Food Chains, Food Webs & Energy Flow
Learn to trace energy from producers to consumers to decomposers, draw food-chain arrows the right way, and explain why only about 10 percent moves up each level.
What you'll do in this lesson
A voice-first session with the Crimsora tutor on Food Chains, Food Webs & Energy Flow, then targeted practice and FRQs — with the tutor adapting to where you get stuck.
What this lesson covers
Every bite of food is really a transfer of energy that started in sunlight. A blade of grass captures sunlight, a grasshopper eats the grass, a shrew eats the grasshopper, and a hawk eats the shrew. Scientists draw that story as a food chain, and when they connect all the overlapping chains in one place, they get a food web.
In this lesson you will learn the three big jobs organisms hold in an ecosystem — producer, consumer, and decomposer — and the one rule that trips up more students than anything else: the arrow points from the organism that is eaten toward the organism that eats it, because the arrow shows which way the energy travels. You will also find out why a meadow can support millions of grass plants but only one or two hawks. The answer is that most energy leaks out of the food chain as heat at every single step.
In this lesson you will learn the three big jobs organisms hold in an ecosystem — producer, consumer, and decomposer — and the one rule that trips up more students than anything else: the arrow points from the organism that is eaten toward the organism that eats it, because the arrow shows which way the energy travels. You will also find out why a meadow can support millions of grass plants but only one or two hawks. The answer is that most energy leaks out of the food chain as heat at every single step.
Producers, Consumers, and Decomposers
Every organism in an ecosystem earns a role based on how it gets its energy.
Producers (also called autotrophs) make their own food. Plants, algae, and some bacteria use photosynthesis to convert light energy into the chemical energy stored in sugar. Producers are the entry point for almost all the energy in an ecosystem — if you are tracing energy, you start here.
Consumers (heterotrophs) cannot make their own food, so they eat other organisms. They are sorted by what they eat: herbivores eat producers (deer, grasshoppers), carnivores eat other consumers (wolves, hawks), omnivores eat both (bears, humans), and scavengers eat animals that are already dead (vultures, crabs).
Decomposers — mostly bacteria and fungi — break down dead organisms and waste. They are consumers too, but they get their own category because they do a special job: they release the nutrients locked in dead tissue back into the soil and water where producers can use them again.
A common mix-up: students call a mushroom a producer because it looks like a plant. Fungi have no chloroplasts and cannot photosynthesize, so they are decomposers. Another mix-up is thinking decomposers sit outside the food chain. They are the final step of every chain, because everything eventually dies and gets broken down.
Producers (also called autotrophs) make their own food. Plants, algae, and some bacteria use photosynthesis to convert light energy into the chemical energy stored in sugar. Producers are the entry point for almost all the energy in an ecosystem — if you are tracing energy, you start here.
Consumers (heterotrophs) cannot make their own food, so they eat other organisms. They are sorted by what they eat: herbivores eat producers (deer, grasshoppers), carnivores eat other consumers (wolves, hawks), omnivores eat both (bears, humans), and scavengers eat animals that are already dead (vultures, crabs).
Decomposers — mostly bacteria and fungi — break down dead organisms and waste. They are consumers too, but they get their own category because they do a special job: they release the nutrients locked in dead tissue back into the soil and water where producers can use them again.
| Role | Energy source | Examples |
|---|---|---|
| Producer | Sunlight (photosynthesis) | Grass, oak tree, algae, phytoplankton |
| Herbivore | Producers | Rabbit, caterpillar, zooplankton |
| Carnivore | Other consumers | Owl, snake, shark |
| Omnivore | Producers and consumers | Raccoon, crow, human |
| Decomposer | Dead matter and waste | Mushroom, mold, soil bacteria |
Reading a Food Chain: The Arrow Rule
A food chain is a single straight-line pathway showing one route energy takes through an ecosystem. Here is a typical one:
algae zooplankton minnow bass osprey
The arrow does not mean "eats." It means energy flows this way. So it always points from the organism being eaten toward the organism doing the eating. Read the chain above as "algae are eaten by zooplankton, which are eaten by minnows."
