High school biology
Food chains & food webs
- Producers, or autotrophs, make their own organic molecules. Consumers, or heterotrophs, get organic molecules by eating other organisms.
- A food chain is a linear sequence of organisms through which nutrients and energy pass as one organism eats another.
- In a food chain, each organism occupies a different trophic level, defined by how many energy transfers separate it from the basic input of the chain.
- Food webs consist of many interconnected food chains and are more realistic representation of consumption relationships in ecosystems.
- Energy transfer between trophic levels is inefficient (with a typical efficiency around ). This inefficiency limits the length of food chains.
Organisms of different species can interact in many ways. They can compete, or they can be symbionts (long-term partners with a close association). Or, of course, they can do what we so often see in nature programs: one of them can eat the other. (Chomp!) That is, they can form one of the links in a food chain.
In ecology, a food chain is a series of organisms that eat one another (so that energy and nutrients flow from one to the next). For example, if you had a hamburger for lunch, you might be part of a food chain that looks like: grass cow human. But what if you had lettuce on your hamburger? In that case, you're also part of a food chain that looks like: lettuce human.
As this example illustrates, we can't always fully describe what an organism (such as a human) eats with one linear pathway. For situations like that, we may want to use a food web, which consists of many intersecting food chains and represents the different things an organism can eat, and be eaten by.
In this article, we'll take a closer look at food chains and food webs, seeing how they represent the flow of energy and nutrients through ecosystems.
Autotrophs vs. heterotrophs
What basic strategies do organisms use to get food? Some organisms, called autotrophs (“self-feeders”), can make their own food – that is, their own organic compounds – out of simple molecules like carbon dioxide. There are two basic types of autotrophs:
- Photoautotrophs, such as plants, use energy from sunlight to make organic compounds (sugars) out of carbon dioxide in photosynthesis. Other examples of photoautotrophs include algae and cyanobacteria.
- Chemoautotrophs use energy from chemicals to build organic compounds out of carbon dioxide (or similar molecules). This is called chemosynthesis. For instance, there are hydrogen sulfide-oxidizing chemoautotrophic bacteria found in undersea vent communities (where no light can reach).
Autotrophs are the foundation of every ecosystem on the planet. That may sound dramatic, but it's no exaggeration! Autotrophs 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. When we're talking about their role in food chains, we can call autotrophs producers.
Heterotrophs (“other-feeders”) such as humans can't capture light or chemical energy to make their own food out of carbon dioxide. Instead, they get organic molecules by eating other organisms or their by-products. Animals, fungi, and many bacteria are heterotrophs. When we're talking about their role in food chains, we can call heterotrophs consumers. As we'll see shortly, there are many different kinds of consumers with different ecological roles, from plant-eating insects to meat-eating animals to fungi that feed on debris and wastes.
Now, we can take a look at how energy and nutrients move through a ecological community. Let's start by considering just a few “who eats whom” relationships – that is, by looking at a food chain.
A food chain is a linear sequence of organisms through which nutrients and energy pass as one organism eats another. Let's look at the parts of a typical food chain, starting from the bottom (the producers) and moving upward.
- At the base of the food chain lie the primary producers. The primary producers are autotrophs, and are most often photosynthetic organisms (such as plants, algae, or cyanobacteria).
- The organisms that eat the primary producers are called primary consumers. Primary consumers are usually herbivores (plant-eaters), though they may be algae or bacteria eaters.
- The organisms that eat the primary consumers are called secondary consumers. Secondary consumers are generally meat-eaters (carnivores).
- The organisms that eat the secondary consumers are called tertiary consumers. These are carnivore-eating carnivores, like eagles or big fish.
- Some food chains have additional levels, such as quaternary consumers (carnivores that eat tertiary consumers). Organisms at the very top of a food chain are called the apex consumers.
We can see examples of these levels in the diagram below. The green algae are primary producers that get eaten by mollusks (the primary consumers). The mollusks then become lunch for the slimy sculpin fish, a secondary consumer, which is itself eaten by a larger fish, the Chinook salmon (tertiary consumer).
