HSC Exam Practice

Sample response. Arrows point from the organism that is eaten TO the organism that eats it — they show the direction of energy and matter flow through feeding, not the direction of movement or attack.

Marking notes. 1 mark for “from prey to predator” or “from eaten to eater”; 1 mark for correctly identifying what the arrow represents (energy/matter flow direction). Award 0 if student states arrows show predator movement toward prey.

Sample response. A food chain shows a single linear pathway of energy transfer from producer through successive consumers to an apex predator. A food web shows multiple interconnected pathways formed by connecting all food chains in an ecosystem into a network. Food chains cannot represent omnivores accurately because an omnivore would need to occupy multiple positions simultaneously; a food web can show omnivores by connecting them with arrows to prey at different trophic levels.

Marking notes. 1 mark for distinguishing linear (chain) vs network/interconnected (web); 1 mark for commenting on the number of pathways (one vs multiple); 1 mark for correctly explaining the contrast in ability to represent omnivores (chain: cannot; web: can, by showing multiple prey connections).

Sample response. (a) Eucalyptus tree: T1, producer — it is an autotroph that captures solar energy via photosynthesis. (b) Koala: T2, primary consumer — it eats the producer (eucalyptus leaves) directly. (c) Wedge-tailed eagle (eating rabbits that eat grasses): T3, secondary consumer — grasses (T1) → rabbits (T2) → eagle (T3).

Marking notes. 1 mark per organism with correct trophic level AND role name. A correct level without the name or vice versa scores 0.5 (accept for 1 mark if meaning is clear). Eagles can also be T4 if rabbits are stated to be T3 prey — accept any chain construction that is internally consistent.

Sample response. Food webs have higher connectivity than single chains — each species is linked to multiple prey and multiple predators. When one prey species is removed, a predator in a food web can switch to an alternative prey species, so its population is maintained. In a single food chain, every species depends on exactly one prey; if that prey is removed, the predator starves and populations collapse up the entire chain. Connectivity provides alternative pathways for energy flow, which is why food webs are resilient and single chains are fragile.

Marking notes. 1 mark for defining connectivity (number of feeding links per species); 1 mark for explaining that high connectivity provides alternative prey (alternative pathways), allowing predators to switch; 1 mark for contrasting with a single chain where no alternative pathways exist, causing cascading collapse.

Sample response. Decomposers (fungi, bacteria) break down dead organic matter from all trophic levels by secreting enzymes and absorbing the products, releasing mineral ions (nitrogen, phosphorus) back into the soil or water. Detritivores (earthworms, woodlice) physically ingest and fragment dead organic matter, increasing surface area for decomposer action. If both groups were removed, dead organic matter would accumulate, mineral nutrients would not be returned to the abiotic environment, and producers at T1 would be unable to absorb sufficient nutrients to grow — energy and matter flow in the entire ecosystem would eventually stop.

Marking notes. 1 mark for correctly describing decomposer role (enzymatic breakdown → mineral nutrient release); 1 mark for correctly describing detritivore role (physical fragmentation, increasing surface area for decomposers); 1 mark for explaining the consequence of removal (nutrient accumulation in dead matter, nutrient starvation of producers, collapse of energy/matter cycling). Accept alternative valid consequences.

Sample response (a) — 2 marks. During the outbreak, the coral polyp population showed the largest decline of the four species studied (approximately −27%). Post-outbreak, the decline continued but at a reduced rate (approximately −8%), indicating partial but incomplete recovery. The coral polyp population remained below its pre-outbreak baseline even in the post-outbreak period.

Marking notes (a). 1 mark for correctly identifying that coral polyps declined during the outbreak (quoting a figure or using “largest/greatest decline”); 1 mark for describing the post-outbreak period correctly (still declining or incomplete recovery, with a figure or relative comparison).

Sample response (b) — 3 marks. During the outbreak, the starfish consumed large amounts of live coral polyps, drastically reducing the amount of coral-dwelling habitat and the coral polyps that small reef fish depend on for food and shelter. This reduced food availability for small reef fish, so small reef fish populations fell, which reduced the prey available for parrotfish that switched to eating them. Additionally, crown-of-thorns starfish cleared coral-covered surfaces, allowing crustose algae to colonise newly bare reef surface. Parrotfish eat crustose algae, so post-outbreak, parrotfish had more food available, allowing their population to recover and even increase above pre-outbreak levels. This recovery illustrates alternative pathway use: parrotfish switched prey from coral-associated food sources to the newly abundant crustose algae.

Marking notes (b). 1 mark for correctly linking the outbreak to coral polyp and small reef fish decline via the food web chain; 1 mark for correctly identifying that crustose algae increased on bare reef surfaces following coral decline; 1 mark for linking crustose algae increase to parrotfish recovery (parrotfish eat crustose algae = alternative food source became more available).

