Year 11 Biology Module 4 · IQ1 Lesson 3 of 18 ~35 min

Food Chains and Food Webs — Modelling Energy and Matter Flow

A food chain is a simplification — a single thread through a complex fabric. Real ecosystems are woven from hundreds of overlapping food chains that form food webs. Understanding how to construct, read and interpret these models is one of the most practical skills in ecology.

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Trophic Pyramid An ecological pyramid showing producers at the base and apex predators at the top, with energy loss at each level. PRODUCERS (T1) 100% energy input PRIMARY CONSUMERS (T2) ~10% energy transfer SECONDARY CONSUMERS (T3) ~1% energy transfer TERTIARY CONSUMERS (T4) ~0.1% energy transfer APEX PREDATORS (T5) ~90% energy lost as heat at each trophic level Energy decreases by ~90% at each trophic level. Biomass and numbers typically follow the same pattern.
Think First

What Would Happen in Kakadu If Saltwater Crocodiles Were Removed?

In Kakadu National Park, saltwater crocodiles are apex predators. They eat fish, birds, turtles and wallabies that come to the water's edge. But they do not eat everything — barramundi, magpie geese and many smaller species are also present.

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?

✏️ Write your answers in your book before reading on.

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
Key Terms — scan these before reading
Food chainA linear sequence showing the transfer of energy and matter from one organism to another.
Food webA network of interconnected food chains showing multiple feeding relationships in an ecosystem.
Trophic levelThe feeding position of an organism in a food chain (producer = T1, primary consumer = T2, etc.).
Apex predatorA predator at the top of a food chain with no natural predators of its own.
OmnivoreAn organism that eats both producers and consumers, occupying multiple trophic levels.
ResilienceThe ability of a food web to resist disruption when one species is removed or reduced.
Core Content
1

Food Chains — Linear Models of Energy Transfer

The simplest way to model who eats whom — but real ecosystems are never this simple

A food chain is a linear sequence that shows how energy and matter move from one organism to the next through feeding. It is a deliberate simplification — a single thread through the complex fabric of an ecosystem. Food chains are useful for teaching basic principles, but they are dangerous if taken as accurate descriptions of real ecosystems.

Food chains and food webs showing trophic relationships

Food chains and food webs showing trophic relationships

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. Always check: does the arrow show who gets eaten by whom? Grass is eaten by the grasshopper, so the arrow points from grass to grasshopper.

Each position in the chain is a trophic level:

  • T1 (Producer): The autotroph that captures energy — grass, algae, phytoplankton
  • T2 (Primary consumer): The herbivore that eats producers — grasshopper, zooplankton, kangaroo
  • T3 (Secondary consumer): The carnivore that eats herbivores — frog, small predatory fish
  • T4 (Tertiary consumer): The carnivore that eats secondary consumers — snake, larger fish
  • T5 (Apex predator): The top predator with no natural predators — hawk, saltwater crocodile, great white shark

Decomposers (bacteria, fungi) and detritivores (earthworms, woodlice) feed on dead organisms from all trophic levels. They are often shown as returning nutrients to the soil, completing the cycle.

Common Error Students draw arrows pointing from predator to prey (hawk → snake). This is always wrong. The arrow shows energy flow: from the organism that is eaten TO the organism that eats it. A quick check: "Can the hawk eat the snake?" Yes. So energy flows snake → hawk.
2

Food Webs — When Multiple Chains Overlap

Real ecosystems are networks, not lines — and that network structure creates stability

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. Food webs are far more realistic models than food chains, and they reveal properties that single chains hide.

Consider a simple Australian wetland food web:

  • Producers: aquatic plants, algae, phytoplankton
  • Primary consumers: zooplankton, aquatic insects, small fish, water snails
  • Secondary consumers: larger fish (barramundi, silver perch), frogs, waterbirds (ducks, grebes)
  • Tertiary consumers: larger predatory fish, 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. This multi-level occupancy is impossible to represent in a single food chain.