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 transfers of energy and nutrients (how many consumption steps) separate an organism from the food chain's original energy source, such as light. As we’ll explore further below, assigning organisms to trophic levels isn't always clear-cut. For instance, humans are omnivores that can eat both plants and animals.
One other group of consumers deserves mention, although it does not always appear in drawings of food chains. This group consists of decomposers, organisms that break down dead organic material and wastes.
Decomposers are sometimes considered their own trophic level. As a group, they eat dead matter and waste products that come from organisms at various other trophic levels (for instance, they would happily consume decaying plant matter, the body of a half-eaten squirrel, and the remains of a deceased eagle). In this sense, the decomposer level kind of runs in parallel to the standard hierarchy of primary, secondary, and tertiary consumers.
Fungi and bacteria are the key decomposers in many ecosystems, using the chemical energy in dead matter and wastes to fuel their metabolic processes. Other decomposers are detritivores (detritus- or debris-eaters). These are usually multicellular animals such as earthworms, crabs, slugs, vultures, etc. They not only feed on dead organic matter, but often fragment it as well, making it more available for bacterial or fungal decomposers.
Examples of decomposers: left, fungi growing on a log; right, an earthworm.
Decomposers as a group play a critical role in keeping ecosystems healthy. When they break down dead material and wastes, they release nutrients that can be recycled and used as building blocks by primary producers.
Food chains give us a clear-cut picture of who eats who. However, some problems come up when we try and use them to describe whole ecological communities. For instance, an organism can sometimes eat multiple types of prey, or be eaten by multiple predators, including ones at different trophic levels. This happens, for instance, when you eat a hamburger patty (cow = primary consumer) with a lettuce leaf on it (lettuce = primary producer).
To represent these relationships more accurately, we can use a food web, a graph that shows all the trophic (eating-related) interactions between various species in an ecosystem. The diagram below shows an example of a food web from Lake Ontario. Primary producers are marked in green, primary consumers in orange, secondary consumers in blue, and tertiary consumers in purple.
The bottom level of the illustration shows primary producers, which include diatoms, green algae, blue-green algae, flagellates, and rotifers. The next level includes the primary consumers that eat primary producers. These include calanoids, waterfleas, and cyclopoids, rotifers and amphipods. The shrimp also eats primary producers. Primary consumers are in turn eaten by secondary consumers, which are typically small fish. The small fish are eaten by larger fish, the tertiary consumers. The yellow perch, a secondary consumer, eats small fish within its own trophic level. All fish are eaten by the sea lamprey. Thus, the food web is complex with interwoven layers.
In food webs, arrows point from an organism that is eaten to the organism that eats it. As the food web above shows, some species can eat organisms from more than one trophic level. For example, opossum shrimp eat both primary producers and primary consumers.
Bonus question: This food web contains the food chain we saw earlier in the article (green algae mollusks slimy sculpin salmon). Can you find it?
Grazing vs. detrital food webs
Food webs don't usually show decomposers (for instance, the Lake Ontario food web above does not). Yet, all ecosystems need ways to recycle dead material and wastes. That means decomposers are indeed present, even if they don't get much air time.
For example, in the meadow ecosystem shown below, there is a grazing food web of plants and animals that provides inputs for a detrital food web of bacteria, fungi, and detritovores. The detrital web is shown in simplified form in the brown band across the bottom of the diagram. In reality, it would consist of various species linked by specific feeding interactions (that is, connected by arrows, as in the grazing food web aboveground). Detrital food webs can contribute energy to grazing food webs, as when a robin eats an earthworm.
The bottom level of the illustration shows decomposers, which include fungi, mold, earthworms, and bacteria in the soil. The next level above decomposers shows the producers: plants. The level above the producers shows the primary consumers that eat the producers. Some examples are squirrels, mice, seed-eating birds, and beetles. Primary consumers are in turn eaten by secondary consumers, such as robins, centipedes, spiders, and toads. The tertiary consumers such as foxes, owls, and snakes eat secondary and primary consumers. All of the consumers and producers eventually become nourishment for the decomposers.
