Energy Flow: Food Chains, Webs & Pyramids
Trace energy from producers to top predators with food chains, webs, and pyramids — and use the 10 percent rule to explain why food chains stay short.
What you'll do in this lesson
A voice-first session with the Crimsora tutor on Energy Flow: Food Chains, Webs & Pyramids, then targeted practice and FRQs — with the tutor adapting to where you get stuck.
What this lesson covers
Every bite of food you have ever eaten traces back to sunlight. Producers capture solar energy, consumers pass a fraction of it along, and decomposers recycle what is left. But energy does not travel through an ecosystem the way water travels through a pipe — most of it leaks out as heat at every single transfer. That leak is the reason a lake can support thousands of tiny algae, hundreds of minnows, a few bass, and maybe one osprey.
In this lesson you will build and read food chains and food webs, keep the arrows pointing the right direction, stack up energy pyramids, and apply the 10 percent rule with actual numbers. By the end you should be able to explain, using energy math rather than guesswork, why food chains rarely have more than four or five links and why top predators are always rare.
In this lesson you will build and read food chains and food webs, keep the arrows pointing the right direction, stack up energy pyramids, and apply the 10 percent rule with actual numbers. By the end you should be able to explain, using energy math rather than guesswork, why food chains rarely have more than four or five links and why top predators are always rare.
Trophic Levels: Who Eats Whom
Energy enters almost every ecosystem through producers (autotrophs) — plants, algae, and photosynthetic bacteria that convert light energy into chemical energy stored in glucose. Producers occupy the first trophic level, meaning the first feeding position in the ecosystem.
Everything else is a consumer (heterotroph). Primary consumers are herbivores that eat producers. Secondary consumers eat primary consumers, and tertiary consumers eat secondary consumers. The same organism can sit at different levels depending on the meal: a bear eating berries is a primary consumer, but a bear eating a salmon is a tertiary consumer. That flexibility is exactly why real ecosystems need webs, not chains.
Decomposers — bacteria, fungi, and detritivores like earthworms — feed on dead tissue and waste from every level. They are not a dead end tacked onto the bottom of a diagram; they are the ecosystem's recycling crew. Decomposers release the chemical energy stored in dead matter and return nutrients such as nitrogen and phosphorus to the soil and water where producers can absorb them again.
Here is where the two big ideas of this unit split apart. Matter cycles; energy flows. The carbon atom in a leaf can be reused indefinitely, passing through a caterpillar, a bird, a fungus, and back into a plant. The energy in that leaf cannot be reused — once it is converted to heat during cellular respiration, it radiates away and leaves the ecosystem for good. That is why ecosystems require a continuous input of sunlight but do not require a continuous delivery of new atoms.
Everything else is a consumer (heterotroph). Primary consumers are herbivores that eat producers. Secondary consumers eat primary consumers, and tertiary consumers eat secondary consumers. The same organism can sit at different levels depending on the meal: a bear eating berries is a primary consumer, but a bear eating a salmon is a tertiary consumer. That flexibility is exactly why real ecosystems need webs, not chains.
Decomposers — bacteria, fungi, and detritivores like earthworms — feed on dead tissue and waste from every level. They are not a dead end tacked onto the bottom of a diagram; they are the ecosystem's recycling crew. Decomposers release the chemical energy stored in dead matter and return nutrients such as nitrogen and phosphorus to the soil and water where producers can absorb them again.
Here is where the two big ideas of this unit split apart. Matter cycles; energy flows. The carbon atom in a leaf can be reused indefinitely, passing through a caterpillar, a bird, a fungus, and back into a plant. The energy in that leaf cannot be reused — once it is converted to heat during cellular respiration, it radiates away and leaves the ecosystem for good. That is why ecosystems require a continuous input of sunlight but do not require a continuous delivery of new atoms.
Food Chains, Food Webs, and Reading Arrows Correctly
A food chain is a single linear pathway of energy transfer, such as grass → grasshopper → shrew → hawk. A food web is a network of interconnected food chains showing the many feeding relationships that actually exist in a community.
The most common mistake students make is drawing arrows backwards. An arrow means "energy flows to," not "eats." So you write grass → grasshopper, even though the grasshopper is the one doing the eating. If you are unsure, read your arrow out loud as "is eaten by" and the diagram will check itself.
