|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 chain||A series of organisms in which energy is transferred to another|
|Food web||A network of feeding interactions, usually consisting of multiple food chains|
|Trophic level||Each step in a food chain or food web|
|Biomass||The 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.
In this illustration, the bottom trophic level is green algae, which is the primary producer. The primary consumers are mollusks, or snails. The secondary consumers are small fish called slimy sculpin. The tertiary and apex consumer is Chinook salmon.
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.
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 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).
Trophic pyramid illustrating the 10% energy transfer rule.
Light energy is captured by primary producers.
Amount of energy stored as biomass:
Primary producers - 20,000 kcal per meter squared per year
Primary consumers - 2,000 kcal per meter squared per year
Secondary consumers - 200 kcal per meter squared per year
Tertiary consumers - 20 kcal per meter squared per year
Quaternary consumers - 2 kcal per meter squared per year
At each level, energy is lost directly as heat, or in the form of waste and dead matter that go to the decomposers. Eventually, the decomposers metabolize the waste and dead matter, releasing its energy as heat also.
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.
Want to join the conversation?
- Does that mean that the decomposers dont have a trophic level?(8 votes)
- At the end of the review here, what does it mean by humans possibly being in a higher lever than secondary consumers?(2 votes)
- At which trophic level would you expect to find only 10% of the energy that was initially stored in the food chain?(1 vote)
- You would find 10% of the mass at the next trophic level up. For example, 10% of a primary producer's biomass is transferred to the primary consumer, and 10% of that primary consumer's biomass is transferred to the secondary consumer and so on. That means that the secondary consumer contains 1% of the initial primary producer's biomass.(2 votes)
- Does the 10% rule apply to decomposers then? Since they can belong to any of the trophic levels, does that mean they get all the energy from said organism? For example, if a decomposer breaks down a lion do they get the same amount of energy as the lion had in real life?(1 vote)
- Yes, I believe so. The reason why is because decomposers make sure that the dops from the producers are used for more things in the ecosyetem.(1 vote)
- Can an organism be in two trophic levels(1 vote)
- Yes--in fact, you are already in multiple trophic levels right now! If an organism is an omnivore, they can occupy multiple levels. For instance, you can eat plants (you're a primary consumer), bugs (you're a secondary consumer), and frogs (you're a tertiary consumer).(1 vote)
- why does it show the prey to the predator energy flow instead of the other way of what gives the predator energy?(1 vote)
- why is there not a trophic level for decomposers?(0 votes)