Food Chains, Food Webs & Energy Flow · Grade 6 Science
Chapter: Ecosystems, Watersheds & ConservationFood Chains, Food Webs & Energy Flow
Learning goals
- I can explain why energy flows through an ecosystem while matter cycles within it.
- I can use the ten per cent figure to calculate energy at each feeding level.
- I can explain why food chains rarely have more than four or five levels.
Readings for this unit
Before this unit
Read the chapter opening and the named observation or diagram. Clarify one word or unit that blocks understanding. Recall the relevant earlier measurement, matter, or energy idea.
Study the unit across three 35–45-minute class sessions. Use the worked examples after an attempt and the supplied records when actual equipment is not available.
Producers, almost always plants or algae, capture sunlight and build sugars from carbon dioxide and water. They are the only entrance: every other organism in an ecosystem is living on energy a producer captured, whether it eats plants directly or eats something that did.
Capture is inefficient from the start. Of the sunlight falling on a field, plants convert only about 1 to 2 per cent into chemical energy in new growth, the rest being reflected, passing through, falling on bare soil or driving evaporation.
Each feeding step afterwards loses far more. Roughly 10 per cent of the energy at one level reaches the next, with the other 90 per cent spent on respiration, movement and body heat, or passing through undigested. That is the single most useful number in this chapter.
Build your understanding
Where this fits
The last lesson described where ecosystems are and what limits them. This one follows the energy through them, and the next follows the matter, which behave in importantly different ways. The 10 per cent figure introduced here is what makes the pyramid shapes in the third section inevitable rather than coincidental.
Use three flexible 40-45-minute sessions: producers and energy capture; the ten per cent rule and where the rest goes; pyramids, chain length and food webs, then the practice set, revision and the exit check. Workshop, review, quiz and exams have separate time.
Part 1 · One way in, one way out
Photosynthesis is the entrance and essentially the only one. Plants and algae use light energy to combine carbon dioxide and water into glucose, releasing oxygen, and everything else in the ecosystem draws on that store. Consumers are classified by how far from it they feed: herbivores eat producers directly, carnivores eat herbivores, and top carnivores eat other carnivores. Decomposers, mainly fungi and bacteria, feed on dead material at every level and are the reason nothing accumulates indefinitely.
The crucial asymmetry is that energy flows through while matter cycles. A carbon atom can go round an ecosystem indefinitely, from air to plant to animal to decomposer and back to air. Energy cannot: at every step a large share becomes heat, which disperses into the surroundings and cannot be recovered, exactly as chapter 3 described for dissipation. That is why an ecosystem needs a continuous supply of sunlight and does not need a continuous supply of carbon, and it is why switching off the Sun would stop everything within a season while the carbon would all still be present. Decomposers deserve more attention than they usually get, because they are the step that closes the matter cycle. Without fungi and bacteria breaking down dead wood, leaves and bodies, the nutrients locked in them would never return to the soil and plant growth would stop within a few seasons. A forest floor processes several tonnes of leaf litter per hectare every year, and the reason you are not wading through centuries of accumulated leaves is that decomposers are keeping pace with the fall.
Pause and explain
Why does an ecosystem need a continuous energy supply but not a continuous supply of carbon?
Check your reasoning after answering
Because energy is degraded at every step while matter is not. A large share of the energy at each feeding level becomes heat that disperses and cannot be used again, so it must be replaced by sunlight. Carbon atoms are passed from air to plant to animal to decomposer and returned to the air unchanged, so the same atoms serve repeatedly.
Part 2 · Where the other ninety per cent goes
The ten per cent figure is an average rather than a law, with real values ranging from about 2 to 20 per cent, and the losses are easy to account for. Most of the energy an animal takes in is spent on staying alive: respiration to power muscles, maintain tissue and in birds and mammals maintain body temperature, all of which ends as heat. Some food is never absorbed at all and leaves as waste. Some parts of an organism are not eaten, such as bones, hooves and the woody parts of plants, and that energy passes to decomposers instead of up the chain.
Those proportions explain a real difference between groups. A cow or a bird spends a great deal of its energy holding its body at around 38 degrees Celsius whatever the weather, so little is left for growth; a snake or a fish does not, and can convert a far higher share of its food into new tissue. That is why fish farming produces more food per unit of feed than cattle farming, and the same arithmetic underlies the observation that a given area of land feeds far more people growing crops than raising animals on it. Size has a related effect that often surprises students. A small mammal such as a shrew loses heat far faster than a large one, because it has much more surface area for its mass, so it must eat almost continuously to keep up; a shrew can starve to death in a few hours. A large animal with the same body temperature loses heat relatively slowly and can go days between meals, which is why the smallest warm-blooded animals have the most demanding lives.
Pause and explain
Why does a fish convert a higher share of its food into growth than a cow does?
Check your reasoning after answering
Because it does not spend energy maintaining a fixed body temperature. A cow holds its body near 38 degrees Celsius whatever the surrounding temperature, and that heat is continuously lost to the environment. A fish takes the temperature of the water around it, so energy that a mammal spends on warmth is available for building tissue instead.
