Food Chains and Food Webs
In 1966, American ecologist Robert Paine removed all Pisaster ochraceus sea stars from an 8-square-metre strip of rocky intertidal at Mukkaw Bay, Washington. Within two years, 15 species collapsed to a monoculture of mussels. A control area 8 metres away, where Paine left the sea stars intact, maintained all 15 species. Paine's experiment proved that food webs — not food chains — determine ecosystem stability, and that a single species can hold the entire web together.
Practise this lesson
Four printable worksheets that build from the foundations up to exam-style questions — start at whatever level suits you.
In Kakadu National Park, saltwater crocodiles are apex predators. They eat fish, birds, turtles and wallabies that come to the water's edge. Before reading on, answer both questions:
Q1: If all saltwater crocodiles were removed from a billabong in Kakadu, predict what would happen to the populations of fish, turtles and waterbirds in that billabong over the next two years. Explain your reasoning.
Q2: A food chain for this billabong might be: aquatic plants → small fish → barramundi → crocodile. But barramundi also eat insects, and crocodiles also eat turtles and birds. Why is a single food chain an inadequate model for this ecosystem?
Know
- How to construct a food chain using standard notation
- The trophic roles: producer, primary, secondary, tertiary consumer, apex predator
- How food webs are constructed from overlapping food chains
- That arrows point from eaten to eater (direction of energy flow)
Understand
- Why food chains are oversimplified models of real ecosystems
- How food web resilience depends on the number of connections
- That matter flows in two directions (along chains and back via decomposers)
- Why omnivores complicate trophic level assignment
Can Do
- Construct and annotate a food web for a given ecosystem
- Predict the consequences of removing one species from a food web
- Identify species that occupy multiple trophic levels
- Explain why food webs are more stable than single chains
Core Content
The simplest way to model who eats whom — but real ecosystems are never this simple
In 1966, Robert Paine marked out two strips of rocky intertidal at Mukkaw Bay, Washington. From one strip he removed every Pisaster sea star, every two weeks, for months. The other strip he left alone. Within two years, the manipulated strip had collapsed from 15 species to one — mussels smothered everything. The control strip held all 15 species. What Paine had destroyed was not a single food chain; he had broken a node in a food web, and the consequences rippled outward to species that had no direct relationship with the sea star at all. A food chain would never have predicted this — but a food web makes it inevitable.
Trophic pyramid showing energy transfer between levels
Standard notation uses arrows to show the direction of energy flow:
grass → grasshopper → frog → snake → hawk
The arrow points from the eaten to the eater — this is the direction energy and matter flow. A common and costly exam error is drawing arrows from predator to prey.
Each position in the chain is a trophic level:
- T1 (Producer): The autotroph that captures energy — grass, algae, phytoplankton
- T2 (Primary consumer): The herbivore — grasshopper, zooplankton, kangaroo
- T3 (Secondary consumer): Carnivore that eats herbivores — frog, small predatory fish
- T4 (Tertiary consumer): Carnivore that eats secondary consumers — snake, larger fish
- T5 (Apex predator): Top predator with no natural predators — hawk, saltwater crocodile
Decomposers (bacteria, fungi) and detritivores (earthworms, woodlice) feed on dead organisms from all trophic levels, returning nutrients to the soil.
Food chain arrows point FROM the eaten TO the eater — this is the direction of energy flow. Trophic levels: T1 producer → T2 primary consumer → T3 secondary consumer → T4 tertiary consumer → T5 apex predator. ~90% of energy is lost at each transfer.
Pause — copy the highlighted arrow rule and trophic level sequence into your book. Drawing the arrow wrong is one of the most common exam mistakes.
In a food chain, the arrow grass → grasshopper represents:
Real ecosystems are networks, not lines — and that network structure creates stability
We just saw that a food chain is a single linear sequence. That raises a problem: real organisms eat multiple prey species and are eaten by multiple predators — a line can't show this. This card answers it → a food web connects all the chains in an ecosystem, and the resulting network structure is what makes the ecosystem resilient.
A food web is constructed by connecting all the food chains in an ecosystem. It shows the full complexity of feeding relationships — which species eat which, how many prey species each predator has, and which species occupy multiple trophic levels.
Consider a simple Australian wetland food web:
- Producers: aquatic plants, algae, phytoplankton
- Primary consumers: zooplankton, aquatic insects, small fish, water snails
- Secondary consumers: barramundi, silver perch, frogs, waterbirds
- Tertiary consumers: herons, egrets, turtles
- Apex predators: saltwater crocodile, white-bellied sea eagle
In this web, barramundi eat both insects (T2) and small fish (T2/T3), placing them at T3 and T4 depending on their meal. Crocodiles eat fish, turtles, birds and mammals — they span T3 to T5.