Food webs matter because they show ecosystem resilience. In a chain, removing one species breaks everything downstream. In a web, a predator with five prey species can shift its diet if one prey population crashes. This is the reasoning you use for any "what happens if" question: follow the arrows outward from the changed species and describe the immediate effects first, then the secondary ones.
Watch for a subtle trap: in a web, an organism's trophic level depends on the path you follow. If a raccoon eats both berries and frogs, it appears at two levels, and questions asking for "the longest food chain in this web" require you to count links along the single longest route, not to average them.
The most common mistake students make is drawing arrows backwards. An arrow means "energy flows to," not "eats." So you write grass → grasshopper, even though the grasshopper is the one doing the eating. If you are unsure, read your arrow out loud as "is eaten by" and the diagram will check itself.
Food webs matter because they show ecosystem resilience. In a chain, removing one species breaks everything downstream. In a web, a predator with five prey species can shift its diet if one prey population crashes. This is the reasoning you use for any "what happens if" question: follow the arrows outward from the changed species and describe the immediate effects first, then the secondary ones.
| Feature | Food chain | Food web |
|---|---|---|
| Structure | One linear path | Many interlinked paths |
| Shows omnivores well? | No | Yes |
| Predicts effect of species loss | Oversimplifies | More realistic |
| Best used for | Counting trophic levels, energy math | Modeling a whole community |
The 10 Percent Rule: Where the Energy Goes
When energy moves from one trophic level to the next, roughly 10 percent is stored in the new level's biomass and about 90 percent is lost. This approximation is the 10 percent rule, and the losses come from three places.
First, cellular respiration. Most of the energy an organism assimilates is burned to power movement, growth, and homeostasis, and that energy exits as heat. Second, incomplete consumption. A deer does not eat the roots or bark of every plant, and a lion abandons hide and bone. Third, egestion and excretion. Undigested material leaves as feces, which passes to decomposers rather than up the chain.
None of this energy vanishes — the second law of thermodynamics is satisfied because the "lost" energy becomes low-quality heat that cannot do biological work. Notice also that decomposers intercept a large share of the chemical energy in uneaten and egested material, which is why decomposer biomass is enormous even though they never appear at the top of a pyramid.
The math is straightforward multiplication. Starting with 20,000 kcal in producers:where is the trophic level. Primary consumers get about 2,000 kcal, secondary consumers 200 kcal, tertiary consumers 20 kcal, and a fourth-level consumer only 2 kcal. Real transfer efficiencies range from about 1 to 20 percent depending on the organism, so treat 10 percent as a working estimate, not a law of nature. Students often lose track of the exponent — count the number of transfers, which is always one less than the number of levels.
First, cellular respiration. Most of the energy an organism assimilates is burned to power movement, growth, and homeostasis, and that energy exits as heat. Second, incomplete consumption. A deer does not eat the roots or bark of every plant, and a lion abandons hide and bone. Third, egestion and excretion. Undigested material leaves as feces, which passes to decomposers rather than up the chain.
None of this energy vanishes — the second law of thermodynamics is satisfied because the "lost" energy becomes low-quality heat that cannot do biological work. Notice also that decomposers intercept a large share of the chemical energy in uneaten and egested material, which is why decomposer biomass is enormous even though they never appear at the top of a pyramid.
The math is straightforward multiplication. Starting with 20,000 kcal in producers:where is the trophic level. Primary consumers get about 2,000 kcal, secondary consumers 200 kcal, tertiary consumers 20 kcal, and a fourth-level consumer only 2 kcal. Real transfer efficiencies range from about 1 to 20 percent depending on the organism, so treat 10 percent as a working estimate, not a law of nature. Students often lose track of the exponent — count the number of transfers, which is always one less than the number of levels.
Ecological Pyramids and Why Top Predators Are Rare
Ecological pyramids are diagrams that quantify a trophic structure. Three kinds appear in biology courses.
An energy pyramid can never be inverted, because a trophic level cannot contain more energy than the level that feeds it. That single constraint answers two classic questions.
Why are food chains short? Each transfer discards about 90 percent of the available energy, so by the fifth level there is typically too little energy left to support a viable breeding population. A fifth-level predator would need an impossibly large hunting range for the calories it needs.
Why are top predators rare? A large predator with high metabolic demands sits on a tiny slice of the original energy budget. Supporting one wolf requires many deer, which require an enormous mass of vegetation. That is also why top predators are the first species to disappear when habitat is fragmented — their energy base shrinks fastest.
