Biology • Year 11 • Module 4 • Lesson 3
Food Chains and Food Webs
Build HSC Band 5–6 extended-response technique on food-web structure, resilience and the consequences of species removal.
1. Extended response — explain why food webs are more useful models than food chains (Band 4–5)
7 marks Band 4–5
Q1. Explain why a food web is a more useful ecological model than a food chain for predicting the consequences of species removal. In your response you must:
- Define both terms and describe what each model shows.
- Use the concepts of connectivity and alternative pathways.
- Explain the term trophic cascade and give one named Australian example.
- Use a specific species removal scenario to illustrate the difference between what a chain and a web would predict.
2. Stimulus-based extended response — dingo removal from Australian grasslands (Band 5–6)
8 marks Band 5–6
Stimulus. Since European colonisation, dingoes have been systematically removed from large areas of south-eastern Australia using baiting programs and the 5,600 km “dingo fence.” Dingoes are the apex predator in many Australian grassland ecosystems; they prey on kangaroos, wallabies, wombats and rabbits. Studies comparing areas with and without dingoes have shown that in dingo-free zones: kangaroo populations increased significantly; overgrazing of native grasses increased; populations of small native mammals declined; and introduced foxes and cats (which dingoes normally suppress through interference competition) increased, further driving declines in small native mammals.
Q2. Analyse and evaluate, using lesson content, why the removal of dingoes had such widespread consequences in Australian grassland food webs. In your answer:
- Describe the trophic position of dingoes in a grassland food web and the number of species they interact with.
- Explain the sequence of changes that followed dingo removal, using the concept of trophic cascade.
- Explain why a single food chain (e.g. grasses → kangaroo → dingo) fails to predict all the consequences described in the stimulus.
- Evaluate whether dingoes are best described as an apex predator, a keystone species, or both. Justify your answer using evidence from the stimulus.
- Assess the implications of this case study for managing introduced apex predators in Australian ecosystems.
3. Evaluate this claim (Band 5–6)
6 marks Band 5–6
“A food web is just a more complicated version of a food chain — it contains the same information as a food chain but with extra arrows that are not really necessary. A food chain is a simpler and therefore better model for ecologists to use when predicting what happens when a species is removed from an ecosystem.”
Q3. Evaluate this claim. Identify which parts are defensible, which are incorrect, and reformulate the claim into a biologically accurate statement using lesson content on food-web structure, connectivity and resilience.
Q1 — Sample Band 5 response (7 marks), annotated
A food chain is a linear sequence showing a single pathway of energy and matter transfer from producer through consumers to apex predator. A food web is a network of interconnected food chains showing all feeding relationships in an ecosystem, including organisms that eat multiple types of prey. [1 — both terms defined]
A food web shows connectivity — the number of feeding links each species has to others. High connectivity means species have multiple prey options and multiple predators. Alternative pathways are additional routes for energy flow: if one prey species declines, predators can switch to another. A food chain shows none of this — every species depends on exactly one prey and one predator. [1 — connectivity and alternative pathways defined with contrast]
A trophic cascade occurs when the removal of one species ripples through multiple trophic levels via changes in predation pressure. For example, removing saltwater crocodiles from a Kakadu billabong (an apex predator, T5) would release barramundi and large fish (T3/T4) from predation pressure, allowing them to increase. This increased predation on small fish (T2) would reduce small fish populations, which would in turn reduce zooplankton predation and increase phytoplankton. The effects cascade across four trophic levels. [1 — trophic cascade defined; 1 — named Australian example with mechanism]
Consider predicting the consequence of removing frogs from this billabong. A food chain (phytoplankton → small fish → barramundi → crocodile) does not include frogs at all, so it predicts no impact. A food web reveals that frogs are also eaten by file snakes and crocodiles; without frogs, file snake populations would decline and crocodiles would need to shift more predation onto barramundi and other prey. The web reveals impacts a chain conceals. [1 — specific scenario contrasting chain and web prediction]
Food webs are therefore more useful than food chains for predicting species-removal consequences because they display the full network of interactions. Food chains are useful for teaching basic principles but dangerous as predictive models in real ecosystems. [1 — conclusion linking usefulness to prediction accuracy]
Marking criteria.
- 1 mark — Defines both food chain (linear) and food web (interconnected network) correctly.
- 1 mark — Defines connectivity and alternative pathways, and contrasts their presence in a web vs absence in a chain.
- 1 mark — Defines trophic cascade correctly (removal of one species creates a chain of effects across trophic levels).
- 1 mark — Gives a named Australian ecosystem example with the cascade sequence (e.g. crocodile removal, dingo removal, reef shark removal).
- 1 mark — Uses a specific species removal scenario to show that a chain fails to predict all consequences but a web does.
- 1 mark — Reaches a justified conclusion that food webs are more useful due to showing full network structure.
- 1 mark (quality mark) — Response uses precise lesson terminology throughout (connectivity, alternative pathways, trophic cascade, resilience) and reasoning flows logically.
