Energy Flow, Food Chains & Food Webs — Print · Grade 7 Science
Chapter: Ecosystems, Matter & EnergyEnergy Flow, Food Chains & Food Webs
Learning goals
- I can explain energy flow, food chains & food webs.
- I can use evidence, a model, and stated assumptions to support a biological explanation.
Before this unit
Use the supplied vocabulary, distinguish an observation from an explanation, and identify any units or model assumptions. Revisit a connected earlier unit if needed; new terms are explained here.
Use three flexible 40–45-minute sessions: core background and Parts 1–2 with their checks; Parts 3–4 and the worked application; then remaining practice, a labeled model, revision, and the exit check. Checks completed during reading can count toward the practice set. Read one short part at a time. Investigations and chapter review/quiz have separate sessions, and flexible weeks allow more time.
In 1995, grey wolves were returned to Yellowstone after a seventy-year absence. The elk, which had been browsing unchecked, began avoiding the open valleys where they were vulnerable. Willow and aspen recovered along the streams. Beavers, which need willow, increased. Songbirds returned to the regrown cover, and the changed vegetation altered the banks of the rivers themselves. Exactly how much of this the wolves caused is still argued about by ecologists, and that argument is part of the lesson — but the case shows what an ecosystem is: a community of organisms and their physical surroundings, connected tightly enough that a change in one place turns up in others nobody was watching.
The connections are feeding relationships, and the roles have names. A producer builds its own food from simple materials using an outside energy source — almost always a plant or alga using light. A consumer obtains energy by eating other organisms, directly or at a remove. A decomposer breaks down dead material and waste, releasing the matter locked in it back into the soil and air. Every organism occupies one of these roles in a given relationship, and a food web is the whole set of them in a place, with arrows showing where energy and matter go.
The arrow convention is worth fixing immediately, because it is the most common source of error: an arrow points from the organism being eaten to the organism doing the eating, in the direction the energy and matter travel. Grass points to rabbit; rabbit points to fox. Read the other way the web becomes nonsense. The second thing to hold onto runs through the entire chapter: matter cycles and energy flows. A carbon atom can go round indefinitely — air, plant, rabbit, fox, decomposer, air again. Energy cannot. It enters as sunlight, passes along the arrows with a large fraction lost as heat at every step, and leaves. Nothing recycles it, which is why an ecosystem needs a continuous supply of light rather than a stock of energy going round.
Build your understanding
Where this fits
The course has worked from cells up to whole organisms and how they reproduce and inherit. This chapter steps outside the organism entirely, to the matter and energy moving between them. The first unit sets out who depends on whom, and the distinction between energy flowing and matter cycling that the whole chapter rests on.
Use three flexible 40–45-minute sessions: core background and Parts 1–2 with their checks; Parts 3–4 and the worked application; then remaining practice, a labeled model, revision, and the exit check. Checks completed during reading can count toward the practice set. Read one short part at a time. Investigations and chapter review/quiz have separate sessions, and flexible weeks allow more time.
Part 1 · Roles are defined by how an organism gets its energy
A producer is defined by building its own food from simple inorganic materials using an external energy source — light for plants and algae, and in a few places chemical energy from rock or vents. A consumer is defined by obtaining its energy from other organisms. The role is about the pathway, not about the kind of organism, which keeps the definitions working in unfamiliar cases.
Decomposers are the role most often left out, and the ecosystem will not work without them. Fungi and bacteria break down dead organisms and waste, which does two things at once: it releases energy for themselves, and it returns the matter — carbon, nitrogen, phosphorus — to forms producers can take up again. Without decomposition the nutrients would be locked in dead material and the cycling would stop.
An organism can occupy different roles in different relationships. A fox eating a rabbit is a consumer; a fox's body after death is material for decomposers. A plant is a producer and also food. Describing roles in terms of a particular relationship, rather than labelling species permanently, is what keeps a web accurate.
Pause and explain
Why are decomposers essential rather than merely tidy?
Check your reasoning after answering
Because they return matter to the system. Breaking down dead organisms and waste releases carbon, nitrogen and other nutrients in forms producers can take up again, which is what allows matter to cycle. Without decomposition those materials would stay locked in dead tissue and producers would run out of what they need, however much sunlight continued to arrive.
Part 2 · Reading arrows the right way
The arrow points in the direction energy and matter move: from the eaten to the eater. Grass → rabbit → fox. Many people draw it the other way, reasoning that the fox goes to the rabbit, and the whole diagram then says the opposite of what is intended.
State the convention when you draw or read one. It costs a line and removes the ambiguity, and it also makes the diagram checkable: anyone can confirm that each arrow runs from a source of energy to something obtaining energy from it.
The arrows carry information beyond direction, but less than people assume. They say that a feeding relationship exists. They do not say how often it happens, how much of the predator's diet it represents, or how much energy actually moves. A thin dependency and a critical one look identical on the page, which matters for the prediction question later in this unit.
Pause and explain
In a food web, which way does the arrow between grass and rabbit point, and why?