This is the single most common error in the whole unit. Students picture a predator lunging at prey and draw the arrow from the hawk to the mouse. But the mouse's stored energy ends up inside the hawk, so the arrow must run mouse hawk. When you check your own diagram, follow one arrow at a time and say out loud, "the energy moves this way."
Each position in the chain is a trophic level. Producers are the first trophic level. Herbivores, also called primary consumers, are the second. Secondary consumers are the third, tertiary consumers the fourth. Chains rarely go past four or five levels, and the reason is energy loss, which the next section explains.
Two more details matter. First, a food chain always starts with a producer, never with an animal — if your chain begins with a rabbit, you left out the plant. Second, decomposers belong at the end, receiving energy from every level, because dead producers and dead consumers alike get broken down. Many textbook diagrams show decomposer arrows curving back from several organisms at once.
algae zooplankton minnow bass osprey
The arrow does not mean "eats." It means energy flows this way. So it always points from the organism being eaten toward the organism doing the eating. Read the chain above as "algae are eaten by zooplankton, which are eaten by minnows."
This is the single most common error in the whole unit. Students picture a predator lunging at prey and draw the arrow from the hawk to the mouse. But the mouse's stored energy ends up inside the hawk, so the arrow must run mouse hawk. When you check your own diagram, follow one arrow at a time and say out loud, "the energy moves this way."
Each position in the chain is a trophic level. Producers are the first trophic level. Herbivores, also called primary consumers, are the second. Secondary consumers are the third, tertiary consumers the fourth. Chains rarely go past four or five levels, and the reason is energy loss, which the next section explains.
Two more details matter. First, a food chain always starts with a producer, never with an animal — if your chain begins with a rabbit, you left out the plant. Second, decomposers belong at the end, receiving energy from every level, because dead producers and dead consumers alike get broken down. Many textbook diagrams show decomposer arrows curving back from several organisms at once.
Food Webs: Many Chains Woven Together
Real ecosystems are messier than a single line. A grasshopper is eaten by a shrew, a frog, a spider, and a bird. A hawk eats snakes, mice, and rabbits. A food web shows all these overlapping food chains in one diagram, giving a far more realistic picture of who depends on whom.
To pull a food chain out of a food web, pick one producer and follow arrows forward, taking one path each time you reach a branch. A web with fifteen organisms might contain a dozen different valid chains.
Food webs matter because they show stability. If a hawk in a simple chain loses its only prey, the hawk starves. In a web, the hawk can shift to other prey and the ecosystem absorbs the shock. The more connections a web has, generally the more resilient the community is to change.
Webs also let you predict the ripple effects of a disturbance, which is exactly what many homework questions ask. Suppose a disease wipes out the rabbits. First look at the arrows pointing away from rabbits: foxes and hawks lose a food source, so those populations likely drop, or those predators eat more mice instead. Then look at the arrows pointing into rabbits: the grass they were eating is no longer being grazed as heavily, so the grass population probably increases. One change spreads in both directions.
Where students go wrong here is stopping after one step. A complete answer traces at least two links — what happens directly to the predators and prey of the affected organism, and then what happens to the organisms connected to those.
To pull a food chain out of a food web, pick one producer and follow arrows forward, taking one path each time you reach a branch. A web with fifteen organisms might contain a dozen different valid chains.
Food webs matter because they show stability. If a hawk in a simple chain loses its only prey, the hawk starves. In a web, the hawk can shift to other prey and the ecosystem absorbs the shock. The more connections a web has, generally the more resilient the community is to change.
Webs also let you predict the ripple effects of a disturbance, which is exactly what many homework questions ask. Suppose a disease wipes out the rabbits. First look at the arrows pointing away from rabbits: foxes and hawks lose a food source, so those populations likely drop, or those predators eat more mice instead. Then look at the arrows pointing into rabbits: the grass they were eating is no longer being grazed as heavily, so the grass population probably increases. One change spreads in both directions.
