. Specialized heterotrophs, called decomposers break down dead organic material and wastes.

Food chains and food webs

Producers form the base of food chains and food webs, and the energy they capture from light or chemicals sustains all the other organisms in the community. Consumers play many different ecological roles, including herbivorous insects, carnivorous animals, and decomposing fungi.
In ecology, a food chain is a series of organisms that eat one another.
Food chain. Image from OpenStax, CC BY 4.0.
Each of the categories above is called a trophic level, and it reflects how many consumption steps separate an organism from the food chain's original energy source, such as light. However, in most ecosystems, energy flow is much more complicated than a linear chain. In this case, a food web can be used to represent these feeding interactions between trophic levels.
Aquatic food web containing multiple trophic levels, from producers (plankton) through tertiary consumers (seals, penguins, seagulls).

Biomass and energy transfer

When energy enters a trophic level, some of it is stored as biomass (as part of organisms' bodies). This is the energy that's available to the next trophic level, since only energy stored as biomass can get eaten.
As a rule of thumb, only about 10% of the energy that's stored as biomass in one trophic level (per unit time) ends up stored as biomass in the next trophic level (per the same unit time).
Image modified from "Ecological pyramid," by CK-12 Foundation, CC BY-NC 3.0.

Common mistakes and misconceptions

  • An organism may not always occupy the same trophic level, depending on the food web. Assigning organisms to trophic levels isn't always clear-cut. For instance, humans are omnivores, meaning they can eat both plants and animals. So they may be considered both primary and secondary (or even higher!) consumers.
  • The arrows in a food web travel from the prey to the predator instead of the other way around. It may seem counterintuitive, but the arrows in a food web or food chain point in the direction the energy is flowing.

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