Food web resilience depends on connectivity:
- High connectivity: Most species have multiple prey species and multiple predators. If one prey species declines, predators can switch to alternatives. The web is resilient.
- Low connectivity: Species depend on only one or two prey species. If that prey declines, the predator has no alternative and its population crashes.
A food web shows multiple overlapping food chains and is more realistic than a single chain. Food web resilience depends on connectivity — the number of feeding links. High connectivity means alternative prey pathways exist, so the web can absorb the loss of one species.
Pause — copy the highlighted food web and connectivity point into your book. The word "connectivity" is the key exam term here.
A food web is more resilient than a single food chain because:
Species that occupy multiple trophic levels complicate models — and stabilise ecosystems
We just saw that food web resilience comes from having multiple feeding pathways. That raises a question: which types of organisms create the most resilience? This card answers it → omnivores are key because they occupy multiple trophic levels, providing flexibility for the whole web.
Omnivores are species that eat both producers and consumers. This places them at multiple trophic levels simultaneously, making trophic assignment impossible with a single label. But omnivory is also a major stabilising force in food webs.
Australian examples of omnivory:
- Barramundi: eat zooplankton and aquatic insects (herbivory) AND smaller fish (carnivory)
- Brush-tailed possum: eats eucalyptus leaves, fruit and flowers AND insects, eggs and small vertebrates
- Many crab species: filter phytoplankton, graze algae AND hunt small invertebrates
Matter flows in two directions through food webs:
- Along food chains: matter moves from producers to consumers through feeding
- Back to the abiotic environment: decomposers break down dead organisms from all trophic levels, releasing mineral nutrients that producers absorb
Omnivores occupy multiple trophic levels simultaneously — they cannot be assigned a single trophic level. They increase food web resilience because they have flexible diets. Matter flows in two pathways: (1) along chains via feeding; (2) back to abiotic environment via decomposers.
Pause — copy the highlighted omnivore definition and the two matter flow pathways into your book.
A barramundi eats both aquatic insects (herbivores) and small fish (carnivores). This means the barramundi is best described as:
Kakadu National Park contains some of Australia's most complex freshwater food webs. A single billabong might contain: water lilies and algae (producers); zooplankton, water beetles and small fish (primary consumers); barramundi, frogs and magpie geese (secondary consumers); herons, turtles and file snakes (tertiary consumers); and saltwater crocodiles (apex predators).
Saltwater crocodiles are not just predators — they are ecosystem engineers. By preying on mid-sized predators, they prevent any one prey population from exploding. If crocodiles were removed, mid-sized predator populations would increase, exerting heavier predation pressure on small fish and waterbirds. The entire community would restructure.
This is precisely why food webs matter: you cannot predict the consequences of removing crocodiles by looking at a single food chain. You need the web — the full set of connections — to see how energy can reroute through alternative pathways.
Using the following species from a coastal wetland, construct a food web by drawing arrows showing who eats whom, then answer the questions.
Species: phytoplankton, zooplankton, aquatic plants, water snails, small fish, silver perch, great egret, freshwater crocodile, detritus, bacteria/fungi.
- Draw the food web in your book, with arrows pointing from prey to predator. Label each organism with its trophic level(s).
- Identify two organisms that occupy multiple trophic levels and explain why.
- If a disease wiped out all the small fish in this wetland, predict what would happen to the silver perch population and explain your reasoning.
Ecosystem A: Grass → grasshopper → frog → snake → hawk. (Single chain only)
Ecosystem B: Grass, shrubs → grasshopper, caterpillar, mouse → frog, lizard, small bird → snake, kookaburra → hawk, dingo. (Web with multiple connections)
- Which ecosystem is more resilient to the removal of the grasshopper? Justify your answer.
- In Ecosystem A, what happens if the frog population is wiped out by disease?
- In Ecosystem B, what happens if the frog population is wiped out? Explain why the outcome differs.
- Which ecosystem better represents a real Australian grassland? Explain.
Which of the following best explains why decomposers are essential to food webs?
A fresh set drawn from this lesson's question bank — feedback shown immediately. +5 XP per correct · +25 XP all correct
Pick your answer, then rate your confidence — that tells the system what to drill next.
ApplyBand 4(4 marks) 1. Construct a food chain for an Australian grassland ecosystem with at least four trophic levels. Label each trophic level and explain the direction of the arrows. Then name one decomposer and one detritivore that would feed on dead organisms in this chain, and explain how they return nutrients to the environment.