Where students go wrong here is stopping after one step. A complete answer traces at least two links — what happens directly to the predators and prey of the affected organism, and then what happens to the organisms connected to those.
Why Energy Shrinks at Every Step
If you counted the energy stored in all the grass in a field and compared it to the energy stored in all the mice, the mice would hold only a small fraction. On average, about of the energy at one trophic level becomes body tissue at the next level. The other never makes it up the chain.
Where does it go? Three places. Most of it is used for cellular respiration — the mouse burns sugar to run, breathe, stay warm, and grow, and that energy leaves as heat. Some leaves as undigested waste; a deer cannot digest every bit of the cellulose it swallows. And some is simply never eaten — roots, bark, bones, and organisms that die of old age go to decomposers instead of predators.
Scientists picture this with an energy pyramid: a wide band of producers at the bottom, narrower bands above. The shrinking width shows shrinking available energy.
This explains two things students are often asked about. First, why food chains are short: after four or five steps there is not enough energy left to support another population. Second, why there are always fewer top predators than prey — a single hawk needs a huge base of grass beneath it.
A misconception worth clearing up: energy is not destroyed, and this does not break the law of conservation of energy. The lost energy still exists as heat spread out in the environment. It is just no longer usable food energy, and unlike matter, it does not cycle back. Energy flows through an ecosystem one way and exits as heat; only matter cycles.
Where does it go? Three places. Most of it is used for cellular respiration — the mouse burns sugar to run, breathe, stay warm, and grow, and that energy leaves as heat. Some leaves as undigested waste; a deer cannot digest every bit of the cellulose it swallows. And some is simply never eaten — roots, bark, bones, and organisms that die of old age go to decomposers instead of predators.
Scientists picture this with an energy pyramid: a wide band of producers at the bottom, narrower bands above. The shrinking width shows shrinking available energy.
| Trophic level | Energy available (kilocalories) |
|---|---|
| Producers | 100,000 |
| Primary consumers | 10,000 |
| Secondary consumers | 1,000 |
| Tertiary consumers | 100 |
A misconception worth clearing up: energy is not destroyed, and this does not break the law of conservation of energy. The lost energy still exists as heat spread out in the environment. It is just no longer usable food energy, and unlike matter, it does not cycle back. Energy flows through an ecosystem one way and exits as heat; only matter cycles.
Putting It Together Without the Common Slip-Ups
When you build or read a diagram, run through a short mental checklist.
Start with a producer. If your chain begins with an animal, energy came from somewhere you did not show.
Check arrow direction on every link, not just the first one. Say "energy goes this way" as you trace.
Name trophic levels by counting from the producer, not by how fierce the animal looks. In grass cricket frog snake, the frog is a secondary consumer even though a snake seems scarier.
Remember that one organism can occupy different levels in different chains. A bear eating berries is a primary consumer; the same bear eating a salmon is a secondary or tertiary consumer. That is why omnivores make food webs so tangled.
Include decomposers. Many diagrams leave them out for simplicity, but if a question asks what happens to the energy and matter in a dead organism, the answer involves decomposers returning nutrients to the soil.
Finally, keep matter and energy separate in your head. Carbon and nitrogen atoms cycle — they are used, released, and reused endlessly. Energy does not cycle. It enters as sunlight, moves up through a few trophic levels, and leaves as heat. Ecosystems therefore need a constant supply of sunlight but not a constant supply of new atoms.
Students most often stumble by reversing arrows, by forgetting that the figure applies between each pair of levels rather than across the whole chain, and by claiming the missing "disappears." Naming heat from cellular respiration as the main destination turns a vague answer into a complete one.
Start with a producer. If your chain begins with an animal, energy came from somewhere you did not show.
Check arrow direction on every link, not just the first one. Say "energy goes this way" as you trace.