Sample response (c) — 2 marks. The single food chain (zooxanthellae → coral polyps → small reef fish → reef shark) does not include parrotfish at all. Because parrotfish are absent from the chain, the model cannot predict any change in parrotfish population under any circumstance. It also does not show crustose algae as a food source, so it cannot represent the switch in parrotfish diet that drove post-outbreak recovery. A food web, by contrast, would show both the parrotfish–crustose algae link and the parrotfish–reef food links, making the recovery pattern predictable.

Marking notes (c). 1 mark for identifying that parrotfish are simply absent from the chain so it cannot make predictions about them; 1 mark for explaining what the chain lacks (the crustose algae–parrotfish link) that prevents it from predicting the dietary switch and recovery pattern.

Sample Band 6 response. A food chain is a linear sequence showing energy and matter transfer from a producer through successive consumers to an apex predator, with arrows pointing from eaten to eater. A food web is a network of interconnected food chains showing all feeding relationships in an ecosystem, including multiple prey and predator connections for each species. [1 mark — both terms defined]

As models of energy and matter flow, both are useful at different levels of complexity. A food chain accurately conveys the direction of energy flow and the concept of trophic levels in an accessible format, making it a strong introductory tool. However, it cannot represent omnivory — a species eating prey at two different trophic levels would need to occupy two positions simultaneously, which is impossible in a linear sequence. A food web can show omnivores by connecting them with arrows to prey at both T1 and T2 (for example, the brush-tailed possum, which eats eucalyptus leaves (T1→T2) and insects (T2→T3)). [1 mark — omnivory contrast with example]

Food chains are also inadequate for predicting trophic cascades. A trophic cascade occurs when removing one species triggers effects that propagate through multiple trophic levels. In a food chain, this appears as complete collapse above the removed species. In a food web, the presence of alternative pathways can buffer the cascade — predators switch to alternative prey, limiting the extent of collapse. The dingo case study in Australian grasslands illustrates this: a food web (grasses → kangaroos + rabbits + wallabies → dingoes; grasses → rabbits → foxes → dingoes) would predict that removing dingoes triggers multiple cascades — herbivore increase, overgrazing, mesopredator (fox and cat) release and small mammal decline — none of which are visible in a single food chain. [1 mark — trophic cascade defined and applied; 1 mark — named Australian example with cascade sequence]

Matter flows in two directions in real ecosystems: along food chains through feeding, and back to the abiotic environment via decomposers and detritivores who feed on dead organisms from all trophic levels. A food web can show decomposers connected to every level; a food chain can only show them at the end. [1 mark — two-direction matter flow addressed]

Food webs are more accurate models of real ecosystems because they show connectivity, alternative pathways and the multi-level interactions that determine resilience. A high-connectivity food web correctly predicts that removing one species has a buffered, limited impact because alternative pathways compensate. A food chain incorrectly predicts total collapse of all species above the removed one. [1 mark — connectivity and resilience contrast]

However, food webs have limitations too: they are complex, difficult to draw completely, and require extensive field data to construct accurately. They can also obscure the basic trophic level concept beneath too many arrows. [1 mark — limitation of food webs acknowledged]

Conclusion: food chains are appropriate as introductory models for explaining energy flow direction and trophic level structure, or for generating initial hypotheses. Food webs are essential for any accurate ecological modelling involving species removal, trophic cascade prediction or conservation management. The choice of model should be matched to the complexity of the question being asked. [1 mark — justified conclusion identifying appropriate use of each model]

Marking criteria.

• 1 mark — Defines food chain (linear sequence, energy flow direction) and food web (interconnected network) correctly.
• 1 mark — Contrasts ability to represent omnivory (chain: cannot; web: can show multi-prey connections) with a named example.
• 1 mark — Defines trophic cascade correctly (species removal triggers multi-level effects) and explains how a web can model it better than a chain.
• 1 mark — Uses a named Australian ecosystem example with a specific trophic cascade sequence (e.g. dingo removal, crocodile removal, Great Barrier Reef starfish outbreak).
• 1 mark — Addresses matter flow: decomposers/detritivores return nutrients to abiotic environment; food webs represent this better than chains.
• 1 mark — Explains connectivity and alternative pathways as the structural feature that makes food webs more resilient and accurate predictors of species-removal consequences.
• 1 mark — Acknowledges limitations of food webs (complexity, data requirements, clarity) for balance in evaluation.
• 1 mark — Reaches a justified conclusion specifying when each model is appropriate (food chain = introductory/simple; food web = predictive/management).