Q2 — Sample Band 6 response (8 marks), annotated
Dingoes are apex predators in Australian grassland food webs, occupying T3–T4. They interact with multiple species: kangaroos, wallabies, wombats, rabbits (prey directly) and indirectly with foxes, cats and the vegetation layer (through prey-mediated effects). This multi-species interaction pattern is precisely what makes their removal have widespread consequences. [1 — trophic position and multi-species interactions identified]
The trophic cascade following dingo removal proceeded in sequence: (i) kangaroos and wallabies increased because their apex predator was removed; (ii) increased kangaroo numbers intensified overgrazing, degrading native grass communities; (iii) with dingoes absent, foxes and cats were no longer suppressed by interference competition, so they increased; (iv) increased fox and cat predation drove declines in small native mammals. The cascade therefore operated both top-down (through direct predation relief) and laterally (through mesopredator release). [1 — trophic cascade sequence described correctly; 1 — mesopredator release identified as a mechanism]
The single food chain grasses → kangaroo → dingo fails to predict most of these consequences. It only shows the direct dingo–kangaroo link and the indirect grass release. It cannot predict fox or cat increases (because those species are not in the chain), small mammal declines (same reason), or the lateral effect of dingoes on competing mesopredators. A food web showing dingoes linked to rabbits, wallabies, foxes, cats, small mammals and the vegetation layer would reveal all these connections. [1 — specific limitations of the single chain identified]
Dingoes qualify as both an apex predator (no natural predators; T3–T4 position) and a keystone species. A keystone species has a disproportionately large effect on its ecosystem relative to its own biomass or abundance. The stimulus shows that dingo removal restructured the entire community — grasses, herbivores, mesopredators and small mammals all changed significantly. This is far greater than expected from removing a species of moderate abundance, matching the definition of a keystone species exactly. [1 — keystone concept applied correctly; 1 — evidence from stimulus used to justify evaluation]
This case study has clear management implications: reintroducing or protecting apex predators in Australian ecosystems can suppress mesopredators (foxes, cats), reduce overgrazing and protect small native mammals simultaneously. Single-species management (baiting one prey or predator in isolation) is unlikely to succeed because trophic cascades mean changes at one level affect species several levels away. Managers must use a food-web perspective, not a food-chain perspective. [1 — management implication with food-web reasoning]
Marking criteria.
- 1 mark — Correctly states dingo trophic position (T3–T4, apex predator) and identifies multiple species they interact with.
- 1 mark — Explains the cascade sequence: dingo removal → kangaroo/herbivore increase → overgrazing → grass decline.
- 1 mark — Identifies mesopredator release: dingo removal → fox and cat increase → small mammal decline.
- 1 mark — Explains why a single food chain cannot predict all these consequences (missing species, no lateral links shown).
- 1 mark — Defines keystone species correctly (disproportionate ecosystem impact relative to abundance/biomass).
- 1 mark — Uses evidence from the stimulus to justify that dingoes are a keystone species (community-wide restructuring from loss of one species).
- 1 mark — Draws a valid management implication using food-web reasoning (e.g. apex predator protection reduces mesopredator impact; whole-web approach needed).
- 1 mark (quality mark) — Response is logically structured, uses precise lesson terminology and reaches an integrated evaluative conclusion.
Q3 — Sample Band 6 response (6 marks)
The claim is partly defensible but mostly incorrect. [1 — overall evaluative judgement]
What is defensible: A food chain is indeed simpler than a food web — it has fewer arrows and is easier to draw and read. Simplicity is a genuine advantage for introducing basic trophic concepts to beginners or for initial hypothesis formation. [1 — correctly identifies the defensible element]
What is incorrect — “contains the same information”: A food web contains substantially more information than a food chain. A food web shows which species have multiple prey and multiple predators, the existence of omnivores, the degree of connectivity, and the alternative pathways that determine food web resilience. None of this is visible in a food chain. The extra arrows are not superfluous — each represents a real feeding relationship that could become critical if another pathway is disrupted. [1 — refutes “same information” with reasoning]
What is incorrect — “better model for predicting species removal”: This is the opposite of the truth. A food chain predicts that removing any species collapses the entire chain above it, which is almost never correct in real ecosystems because alternative prey exist. A food web shows the alternative pathways that buffer against species loss, giving a far more accurate prediction of what actually happens. The lesson’s Activity 2 (Ecosystem A vs B) directly demonstrated that single-chain ecosystems are fragile and unrealistic while web ecosystems are resilient. [1 — refutes “better model for prediction” with reasoning]
Biologically accurate reformulation: “A food chain is a useful simplified model for introducing the concept of trophic levels and energy flow, but it is an inadequate model for predicting the consequences of species removal. A food web provides far more accurate predictions because it shows the full network of feeding relationships, including connectivity, alternative pathways and omnivory, all of which determine how resilient an ecosystem is to species loss.” [1 — defensible reformulation using lesson terminology]
Marking criteria.
- 1 mark — States an overall evaluative judgement (e.g. “partly defensible but mostly incorrect”).
- 1 mark — Correctly identifies the defensible element (simplicity as a teaching tool).
- 1 mark — Correctly refutes “contains the same information” by specifying what a food web shows that a chain does not (connectivity, omnivory, alternative pathways, resilience structure).
- 1 mark — Correctly refutes “better model for predicting species removal” by explaining how food chains overpredict collapse while food webs can model resilience.
- 1 mark — Reformulates the claim into a biologically accurate alternative statement that preserves the grain of truth (chains are simpler) while correcting the errors.
- 1 mark — Uses precise lesson terminology throughout (connectivity, alternative pathways, resilience, trophic cascade or omnivory) in the evaluation and reformulation.