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. The web is fragile.
HSC Tip When asked to explain why food webs are more useful than food chains, use the word "connectivity." A food web shows multiple feeding relationships, which allows you to predict what happens when one species is removed. A food chain cannot show alternative prey or predators.
3

Omnivores, Matter Flow and Food Web Resilience

Species that occupy multiple trophic levels complicate models — and stabilise ecosystems

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 not just a taxonomic inconvenience — it is a major stabilising force in food webs.

Australian examples of omnivory:

  • Barramundi: eat zooplankton and aquatic insects (herbivory / primary consumption) AND smaller fish (carnivory / secondary consumption)
  • Brush-tailed possum: eats eucalyptus leaves, fruit and flowers (herbivory) AND insects, eggs and small vertebrates (carnivory)
  • Many crab species: filter phytoplankton, graze algae, scavenge dead matter AND hunt small invertebrates

Omnivores increase food web resilience because they provide "alternative pathways" for energy flow. If one prey species crashes, the omnivore switches to another. This buffers the ecosystem against fluctuations in any single species.

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

This dual flow is critical: without the return pathway via decomposers, matter would accumulate in dead biomass and producers would starve.

Real-World Anchor — Kakadu National Park Billabong

What Would Happen in Kakadu If Saltwater Crocodiles Were Removed?

Kakadu National Park in the Northern Territory 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 (large fish, turtles), 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. Some prey species would decline; others might increase if their competitors were reduced. 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.

Priority Misconceptions — Food Chains and Food Webs

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"Arrows in a food chain point from predator to prey." — Arrows always point from the organism that is eaten TO the organism that eats it — the direction of energy flow. A quick check: "Can the hawk eat the snake?" Yes. Arrow: snake → hawk.

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"Removing one species from a food web only affects the species directly above and below it." — Trophic cascades can affect species multiple levels away. Removing an apex predator can reshape the entire community through indirect effects.

Image Slot 1: 7-species Australian wetland food web diagram. Nodes: algae/aquatic plants (T1, green), aquatic invertebrates (T2, blue), small fish/tadpoles (T2/T3, light blue), silver perch (T3, orange), great egret (T4, red), freshwater crocodile (T4, dark red), bacteria/fungi (decomposers, grey). Arrows from prey to predator. Decomposer nodes receive arrows from all levels.

Image Slot 2: Comparison diagram — single food chain (linear, 4 organisms) vs food web (network, 8+ organisms with multiple connections). Labels showing: "Food chain: one pathway" vs "Food web: multiple pathways = resilience". Highlight what happens when one species is removed in each model.

Copy Into Your Books

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Food Chain Notation

  • Arrows point from eaten TO eater (energy flow direction)
  • Producer → primary consumer → secondary consumer → tertiary consumer → apex predator
  • Decomposers feed on all trophic levels

Food Web vs Food Chain

  • Food chain: single linear pathway
  • Food web: multiple interconnected chains
  • Food webs show resilience through alternative pathways

Trophic Levels

  • T1: producers (photoautotrophs)
  • T2: primary consumers (herbivores)
  • T3: secondary consumers (carnivores)
  • T4: tertiary consumers
  • T5: apex predators

Resilience and Omnivory

  • Multiple prey species = resilience
  • Omnivores occupy multiple trophic levels
  • Matter flows along chains AND back via decomposers
Activities
Sort + Classify — Activity 1

Construct and Analyse a Food Web

Using the following species from a coastal wetland, construct a food web by drawing arrows showing who eats whom. Then answer the questions below.

Species: phytoplankton, zooplankton, aquatic plants, water snails, small fish, silver perch, great egret, freshwater crocodile, detritus (dead organic matter), bacteria/fungi.

1 Draw the food web in your book, with arrows pointing from prey to predator. Label each organism with its trophic level(s).

✏️ Draw the food web in your book.