Check your reasoning after answering
From grass to rabbit, because the arrow shows the direction energy and matter travel — from the organism being eaten to the one eating it. Drawing it the other way reverses the meaning of the whole diagram. Stating the convention alongside the web makes it checkable by anyone reading it.
Part 3 · From single chains to a web
A food chain is a single path: grass to rabbit to fox. A web is what you get when you draw all the chains in one place together, and it looks different, because most organisms eat several things and are eaten by several things. That branching is the important structural fact, and a single chain gives a misleading picture of how a real community behaves.
Branching provides alternatives, which makes a web more stable than a chain. If insects decline in a web where mice eat both insects and seeds, mice have another route; in a chain with no alternative, the effect would pass straight through. More connections generally mean more resilience, which the biodiversity unit in the next chapter develops.
Branching also makes prediction harder, and honestly so. A change at one point can propagate in several directions at once, with effects that may partly cancel. The Yellowstone case is argued over precisely because the web is complicated: the wolves' return coincided with other changes, and separating their contribution from drought, from the bear population, and from recovering beaver numbers is genuinely difficult.
Pause and explain
Why is a food web more stable than a single food chain?
Check your reasoning after answering
Because branching provides alternatives. If one food source declines in a web, consumers that also eat something else can shift, so the effect is absorbed rather than passed straight along. In a chain with no alternatives, a loss at one point passes directly to everything above it. More connections generally mean more resilience.
Part 4 · Energy flows one way; matter goes round
Follow a carbon atom. It enters a plant from the air as carbon dioxide, becomes part of a sugar and then a leaf; a rabbit eats the leaf and the atom becomes rabbit tissue; a fox eats the rabbit; the fox dies and decomposers release the atom to the air again. The same atom can make that circuit indefinitely. Matter cycles, because there is a return path.
Now follow the energy. Sunlight is captured in a sugar; the rabbit obtains some of it; much of the rest was used by the plant's own respiration and lost as heat. At every step the same thing happens, and heat released to the surroundings cannot be recovered to do work. There is no return path for energy, which is why an ecosystem must be continuously supplied with light.
The practical consequence is a test you can apply to any ecological claim. If an account has energy going round a loop, it is wrong. If it has matter being used up and gone, it is usually wrong too. Keeping the two in separate columns — matter cycling, energy flowing one way and leaving as heat — resolves most of the confusion in this chapter before it starts.
Pause and explain
Why does an ecosystem need a continuous supply of sunlight when it does not need a continuous supply of carbon?
Check your reasoning after answering
Because matter has a return path and energy does not. Carbon atoms cycle between air, producers, consumers and decomposers indefinitely, so the same atoms are reused. Energy enters as light, passes along the feeding arrows, and leaves as heat at every transfer, with no way back. It must therefore be replaced continuously.
Apply the whole unit · Predict from a supplied web
Illustrative web, not a surveyed ecosystem: grass feeds insects and rabbits; insects and seeds feed mice; mice and rabbits feed foxes. A decline in insects is recorded, and no counts are given for the other resources.
Your task
Trace one pathway the insect decline could affect, and explain why the exact change in mice or foxes cannot be determined from this web.
What your response should show
- Explain the relevant biological process accurately.
- Use the supplied evidence and show calculation steps with units where needed.
- Connect a labeled model or mechanism with the claim.
- State an assumption, evidence limitation, or useful next observation.
Response guidance — after your attempt
Trace the pathway: fewer insects means less food reaching mice along that arrow, which could reduce mouse numbers and in turn reduce food reaching foxes.
Identify the alternatives the web shows: mice also eat seeds, and foxes also eat rabbits, so each has another route.
State what the web omits: arrows show that a relationship exists, not how much of each diet it supplies or how strongly each consumer depends on it.
Name what would be needed: counts of seeds and rabbits over the same interval, and a measure of what proportion of each consumer's diet each source provides.
One affected pathway runs grass → insects → mice → foxes. A decline in insects reduces the food arriving at mice along that arrow, which could reduce mouse numbers, and fewer mice could in turn mean less food reaching foxes. That is a plausible chain of effects and it is as far as the web supports going. The exact change in mice cannot be determined, because mice also eat seeds, and the web does not say what proportion of a mouse's diet comes from insects rather than seeds, nor whether seeds are plentiful or scarce over the same interval. If seeds are abundant, mice may be barely affected; if insects were most of their intake, the effect could be large. The same reasoning applies one step further up: foxes also eat rabbits, so even a substantial fall in mice might be offset, and rabbit numbers are not recorded either. The general limitation is that an arrow in a food web states that a feeding relationship exists and nothing about its strength — a critical dependency and an occasional meal look identical on the page. To make a quantitative prediction you would need the proportion of each consumer's diet supplied by each source, counts of the other resources over the same interval, and some measure of how quickly each population responds, since effects move through a web with delays. Without those, the honest statement names the direction of a possible effect and the alternatives that could absorb it.
Scientific references for this unit
Parent / teacher teaching notes
Ask for a because-statement and a labeled relationship, not just a memorized word. Work through one calculation step at a time, permit oral or diagram responses, and use flexible sessions for difficult reading. Compare the learner’s actual evidence with the supplied model; do not reward copying or agreement with a personal preference. No private family, genetic, or medical records are required.