Name trophic levels by counting from the producer, not by how fierce the animal looks. In grass cricket frog snake, the frog is a secondary consumer even though a snake seems scarier.
Remember that one organism can occupy different levels in different chains. A bear eating berries is a primary consumer; the same bear eating a salmon is a secondary or tertiary consumer. That is why omnivores make food webs so tangled.
Include decomposers. Many diagrams leave them out for simplicity, but if a question asks what happens to the energy and matter in a dead organism, the answer involves decomposers returning nutrients to the soil.
Finally, keep matter and energy separate in your head. Carbon and nitrogen atoms cycle — they are used, released, and reused endlessly. Energy does not cycle. It enters as sunlight, moves up through a few trophic levels, and leaves as heat. Ecosystems therefore need a constant supply of sunlight but not a constant supply of new atoms.
Students most often stumble by reversing arrows, by forgetting that the figure applies between each pair of levels rather than across the whole chain, and by claiming the missing "disappears." Naming heat from cellular respiration as the main destination turns a vague answer into a complete one.
Key terms
- Producer.
- An organism such as a plant or alga that makes its own food, usually by photosynthesis; the first trophic level and the entry point of energy into an ecosystem.
- Consumer.
- An organism that gets energy by eating other organisms. Subtypes include herbivores, carnivores, omnivores, and scavengers.
- Decomposer.
- A bacterium or fungus that breaks down dead organisms and waste, releasing nutrients back to the soil and water for producers to reuse.
- Food chain.
- A single-path diagram showing one route energy takes through an ecosystem, with arrows pointing from the organism eaten to the organism that eats it.
- Food web.
- A diagram of many interconnected food chains, showing the realistic, overlapping feeding relationships in a community.
- Trophic level.
- A feeding position in a food chain, counted from the producers upward: producers, primary consumers, secondary consumers, and so on.
- Energy pyramid.
- A diagram whose bands narrow going upward to show that less usable energy is available at each higher trophic level.
- Ten percent rule.
- The generalization that only about of the energy at one trophic level is stored as body tissue at the next; the rest is lost mainly as heat from cellular respiration.
Worked example
A prairie food chain is: prairie grass grasshopper meadow lark hawk. The prairie grass in one plot stores 60,000 kilocalories of energy. (a) Using the ten percent rule, estimate the energy available to the hawk. (b) Identify the trophic level of the meadow lark. (c) Explain what happened to the energy that did not reach the hawk.
Part (a). Apply the ten percent rule once for each arrow, and count the arrows: grass to grasshopper, grasshopper to lark, lark to hawk. That is three transfers.
Grasshoppers: kilocalories.
Meadow larks: kilocalories.
Hawk: kilocalories.
You can also do it in one step: kilocalories. A frequent error is multiplying by only once, or counting organisms instead of arrows — four organisms means three energy transfers.
Part (b). Count from the producer. Grass is the first trophic level (producer). The grasshopper is the second (primary consumer). The meadow lark is the third trophic level, a secondary consumer. The hawk is the fourth, a tertiary consumer.
Part (c). Of the 60,000 kilocalories in the grass, only 60 reach the hawk. The missing energy was not destroyed. Most of it was used by the grasshoppers and larks for cellular respiration — moving, growing, and maintaining body temperature — and left their bodies as heat. Some passed out as undigested waste, and some was never eaten at all: grass roots, dead insects, and birds that died of other causes went to decomposers instead of being passed up the chain. Because energy leaves as heat rather than cycling back, a fifth trophic level above the hawk would have only about 6 kilocalories to work with, which is why this chain stops where it does.
Grasshoppers: kilocalories.
Meadow larks: kilocalories.
Hawk: kilocalories.
You can also do it in one step: kilocalories. A frequent error is multiplying by only once, or counting organisms instead of arrows — four organisms means three energy transfers.
Part (b). Count from the producer. Grass is the first trophic level (producer). The grasshopper is the second (primary consumer). The meadow lark is the third trophic level, a secondary consumer. The hawk is the fourth, a tertiary consumer.