Part 3 · Why the chain runs out
Follow the arithmetic and the consequence is forced. Suppose 10,000 kilojoules reaches the plants in an area each day as captured energy. The herbivores eating them receive about 1000 kJ, the carnivores eating those about 100 kJ, and a predator eating those carnivores about 10 kJ. A fifth level would be left with about 1 kJ, which is not enough to sustain a population of any animal large enough to hunt the fourth. Food chains are therefore almost never longer than four or five levels, not because of any biological rule but because the energy has run out.
The same arithmetic explains why top predators are rare and why they need such enormous territories. A single wolf pack may range over several hundred square kilometres, and a tiger over a hundred, because that is the area required to support the prey populations that support them. It also explains why top predators are so vulnerable: a reduction in plant growth at the bottom is multiplied by the transfers, so a 20 per cent drop in vegetation can wipe out the top level entirely, which is why they are usually the first species lost when a habitat shrinks. Aquatic systems can support slightly longer chains for a reason worth knowing. Their producers are mostly microscopic algae that are eaten whole, with no woody stems or roots left uneaten, so the transfer at the first step is more efficient than on land. An ocean chain may run five or six levels from algae to tuna, which is why the largest marine predators can exist at all when a comparable land chain would have run out of energy.
Pause and explain
Why are food chains almost never longer than four or five levels?
Check your reasoning after answering
Because only about a tenth of the energy passes each step. Starting from 10,000 kJ at the producers, the levels above receive about 1000, then 100, then 10 kJ, so a fifth consumer level would have roughly 1 kJ available. That cannot support a population of animals large enough to prey on the level below, so the chain stops.
Part 4 · Chains are a simplification
A food chain showing grass to rabbit to fox is a model in the sense chapter 1 defined, useful and deliberately wrong. Almost no animal eats only one thing: a fox eats rabbits, voles, beetles, earthworms, birds, fruit and whatever it finds in a bin, and a rabbit is eaten by foxes, buzzards, stoats and badgers. Drawing all of those relationships gives a food web, which is messier and far closer to the truth.
The difference matters when something changes. A chain predicts that removing rabbits removes foxes, while a web predicts the foxes switch to voles, whose numbers then fall, affecting whatever else eats voles. Webs also show why the effects of a removal are hard to predict: when sea otters were hunted almost to extinction along the North Pacific coast, the sea urchins they ate increased enormously and grazed the kelp forests down to bare rock, which removed habitat for dozens of fish species. Nobody had predicted that hunting one animal would destroy a forest, and the connection runs through three levels of a web. Some species turn out to matter far more than their numbers suggest, and ecologists call these keystone species. Otters are one; wolves in Yellowstone are another, where their return after 70 years changed where elk grazed, which let willow and aspen recover along the rivers, which brought back beavers and changed the shape of the streams themselves. Identifying which species hold a web together is one of the central practical problems in conservation, and it is rarely the largest or the most obvious one.
Pause and explain
Why did removing sea otters destroy kelp forests?
Check your reasoning after answering
Because the effect travelled through the web rather than stopping at the otters. Otters eat sea urchins, so with the otters gone the urchin population grew enormously, and urchins graze on kelp. The kelp forests were eaten down to bare rock, removing habitat for many fish species, which is an outcome a simple chain diagram would not have suggested.
Apply the whole unit · Work the pyramid
In a grassland, the plants capture 20,000 kJ per square metre per year. Energy transfer between levels averages 10 per cent. The chain runs grass, grasshoppers, shrews, owls.
Your task
Calculate the energy at each level, and use the result to explain two observations about the ecosystem.
What your response should show
- Calculates all four levels correctly from the 10 per cent figure
- Explains owl numbers by energy available rather than only by body size
- Carries the 20 per cent reduction through to the top level
- Explains why a small absolute loss matters at the top of a chain
Response guidance — after your attempt
Calculate the energy available at each of the four levels.
Explain what this predicts about the number of owls compared with grasshoppers.
Predict what a 20 per cent fall in plant growth would do to the owls, and explain why.
The grass holds 20,000 kJ per square metre per year. At 10 per cent transfer the grasshoppers receive about 2000 kJ, the shrews about 200 kJ, and the owls about 20 kJ. That is a thousandth of what the plants captured. The arithmetic predicts far fewer owls than grasshoppers, and not simply because owls are larger: each owl needs enough energy to live on, so the 20 kJ per square metre must be spread across a very small number of individuals, each ranging over a large area. A 20 per cent fall in plant growth takes the grass to 16,000 kJ, and the proportional loss carries through every level, so the owls fall to about 16 kJ. The absolute loss looks small, 4 kJ, and it is a fifth of everything available to them, which may be enough to leave the area unable to support a breeding pair at all. That is why the top of a food chain is the first part to disappear when conditions at the bottom worsen.