The key idea
An ecosystem includes organisms and their physical environment. Producers build organic molecules using an energy source, commonly sunlight in the ecosystems studied here. Consumers obtain organic matter by eating organisms or their products. Decomposers use dead material and wastes, transferring matter into other forms. These are functional roles, not ranks of importance.
Worked examples
Example 1 · Correct a reversed web
- Check the convention: arrows run from the organism eaten to the organism eating it, following energy and matter.
- Redraw accordingly: grass → rabbit → fox.
- Correct the wording: a rabbit eats grass rather than hunting it, and the arrow records the transfer of energy and matter, not an act of pursuit.
Follow the energy. If the arrow does not point the way the energy goes, it is backwards.
Example 2 · Say what a web cannot predict
- Identify what the web shows: that foxes feed on both, and nothing about the proportions.
- Consider the alternative route: if mice are plentiful, foxes may shift towards them and be little affected.
- Consider the other direction: fewer foxes eating rabbits could let rabbits recover, and more fox pressure on mice could reduce mice.
An arrow is a relationship, not a quantity. Magnitudes need measurements.
Avoid these mistakes
- Mistake: arrows point from the eater to the eaten. They follow energy and matter, from the organism eaten to the one eating it — grass to rabbit to fox.
- Mistake: energy cycles round the web. Matter cycles; energy enters as light and leaves as heat at every transfer, which is why continuous sunlight is needed and continuous carbon is not.
- Mistake: decomposers are optional tidying. They return nutrients to forms producers can use, without which cycling stops even with sunlight still arriving.
- Mistake: a web lets you predict how much a population will change. An arrow shows that a relationship exists, not its strength; magnitudes need diet proportions and counts.
Check your understanding
Why is a decomposer not outside the food system?
Discuss the answer
It obtains and transfers organic matter and energy from dead material and wastes within connected pathways.
Which way should an arrow point from an eaten seed to a seed-eating bird?
Discuss the answer
From seed to bird, because the bird receives matter and stored energy from the seed.
Words to know
- ecosystem
- A community of organisms together with their physical surroundings, connected closely enough that a change in one part appears in others.
- producer
- An organism building its own food from simple materials using an outside energy source, almost always light.
- consumer
- An organism obtaining energy by eating others; the role describes a relationship rather than labelling a species permanently.
- food web
- All the feeding relationships in a place drawn together, with arrows running from the eaten to the eater.
- decomposer
- An organism breaking down dead material and waste, returning carbon and nutrients to forms producers can use again.
Extend your scientific explanation
Compare a second supplied example or the linked educational reference. Identify a connection, a model limit, and an additional observation that would test the explanation.
In the real world
Use a clear question, reliable observations, and a biological mechanism to evaluate a claim. State what remains uncertain before applying a classroom model to a different setting.
Exit check
Do decomposers recycle all usable energy back to plants?
Check your explanation after answering
No. They recycle matter into other forms while energy transfers and much dissipates as heat.
Explain your reasoning without notes. If a key idea is missing, revisit the lesson and try again.
Problem 1
Answer and explanation
It obtains and transfers organic matter and energy from dead material and wastes within connected pathways.
Problem 2
Answer and explanation
From seed to bird, because the bird receives matter and stored energy from the seed.
Problem 3
Answer and explanation
It may use other resources, migrate, or respond over time; the web shows possible connections rather than an immediate fixed outcome.
Problem 4
Answer and explanation
No. They recycle matter into other forms while energy transfers and much dissipates as heat.
Problem 5
Answer and explanation
The insect decline can affect the mouse pathway and indirectly foxes, but mice also have seeds and foxes also have rabbits. The web does not give dependence strengths, alternative resource changes, movement, or timing. It supports a conditional mechanism, not an exact population forecast.
Follow insect→mouse→fox under the stated arrows.
Identify seeds as another mouse resource and rabbits as another fox resource.
Request counts, rates, and interval information.
Response criteria
- Explain the relevant biological process accurately.
- Use the supplied evidence and show calculation steps with units where needed.
- Connect a labeled model or mechanism with the claim.
- State an assumption, evidence limitation, or useful next observation.
Problem 6
Answer and explanation
It can eat resources from different positions, such as both producers and other consumers. The web must show its actual resource relationships.
Response criteria
- Explain the relevant biological process accurately.
- Use the supplied evidence and show calculation steps with units where needed.
- Connect a labeled model or mechanism with the claim.
- State an assumption, evidence limitation, or useful next observation.
Problem 7
- Grass hunts rabbits
- Rabbit receives food matter and energy from grass
- Rabbits create sunlight
- Grass becomes a whole rabbit instantly
Answer and explanation
Rabbit receives food matter and energy from grass
The arrow tracks transfer from the eaten resource toward the consumer.
Problem 8
Answer and explanation
Feeding rates, dependence on each resource, initial counts, movement, and environmental conditions are examples of needed information.