The same logic explains a human application: a hectare of farmland feeds far more people when the grain is eaten directly than when it is fed to cattle first, because the beef pathway adds an extra trophic transfer and throws away about 90 percent of the crop's energy along the way.
| Pyramid type | What each level measures | Can it be inverted? |
|---|---|---|
| Energy | kcal or joules per area per year | Never |
| Biomass | Dry mass of living tissue | Rarely (open ocean, where fast-reproducing phytoplankton support more zooplankton mass at any instant) |
| Numbers | Count of individual organisms | Yes (one oak tree supports thousands of insects) |
Why are food chains short? Each transfer discards about 90 percent of the available energy, so by the fifth level there is typically too little energy left to support a viable breeding population. A fifth-level predator would need an impossibly large hunting range for the calories it needs.
Why are top predators rare? A large predator with high metabolic demands sits on a tiny slice of the original energy budget. Supporting one wolf requires many deer, which require an enormous mass of vegetation. That is also why top predators are the first species to disappear when habitat is fragmented — their energy base shrinks fastest.
The same logic explains a human application: a hectare of farmland feeds far more people when the grain is eaten directly than when it is fed to cattle first, because the beef pathway adds an extra trophic transfer and throws away about 90 percent of the crop's energy along the way.
Key terms
- Producer (autotroph).
- An organism that converts light or chemical energy into organic compounds, forming the first trophic level and the energy entry point of an ecosystem.
- Trophic level.
- An organism's feeding position in a food chain, counted from producers (level 1) upward through primary, secondary, and tertiary consumers.
- Decomposer.
- A bacterium, fungus, or detritivore that breaks down dead organic matter and waste, releasing its stored energy as heat and returning nutrients to producers.
- Food web.
- A network of interconnected food chains that shows the multiple feeding relationships within a community, including omnivores.
- 10 percent rule.
- The generalization that only about 10 percent of the energy at one trophic level is stored as biomass in the next level; the rest is lost to respiration, egestion, and uneaten parts.
- Energy pyramid.
- A diagram showing the energy available at each trophic level per unit time; it always narrows upward and can never be inverted.
- Biomass.
- The total dry mass of living tissue at a trophic level, often used as a proxy for stored chemical energy.
- Primary productivity.
- The rate at which producers convert light energy into chemical energy in an ecosystem, setting the ceiling for all levels above.
Worked example
A meadow's grasses capture 150,000 kcal of energy per square meter per year. The food chain is grass → grasshoppers → shrews → snakes → hawks. Assume 10 percent transfer efficiency. (a) How much energy reaches the shrews? (b) How much reaches the hawks? (c) A hawk needs about 15 kcal per square meter per year of available energy to sustain a breeding population in this meadow. Can the meadow support hawks? Explain using energy flow.
Step 1: Label trophic levels. Grass is level 1, grasshoppers level 2, shrews level 3, snakes level 4, hawks level 5.
Step 2: Count transfers, not levels. Getting from grass to shrews takes two transfers, so use . Grasshoppers receive kcal, and shrews receive kcal per square meter per year.
Step 3: Continue to the hawks. Snakes get kcal. Hawks get kcal. Using the formula directly, kcal.
Step 4: Interpret part (c). The available energy at level 5 is exactly 15 kcal per square meter per year, which just meets the stated requirement — meaning this meadow sits right at the edge of what a fifth-level predator can survive on. Any drop in primary productivity, a drier year, or a transfer efficiency below 10 percent would push the hawks below their threshold.
Step 5: State the biological conclusion. Because roughly 90 percent of energy is lost at each transfer to respiration, undigested waste, and uneaten tissue, only one ten-thousandth of the grass energy remains at level 5. This is why food chains rarely extend beyond four or five links and why the top predator in any ecosystem is present in very small numbers.
Step 2: Count transfers, not levels. Getting from grass to shrews takes two transfers, so use . Grasshoppers receive kcal, and shrews receive kcal per square meter per year.
Step 3: Continue to the hawks. Snakes get kcal. Hawks get kcal. Using the formula directly, kcal.
Step 4: Interpret part (c). The available energy at level 5 is exactly 15 kcal per square meter per year, which just meets the stated requirement — meaning this meadow sits right at the edge of what a fifth-level predator can survive on. Any drop in primary productivity, a drier year, or a transfer efficiency below 10 percent would push the hawks below their threshold.