Scientific references for this unit
- US Geological Survey: sea otters, urchins and kelp forest ecosystems
- National Oceanic and Atmospheric Administration: marine food webs and energy transfer
- US Department of Agriculture: feed conversion in livestock and aquaculture
- National Park Service: trophic cascades and predator removal
Parent / teacher teaching notes
Begin with the literal observation, quantity, or diagram relationship. Ask the learner to explain why the evidence supports the claim, check a relevant unit or boundary, then discuss an alternative. The supplied numbers are illustrative, not real observations. A parent can read aloud and allow a spoken rehearsal before writing. Judge science and evidence rather than political, faith, or family agreement. Use the complete data option whenever an actual procedure is unavailable or unsuitable.
The key idea
Energy enters an ecosystem only through producers and leaves it as heat at every step, with roughly a tenth passing from one feeding level to the next. That single fraction explains why food chains are short and why large predators are rare.
Worked examples
Example 1 · Trace a food web and energy model
- Trace resource to consumer.
- Divide the modeled larvae amount by algae amount.
- Name an alternative route or missing population record.
Example 2 · Transfer to a different case
- Identify the distinction and conditions in this new case.
- Use the relevant mechanism or quantity; preserve units and the chosen boundary.
- Explain why the answer fits and what further evidence would be needed.
Example 3 · Build a web from several relationships · a focused example
- State the literal condition or observation in the example.
- Apply the distinction explained in this teaching part.
- Check whether the claim stays within the given evidence.
Avoid these mistakes
- Misleading claim: Food-web arrows always point from predator toward prey.
- Misleading claim: Every feeding transfer has exactly the same universal percentage.
- Omitting units, measurement timing, or a model boundary makes the evidence hard to interpret.
- Treating illustrative classroom data as real field measurements or copying a model is not an independent explanation.
Check your understanding
Why does the arrow point toward the rabbit?
Discuss the answer
The rabbit receives matter and chemical energy by eating the grass under the stated convention.
What does the second route change?
Discuss the answer
It adds an alternative resource pathway, so removing one prey does not automatically describe all food available to the fox.
Does the remaining 900 units vanish?
Discuss the answer
No. They follow other pathways, including uneaten material, waste, activity, and thermal transfers; the model must account for them.
Can a web alone predict exactly how many foxes remain?
Discuss the answer
No. Quantitative prediction requires population, feeding, movement, and time information beyond the drawn links.
Sustained scientific explanation and revision
These activities are scheduled in the parent plan. Save a first response, a correction, and the reason for the change in the science notebook.
Activity 1 · Food Chains, Food Webs & Energy Flow · Model and explain
Read A Clearer Stream Is Still a Question and the linked unit teaching; use its complete supplied case and diagrams.
Your task
For Food Chains, Food Webs & Energy Flow, study Parts 3–4 and attempt the worked case: Explain arrow direction, calculate the stated fraction, and identify a limit on population prediction. Explain the steps before opening the model.
Success criteria
- An accurate scientific claim answering the actual question.
- Relevant evidence or a labeled model with quantities and units when applicable.
- Reasoning that connects evidence with the mechanism rather than only repeats it.
- A specific method, boundary, scale, or alternative-explanation limit.
Hint
Begin with the actual observation, quantity, or diagram relationship. Explain how it bears on the question before making a broad claim.
Response guidance — discuss after your attempt
The arrows show receiving organisms obtaining matter and energy from resources. 80 ÷ 800 = 10% in the stated biomass-transfer model, not a universal percentage. The second fish provides another feeding route, and the web lacks population sizes and rates; it cannot specify an exact future bird count.
Activity 2 · Food Chains, Food Webs & Energy Flow · Practice, revise and exit
Read A Clearer Stream Is Still a Question and the linked unit teaching; use its complete supplied case and diagrams.
Your task
For Food Chains, Food Webs & Energy Flow, complete practice questions 1–6 and both choice checks, then answer independently: A learner draws bird → fish to show that the bird eats fish. How should it be corrected under the course convention? Save a correction explaining one changed claim or method.
Success criteria
- An accurate scientific claim answering the actual question.
- Relevant evidence or a labeled model with quantities and units when applicable.
- Reasoning that connects evidence with the mechanism rather than only repeats it.
- A specific method, boundary, scale, or alternative-explanation limit.
Hint
Begin with the actual observation, quantity, or diagram relationship. Explain how it bears on the question before making a broad claim.
Response guidance — discuss after your attempt
Draw fish → bird, showing the direction of matter and energy transferred through feeding. State the convention so the arrow can be interpreted.
Words to know
- producer
- An organism that builds organic matter using an energy source rather than feeding on organic food.
- food web
- A model of connected feeding relationships among organisms.
- evidence
- A relevant observation, measurement, or record used to evaluate a claim.
- system boundary
- The chosen limit specifying what is included and which exchanges cross it.
- uncertainty
- A stated limit in measurement or knowledge, distinct from a careless mistake.
In the real world
Eating lower on the food chain — more plants and less meat — passes along more of the sun's original energy, which is why plant-based meals generally use far less land and water per calorie of food.
Exit check
A learner draws bird → fish to show that the bird eats fish. How should it be corrected under the course convention?
Check your explanation after answering
Draw fish → bird, showing the direction of matter and energy transferred through feeding. State the convention so the arrow can be interpreted.
Explain your reasoning without notes. If a key idea is missing, revisit the lesson and try again.