Energy transfer efficiency limits food chain lengths
Energy is transferred between trophic levels when one organism eats another and gets the energy-rich molecules from its prey's body. However, these transfers are inefficient, and this inefficiency limits the length of food chains.
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). This rule of energy transfer is a good thing to commit to memory.
As an example, let's suppose the primary producers of an ecosystem store of energy as biomass. This is also the amount of energy per year that's made available to the primary consumers, which eat the primary producers. The rule would predict that the primary consumers store only of energy in their own bodies, making energy available to their predators (secondary consumers) at a lower rate.
This pattern of fractional transfer limits the length of food chains: after a certain number of trophic levels (generally, ), there is too little energy flow to support a population at a higher level.
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.
Why does so much energy exit the food web between one trophic level and the next? Here are a few of the main reasons for inefficient energy transfer:
- In each trophic level, a significant amount of energy is dissipated as heat, as organisms carry out cellular respiration and go about their daily lives.
- Some of the organic molecules an organism eats cannot be digested and leave the body as feces (poop) rather than being used.
- Not all of the individual organisms in a trophic level will get eaten by organisms in the next level up. Some instead die without being eaten.
The feces and uneaten, dead organisms become food for decomposers, who metabolize them and convert their energy to heat through cellular respiration. So, none of the energy actually disappears – it all winds up as heat in the end.
Want to join the conversation?
- how does decomposition work in a dessert.(6 votes)
- In the desert, the dead thing just rots away or is eaten by a scavenger (most the time).(7 votes)
- I noticed that the producers were referred to as Primary Producers. Are there such thing as secondary consumers?(2 votes)
- Yes, there is because they come right after the primary consumers.(10 votes)
- Is there any other producer than the primary producer?(4 votes)
- technically the sun, but its not physically apart f the ecosystem(1 vote)
- Then what are the onivores what is their part(1 vote)
- they can be found everywhere in the food web they just eat different things.(2 votes)
- why are their a food web and a food chain in life(3 votes)
- In short, because without them life would not, could not, exist. Each level of consumer needs a lower level of consumer or a producer to get them energy it needs to live. The only way there could not be a food chain, is if we were all autotrophs. However, if this were to occur, there would be such a large build up, it seems, of waste from the natural death of organisms that living organisms would eventually become extinct.
The world is made perfectly to run itself, so that large changes like the removal of all heterotrophs could be catastrophic. Think what an intelligent mind must created such a perfect utopia that a bunch of atoms could make up the beautiful and stable world which you and I live in today!(4 votes)
- if the things they use to collect sunlight are under their leaves how do they collect sunlight?(2 votes)
- they bend dude. they do this cool flip like thing with the leaves, but theyre shy so they dont do it in front of human eyes(2 votes)
- In this article, it says that autotrophs are the foundation of every ecosystem on the planet. But, are chemoautotrophs dependent off of autotrophs?(2 votes)
- I am still a little confused about why the primary consumers produce more kcal per square meter per year. I would think that even though there are on average more blades of grass per square meter than a hawk for example (as shown above), but does the hawk not need to consume more food than the grass? Or is it that only so much of that one square meter's energy from the grass get passed on to the hawk, though it still consumes more food then the grass? I apologize if this does not make sense.(2 votes)
- are possums decomposers(2 votes)
- so eating a plant gives us more energy compared to eating a secondary consumer for example?(2 votes)
- No, it is just more efficient. Many many more plant biomass went into feeding that primary and secondary consumers before you get a fraction of the original energy that went to earlier consumers. They have already done the work of gathering that energy and passing it on to you, so effectively they are the same total energy to you in the end.
If you a always a primary consumer, it would require less producers to sustain you long term.(0 votes)