2 Identify two organisms that occupy multiple trophic levels and explain why.

✏️ Answer in your book.

3 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.

✏️ Answer in your book.
Analyse + Connect — Activity 2

Food Web Resilience Analysis

Two ecosystems have the following food web structures:

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)

✏️ Answer all parts in your book.
Multiple Choice
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Test Your Understanding

UnderstandBand 3

1. In a food chain, what does the arrow represent?

A
The direction a predator moves when hunting prey
B
The direction of energy and matter flow — from the organism that is eaten to the organism that eats it
C
The sequence of evolution from simple to complex organisms
D
The migration path of animals between habitats
UnderstandBand 3

2. Which feature most clearly distinguishes a food web from a food chain?

A
A food web includes only producers and consumers; a food chain also includes decomposers
B
A food web is shorter than a food chain
C
A food web shows multiple interconnected feeding relationships, while a food chain shows only a single linear pathway
D
A food web exists only in aquatic ecosystems; food chains exist only on land
AnalyseBand 4

3. A barramundi in a northern Australian river eats both aquatic insects and smaller fish. Which statement best describes its trophic position?

A
The barramundi occupies a single trophic level because each individual fish only eats one type of prey
B
The barramundi is a producer because it converts insect protein into fish biomass
C
The barramundi is always a tertiary consumer because it is a large predatory fish
D
The barramundi occupies multiple trophic levels — it is a secondary consumer when eating herbivorous insects and a tertiary consumer when eating carnivorous small fish
UnderstandBand 3

4. Why are food webs generally more resilient to species removal than single food chains?

A
Food webs contain alternative pathways for energy flow, so predators can switch to different prey if one species declines
B
Food webs have fewer trophic levels, so less energy is lost between producers and apex predators
C
Food webs only include resilient species, while food chains include fragile species
D
Food webs are protected by law, while food chains are not
EvaluateBand 5

5. A student constructs a food chain for a coral reef: zooxanthellae → coral polyp → parrotfish → reef shark. They then claim this chain accurately represents the reef ecosystem. Evaluate this claim using your knowledge of food webs, omnivory and the role of decomposers.

A
The claim is correct — a single chain is sufficient because every reef organism ultimately depends on this pathway
B
The claim is partially correct but the chain should include decomposers at every trophic level instead of just at the end
C
The claim is incorrect: the chain omits the zooxanthellae-coral symbiosis mechanism, ignores that parrotfish also eat algae directly (not just coral), omits dozens of other species (crustaceans, molluscs, other fish), and fails to show alternative pathways that give the reef resilience. A food web is required for an accurate model
D
The claim is incorrect because reef sharks do not eat parrotfish — they only eat larger predatory fish
Short Answer
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Short Answer Questions

ApplyBand 4

6. 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. 4 MARKS

✏️ Answer in your book.
AnalyseBand 4–5

7. 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. 5 MARKS

✏️ Answer in your book.
EvaluateBand 5–6

8. 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 (disproportionate impact relative to abundance). 6 MARKS

✏️ Answer in your book.

Revisit Your Thinking

Return to your Think First responses at the start of this lesson.

Comprehensive Answers

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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). This is because they are opportunistic feeders that consume prey from multiple trophic levels.

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. This illustrates food web resilience through alternative pathways.

Activity 2 — Resilience Analysis

(a) Ecosystem B is more resilient. It has multiple herbivores (grasshopper, caterpillar, mouse) and multiple predators at each level, so the 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 or leave. The hawk would also lose its food source. The entire chain above frogs collapses.

(c) In Ecosystem B, if frogs are wiped out, snakes can still eat lizards and small birds. Kookaburras can eat lizards and mice. The hawk can eat snakes and kookaburras. Multiple alternative pathways maintain energy flow. The outcome differs because Ecosystem B has connectivity — alternative prey species that buffer the loss of frogs.