Part (c). Of the 60,000 kilocalories in the grass, only 60 reach the hawk. The missing energy was not destroyed. Most of it was used by the grasshoppers and larks for cellular respiration — moving, growing, and maintaining body temperature — and left their bodies as heat. Some passed out as undigested waste, and some was never eaten at all: grass roots, dead insects, and birds that died of other causes went to decomposers instead of being passed up the chain. Because energy leaves as heat rather than cycling back, a fifth trophic level above the hawk would have only about 6 kilocalories to work with, which is why this chain stops where it does.
Practice questions
In the food chain oak leaves caterpillar warbler hawk, what does the arrow between the caterpillar and the warbler mean?
- The caterpillar hunts and eats the warbler.
- Energy stored in the caterpillar is transferred to the warbler that eats it.
- The caterpillar and warbler compete for the same oak leaves.
- The warbler helps the caterpillar find food.
Answer: Energy stored in the caterpillar is transferred to the warbler that eats it.
Arrows in food chains show the direction of energy flow, always pointing from the organism that is eaten toward the organism that eats it. Since the arrow runs caterpillar to warbler, the warbler is the eater and the caterpillar's stored chemical energy moves into the warbler's body. The choice about the caterpillar eating the warbler reverses the arrow, which is the most common mistake in this topic.
A pond food web includes algae, zooplankton, minnows, bass, and herons. Suppose a pollutant kills most of the zooplankton. Predict two effects on other populations in the web and explain your reasoning using arrow directions.
Answer: Algae would likely increase because fewer zooplankton are grazing on them, and minnows would likely decrease because they have lost their main food source; the drop in minnows could then reduce the bass population as well.
To predict ripple effects, look both directions from the affected organism. Arrows pointing into zooplankton come from algae, so removing the grazer lets the algae population grow. Arrows pointing out of zooplankton go to minnows, so minnows lose energy input and decline. Then follow one more step: fewer minnows means less food for bass, so bass may decline or shift to other prey. A complete answer traces the effect at least two links and names the arrow direction as the reason.
An energy pyramid shows 80,000 kilocalories at the producer level. Using the ten percent rule, how much energy is available to a secondary consumer, and where did the rest of the energy go?
Answer: About 800 kilocalories; the rest was lost mostly as heat from cellular respiration, plus undigested waste and parts never eaten.
Two transfers separate producers from secondary consumers: producer to primary consumer, then primary consumer to secondary consumer. So kilocalories for primary consumers, and kilocalories for secondary consumers. The roughly kilocalories that did not arrive were not destroyed — organisms burned most of that energy in cellular respiration and released it as heat, some exited as undigested waste, and some was in tissue that predators never consumed and decomposers broke down instead.
FAQ
- Which way do food chain arrows point, and why do people get it backwards?
- Arrows point from the organism that is eaten to the organism that eats it, because they show the direction energy travels. People get it backwards because they picture the predator attacking and draw the arrow from hunter to prey. Fix it by reading each arrow as "energy goes this way," not "eats."
- What is the difference between a food chain and a food web?
- A food chain shows one single pathway of energy, such as grass to rabbit to fox. A food web combines many overlapping chains into one diagram, showing that most organisms eat several things and are eaten by several things. Webs are more realistic and show why ecosystems with many connections are more stable.
- Why do food chains usually have only four or five levels?
- Because only about of the energy at one level is stored as tissue at the next. After four transfers, less than one hundredth of one percent of the original energy remains, which is not enough to support another population of predators.
- Are decomposers part of the food chain?
- Yes. Decomposers such as bacteria and fungi feed on dead organisms and waste from every trophic level, so energy and matter from producers and consumers alike pass through them. They also return nutrients to the soil and water so producers can build new tissue, which links food chains to the cycling of matter.
Learn this with a teacher, not a page
The Crimsora tutor teaches Food Chains, Food Webs & Energy Flow live — explaining on a whiteboard, asking you questions, and adapting to where you get stuck.