Step 5: State the biological conclusion. Because roughly 90 percent of energy is lost at each transfer to respiration, undigested waste, and uneaten tissue, only one ten-thousandth of the grass energy remains at level 5. This is why food chains rarely extend beyond four or five links and why the top predator in any ecosystem is present in very small numbers.
Practice questions
In the food web arrow "algae → water flea," what does the arrow represent?
- The water flea provides nutrients to the algae
- Energy flows from the algae to the water flea
- The two organisms compete for the same resources
- The algae decompose the remains of the water flea
Answer: Energy flows from the algae to the water flea
Arrows in food chains and webs always point in the direction of energy transfer, from the organism being eaten toward the organism eating it. Reading the arrow as "is eaten by" gives "algae is eaten by water flea," which is correct. Students who read arrows as "eats" reverse every relationship in a web and then misidentify producers as top predators.
A pond ecosystem's phytoplankton produce 80,000 kcal per square meter per year. Calculate the energy available to the secondary consumers, then explain why a fourth-level consumer such as a large heron is uncommon in this pond.
Answer: Secondary consumers receive about 800 kcal per square meter per year; a fourth-level consumer would have only about 80 kcal available, too little to support many large-bodied individuals.
Phytoplankton are level 1 with 80,000 kcal. Primary consumers (zooplankton) get 10 percent, or 8,000 kcal. Secondary consumers get 10 percent of that, or 800 kcal — two transfers, so . A third transfer leaves 80 kcal, and that is what a fourth-level consumer such as the heron has to live on: . A heron has a large body and a high metabolic rate, so 80 kcal per square meter per year supports only a handful of individuals over a very wide foraging area. The energy losses to cellular respiration, egestion, and uneaten tissue at each step are what make high trophic levels so sparsely populated.
A pyramid of numbers for a forest shows 1 oak tree at the bottom level and 5,000 caterpillars at the next level, giving an inverted shape. Explain why this does not violate the rule that an energy pyramid can never be inverted.
Answer: A pyramid of numbers counts individuals, not energy. One massive oak contains far more stored energy and biomass than 5,000 small caterpillars, so the energy pyramid still narrows upward.
Body size varies enormously between trophic levels, so counting heads can produce an inverted pyramid of numbers. Energy, however, is constrained by thermodynamics: the caterpillars can only obtain energy that the oak already captured, and they lose about 90 percent of what they assimilate to respiration and waste. Measure the oak's stored energy in kilocalories and it dwarfs the caterpillars' total, so the energy pyramid keeps its normal upward-narrowing shape. This is a good reminder to check which quantity a pyramid is measuring before interpreting its shape.
FAQ
- Where do decomposers belong on an energy pyramid?
- Decomposers do not sit on a single level, because they receive energy from dead matter and waste produced at every trophic level. Many textbooks draw them as a bar alongside the pyramid or as arrows leaving each level. They are essential to the ecosystem's nutrient cycling, but the energy they release still ends up as heat, so it does not return to the pyramid.
- Is the 10 percent rule exact?
- No. Measured transfer efficiencies in real ecosystems range from roughly 1 percent to 20 percent, depending on the organisms involved. Ectotherms like insects and fish transfer energy more efficiently than endotherms, which burn a large share of their calories maintaining body temperature. The 10 percent figure is a useful average that makes classroom calculations manageable and correctly predicts the general shape of pyramids.
- Why is energy said to flow while matter cycles?
- Atoms of carbon, nitrogen, and oxygen can be used over and over as they pass between organisms and the environment. Energy cannot. Each time an organism does biological work, some usable chemical energy is converted to heat that radiates out of the ecosystem, so ecosystems need a constant new supply of sunlight but not a constant new supply of atoms.
- How do I count trophic levels in a food web with several paths?
- Follow one path at a time from a producer to the organism in question and count the arrows. If a question asks for the longest food chain, trace the route with the most links. If an organism appears at two levels, say so directly — omnivores genuinely occupy more than one trophic position, and describing both is the accurate answer.
Learn this with a teacher, not a page
The Crimsora tutor teaches Energy Flow: Food Chains, Webs & Pyramids live — explaining on a whiteboard, asking you questions, and adapting to where you get stuck.