AnalyseBand 4–5(5 marks) 2. Explain why a food web is a more useful model than a food chain for predicting the consequences of species removal. In your answer, use the concepts of connectivity, alternative pathways and trophic cascades, and refer to a specific Australian ecosystem example.
EvaluateBand 5–6(6 marks) 3. Using the Kakadu billabong as a case study, evaluate whether the removal of saltwater crocodiles would cause more harm to the food web than the removal of barramundi. In your answer, consider the trophic position of each species, the number of species they interact with, and the concept of keystone effects.
Show all answers
Multiple Choice
MC answers and full explanations are shown inline as you complete each question.
Activity 1 — Construct a Food Web
1. Producers (T1): phytoplankton, aquatic plants. Primary consumers (T2): zooplankton, water snails, small fish (herbivorous). Secondary consumers (T3): silver perch, small fish (carnivorous). Tertiary consumers (T4): great egret. Apex predators (T4/T5): freshwater crocodile. Decomposers: bacteria/fungi feeding on detritus from all levels.
2. Small fish occupy T2 (when eating phytoplankton) and T3 (when eating zooplankton). Silver perch occupy T3 (eating herbivorous fish) and potentially T4 (eating carnivorous small fish).
3. Silver perch would decline because small fish are a major food source. However, if silver perch can switch to eating more aquatic insects or zooplankton directly, the decline might be partial rather than catastrophic — illustrating food web resilience through alternative pathways.
Activity 2 — Resilience Analysis
(a) Ecosystem B is more resilient. Multiple herbivores mean removal of grasshoppers does not eliminate all prey for frogs, lizards and small birds.
(b) In Ecosystem A, if frogs are wiped out, snakes have no alternative prey and would starve. The hawk would also lose its food source. The entire chain above frogs collapses.
(c) In Ecosystem B, snakes can still eat lizards and small birds. Multiple alternative pathways maintain energy flow — the outcome differs because Ecosystem B has connectivity.
(d) Ecosystem B better represents a real Australian grassland because real ecosystems always have multiple species at each trophic level, omnivores, scavengers, and decomposers.
Short Answer Model Answers
Q1 (4 marks): Example chain: grasses (T1) → kangaroo (T2) → dingo (T3) → wedge-tailed eagle (T4) [1 mark]. Arrows point from eaten to eater — energy flows from grass to kangaroo when eaten [0.5 marks]. Decomposer: fungus — secretes enzymes, absorbs nutrients, releases mineral ions [1 mark]. Detritivore: dung beetle — fragments material, increases surface area for fungi [0.5 marks]. Both return nitrogen and phosphorus to the soil as inorganic ions [1 mark].
Q2 (5 marks): Food chains are inadequate because removing any species collapses the entire pathway [1 mark]. Connectivity = number of feeding links each species has — high connectivity means multiple prey and predators [1 mark]. Alternative pathways buffer the loss of any single species [1 mark]. Trophic cascades: removing dingoes allows kangaroos to increase, which overgrazes vegetation, reducing ground-nesting bird habitat [1 mark]. Australian example: Kakadu billabong — removing crocodiles restructures the entire community through indirect effects; single chain cannot predict this [1 mark].
Q3 (6 marks): Crocodiles are apex predators (T4/T5) interacting with fish, turtles, waterbirds and mammals [1 mark]. Barramundi are secondary/tertiary consumers (T3/T4) interacting with insects, small fish and crustaceans [1 mark]. Crocodiles interact with more species across more trophic levels [1 mark]. Keystone concept: species with a disproportionately large impact relative to biomass — crocodiles fit this definition [1 mark]. Removing crocodiles triggers top-down trophic cascades: mesopredators increase, exerting heavier pressure on their prey [1 mark]. Evaluated conclusion: removing crocodiles causes more harm — they are keystone apex predators whose loss triggers multi-level cascades; barramundi's role is more substitutable [1 mark].
Five timed questions on food chains, food webs and trophic levels. Beat the boss to bank a tier.
Enter the arenaRobert Paine's 1966 Mukkaw Bay experiment showed that removing a single sea star from one 3-metre strip caused 15 species to collapse to 1 within two years — while an identical 8-square-metre control strip maintained all 15 species. This outcome is only predictable from a food web model, not a food chain. The sea star controlled mussel abundance by direct predation; without it, mussels outcompeted all other sessile species for rock space. Every species connected to the sea star through the web — including species that never touched a sea star directly — felt the effect.
Return to your Think First response. Could you now trace Paine's cascade: sea star removed → mussels increase → [which species lost space?] → [what happened to the web]? Write the rule for arrow direction in food chains from memory.