(d) Ecosystem B better represents a real Australian grassland because real ecosystems always have multiple species at each trophic level, omnivores, scavengers, and decomposers. No natural ecosystem consists of a single linear chain.

Multiple Choice

1. B — Arrows show energy flow from eaten to eater. Options A, C and D are biologically meaningless in this context.

2. C — Food webs show multiple relationships; chains show one. Option A is wrong — both can include decomposers. Option B is wrong — webs are typically more complex, not shorter. Option D is wrong — both exist in all ecosystems.

3. D — Barramundi are omnivores occupying multiple levels. Option A is wrong — individual fish eat multiple prey types. Option B is wrong — barramundi are not producers. Option C is wrong — trophic level depends on prey, not size.

4. A — Alternative pathways provide resilience. Options B, C and D are incorrect or nonsensical.

5. C — The chain is oversimplified, omits key mechanisms and species, and cannot show resilience. Option A is wrong — chains are insufficient. Option B is wrong — decomposers are not the main issue. Option D is wrong — reef sharks do eat parrotfish.

Short Answer Model Answers

Q6 (4 marks): Example chain: grasses (T1, producer) → kangaroo (T2, primary consumer/herbivore) → dingo (T3, secondary consumer/carnivore) → wedge-tailed eagle (T4, tertiary consumer) [1 mark]. Arrows point from eaten to eater — from grass to kangaroo (energy flows from grass to kangaroo when eaten), from kangaroo to dingo [0.5 marks]. Decomposer: fungus (e.g. mushroom) breaking down dead kangaroo — secretes enzymes, absorbs nutrients, releases mineral ions [1 mark]. Detritivore: dung beetle ingesting kangaroo droppings — fragments material, increases surface area for fungi [0.5 marks]. Both return nitrogen and phosphorus to the soil as inorganic ions that grasses absorb [1 mark]. Total: 4 marks.

Q7 (5 marks): Food chains are inadequate because they show only one linear pathway — if any species in the chain is removed, the entire pathway collapses and the model cannot predict what happens [1 mark]. Connectivity refers to the number of feeding links each species has — high connectivity means species have multiple prey and multiple predators [1 mark]. Alternative pathways are additional routes for energy flow — if one prey species declines, predators can switch to another prey, preventing population crashes [1 mark]. Trophic cascades occur when the removal of one species triggers effects that ripple through multiple trophic levels — for example, removing dingoes from Australian grasslands allows kangaroo populations to increase, which overgrazes vegetation and reduces habitat for ground-nesting birds [1 mark]. Australian example: the Great Barrier Reef food web has hundreds of interconnected species. A single chain (phytoplankton → zooplankton → small fish → barramundi → shark) cannot predict what happens when crown-of-thorns starfish outbreaks reduce coral cover, because the web shows alternative energy pathways through algae, sponges and other invertebrates [1 mark]. Total: 5 marks.

Q8 (6 marks): Crocodiles are apex predators occupying T4/T5. They interact with many species: fish (barramundi, catfish), turtles, waterbirds (magpie geese, herons), and mammals (wallabies) [1 mark]. Barramundi are secondary/tertiary consumers (T3/T4) interacting with fewer species: insects, small fish, crustaceans [1 mark]. Crocodiles interact with more species across more trophic levels, so their removal would affect a larger portion of the web [1 mark]. The keystone concept states that some species have a disproportionately large impact on ecosystem structure relative to their biomass or abundance [1 mark]. Crocodiles fit this definition: they are relatively rare (low biomass) but their predation on mid-sized predators prevents any one prey population from exploding. This is a top-down trophic cascade effect [1 mark]. Evaluated conclusion: removing crocodiles would cause more harm than removing barramundi because crocodiles are keystone apex predators whose loss would trigger multi-level trophic cascades. Barramundi are important but their ecological role is more substitutable — other predatory fish could partially compensate for their loss [1 mark]. Total: 6 marks.

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